BIOLOGY OF THE ATLANTIC MACKEREL (SCOMBER SCOMBRUS) … · 2008-06-24 · BIOLOGY OF THE ATLANTIC...

BIOLOGY OF THE ATLANTIC MACKEREL (SCOMBER SCOMBRUS) OFNORTH AMERICA

PART II: MIGRATIONS AND HABITS

By OSCAR ELTON SETTE, Aquatic Biologist

The commercial catch of mackerel, Scomberscombrus Linnaeus 1758, along the Atlantic coast ofNorth America has fluctuated widely (Sette andNeedler 1934) owing to similarly wide changeseither in abundance or in availability of the fish tofishermen. Since such fluctuations vitally affectboth the fishery and the trade in its products, andalso because they confuse the conservation problem,the United States Bureau of Fisheries (now a part ofthe Fish and Wildlife Service) in 1925 undertook aninvestigation of the causes of these fluctuations.The work involved not only studies of the fluctuations, but also of the many phases of life history andhabits which had to be understood to interpret theobservations of the changes in catch.

This report is one of several resulting from themackerel investigations. In it there have beencollected the facts that pertain to habits and migrations, particularly those that are pertinent to theunderstanding of changes in abundance or availability. The first number of this series of reports(Sette 1939) was on the early life history withspecial reference to mortality; others will be on ageand rate of growth and on fluctuations in abundance.

In considering the subjects included in this paperit is necessary to draw on results which are to bereported later. This is particularly true withrespect to the ages of certain size categories ofmackerel. To a limited extent the data have appeared in preliminary reports (Sette 1931,1932,1933,and 1934) but for the most part the technical detailsare to be included in reports now in preparation andas yet unpublished.

ACCOUNT OF INVESTIGATIONS

The major conclusions of this report rest on thesize composition of the mackerel population as

determined from measurements of individual fish inthousands of samples drawn from the commercialcatch at the principal ports of landing. The collection of data began in 1925 after part of that fishingseason had elapsed. Much of the work in that yearwas preliminary in nature and not strictly comparable with subsequent observations. During theensuing 10 years, 1926 to 1935, the program wascarried out consistently so that data are comparableand the present report is confined to this period,except for the inclusion of certain data from tagginginitiated in 1925.

The interviewing of fishermen for catch-date andlocality and the samplil,lg was done by Magnus I.Gregorsen in 1925, R. A. Nesbit in 1926, E. W.Bailey in 1927 and 1928, and by F. E. Firth in subsequent years. In many of the seasons R. A. Goffincontributed many samples from minor ports,principally Woods Hole, Mass., and also assisted intagging experiments at that place. .

Tagging of mackerel was recommended by theNorth American Council on Fishery Investigationsand initiated under the supervision of Wm. C.Schroeder early in the 1925 mackerel fishing season,and after I undertook an investigation of themackerel in all its phases during midseason of thatyear the tagging program was transferred to me.After completing the 1925 tagging season and uponcomparing the size composition of the tagged fishwith that of the samples taken from catches landedby mackerel vessels at Boston, it was obvious thatthe population from which the fish were drawn fortagging differed strikingly from the populationupon which the vessel fishery was based.

This was not particularly surprising inasmuch asthe tagging utilized fish from alongshore traps andpound nets whereas the vessel catch came mostly

251

252 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE

from offshore schools. Since the vessel catch was byfar the major element in the mackerel fishery as a",hole (Sette and Needler 1934: 14) and presumablyconsisted of fish that were representative of themain population whereas the trap and pound-netcatch presumably represented an inshore fringe ofthe main population, it was considered unlikely thattagging returns would be representative of the migrations of the population as a whole, or even of a veryimportant segment of the whole population.

It also became apparent by the end of the firstseason's tagging that the tags were injuring the fish,with unknown effects on their survival and theirmigratory pattern.

For these reasons the emphasis on tagging wasshifted from large-scale releases to small-scale experimental work directed toward improvement of tagsand exploring the possibilities of tagging fish fromthe offshore population. The details of these experiments, in which I was ably assisted by R. A. Goffinin getting and caring for the fish in captivity andby R. A. Nesbit in developing ideas for devising andtesting various tags, are given in appendix B.

The Biological Board of Canada kindly furnishedrecords of mackerel tagged in Canadian waters andrecaptured off the United States coast.

In the meantime the major activity of the investigation, aimed at discovering the causes forfluctuations in the mackerel catch, including theinterviewing of fishermen, the measuring of samplesof their catch and the collecting of catch recordssuitable for abundance indices proceeded regularly.By 1935, partial analysis of these data appeared toafford insight into many phases of mackerel biologyand it was decided to report upon the materialaccumulated up to the end of the 1935 season.

In studying this wealth of material I have had theable assistance of Mildred S. Moses in preparingtabulations and performing computations, the helpful counsel of Henry B. Bigelow, and the use offacilities at the Harvard Biological Laboratories.

In 1937 the study of this subject was interruptedby other duties and could not be resumed until 1947,with facilities at the Stanford University's School ofBiology and ",ith the counsel of Willis H. Rich, atwhose suggestion and encouragement the investigation was originally started in 1925.

SYNOPSIS OF RESULTS

The mackerel is found in the western Atlanticfrom North Carolina to the Straits of Belle Isle and

is sufficiently abundant for commercial fishing fromthe Chesapeake Capes on the south to the MagdalenIslands and the Gaspe Peninsula on the north.During the season of fishing it is most abundant inthe open waters of the inner third or half of thecontinental shelf.

The mackerel appears in April near the southerlyend of its range and by July is found from southernNew England to the Gaspe coast. In September itbegins to disappear from the most northerly regionsand in December it vanishes from all places. Duringthe summer season the smaller and younger sizesare usually found closer to the shore line than theadults.

When mackerel disappear in the fall they go southward and offshore to the zone of warm water whichflanks the outer edge of the continental shelf andduring wintertime occupy this relatively narrowstrip of water running more or less parallel to shore,but some 20 to 100 miles distant from it, from CapeHatteras northward surely to the southern edge ofGeorges Bank and possibly as far as Sable Island.While there they probably occupy middepths and soare seldom seen or caught. In this location theirfood supply probably is uncertain and may dependon local swarms of plankton whose occurrence isirregular.

The pronounced schooling habit of the mackerelis dependent on a special tropism involving vision,and hence schools may disband and reform according to diurnal variations in light. Luminescenceprobably is important in keeping schools togetherat night in the spring and fall. Schooling tends tobe according to sizes, perhaps owing to a connectionbetween size and swimming ability. This in turn isprobably dependent on the ratio of volume to surfacewhich of course increases with size of body.

During spring, summer, and fall, the mackerelstay in the warm surface layer of the ocean becausethey are prevented from descending below thethermocline by the comparatively low temperatureof the underlying waters. Variation in availabilityto fishermen, depending as it does on sighting schoolsat the surface, is therefore probably dependent onthe varying depth of the thermocline. Fishing isbest moderately close to shore where in summer thethermocline lies only 15 to 20 meters (8 to 11 fathoms)deep, and, as a rule, poorer farther offshore wherethe thermocline may be as deep as 40 to 50 meters(22 to 27 fathoms).

Mackerel feed principally on plankton but the

MIGRATIONS AND HABITS OF THE ATLANTIC MACKEREL 253

possibility that the larger individuals may in latesummer subsist mainly on young fishes should beexamined. Feeding is so much better in the summertime than in winter that the fat content ofmackerel increases from a minimum in April to amaximum in August.

Two subdivisions are detectable in the westernAtlantic mackerel population: A southern and anorthern contingent which perform different springmigrations, occupy different areas in the summertime, and withdraw in the fall by different routes.The southern contingent comes from its offshorewinter habitat toward the Virginia, Maryland, andNew Jersey coasts in April, thence migrates northeastward to occupy the western part of the Gulf ofMaine in summer. The northern contingent migrates toward the southern New England coast inMay and thence goes northeastward to occupy theGulf of St. Lawrence in summer. During thespring migration both contingents are joined byadditional members of their kind which move fromoffshore directly toward the coast joining the mainbodies as they pass along on their northeastwardjourney. For a short while in May both contingentsare together in the area off southern New England,otherwise their courses are fairly independent. Inthe fall migration, both contingents approximatelyretrace their spring courses in returning to the winterhabitat; but the northern contingent travels throughmore westerly waters in fall than in the spring,passing through the western part of the Gulf ofMaine, and then disappearing off Cape Cod. Thesouthern contingent, on the other hand, disappears,sometimes north of and sometimes west of NantucketShoals. The disappearance of both contingentsnorth of the areas of their spring appearance may bedue to their descent to deeper levels as the thermocline is lowered or obliterated by autumnal chilling.

DISTRIBUTION

RANGE

The mackerel is found on both sides of the Atlanticin the Northern Hemisphere, extending from theMediterranean Sea to Norway in the eastern Atlanticand from North Carolina to Newfoundland in thewestern Atlantic. Since those of the eastern Atlanticare racially distinct from those on the western side(Garstang 1898), we need not consider them here.

The southernmost record on this side of the At-

Iantic is of two individuals taken in a pound netnear Beaufort, N. C. The northern limit is theStrait of Belle Isle. l Reports of mackerel along thesouth and west coasts of Newfoundland are notuncommon, but occurrence seems not to be consistent enough to support a regular fishery formackerel in Newfoundland. The region habituallyoccupied (in the fishing season) is from the Chesapeake Capes on the south to the Magdalen Islandsand the Gaspe Peninsula in the Gulf of St. Lawrenceon the north; in other words, between the thirtyseventh and forty-ninth parallels of north latitude.

Although the mackerel is distinctly an open-seaspecies, it is rarely found beyond the \\uters overlying the continental shelf; and while mackerel havebeen found at one time or another in the watersoverlying the entire shelf, the greatest concentrationsduring the fishing season appear to be within itsinner third or half. Often mackerel are found veryclose to the shore line, occasionally even inside ofharbors and inner estuaries. Usually it is only thesmall sizes that are found in the semi-enclosed waters,the adults generally keeping to the open water,though they too enter some of the more or less openbays in the spring.

It is difficult, if not impossible, to determinewhether the species is more abundant in the southernhalf of its range (that is, off the coast of the UnitedStates) than it is in the northern half of its range(that is, off the coast of Canada). Of the totalannual catch along the North American coast inrecent years, more than two-thirds have been takenoff the coast of the United States and less than onethird off the coast of Canada; but this does notnecessarily reflect the relative abundance, becausethe principal methods of fishing and also the intensity of fishing differ widely in the two countries.In the United States there is a fishery by pound netsand traps along shore, a minor offshore fishery usingdrift gill nets, and also a much more important offshore fishery using purse seines. In Canada fishingis confined almost entirely to pound nets, traps, andgill nets operated almost exclusively in inshorewaters. It is likely that the international boundary,extended seaward, would divide the mackerel population into parts that are more nearly equal thantotal catch statistics indicate.

1 Hearsay evidence cited by Goode, Collins, Earle, and Clark (1884: 3-4) ofOCcurrence farther north along Labrador has yet to be confirmed by authenticrecords of capture.


SEASONAL CHANGES IN DISTRIBUTION

Along the Atlantic seaboard of North Americathe mackerel is a seasonal visitor, appearing in thespring, remaining during summer and autumn andthen disappearing. Judging from the location ofcatches, mackerel appear first early in April about30 to 40 miles offshore abreast of the coast linebetween Chesapeake and Delaware Capes. Soonthey approach closer to the coast and during Apriland May they are found successively farther upcoast until they reach southern New England. Atthis time or shortly afterward they also appearalong the Nova Scotian cOast. During the ensuing

2 to 4 weeks they disappear from the waters southof Cape Cod and spread throughout the westernportions of the Gulf of Maine and the MaritimeProvinces of Canada up to the Gaspe Peninsula,where they remain until sometime in September,During that month they begin to disappear fromthe most northerly region and withdrawal proceedsfrom north to south during October and Novemberuntil finally in December they disappear from allcoastal waters. These changes in distribution arecharted, by months, in figure 2.

While the above description holds true for mackerel generally, there are differences that should be

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I. BAY OF ISLANDS2. CAPE ANGUILLE3. LAURENTiAN CHANNEL4. ~AGDALEN ISLAND5. STRAITS OF CANSO6 PRINCE EDWARD ISLAND7 CAPE BRETON8. CAPE CANSO9. HALIFAX

10 CAPE SABLEII. BROWNS BANK12. BAY Of FUNDY13. CASCO BAY14. CAP,E ELIZABETH15. CAPE ANN16. BOSTON17. MASSACHUSETTS BAYlB. CAPE COD BAY19.,WOOD END

20, CAPE· COD2 I CHATHAM22. NANTUCKET SOUND23. WOODS HOLE24. BUZZARDS BAY25. NO MANS LAND25.1 CAPE GASPE25.2 SABLE ISLAND

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eo'e5'L.L~L---""':.L-_.L-_.L.,._-'-_-'-_-'-_-'-_-'--_...J.._...J.._~_-'-_-L_-l._-'_-'_--J'----J,----l3,'

75- 70·

FIGURE l.--Geographic features, landmarks, and delineation of statistical areas mentioned in this report. The statistical areas arethose adopted by the North American Council on Fishery Investigations except for the lettered subareas of area XXVIII which wereadopted for the purpose of this report, only, and have no official status. The broken line marks the loo-fathom contour.


FIGURE 2.-Approximate seasonal distribution of the mackerel as indicated by location of the commercial fishery in the various monthsof the fishing season.

noted in the distribution of various size categories:(1) Juveniles, (2) yearlings, and (3) adults.

The juvenile sizes are fish from the current spa\\'ning season, hence less than a year old, and range

from 2 inches up to about 8~ inches in length.Early in summer they are too small to be retainedby the meshes of commercial nets, but toward latesummer and fall, though not sought after, they are

~56 FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE

caught incidentally. To fishermen they are knownas "tacks" and "spikes." Drift-gill-netters nevertake them in quantities, though we have an occasional sample of fish whose teeth caught in the twineof gill nets. Purse seiners usually avoid thembecause they plug the meshes of the seine, sometimes causing loss of gear. Some of the largest sizesof juveniles are, however, caught by purse seinerslate in fall. Pound nets, traps, and weirs are theform of gear taking them most consistently. Theschools of juveniles are deflected by the coarsemeshed leader of these forms of gear and turn offshore into the fine-meshed pound, where the smallerones may be taken by dip net before they slip outthrough the meshes while the pound is hauled; thelarger ones, of course, are retained through thehauling process and regularly form a part of thecommercial catch. It is the catch by this form ofgear that provides most of the information on distribution of juveniles. Due to the selective natureof fishermen's catches of those small sizes, the conclusions must be inferential.

Their distribution early in summer is probablydetermined largely by the location of the grounds onwhich they were spawned and on their subsequentdrift from these grounds (Sette 1939: 83-191). InUnited States waters, they are found most consistently along the shore from Long Island to CapeAnn. The maximum concentrations appear alongthe southern shore of Massachusetts, though somehave been occasionally taken along the coast of NewJersey and the coast of Maine. Doubtless, such assurvive on the spawning grounds of the Gulf of St.Lawrence would be found along the shores of thatGulf, presumably mostly along its easterly portions,but published records of this are lacking.

Although the available records of occurrence arealmost entirely from along the very shore line, thismay be because there is no form of gear employedoffshore which will catch the juveniles. Late fallcatches of these sizes by purse seiners sometimeshave been at a moderate distance from shore (up to20 miles) and it is probable that large bodies of thesesmall mackerel exist offshore as well as inshore.

In the inshore locations, the juvenile mackerelseem to stay all summer, into late fall and evenearly winter, catches of them having been made aslate as December. From their distribution, there islittle indication of any extensive migrations beforetheir disappearance.

Yearling mackerel range from about 20 centi-

meters (8 inches) in the spring to about 35 centimeters (14 inches) in late fall. They are called"blinks" and "tinkers" by fishermen and in thetrade. The term "blinks" is usually applied to thesmaller ones, "tinkers" to the larger ones of thiscategory.

During summer and fall, their distribution parallelsthat of the adults (p. 254) but their appearance inthe spring is usually later than that of the adults.Occasionally schools are taken during the spring runof adults in both pound nets and purse seines (driftgill nets almost never take them at any season); butit is not until July and August that they are takenregularly in large numbers. From that time onwardthey are taken all along the coast from southernMassachusetts to Maine. Although samples ofyearlings have been secured from PassamoquoddyBay and from the vicinity of Halifax (Pennant,Nova Scotia), Dr. Cox found no "small" mackerelat the Magdalen Islands in 1925 (North AmericanCouncil on Fishery Investigations, 1932, p. 27) andsamples taken during this investigation from thecatches of United States mackerel purse seinersfishing off the Nova Scotian coast have never contained yearling mackerel. It is likely that yearlingsare much less abundant, as a rule, off Canada thanoff the United States. Like the juveniles and theadults, the yearlings disappear from coastal watersin late autumn and early winter.

The adult mackerel are known simply as mackerelby the fishermen, sometimes with the qualifyingadjectives "medium" or "large." They are fish of35 centimeters (14 inches) and upward and includeall aged 2 years and older. They are the mostdesirable sizes and usually form the bulk of thecatch. Their distribution corresponds with thegeneral description at the beginning of this section.

WINTER HABITAT

LOCATION

Whence the mackerel come in the spring andwhither they go in the autumn have been subjectsof conjecture for many years. Bigelow and Welsh(1925: 197) surmised that they winter "on the upperpart of the continental slope at a depth rather greaterthan the otter trawlers reach-say at 100 to 200fathoms-but so close at hand that odd fish stray orremain on the banks." The present available datasupport this view as to the general winter location.It suggests, however, somewhat different conclu-


sions as to the depths inhabited in the wintertime.That the late autumn chilling of the water drives

the mackerel from their customary summer hauntsappears so obvious that most investigators haveaccepted this assumption as fact. It is true thatthe months during which the mackerel are absentare the coldest months of the year but there is noexperimental evidence as to the minimum temperature that can be withstood by the species. As faras observational evidence is concerned, mackerelhaTe been found in abundance in temperatures aslow as 8° C. (fig. 14). They are often present inwater of 7° C. in sufficient numbers to make commercial fishing profitable. Below this temperaturethey have been taken only as stragglers in Americanwaters where there is record of one occurrence inwater as cold as 4.5° C.2 Thus it appears that theAmerican mackerel prefers temperatures above 8°c., that it frequently tolerates temperatures down to7° C., and that its toleration may extend to temperatures as low as 4.5° C.

This being true, the winter temperature in thenorthern portion of its range, that is in the Gulf ofSt. Lawrence and along the inner portions of thecontinental shelf of Nova Scotia where ice oftenforms in the wintertime, is certainly too low for thisspecies. The inner parts of the Gulf of Maine withwinter temperatures from 2° to 3° C. must also betoo cold for mackerel. South of Cape Cod in coolwinters when temperatures of 2° to 4° C. usuallyprevail over the inner half of the continental shelf,the mackerel should be normally absent; but thereare instances such as the winter of 1932 (Bigelow1933: 8-27) when water as warm as 7° C. persistedthroughout the winter almost to the shore line andnorth nearly to New York Obviously, temperaturealone cannot explain the absence of mackerel fromthese waters during such exceptionally warm winters,and thus we may not assume their disappearance inthe fall to be a simple direct response to temperature.

Yet it is reasonable to look for their winter habitatwhere temperatures approach those prevailing intheir summer habitat. Waters with such temper-

2 In European waters. large quantities have been taken by trawlers in northernparts of the North Sea, notably Great Fisher Bank, in the wintertime when 6°and 7° C. water prevails on those grounds, and in the English Channel wherethey are also trawled in the wintertime, the temperatures according to Bullen(1908: 284-285) are between 8° and 9° C. However, the European mackereldiffers structurally from the Amorican mackerel, sufficiently to be regarded asracially distinct, so it may be physiologically different as well. Hence, it is wisenot to lay much stress on the evidence provided by the European representativesof the species.

atures flank the North American coast from 30 to100 miles offshore, their inner border lying near theedge of the continental shelf where depths increaserapidly beyond the 100-fathom contour (fig. 3).It is not necessary to look farther than this, formackerel have never been found south of Beaufort,N. C., or far enough beyond the continental shelf toindicate that they wander far out into truly oceanicwaters.

The constancy in location and warmness of thisflanking zone of water as a regular winter home forthe mackerel is of particular significance. Althoughthe temperatures of each profile in figure 3 pertainto only 1 year, it is highly probable that the warmzone has the same position year after year. This iscertainly true along the continental edge betweenCape Cod and Cape Hatteras. This region has beenexamined hydrographically in five different winterswith very little variation except in the unusuallywarm winter of 1932. In that year nearly theentire continental shelf south of the middle of NewJersey was covered with water 7° C. or higher, buteven then the temperatures at the edge of the shelfwere very little different than in cold winters.Hydrographical surveys of the southern edge ofGeorges Bank in the winter have been less frequent,but examination of early spring conditions in 1929,1930, and 1931 reveal no striking variations.3 Itmay be assumed, therefore, that the mackerel canalways find temperatures surely suited to its existenceat one depth or another by moving offshore to aboutthe continental edge along the southern part ofGeorges Bank or to the outer third d the continentalshelf between Cape Cod and Cape Hatteras, andpossibly suited to its existence along the edge of theNova Scotian Banks. It remains to be seen whatdirect evidence there may be of the actual presenceof mackerel in this warmer zone.

The occasional capture of mackerel incidental tothe fishery for other species in the winter has beenreported often in the literature. Goode, Collins,

3 Unfortunately, there are no data on an extremely cold winter when it ispossible that the warm zone may shift to a more offshore position. The disap~

pearance of the tilefish in 1881 (Bigelow and Welsh 1925: 354) has been thoughtto have been caused by such a shift and if this supposition is correct, it mustmean that the warm zone shifts far enough offshore so that it does not come intocontact with the sea bottom at the continental edge. However, just as thetilefish disappearance may be taken as evidence of possible offshore shifting ofthe warm zone in severe winters, it also constitutes evidence that such shifts areextremely rare, for as far as is known this has happened only once during thepast century. Even then it apparently had no effect upon the mackerel whichmay have been wintering on or near the tilefish grounds, for mackerel reappearedin normal numbers during the summer following the tilefish disappearance.

258

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40·

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.45

LL-:::"-~7;t;5:O.----------:7o!;O:O.-----------;6~5:O.----------6;;.let;:;;;---------I350

FIGURE 3.-Winter captures of mackerel (triangles) and winter temperatures. The isotherms mark the greatest inshore extension ofwater of designated temperature; insets A to D represent a width of 80 miles, E to K a width of 35 miles, and aU represent depthsof 400 meters (218 fathoms). Light shadi'lg designates temperature above 6° C. and heavy shading temperatures above 10° C.Sources of temperature data are: Section A, C. G. S. Acadia Stations V, 6-8-9-10, May 30, 1915 (Bjerkan 1919: 384). Sections Band C, Atlantis Stations 2510-11-12-15-16-17, March 6 and 15, 1936 (BuU. Hydrographique Cons. Perm. Internat. pour de l'Explor.de la Mer). Sections D and E, Albatross II Stations C20044-45-46-47--66-67--68--69, March 11-12 and 22-23, 1920 (D. S. Bureauof Fisheries Rept. 1921: 154, 160). Section F to K, Albatross II Stations 20618-19-20-21-22-23-24-25-26-28-29-30-31-32-3435-36-37-40-41-42-43, Feb. 5-10, 1930 (Bigelow 1933: 113).

Earle, and Clark (1884: 98) list 7 or 8 taken in a gillnet on Georges Bank on a January 3 or 4, a numbertaken by a schooner on Georges Bank, in March1856, tinkers taken from the stomachs of cod, sometimes 5 or 6 from one fish, and used for bait onGeorges Bank in February 1878; 30 caught on atrawl line set on Middle Bank in January 1868 or1869, and "two fine fat fresh mackerel were foundamong the kelp at Green Cove on Friday, December28, 1878," reported by the Yarmouth (Nova Scotia)

Herald. Bigelow and Welsh (1925: 196) give additional instances of mackerel taken from cod stomachson Georges and La Have Banks and off the coast ofNew Jersey in winter, also occasional catches byotter trawlers in the South Channel and on GeorgesBank in February and March. Such records cannow be augmented materially by instances that haveaccumulated during the course of the present investigations. These are listed in table 1, and theirpositions appear in figure 3.


TABLE i.-Winter records of mackerel

Locality DateDepth(fathoms)

Quantity Size Remarks

-----------·------[------1---·1----------------------------------~.

Jan. 9-16, 193L 12-15 centimetera Lot of pollock containing a number ofsmall mackerel.

Lat. 40°40' N.,long. 69°50' W Jan. 5-13,1932-- _Lat. 40°40' N.,long. 69°20' W do _Lat. 40°30'-40' N.,long. 69°20' W do _Lat. 40°40' N.,long. 69°20' W do _

Southeast part of Georges Jan. 10, 193L__ 27Southeast" east Highland Lighc Jan. 16, 193L__ 28-3078 miles southeast H south Highland Lighc __ Jan. 21, 193L__ 60Lat. 40°50' N.,long. 68°00' W Jan. 22, 193L _

Southwest Georges Mar. 25, 193L _Do do _

Georges Bank Dec. 21, 193L _Do Jan. I, 1932- _

Of! South Shoal Lightship Jan. 7-14,1932-- _

gill

Fishing boats report catching a numberof mackerel of mixed sizes. 1 steamergot as many as 200 or 300.

Debris and mucus in stomach.In stomach of a polloc;k.Stomach empty.

Very thin.Very thin; stomach empty.Very slender; much sand on lIs,

rakers, and mouth.

Found alive in stomachs of hake andpollock.

Weighed 2 pounds in round condition.In pollock; cod and haddock also eat

ing them.

In stomach of a haddock.

Small _48.5 centimeters- _Tinker _44 centimeters _Tinker _IH pounds _32.5 centimeters _31 centimetcrs _30 centimeters _

IH pounds . _13-15 centimeters _About 2" pounds _41 centimeters _

L 21 centimeters _12 17.5-21 centimeters _L 44.2 centimeterL _19 12-20 centimeters- _

L _174 17--40 centimeters Temperature 4.50 C.66 . - - - ---12 11 were in stomach of 1 pollock; 1 in

stomach of another.

40__ ___ ___ __ TmkeL _L 19 centimeters _65 pounds _L_____________ Large _200 or 300 Mixed . _

L _Many _35 _L _

36 _L _3 _L _6 _100 pounds _L _L _L _

2_ _____ _____ Large c_Few __ SmalL _100 pounds • _95 pounds 26.8-28.7 centimeters __100 pounds Tinker _75-100 pounds __ Small _60 pounds _5 15-20 centimeters- _

30-4253-5550-60

60

Jan. 8, 1935 _Feb. 14, 1935 _Mar. 13, 1935 _Mar. 28, 1935 _

Emerald Bank _North of Emerald BanL _Lat. 43°15' N.,long. 61000' W _Browns Bank _

In the offing of New England:Georges BanL Jan. 18, 1929 _Lat. 41°11' N.,long. 69°()()' W Feb. 9, 1929____ 50

Do do__ __ 5060 miles southeast Highland LighL Mar. 12, 1929___ 70

Do do__ __ __ ___ 707 miles south of South Shoal Lightship Dec. 23,1929--- _Southeast part of Georges Bank Jan. 21, 1930___ 4290 miles southeast Highland LighL do_________ 3880 miles southeast Highland LighL Mar. 31, 1930___ 36

In the offing of Nova Scotia:Western Bank, 50 to 100 miles west of Sable

lsland.Do____ __ __ __ __ __ ___ __ _______ _ _do _Do do _

La Have Bank- Dec. 12, 193L _Lat. 43°10' N.,long. 61°40' W____________ Feb. 3, 1932-___ 4-5

t:: !~~52;,~:,II~:~.6S~P~,~---~~:::::::: : }Feb. 15, 1932--_ 3HOLat. 43°05' N.,long. 64°25' W do_________ 50-60Lat. 43°20'-30' N.,long. 62°5'-15' W do_________ 50-70Lat. 43°55' N.,long. 64°25' W Feb. I, 1934____ 70

Lat. 41°35' N.,long. 69°40' W Feh. 15, 1932___ 60-75 35 pounds _Lat. 41°20'-30' N.,long. 68°30'-50' W do_________ 70 12 _

Lat. 41010'-30' N.,long. 69000'-10' W do { 4518 _~_b_o_d~~::::~:::Georges Bank do About 50 _

Do Feb. 19, 1932 _

50 miles east by south Chesapeake Lightship__ Feb. 19, 1933_ __ 32

Feb. 17, 1932_ __ 38-{)0Feb. 25, 1932___ 38-40Apr. 7, 1932____ 35~lUJan. 4, 1933 .. __Jan, 12, 1933 ___ 45

Thin and in poor flesh.Very large and fat.Large, in good condition, showed fat

ness.Very thin. Crustacea and worms in

stomach.Very large and fat. Stomachs crammed

full of shrimplike crustacea 1 inchlong.

Stomach partially filled with fragmentsof copepods, Euthe.misto, and unidentified fragments.

Large and tinker A number of vessels. caught mackerelboth large and tinkers. A largeschool of large mackerel was sighted.

31.5 centimeters _

SmaiL _SmalL _25.5-42.5 centimeters __44.5 centimeters- _30-35 centimeters _

50 centlmeters- _

33 centimeters _

SmaIL _Large _Tinker _Large _

SeveraL _L _18 _L _L _

L _

1 10 centimeters uSpike."

120 pounds _L 31.6 centimeters Temperature 10.50 c.18 17.1-20.3 centimeters __2-. 30.7 and 34.4 centi-

meters.L 19.2 centimeters _290 pounds _300 pounds _50 pounds All sizes _4. __ ___ __ Medium _51- 14--24 centimeters Temperature 7.3 0 c.9 In hake stomachs; 4 in I, 3 in I, and 2

in 1.L SmaIL In hake stomach.L __ ___ __ ___ __ _ 25 _

6 SmaIL 3 to pound.3 SmalL 2 in hake stomachs.

45

40

52

2935-45

27

5230

57

43-5942-46

Mar. 17, 193L__

Feb. 2-3, 1932-_Feb. 13, 1932- __

Dec. 7, 1932-___ 27Dec. 12, 1932 _Dec. 23, 1932 _Dec. 5, 1934 _Jan. 9,1935----Dec. 3, 1935 _

Mar. 2, 193L _

Do do _180 miles south of Cape May, 30 miles east of Mar. 10, 193L__

Bodie Island.70 miles south ~ west Winterquarter Light

ship.52 miles east by south Cape Charles _45-50 miles east by south Chesapeake Light-

ship.65 miles east-northeast Chesapeake Lightship_45 miles east by south Chesapeake Lightship __20 miles southeast Winterquarter Lightship _Lat. 36°50'-60' N., 10nK. 74°30'-40' W _60 miles east by north 72 north, Chesapeake

Lightship.40 miles east H south Chesapeake Lightship __ Feb. 3, 1933- _

62 miles east-southeast Cape May ---,1933 _

South ChanneL Mar. 23, 1932 _Lat. 41°10'-20' N.,long. 67°10'-20° W Dec. 3, 1932-___ 26Lat. 41°10'-20' N.,long. 67°20'-30° W do_________ 27-31Lat. 41°20'-30' N.,long. 67°10'-20' W Dec. 6,1932---- 27

Lat. 41"10'-20' N.,long. 67°10'-20' W _Nantucket Shoals _

Do _20 miles southwest of No Man's Land _Southeast part of Georges Bank _Lat. 40°41' N.,long. 69°40' W _

In the offing of the Middle Atlantic States:Lat. 38°13' N., long. 73°49' W _


TABLE I.-Winter records of mackerel-Continued

Locality DateDepth(fathoms)

Quantity Size Remarks

----------------------------1---------------------------.----

Feb. 18, 1934___ 60 L 2 pounds Thin.

Mar. 3, 1934____ 51-60 L 37-45 centimeters _

Feb. 8, 1935____ 58 20 pound"- _

Feb. 14, 1935___ 90 2 tinkers, 1 large _Mar. 2,1935____ 55 11 pound"- 29-44 centimeters _Mar. 28, 1935 35 pounds 1-2 pounds _

In the offing of the Middle Atlantic States-Con.100 miles northeast by east Chesapeake Light

ship.58 miles east by north-east by north H north

Chesapeake Lightship.45 miles east ~~ north Chesapeake Lightship __106 miles northeast by east Chesapeake Light

ship.70 miles east by north % north Chesapeake

Lightship.63 miles east by north % north Chesapeake

Lightship.65 miles east-southeast Five Fathom Light-

ship.East-southeast Cape Henry _55 miles east by north Chesapeake Lightship__100 miles southeast Cape May _

Mar. 4, 1934 _Mar. 12, 1934 _

Mar. 23, 1934 _

Mar. 28, 1934 _

5843-50

60-65

55-63

L 46 centimeters- _10 pounds _

7 29-39 centimeters _

Very thin.

The significance of these records must be weighedin relation to the distribution of fishing in the wintertime, for the lack of records from any particular areawould be meaningless unless fishing took place inthat area. Thus, the lack of winter catches alongthe southern edge of Georges Bank is due to thefailure of fishermen to trawl there.4 Similarly, thedearth of any winter records between the westernportion of Georges Bank and the offing of DelawareBay has no significance because no fishing takesplace near the edge of the shelf in this sector duringthe wintertime. From about the offing of DelawareBay to Cape Hatteras, on the contrary, numerousotter trawlers fish intensively during the entirewinter along the continental edge and accordinglythere are a number of instances in which mackerelwere caught on these grounds. Thus, where fishingtakes place in the warm zone in the wintertime,mackerel appear in the catch, and only portions ofthe warm zone that are not fished in the wintertimefail to contribute winter mackerel records.

There are, however, two features of this series ofwinter records that are contrary to the theory thatthe warm zone constitutes the winter habitat ofmackerel. These are, first, the numerous specimensof mackerel taken on Georges Bank and along thecoast of Nova Scotia considerably inshore of thewarm zone and in water that presumably was muchcolder than is considered suitable for mackerel, andsecondly, that even in the sector between the offingof Delaware Bay and Cape Hatteras where mackerelhave been taken by otter trawlers, the numbersencountered are so very few that they cannot be

• During the period 1931-33 the location of fishing during each trawler tripwas ascertained in connection with the Bureau of Fisheries investi~ation of thehaddock fishery. Amon!! the thousands of fishin!!locations which were recordedonly one fell within the area bounded by the 6° C. isotherm and the edge ofthebank.

taken as representing the main body of the mackerelpopulation.

Since it is hardly likely that there are warm poolsor lateral extensions of warm water along the bottomon Georges Bank (hydrographers have never en·countered them), the records of mackerel well up onthe bank must be accepted as evidence of theirpresence in rather cold water; indeed one of thecatches was made in water that tested 4.5 0 C. atthe time. Either our notions of the temperaturestolerated by mackerel are erroneous, or for briefperiods of time small groups may stray away fromthe main population. That these records of straymackerel exist is more likely owing to the thoroughness with which the waters there are dredged by thetrawlers rather than to the occurrence of quantitiesof mackerel.

Winter catches of mackerel in the sector in theoffing of the Middle Atlantic coast between DelawareBayand Cape Hatteras nearly all fall within the warmzone at the continental edge as might be expected.Although their location agrees well with the theoryset forth above, the number of records and thequantities of fish taken are far too low for a regionsupposed to harbor the main bodies of mackerel inthe wintertime and where winter trawling is intensively practiced. This can hardly be attributed todeficiencies of the otter trawl as a means of catchingmackerel for mackerel are regularly taken by trawlin the wintertime along the slope of the North Seaplateau toward the Norwegian Channel and in theEnglish Channel. Hence it must be concluded thatthe American mackerel are not concentrated nearbottom in the .vintertime.

It is also apparent that mackerel are not at thesurface in the wintertime as has often been remarkedupon in the literature. Exceptional reports of the


sighting of such schools as have appeared in theliterature may be discounted (Bigelow and Welsh1925: 196) on the basis that the reports are notauthenticated by specimens from such schools, theinference being that some other scombroid or evenclupeoid species was concerned. Furthermore,mackerel fishermen often have striven to extend thefishing season by searching for fish earlier in thespring and later in the fall than the regular season.During the 10 years of this investigation when theactivities of the mackerel fleet were under closeobservation, seiners have scouted in early springfarther to the south and farther offshore than thearea in which the first catches are customarily made.In the fall also, they have often persisted in lookingfor mackerel some weeks after the final catches weremade, and although such searches extended fartheroffshore and farther southward than the ordinaryrange of the fishery, they have been in vain.5 Thentoo, the trawlers that frequent the warm zone in theoffing of the Chesapeake Capes each winter wouldsurely recognize mackerel schools if they saw any,for these same fishermen, as a rule, engage in mackerel fishing in the summertime and would not onlybe quick to report any schools sighted in the wintertime but also would very likely outfit for seiningand try to catch them, for the winter prices wouldmake the fishery quite lucrative if considerablequantities could be caught. Hence it must be concluded that mackerel in disappearing in the fall,sink below the surface 6 and if they reappear at thesurface subsequently, it is only for short periods oftime and at infrequent intervals.

In summary, it may be concluded that the winterhome of the mackerel is in the warm zone along thecontinental edge from Cape Hatteras to the middleof the southern edge of Georges Bank, for here thewater is surely warm enough and enough stragglershave been taken to indicate that the main populationis nearby. It may possibly extend even as far to

6 In December of 1932, for instance, schools of mackerel were reported fromthe easterly portions of Georges Bank and at least one seiner cruised to thesegrounds but failed to find the mackerel. Again in December 1933, the schoonerOld Glory, Capt. Frank Foote, sailed to the southerly offshore grounds for mackerel and although a school was sighted 25 miles southeast of Fire Island Lightship, deficiency of the gear prevented a catch, and when the vessel reoutfittedand returned some days later, no mackerel schools were to be found.

eA bit of experimental evidence on this point is afforded by the action ofmackerel held in a large outdoor pool at Woods Hole in 1932. These frequentlywere schooling at the surface during summer months but toward the end ofSeptember when the water cooled they stayed well below the surface and eVenwhen food was thrown on the water, appeared reluctant to rise to the surfaceas formerly was their habit. Unfortunately, the occurrence of a heavy rainflooding the harbor and making the water markedly turbid at this time leadsto uncertainty as to whether turbidity, decrease in salinity, or lower temperatures caused the change in habit.

the northeast as the offing of Sable Island, for 6° C.water extends that far and stragglers have oftenbeen taken on the Nova Scotian banks in the wintertime. Although present in this zone of warm water,they do not regularly appear at the surface nor dothey stay close to the bottom. In all probabilitythey are in middepths, perhaps above rather thanbelow about 100 fathoms. Whether their schoolinghabits are preserved in the wintertime or whetherthe individuals are widely scattered, must remain amystery probably until means of fishing the middepths have been devised.

PROBABLE CONDITIONS OF EXISTENCE IN THEWINTERTIME

Whether or not food is of any consequence to themackerel in wintertime is called into question byearly theories of hibernation of the mackerel whichinclude such fanciful suppositions as inspired thestatement of a French Admiral whom Ehrenbaum(1914: 10) quotes as declaring that "his men had seenthousands of mackerel in the bays of the Greenlandcoast in the spring, the fish having buried theirheads in the mud, and hibernating in that position,as a result of which they became blind, and werethus very easily caught." Needless to say, suchevidence of the mackerel's food requirements in thewintertime need not be taken seriously, sincemackerel are now known not to inhabit Greenlandwaters; but Ehrenbaum, whose extensive study ofthis species entitles his views to great respect was notconvinced that the European mackerel may not bein at least semihibernation during the wintertime,for he stated (1914: 13):

There is thus no longer any doubt that the mackerel at certainseasons of the year seek the bottom, and the lower water layers,and although they do not appear to spend the whole of thisperiod in passive hibernation, but rather to be, at times, eager forfood, it is nevertheless highly probable that they hibernate for apart of their stay at the bottom, viz, from November to January,when, according to the observations of Irish investigators, asalso my own, the taking of food is as a rule suspended. Duringthis time, the stomachs of mackerel taken with the trawl arefound to be entirely empty, and not until February and Marchdo we find a slowly increasing percentage of fish containing food.

Bigelow and Welsh (InS: 197), commenting onEhrenbaum's view of mackerel hibernation, say:

It is not likely, however, that the American mackerel do so,though they may be semitorpid or at least very sluggish duringthe cold season, the presence of mackerel in the stomachs of otherfish as well as the fact that they sometimes have food in their ownstomachs in midwinter, proving that they move about more orless even then, though they certainly feed very little, for not only


are most of the European fish trawled at that season empty, butEuropean and American mackerel alike are thin when theyreappear in the spring.

I agree with Bigelow and Welsh that there mustbe some feeding activity in the wintertime. If, asis supposed, mackerel at this season are at middepths, some activity would be required and someenergy consumed in maintaining this position. Thatsome of the needed energy is acquired currently byfeeding is indicated by the food in the stomachsreported by Bigelow and Welsh, and observed byF. E. Firth during the present investigations. Thatthey fare less well in winter than in summer and areforced to draw on stored energy, is proved by the lowaverage fat content in early spring (p. 268). It alsoappears that not all portions of the population areequally successful (or unsuccessful) in obtainingfood in the wintertime, for of the winter-caughtspecimens that came to the hands of F. E. Firthduring the course of the present investigation, somewere fat and some were lean. This suggests thatfood is not distributed uniformly, or that the concentrations of mackerel and of mackerel food do notalways coincide.

What lit~le is known about plankton (the principalfood of mackerel) is consistent with the view thatfood is generally scarce in this area in the winter andthat it may be spotty. In the Gulf of Maine,Bigelow (1928: 190-191) found Calanus ftnmarchicus(the dominant plankton species of the region andthe most important food organism for mackerel) soscarce in February and March (1920) as to yieldhauls of only 3,900 per square meter on the average.Whereas in May and June (1915) the average forall stations was 86,000 per square meter. Thus, inthe Gulf of Maine, the winter populations of Calanusftnmarchicus appeared to be less than one-twentiethas large as the summer population. On the continental shelf between Cape Cod and Chesapeake Bay,this copepod is also very scarce in the wintertime.Bigelow and Sears (1939: 306) found tow-net catchesto be only one-eighteenth as large in February as inApril 1930, and only one-twentieth-ninth, one-ninth,and one-tenth as large in February as in May of1930, 1931, and 1932, respectively.

Not only is there a general scarcity of planktonin the wintertime, but the waters at the outer edgeof the continental shelf where the mackerel are supposed to winter, have as little plankton as the watersfarther inshore, and south of Cape Cod, even less.However, among all the plankton tows taken along

the edge, both by Bigelow and by ourselves, therewas one at the southern edge of Georges Bank thatyielded 103,000 Calanus per square meter (Bigelow1926: 190-191) which is an abundance comparableto spring or early summer in the Gulf of Maine.Bigelow regarded this as a local swarm. Towingsalong the continental shelf in the wintertime haveneither been closely enough spaced to indicate howmany such swarms exist at a given time nor havethey been made at enough different times duringwinter to prove or disprove their existence as acharacteristic winter phenomenon. Their occasionaloccurrence, suggested by the catch at the southernedge of Georges Bank, would not only enhance thesuitability to mackerel of continental edge watersin the wintertime but would also account for thefatness of some winter-caught mackerel despite theleanness of most.

SCHOOLING HABITS

MECHANISM OF SCHOOLING

One of the most characteristic habits of the mackerel is its tendency to associate in dense schools. Onthe basis of observations and experiments principally on Pneumatophorus {{rex, a related species ofsimilar schooling habit, Parr (1927) evolved thetheory that the school is maintained by simple reactions which "may be regarded as automaticallycontrolled by a special kind of tropism giving responses of approach and adjustment of direction tothe stimulus of a perceived prospective companion."That perception is by visual means appears adequately proved by Parr's observations and experiments and has particular significance in connectionwith the formation and stability of schools.

As Parr pointed out, if the aggregation into schoolsdepends on vision, it should take place during thedaytime and the schools should be broken downduring every sufficiently dark night. If this is true,the nightly reshuffling of individuals should tend tokeer the population homogeneously mixed. Atcertain seasons, ho",ever, the break-down of schoolsobviously does not take place at night, for purseseiners locate and catch schooled mackerel at nightboth in the springtime and in autumn. At theseseasons the schools are located by the luminescencewhich is associated with them. This occurrence ofschools at night need not be contrary to Parr'stheory, for obviously the luminescence may be aseffective as daylight in permitting the visual per-


ception necessary for schooling. However, thereshould be a tendency toward greater permanence ofschools in spring and autumn, which might bereflected in greater divergence between schools inrespect to certain characters such as size-composition. This has not been so obvious for me to havenoted it, but admittedly the data have not beencollected or analyzed in a way which would beadequate to demonstrate this point.7

ADVANTAGES OF THE SCHOOLING HABIT

In addition to his inquiry as to the method inwhich the schools are maintained, Parr (1927: 31)discussed the usefulness to the species of the schooling habit, pointing out that if individuals of a muchpreyed-upon species like sprat or herring traveledaround separately, "scarcely a single one of themwould escape the enemies sufficiently long to beable to propagate, while the occurrence of a greatnumber of specimens united in schools among scattered enemies may give a certain percentage achance to survive and continue the existence of thespecies." This, however, overlooks the fact thatthe enemies of schooling fish often are bandedtogether and thus tend to overcome this advantageof their prey.

In view of this, it appears to me more reasonableto suppose that the advantage of schooling, if any,would lie in the increased ability to capture preyrather than to elude predators. This is suggestedby observation on the feeding activity of mackerelheld in confinement in an outdoor pool open to tidalcirculation (p. 352). When food was offered to thesemackerel in the form of ground squid or fish, i. e.,in relatively large particles compared with plankton,the mackerel darted toward certain particles andsecured them individually. When feeding thus theschooling habit was broken down and although theindividuals congregated in the vicinity of food theiractions were not coordinated. On the other hand,when not so-feeding, the mackerel collected in adefinite school coursing around the pool in a fairdegree of unison though even at such times theiractions were less concerted than when feeding onplankton. Such feeding took place very close to

1To do this would require that each sample be sufficiently large to describe thesize (or age) composition of a school. With limited resources it seemed better totake small samples from each of many catches (schools) rather than largersamples from fewer catches (schools). However, even these small samples mightyield information if studied by statistical methods developed for quality control(Shewhart 1931).

the surface, usually in calm weather and often inthe early morning or late evening. At least it wasmost easily observed at such times.

When feeding on plankton the mackerel assembledin a much more compact school than at other times.The school swam in a path describing a small circleor ellipse perhaps 8 or 10 feet in diameter andlying in an inclined plane, the upper limb of theellipse touching the surface, the lower limb about2 or 3 feet deep. On the descending segment theindividuals swam vigorously, perhaps at twice thespeed customary in coursing movements, obviouslygetting up speed. As they returned up the ascendingsegment of the ellipse, they opened their mouths tothe fullest extent and extended their operculumswidely, obviously to pass the maximum of waterpast the gill rakers. In this condition the schoolformed a group of miniature tow-nets spaced hardlymore than their own diameter apart. If it besupposed that copepods (the principal element intheir diet) are capable of darting 1 or several centimeters at a time through the water, as observationindicates they do, they might elude one such miniature tow-net; but with a group of miniature townets as closely spaced as these, the success of acopepod in eluding one of them would frequentlyonly put it in the path of another. Thus mackerel,acting in concert probably would average morecopepods each than if they acted individually.

This theory supposes that the copepods and otherplanktonic food organisms are capable of detectinga mackerel at a small distance, and it needs supportfrom critical experiments or observations indicatingthe sensory ability of a copepod in such a situation.If they do not possess this faculty the individualmackerel would be at no disadvantage as comparedwith a school in catching copepods.

Despite the lack of proof that schooling is an advantage in plankton feeding, there is strong indication that schooling is related to this method offeeding in the fact that schooling is so prevalentamong species whose principal food is plankton.Furthermore, those schooling fishes (bluefish, tuna,bonito) which do not subsist primarily on plankton,feed instead mainly upon schooling fish, crustaceaor cephalopods (menhaden, herring, sardines, euphausids, squid). Here the relation of predator toprey is essentially similar, i. e., a menhaden eludingone bluefish would be in the path of another, ifthe bluefish were in schools.


SCHOOLING ACCORDING TO SIZE

A further feature of schooling, significant to thestudy of age composition, is the tendency of individuals of the same size to school together. Fishof the year, as far as we know, always school separately from the rest. Yearlings usually do, but,judging from samples, may sometimes join schoolsof adults, especially when the latter are predominately in their third year. The adults-third-yearfish and upwards-seem not to separate themselvesaccording to age or size in any sharply definedmanner. Nevertheless there is often enough difference in the size distribution among samples fromdifferent catches to suggest some tendency ofmackerel in a given size range to band together inschools distinct from those of another but overlappingrange.

A physical explanation of the tendency formackerel of different sizes to form separate schoolsis suggested by the activities of fish whose swimmingwas timed as they circled around the live car inwhich they were enclosed. Among several dozenmackerel thus observed together, there were twoyearlings, while the remainder were of juvenile size.The juvenile mackerel schooled together travelingin circuits around the enclosure at the rate of 10feet per second and keeping in the middle or upperlevels. The two yearling mackerel traveled in company with each other around the enclosure at therate of 19 feet per second, i. e., distinctly faster thanthe small ones, always keeping below the small ones,sometimes circling in the same direction, sometimesin the opposite.

There is a simple explanation for this difference inspeed if the work performed by the mackerel is proportional to its weight and if it serves mainly toovercome friction between water and the surface ofthe fish.8 Then large fish should move fasterthrough the water because the weight of musculatureincreases as a cube of length and the area of thesurface only as the square. So, with less surfacefriction to overcome per gram of muscle in large fishthan in small the "cruising" speed of the formershould exceed the latter for a given output of energyper unit weight of muscle.

8 The work done in displacing water as the mackerel moves should increase inproportion with the weight, if stream-lining is equally efficient in all sizes. Butsince musculature is also proportional to the weight, the expenditure of the sameamount of energy per unit weight at a given speed should accomplish the displacement of water for small as well as large fishes. Therefore, the work ofdisplacing water should not differentially affect the speed of small as comparewith large fishes.

Moreover, there probably are lower and upperlimits on swimming speed imposed by the inherentcapacity of the mackerel's physiological processesand these are reinforced or perhaps even narrowedby special features of the physiological and physicalsystems involved in the mackerel's swimming.

For instance, a lower limit on swimming speed isimposed by the mackerel's respiratory requirements.F. G. Hall (1930) found that the mackerel dependson swimming to produce sufficient flow of water pastits gills for its respiration. This no doubt accountsfor the generally observed facts (1) that mackerelare always swimming and never at rest and (2) thatwhen the scope of swimming movements is restrictedby putting them into small aquaria they soon die.Thus there must be a certain limit below which theswimming may not fall without disequilibrium between respiration and metabolic requirements. Although the existence of such a lower limit wasestablished by Hall's experimental demonstrationthat respiration of the mackerel depended onswimming, he neither located this limit nor determined whether or not it varied as a function of size.

An upper limit would be imposed by the amountof energy the mackerel may expend in swimmingwithout causing disequilibrium in its metabolicsystem If resistance to passage of a mackerelthrough the water depends not only on its surfacevolume ratio but also is a function of speed such thatthe resistance increases more than proportionallywith speed, then the upper limit would tend to besharpened. An inordinate amount of energy wouldbe required to swim even moderately faster thanthat point at which the energy expended on swimming is currently replenished and exhaustion wouldquickly ensue.

Obviously the swimming rate is dependent on anumber of physical and physiological interactionsand my single set of observations on the swimmingrates of the two sizes of mackerel is hardly sufficientto prove that the rate is dependent primarily onsize. Indeed the considerable range in sizes of individual fish found in a single school argues against it.

With speed directly dependent on size one mightexpect the sorting, by sizes, to be fairly precise, foreach size of fish would have a particular speed differing from that of other sizes, and only fish of onesize could stay together. Actually, rather diversesizes are found in the same school. This can occurif the smaller fish put forth relatively more exertionthan the larger ones. This probably takes place


within a moderate size range. The size compositionin a school perhaps is in a state of dynamic equilibrium where the tropistic tendency for aggregationcausing uniform speed is opposed to the physicaltendency toward different swimming speeds. Withincertain size ranges the former tends to keep theindividuals together, while the latter tends to separate them. This would produce the effect observed:that mackerel school together according to size butthat schools contain individuals of enough diversityin sizes to provide extensive overlapping in the sizerange.

VERTICAL DISTRIBUTION

The American mackerel is generally regarded as asurface fish because practically all of the catch istaken at or near the sea surface. But at times someof them are on bottom for they are occasionallytaken by trawlers in autumn and winter, and itcannot be assumed a priori that they may not alsoinhabit intermediate depths. It has already beenindicated (p. 261) that in winter most of the mackerelprobably inhabit mid-depths. What their lowermostlimit is in summer must be determined by indirectmeans, for gear that is effective in mid-depths has yetto be developed.

Bigelow and Welsh (1925: 195) were of the opinionthat "there is no reason to suppose that they everdescend more than a few fathoms during their[summer] stay, the supply of small crustaceans onwhich they feed being invariably richer above thanbelow 50 fathoms depth in the Gulf of Maine."The vertical gradient of temperature in the summertime, in my opinion, affords additional reasonto suppose that they stay in the upper levels andthat the temperature influence would tend to keepthem even nearer the surface than 50 fathoms, which,after all, is a considerable depth from the standpointof fishermen using surface gear such as the purseseme.

Temperatures in the western part of the Gulf ofMaine during July 19'32 (fig. 4) prove the existenceof a very pronounced thermocline in the regionwhere mackerel were being caught at the time. Ata typical station (A) the temperature was 16° C.(60° F.) at the surface and at 10 meters, but fell to8° C. (46° F.) at 20 meters, and to 6° C. (42° F.) at30 meters. Although other stations varied from thisin detail, all had temperatures above 13° C. (55° F.)at the surface and all had temperatures below 7° C.(45° F.) at the 30-meter level. At most of them

856618 0 --50-----2

the temperature gradient crossed the 7° C. line nearthe 20-meter level.

While mackerel have been found in temperaturesas low as 4° or 5° C., it is not likely that they wouldvoluntarily enter or stay in water of temperatureslower than 7° or 8° c., for they are rarely found insurface waters as cold as this and only in the wintertime have they been found at any level in lowertemperatures. Hence it is likely that the thermocline in the summertime forms a barrier or floor,constituting a lower limit of depth-range.

This floor may move up and down during theseason; and its depth varies from place to place. Itis formed by the warming of surface layers in thespring and summer and destroyed by their chillingin autumn. During the season when it is in existence, stormy periods lower it and calm warm periodsraise it. It tends to be higher in inshore areas andlower in offshore areas; but vertical turbulence,which may attend currents or be induced by storms,modifies this rule. Judging from such few of thetemperatures given by Bigelow (1926: 978-997)as are pertinent,9 the 20-meter level shown in figure4 is fairly typical for the western part of the Gulf ofMaine in early summer. Later in the season andfarther offshore the thermocline tends to be deeper,perhaps with 40 or 50 meters as the lower limit.The probability that the mackerel are located atlevels too deep to show at the surface in offshorewaters where the thermocline lies deeper may explainthe dearth of catches (p. 297) from over the centraldeeps of the Gulf of Maine. However, it is alsopossible that they seldom occur at any depth inthat area.

In summary, it appears that the vertical range ofthe mackerel is limited by temperature. Duringthe height of the fishing season and throughout themajor portion of the fishing area, they must be keptwithin 15 to 20 meters (8 to 11 fathoms) of thesurface. At certain times and places they are freeto descend to greater depths, but probably not muchbelow 40 or 50 meters (22 to 27 fathoms) and usuallynot that deep.

From the standpoint of studying fluctuations inabundance and age composition, the additionalquestion arises: Does their vertical distributionrender all of the mackerel accessible to fishermen atall times during the summer, or only part of them

~ Unfortunately, most of the serial temperatures did not include observationsbetween the surface and 40-50 meters, hence, do not fix the position of thethermocline except to indicate that it was above rather than below 50 meters


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40

6 8 10 12 14 16 18 20TEMPERAT URE.o C.

FIGURE 4.-Temperature gradients (vertical) in mackerel fishing waters. At the left are shown the areas where many catches weremade (heavy shading), the areas where few were made (lighter shading), and the places where the temperatures were taken (lettereddots). On the right are the temperature gradient curves for each of the lettered positions. The catch data refer to the period,July 16-31, 1932, and the temperatures were taken July 22 and 23, 1932.

part of the time? Purse seines of ordinary sizereach down to 20 fathoms, but their effectivenessdepends not only on how deep they reach but alsoon how deep the schools can be seen and thus locatedbefore the seine is set.

In the daytime the schools are betrayed by arippling of the water if they are at the surface, orby dark, shadowy, and sometimes reddish patches

in the water if they are somewhat below the surface.How deep they may be detected depends on theheight of the observer above the water, the qualityof illumination, the roughness of the sea surface, theturbidity of the water, the keenness of vision andthe alertness of the observer. Thus the depth atwhich they can be located is highly variable and notreadily ascertained. It is reasonable to believe that


it seldom exceeds 10 fathoms and is usually lessthan that in the daytime.

On moonless nights, when the schools are visibleas a luminescent patch in the water, it is probablethat they can be seen at greater depths, becausethere is no interference from surface reflection.One instance illustrating the depth to which suchluminescence is visible was reported by F. E. Firthin a letter written April 16, 1935, as follows: "Several, about 7, vessels went out Friday noon andreturned Saturday P. M. without success." CaptainFirth stated, "Saw a school firing deep in the water,made a set, pursed seine, got nothing, and the fishwere still visible under the seine." He said, "thefish must have been down 25 fathoms, for his seinereaches down 22 fathoms. Water firing exceptionally welL" This was, undoubtedly, an instance ofremarkably good visibility. There is considerablevariation in how well "the water fires"-to usefishermen's parlance-hence it cannot be expectedthat schools would always be seen at such greatdepths. It seems reasonable, however, to supposethat they can usually be seen down to a depth of10 fathoms during night fishing.

Taking these considerations together, it is probablethat the fishery is effective throughout the verticalrange of the species only when favorable visibilitycoincides with a shoal thermocline. At other times,which must be frequent, a substantial portion of themackerel population is inaccessible to the fishery onaccount of poor visibility, or because the fish aretoo deep, or combinations of these two impedimentsto the sighting of schools.

Although it is not possible to express the effect ofvertical distribution quantitatively, it is obviousthat the variations in the success of fishing may bemodified considerably by the shifting up and downof the thermocline. When it is close to the surface,say within 10 fathoms, fishing should be good becausenearly all of the population should be within verticalrange of the fishing method. Unfortunately, serialtemperature records are inadequate for determiningthe correlation between position of the thermoclineand success of the fishery, but perhaps it is significant that fishing is less uniformly successful inspring and fall (when the thermocline is less welldefined) than in summer, and that even when fishermen sight mackerel in offshore waters where thethermocline is deeper it is only rarely that goodcatches are made there regularly over extendedperiods of time. The bathythermograph, developed

after the close of this investigation, offers a newinstrument for examining vertical temperaturegradients speedily and in detail. Its application tothis problem might demonstrate relationships thatwould be of high practical value in actual fishingoperations as well as serviceable in biologicalresearch on population abundance and relatedsubjects.

According to direct observations on mackerel incaptivity during the present investigation andjudging from fishermen's reports, the larger mackereltend to swim deeper than the smaller ones. This isparticularly true in mid and late summer. At suchtimes fishermen often report that the schools of largeindividuals are deep and "hard to stop," meaningdifficult to encompass with the seine. Accordingly,there may be a tendency toward catching a largerproportion of small mackerel than of large oneswhenever and wherever the thermocline is relativelydeep. This probably is the explanation of the"disappearance" of the large mackerel in the latesummer of many seasons (p. 268). For these reasonsit is probable that the fishermen's catch in theaggregate undersamples the larger mackerel andoversamples the smaller ones within the range ofcommercially desirable sizes.

This is one aspect of mackerel behavior amongmany others which may be grouped together underthe heading of ·'availability." By this is meant allof the various elements in the behavior of the fishand of the fishermen which cause the catch to be outof proportion to the stock of fish. With pelagicfishes such as the mackerel, where the vertical aswell as the horizontal extent of distribution affectsthe quantity caught, there is opportunity for availability to have a much more pronounced effect onthe quantity caught than with nonpelagic fishes;and there is evidence that effects of availabilityextend also to the size categories caught.

It has become standard procedure in studyingmarine fish populations to use the commercial catchper unit of fishing effort as an estimate of abundanceand the size composition or age composition of thecommercial catch as an approximation of the size orage composition of the stock. This has worked well,notably with demersal fishes. With pelagic fishesthe element of availability is so strong that it issafer to assume that the catch per unit of effortonly indicates apparent abundance-not abundanceitself, also that the distribution of sizes or ages inthe catch registers something other than the size or


TABLE 2.-oil content of mackerel

MIGRATION OF ADULT MACKEREL

1 This value was derived from Stansby and Lemon's (1941) table 4, by takingthe simple average of the percentage of oil content in the 31;..2- to 36~2-inch,

36>2- to 38-inch, and 39- to 42-inch size categories. The number of fish in thesample was not given.

That mackerel migrate seasonally is generallyaccepted, but concerning the direction and extent oftheir travels there are two schools of thought-onethat they migrate great distances from north tosouth when they leave the coast in the fall andback again in the spring; the other, that they sinkand move directly out to deeper water in the falland merely rise and move inshore in the spring.The controversy between the schools was lively inthe latter part of the nineteenth century in connection with the dispute between the United States and

3.94.88.09.09.8

16.810.617.519.26.5

10.1115.7

8.2'19.5

Oil content, percentage

Maximum Minimum Average

Numberof fish 1---,-----....,,--

analyzedDate fish were caught

additional information communicated to me by Dr.Stansby, are summarized in table 2 While the dataleave no doubt as to substantial fattening duringthe spring and early summer months, with the oil inthe flesh increasing from about 4 percent in Aprilto nearly 20 percent in August, the course of eventsduring the remainder of the season is not clear.The values seem to fluctuate from sample to samplethrough a range from 6 to 19 percent. This, together with the wide variation between individualswithin samples indicated by the minimum and maximum values, suggests that there is considerabledifference in the success of individuals and groups ofindividuals either in feeding sufficiently or uponsufficiently nutritious food to provide an excess, overmetabolic requirements, for fat storage. With someof the high oil content values attained as early asAugust, it also appears that feeding, on the whole,is usually better prior to August than after. However, the wide variation precludes any conclusionas to whether there is an average gain or loss of fatthrough the late summer and autumn months.

Apr. 18, 1935-_______________ 13 _Apr. 22, 1935-_______________ 8 7.6 2.7May 3,1935_________________ 4 _May 21, 1935________________ 4 _

Hr;K~fjti~=============== -------1:- =====j;:i= ======i:i=Aug. 13, 1934._______________ 7 25.6 10.6Aug. 15, 1934________________ 7 21. 6 16.0Sept. 11, 1934._______________ 20 11. 4 2.0Oct. 1, 1934.________________ 25 16.2 3.0Oct. 20,1934. --- _Nov. 17, 1933________________ 12 15.2 2.2Nov. 17, 1934 _

According to Bigelow and Welsh (1925: 201), theAmerican mackerel feeds chiefly on plankton, ofwhich copepods form the dominant part, and amongthe copepods Calanus finmarchicus is by far the mostimportant. In Europe the same is true in springand early summer but in late summer and autumnthe mackerel there turns its attention more to smallfishes of various species.

Present observations, admittedly limited in extent, agree with those of Bigelow and Welsh. It issuggested, however, that the difference between thefeeding habits of the mackerel in American watersand those in European waters in late summer maybe more apparent than real, for we have found thatthe larger ones usually are not caught in quantityin late summer in American waters, and it may bethat their search for larger food animals like euphausids and young fish leads them away from surfaceinshore waters at this time. Examination of stomach contents of such large mackerel as are occasionally caught offshore and in deeper water in latesummer should be instructive on this point.

Whatever mackerel eat they are more successfulin obtaining food after they have reached coastalwaters in the spring than during their winter stayalong the edge of the continental shelf. In Aprilwhen mackerel first appear on the fishing groundstheir fat content is very low and it increases markedly during ensuing months, according to analysisof the oil content of the flesh by Stansby and Lemon(1941: 10-11). Their values, supplemented by

FOOD

age distribution of the general stock in the sea.When these more limited assumptions are adopted,most of the established techniques for studyingthe dynamics of recruitment, natural mortality,and catch mortality of fish populations are no longerapplicable. If they are applied, nonetheless, theyare likely to lead to anomalous results.

In view of this, it appears likely that progress inunderstanding the dynamics of the mackerel population will be impeded until more is learned aboutthe reactions of the mackerel to its environment andthe quantitative effects these have on commercialcatches as samples of the abundance and size composition of the mackerel stock. The discussions ofthis and the preceding sections are intended to pointout some of the features which appear significantand some of the lines of study which might provefruitful of results.


Canadian Governments concerning the right ofUnited States fishermen to participate in themackerel fisheries in Canadian waters (Goode, Geo.B., et al., 1884: 95). With both sides basing theirargument on fragmentary data largely from testimonyof unscientific observers, the question was in therealm of conjecture and remained there at leastuntil 1908 (Kendall 1910: 293). Latterly, with morefacts at hand, with respect to European as well asAmerican mackerel, there was a decided leaningtoward the school favoring the on-and-off-shore asagainst the north-and-south migration (Bigelowand Welsh 1925: 191).

With the more extensive, systematic, and detailedinformation available from the present studies, itnow appears that neither school was wholly wrong orwholly right, for a critical comparison of all evidencepoints definitely toward the existence of a complexcombination of the two (or three, if we include thevertical) sorts of movement. The general courseof the migrations is diagrammatically charted inii.gures 5 and 6. Proof of the essential correctnessof the routes shown requires consideration of theirwinter habitat, the existence of two migratingpopulations, a northern and a southern contingent,and various other relations which will be taken upin detail. But for the convenience of those whomay not be interested in proofs and details, a summary of the migration will be given here.

SUMMARY DESCRIPTION OF MIGRATION

Although both the northern and the southerncontingents are supposed to spend the winter in thezone of warm water, some thirty to one hundredmiles out to sea along the continental edge fromVirginia to Nova Scotia, it is probable that themembers of the southern contingent tend to be atthe southerly end of this zone, and those of thenortherly contingent at the northerly end.

The southern contingent first appears in the surface waters overlying the continental shelf somewherebetween Cape Hatteras and the offing of DelawareBay, and usually in the early days of April. Thoughat first some thirty to fifty miles offshore, theysoon come closer inshore occupying the inner thirdor half of the continental shelf which is about fiftymiles broad at Delaware Bay. From here they movenorthward and eastward at a rate not faster thanthe progressive northerly warming of the surfacewater and reach the offing of southern New Englandduring the month of May.. During the northerly

journey they are joined by additional schoolsmoving in from the edge of the continental shelfin wavelike incursions. Although the southerncontingent always tarries a month or more in thevicinity of southern New England, toward the endof June or early in July its members make their wayaround Nantucket Shoals and reach the Gulf ofMaine where they make their summer sojourn.

The northern contingent makes its appearanceduring the latter half of May forming a waveadvancing toward the coast along a broad front,perhaps from Hudsonian Channel eastward. Thewestern end of this wave strikes the southern coastof New England, the middle portion, southernNova Scotia and the eastern, the more easterlyportions of Nova Scotia. Once inshore, the members of this contingent migrate along shore. Thosethat strike the coast of southern New England mixtemporarily with the southern contingent amongwhich they are detectable by their different (usuallylarger) sizes. But after staying only a week or twothey separate from the southern contingent andtoward the end of May some filter into MassachusettsBay, but the major portion are next to be found onthe Nova Scotian coast reaching there during theearly days of June about the same time as those thatapproached that coast directly. During June, therun is heavy along the entire length of the NovaScotian coast, Cape Breton, and eastern portionsof the Gulf of St. Lawrence. During this Junerun there are perhaps additional minor waves ofmackerel coming shoreward from the outer edge ofthe Nova Scotian shelf if any have wintered in themore chilly waters of this region (p. 261). Most ofthe northern contingent probably summers in theGulf of St. Lawrence though part may remain alongthe coasts of Maine, Nova Scotia, and Capte BretonIsland.

In withdrawing from the coastal areas in the fall,the movements of the two contingents, for the mostpart, are simply the reverse of their approach in thespring. The southern contingent in retiring from theGulf of Maine goes southeastward past Cape Codand in some years then trends westerly off No Man'sLand and Block Island. This usually take place inSeptember or October. About the same time, thoughsometimes earlier and sometimes later, the northerncontingent begins retiring from Canadian waters.In doing so, a large portion, if not all, passes throughthe Gulf of Maine providing the basis for the latefall fishery off Cape Anne in October, November,


and December. They, too, leave the Gulf of Maineby going toward the offing of Cape Cod, but havenever been observed southerly or westerly of thatarea. During the fall migration, as during the springmigration, there is a brief period when the two contingents are mixed. The fall withdrawal differsfrom the spring approach mainly by the mixing of

the two contingents north rather than south ofNantucket Shoals; the disappearance of each contingent while still well north of the points at whichthey first appeared in the spring; and the occupationby the northern contingent of the western portionof the Gulf of Maine to a greater extent and for alonger period in the fall than in the spring.

50"

40·

l.:..::.-l...t:-::I~--.I_....J._....L_...J.._~:--...l..._..I..._.l..._.L---:~-'l.---.lL.-....J._....L--:::I:-.:-....L._...l...-~-.J.--:55·

FIGURE 5.-Diagrammatic representation of the spring migration. The number of arrowshafts is roughly proportional to the relativenumber of mackerel believed to traverse the several localities. The number of arrowheads is roughly proportional to the relativeamount of commercial catch taken in the several areas. Lines of dashes indicate weak evidence as to the origin or route of migration.

BIOLOGY OF THE ATLANTIC MACKEREL (SCOMBER SCOMBRUS) … · 2008-06-24 · BIOLOGY OF THE ATLANTIC...

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