MFR PAPER 1126

The Purse-Seine Fisherytor Bluefin Tuna in--th:eNorthwestern Atlantic Ocean

--~

ABSTRACT- The history of the purse-seine fishery for bluefin tuna inthe northwestern Atlantic Ocean is reviewed, and events that contributedto fluctuation of the catch are discussed. The fishery began in 1958 andproduced 5,770 mt (metric tons) at its peak in 1963. The catch declinedfrom 4,290 mt in 1970 to about 1,780 mt in 1973. The 1973 catch was landedby a fleet with about 4,900 mt carrying capacity, which was in excess of thecatch. Average length of bluefin tuna in the purse-seine catch decreasedfrom about 140 cm in 1960 to about 89 cm in 1973, owing in part to a south­ward expansion of the fishing grounds into areas where small bluefin tunaare more available. Prior to 1962, the fishing grounds were centered inCape Cod Bay. Currently it is centered off New Jersey-New York. Thenominal catch rate of Class 3 seiners has fluctuated, but it appears thatthe prevailing trend in the 1970's is downward with the current rate at alow level.

northwestern Atlantic and describessome events lhal had a bearing onfluctuation of the catch.

Figure l.-Canedlan end U.S. commercial catch o' bluelln luna 'rom the Atlantic Ocean: A, theyear 1958 when the purse-aelne Iishery firat atarted; B, Ihe year 1963 when large ulnera (> 181lona capeclty) firat entered the IIshery. The 1973 catch.. are preliminary and only includea purae­aelne cetchea.

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Gary T. Sakagawa is a fisherybiologist with the Southwest Fish­eries Center, National MarineFisheries Service, NOAA, La Jolla,CA 92037.

Purae aelnlng by commercial fiahermen andtrolling by recreallonel lIahermen lor bluellntuna off the eaat coaat 01 the United Statea.U.S. commercial and recreellonal lIahermencompete lor bluelln tuna during the aummerand lall montha offahore Irom New Jeraey toMaine.

However. the fishery, as we know ittoday, essentially began in 1958 whenthe Silver Mink, a 41-ton carryingcapacity U.S. purse seiner, demon­strated in Cape Cod Bay, Mass. thatbluefin tuna could be caught commer­cially (Squire, 1959; Wilson, 1965).From 1958 to about 1962 only small,U.S. seiners « 92 tons carrying capac­ity). based in New England ports. par­ticipated in the fishery, which wascentered in Cape Cod Bay. 1n 1962,the fishery expanded southward as faras North Carolina (Wilson, 1965). In1963, Canadian seiners (100 tonscapacity) and large, California- andPuerto Rico-based seiners (> 363 tonscapacity) entered the fishery. These

CANADAU.S.

TOTAL

2

4

7

6

....... ,- , , : .~;:..:.:::... _. \ ·"'··I :',;,;,;~

~9t=3t:'O±±±19=t3:!5::::D::::J1L::94F=O:l::±::i:::±19:f4:...5JJCIi:9+==50tt:::t::±19~5~5~/·~=19+=60:l:::l::~19-t6~5:::I::d...19+-70J...l::::u

YEAR

THE FISHERY

Canada and the United States arethe only participants in the purse­seine fishery for bluefin tuna in thenorthwestern Atlantic Ocean. As earlyas 1937 purse seines were experimen­tally used to catch bluefin tuna in thenorthwestern Atlantic (Murray, 1952).

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INTRODUCTION

'Tons refer [0 metric tons in this report.

The northern bluefin tuna (Thunnusthynnus thynnus) was considered anuisance by U.S. fishermen of theAtlantic coast until at about the turnof t his century a domestic market wasdeveloped, first for the oil then onlyfor the meat (Bigelow and Schroeder,1953). Traps. harpoons. and handlineswere the primary gear used to catchbluefin tuna in the northwestern Atlan­tic before 1958. The total Canada­U.S. catch averaged about 900 tons l

annually in 1942-57. In 1958, purse­seine fishing was introduced (Squire,1959; Wilson, 1965), and the totalcatch (all gear) increased to an average 'bof 2,800 tons annually (Fig. I), butthe catch has fluctuated considerably.In fact, the total catch since J 969declined from 4,800 tons in 1970 to2,400 tons in 1972. This report re­views the development of the purse­seine fishery for bluefin tuna in the

1

1961

1965

1969

MJJASON

1968

1972

1\ 1960II1\

\,,

MJJASON

Tabl. 1.-Pur.....ln. catch•• (Can.dlen endU.S. combined) of blu.f1n and sklPJ.ck tune Inthe northwe.t.rn Allentlc Ocean, 1951-73.

Catch (metric tons)Number

Year of seiners Bluefin l Skipjack' Total

1958 1 138 0 1381959 1 781 0 7811960 1 277 0 2771961 2 903 0 9031962 7 3,767 434 4.2011963 18 5.770 2.899 8.6691964 21 5.167 4.974 10.1411965 13 3.331 1793 3.5101966 7 1.006 1493 1.1551967 11 2.083 3 2.0861968 5 688 23 6901969 4 1.118 1033 1.2211970 8 4,287 6503 4,9371971 12 3.825 6103 4,4351972 11 2.137 443 2.1811973 7 1,781' 35' 1.816'

'Sources: Aloncle et al (1972). Fishery Statisticsof the United States (1960-69. 1971·73) and Statis­tics and Market News Division, National MarineFisheries Service, Washington, DC 20235. Vir­tually all bluefin tuna are landed in the yearcaught, and landings are considered catches inthis report.'Estimated catch statistics from Mather. F. J. III.Woods Hole Oceanographic Institution. WoodsHole, MA 02543 (pers. commun.) except thosefrom logbooks (see footnote 3).3Estimates from logbooks.4Preliminary estimates.

197\

1959

I',, ,I ,

/ ' ......) CANADA - U.S.

... PURSE SEINE

MJJASON

1966

CANADA - U.S.ALL COMMERCIAL FISHEAI(S

70

60 1970

50

40

30

20

10

O~..........'-+-r---MJJASON

70

60 1958~

50 /\CATCH

40I I

30

20

10

0

1962

I­ZWUa::: 70Wa.. 60

50

:r 2U....<l

u O'-::'~'-c::'~-:=:;:-"-:...::':74-'---:;"~6;:-6 -'---:;1'~6.;:-'-'::"'='1o:--'--::,.~n:-'YEAR

Figure 2.-Total Canada·U.S.',commerclal catchof Allanllc bluefln lune by lhe pur..·.elnefI.hery.

Figure 3.-Percent distribution of catch ofblu.fln tune and fI.hlng .ffort for purse••In.r. partlclpetlng In the blu.fln tune fI.h·ery of the northwe.tern Allantlc.

TOTAL CATCH

- .~2

'E 6

E'" .Q

The total Canadian-U.S. landings(all gear) of bluefin tuna from thenorthwest Atlantic historically fluc­tuated around 900 tons annually until3 years after the purse-seine fisherybecame established in 1958 (Fig. I).The total landings then increasedsharply from 1,200 tons in 1961 to

'Length measurements are straight lioe dis­tances from tip of snout to fork of tail.

3M~ther, F. J. 1II. and H. A. Schuck. Con­tribution to the biology of the bluefin tuna(Thunnus Ihynnus) of the western NorthAtlantic. Part I. Age and growth, distribu­tion, and migrations. Manuscript, J77 p.Woods Hole Oceanographic Institution,Woods Hole, Massachusells 02543.

·Sakagawa. G. T., and A. L. Coan. A reviewof some aspects of the bluefin tuna (ThynnusIhYnllUS rhynnus) fisheries of the AtlanticOcean. Manuscript, 83 p. National MarineFisheries Service, Southwest Fisheries Center,La Jolla, California 92037.

events transformed the fishery into onewith cosmopolitan participation withvessels fishing for bluefin tuna as wellas the tropical tunas, yellowfin (T.

albacares) and skipjack (Kafs/./wonuspelamis), that occur in fishable quan­tities in some years off the east coastof the United States (Wilson, 1965).

Unlike the trap, harpoon, and hand­line fisheries that generally catch large'bluefin tuna (> 122 cm long2), thepurse-seine fishery primarily catchesschooled bluefin tuna, 50-122 cm long,which are sold to canneries for process­ing into canned tuna. Fish largerthan about 140 cm long are generallyundesirable for canning and are soldto fresh-fish markets. (Currently, Ja­pan is an important market for thelarge fish.) In anyone year, not morethan about 16 percent of the purse­seine catch, in weight, has been largebluefin tuna.

The purse-seine fishery operateswithin a narrow belt that extendsabout 220 km from the coast. Fisher­men claim that not many schools arefound beyond that region. Mather andSchuck3 indicated that schooled blue­fin tuna are found in the northwesternAtlantic between Cape Charles, Va.and Newfoundland, but primarily be­tween Cape Charles and Cape Cod,Mass.

Through the years the use of land­based aircraft to assist seine fisher­men in locating and setting on schoolsof bluefin tuna has become popular.Virtually all the small seiners andsome large seiners currently use air­craft in fishing operations. The techni­que is quite effecti ve in that it increasesthe searching range and fishing suc­cess of the vessels. This is probablya major factor contributing to theability of the small vessels to consis­tently catch a large share of bluefintuna from the northwestern Atlantic(Sakagawa and Coan4 ).

2

·,' ."JUNE 1963 JULY 1963 AUGUST 1963

CATCH (METRIC TONS)BLUEFIN TUNA 637SKIPJACK TUNA 0

CATCH (METRIC TONS)~""~BLUEFIN TUNA 91 7SKJPJACK TUNA 142 0

...- 0--.l.-~.",----\

oo

o 0o 0

CATCH (METRIC TONSj~""tBLUEFIN TUNA 3064SKIPJACK TUNA 17':;> . ~

-4.",1\-----joo

1Y''---+-+-+---1r--+-+--I ,..

Figure 4.-Percenl celch o' blue'in and Iklp­jeck lune. by 1° er.e "'Ilhln month for lhepur••-.eine f1ahery 01 lh. norlh"'.llern Allan­lie, 1963.

;+--+--+-----+--+-----140"

19,-i--+--j--+-----+---j-· --j----jw

...."...CATCH (METRIC 'tONS:BLUEFlN TIMA 490SKIPJACK TIMA 0

."OCTOBER 1963SEPTEMBER 1963

~"-!~CATCH (METRIC TONS)

BLUEFlN TIMA 612SKIPJACK TUNA 1006 41 0-- 0 0

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8 67 2!l

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0

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6,.200 tons in 1963, decreased to 1,100tons in 1968. increased again to 4.800tons in 1970, and declined to 2,400tons in 1972 (Fig. I).

Since 1960, more than 90 percentof the total annual landings of bluefintuna made by Canadian and U.S.fishermen has been attributed to thepurse-seine fishery. The purse-seinecatch of bluefin tuna was 140 tons in1958; it peaked in 1963 at 5.770 tons,declined substantially in 1968. androse to another peak of 4.290 tons in1970 (Fig. 2). The catch then declinedto about 1,780 tons in 1973.

Skipjack tuna and small amountsof yellowfin tuna are also caught inthe bluefin tuna fishery. The catchesof tropical tunas, however, are usuallya minor part of the total catch, al­though in 1964 the skipjack tuna catchof 4,970 tons was nearly equal to thebluefin tuna catch of 5.170 tons(Table I).

The fishery operates during a shortseason, June through early October,with virtually all (90 percent or more)of the catch and effort concentratedin July and August (Fig. 3). Weather isbelieved to be an important determin-

ing factor for the short season. Avail­ability is another. Apparently. changesin migration patterns influence theavailability of bluefin tuna to the purse·seine fishery (Mather, Rothschild, Paul­ik, and Lenarz'). Wilson (1965) statedthat the purse-seine fishing season wasincreased by ex.pansion of the fisheryinto southern fishing grounds, whereschooled fish are available earlier in

5Mather. F. J. III, B. J. Rothschild, G. J.Paulik, and W. H. Lenarz. Preliminary analy­sis of bluefin tagging data. Manuscript, 27 p.National Marine Fisheries Service. South­west Fisheries Center, La Jolla. California92037.

3

70'

cO'

JUNE 1966

CATCH (METRIC TONS)BLUEFIN TUNA 0SKIPJACK TUNA 0

cO"

70'

JULY 1966

CATCH (METRIC TONS)"''''ff

BLUEFIN TUNA 312

SKIPJACK TUNA 7~.~.<~

""'~~ ':;4.J

r:~:;"~ 0~.,..~""O 0

~ 0 0

fo.S:,) 5819

~__'.3J 4;Z; 81

!

~~/

70"

CO'

AUGUST 1966

CATCH (METRIC TONS)

BLUEFlN TUNA 550SKIPJACK TUNA 57

t+--t--+-+---+---1co'oo

oo

oo

IYr--+--+--+--+--+---+---1w

70"

c,

cO"

SEPTEMBER 1966

...,,~!.p'CATCH (METRIC TONS)

BLUEFIN TUNA 145 <SKIPJACK TUNA 19 0

...- 0

'."~'~, 0

....--~ ..;, '0 ~... 0

~'''''F"v 55 1079 21

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l/J~1{)

c,'

OCTOBER 1966

CATCH METRIC TONSBLUEFIN TUNA 0SKIPJACK TUNA 0

;+---+---+--+--+----1co'

1<,.,'-----+--+---+--+--+---+---1...70"

Flgur. 5.-Perc.nt cetch ot bluefln .nd .klp·j.ck tun•• by 10 .r•• within month lor th.pu r..•••ln. II.h.ry 01 the northw••ternAII.nllc, 1966.

the year than in the region north ofCape Cod.

During the early years. the 1950'sand early 1960's, of the purse-seinefishery, a greater amount of the sea­son's catch was made in Septemberand October. The fishery during theearly years was centered in Cape CodBay with little effort expended southof Martha's Vineyard (Wilson, 1965).In 1962, the fishery expanded southof Martha's Vineyard. Currently mostof the bluefin tuna are caught in Julyand August off New Jersey and NewYork, perhaps before they can travel

northeastward to be available in morenorthern waters In September orOctober (Sakagawa and Coan, Foot­note 4; Mather et ai, Footnote 5).Most current catches in Septemberand October are made in Cape CodBay and consist of fish primarily largerthan about 122 cm long. Generally,Cape Cod Bay is considered a goodfishing ground for large bluefin tuna(Wilson, 1965).

The fishery operates within a smallgeographic area and the catch is ob­tained from only a few "key" I° X 1°areas, those in which 20 percent or

4

more of the bluefin tuna are caught inanyone month. The percent catchesin lOx 1° areas by months for 3years, 1963, 1966, and 1973 whichrepresent years of peak catch (1963),low catch (1966), and recent catch(1973), are shown in Figures 4, 5,and 6. In June, there is not muchfishing and usually only one or two keyareas off the New Jersey coast arefished by the purse-seine fleet. In July,schooled bluefin tuna are caught inseveral key areas off New Jersey andLong Island, N.Y. between lat. 38°Nand 41°N. In August and September,

fj-'-"-t--I--+-+-----i.o·

11,·,--I---I--+---I--I--+-----j,,·

". 70" "" 70" "" 10".,. .,.JUNE 1973 JULY 1973 AUGUST 1973

CATCH (METRIC TONS) CATCH (METRIC TONS) CATCH (METRIC TONS)

BLUEFIN TUNA 0 BLUEFIN TUNA 317 BLUEFIN TUNA 1195SKIPJACK TUNA 0 SKIPJAC K TUNA 9 SKIPJACK TUNA 21

10"

Figure S.-Percent cetch of bluefln end .klp­jeck tune. by 1° eree within month lor thepurae-aelne lishery 01 the northwe.tern AUen­lie. 1973.

10'

;+--+--11--+-+----1.0"

".It,.;'---I---I--+---I--+-+------l".

.,.

CATCH (METRIC TONS)

BLUEFIN TUU\ 174SKIPJACK~ 0

OCTOBER 1973SEPTEMBER 1973

""'"I.p1CATCH (METRIC TONS)

BLUEFIN TUNA 96SKIPJAC K TUNA 5...-~-

71-',-r-""'" O'~

~....&.,Y 38

0 100

~/~I;--J

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.,

'0"

the key areas are primarily off LongJsland. although in 1963. substantialcatches were made in a more northernarea, off Boston, Mass. in September.Very little fishing is done in Octoberbecause of poor weather conditionsand availability. Catches during thatmonth generally have been made offRhode Island and Massachusetts andconsist of Iarger fish (> 122 cm).

FISHING EFFORT

The number of vessels in the blue­fin tuna purse-seine fishery increasedfrom I boat in 1958 to 21 boats in

)964. and thereafter fluctuated be­tween 4 and 13 boats, averaging about9 boats fishing per year (Table 2).

Currently there are two majorgroups of seiners that participate inthe fishery (Table 2). One group isseiners 92-181 tons capacity basedprimarily in New England ports whichfish regularly for bluefin tuna in thenorthwestern Atlantic. Their tripsseldom last more than a week. Thesecond group is of seiners 364-1,000tons capacity based in California,Puerto Rico, and New Brunswick,which fish irregularly In the north-

western Atlantic for tunas. Usuallythese large vessels enter the fisheryonly when tropical tuna fishing in theeastern Pacific or in the eastern Atlan­tic is poor. They are capable of re­maining at sea for several months andusually return to their home ports tounload or for provisions.

Although there was a decrease of14 boats (67 percent) in the fisheryfrom 1964 to 1973. the carrying capac­ity of the fleet decreased only by 40percent; the bluefin tuna catch de­creased by 66 percent over the sameperiod. The total carrying capacity of

5

Table 2.-Numbers and classas 01 purse seiners participating In the bluelin tuna IIshery 01 the north­western Atlantic Ocean, 1958-73. 1

Sakagawa and Coan (Footnote 4)estimated the age composition of thepurse-seine catch for 1960-72. I esti­mated the composition for the 1973catch using that method. The resultsshow three phases in the developmentof the purse-seine fishery (Fig. 7). Thefirst was in 1960-62 when age groups

AGE COMPOSITIONOF CATCH

fishery, however, such as the entry oflarge seiners, the increased use of air­craft, and the southward expansion ofthe fishing grounds, have affected thefishing efficiency of the boats and thenature of the effort statistics collectedthrough the years. For example, a unitof nominal fishing effort of a Class 3seiner (92-181 tons) is not equal interms of mortality generated to a unitof nominal fishing effort of a Class 6seiner, (> 364 tons), since Class 3seiners more often use aircraft to aidin searching for schools and usesmaller seines than do Class 6 seiners.Furthermore, Class 3 seiners wereformerly in the unique position ofbeing able to fish in both deep andshallow waters whereas larger seiners,with deeper nets, were able to fishonly in deep water (Wilson, 1965).There has also been a change in thefishing effectiveness of perhaps allboats as the captains and crews be­came more proficient in the fishingtechniques (Wilson, 1965), but partic­ularly in that of Class 3 seiners asthey increased their dependence onaircraft to locate schools and to assistin positioning the net. Because ofthese changes in the efficiency of thevessels, nominal fishing effort for thefleet is not a reliable measure of effec­ti ve effort from one year to another.

1971

1970

::~ ~ 1973

2~tA~ i" iOI234567B91011121314+

BO

L60

~ 1972

120

100

leo

160

140

'00

BO

60

40

120

1969

1967

1966

3°ll20

10

o 0 I 2 3 456"7"'8"'9""IO~I~II"'2-13~'4 +

40

20

o

40lL.:30

20 196B

'0

o

140

120

100

BO

60

40

20

o

120

100

eo

601963

1964

1965

120

'00

so60

40

20

o01234567 S9101112l314+

100

so

:~1960100J ~~1961

::lL:962

10

o

)(

(J)

a:wm~:JZ

IUf­<4u

otlo

EstimatedtOlal Class (carrying capacityZ)

carrying 1 2 3 4 5 6Year capacity' (0-45) (46-91) (92-181) ( 182·272) (273·363) (364 +) Total

1958 40 11959 40 11960 40 11961 100 1 21962 1.000 2 2 1 71963 5,300 4 3 4 6 181964 8.100 4 3 2 2 9 211965 3,400 4 3 1 4 131966 900 2 3 1 71967 5,100 1 3 6 111968 700 1 3 51969 400 1 3 41970 3.500 1 3 4 81971 8.200 3 8 121972 6.200 3 7 111973 4,900 3 4 7

'Data are from the loltowing sources: 1958-63 'rom Wilson (1965). 1964·73 from Mafher F. J. III,Woods Hole Oceanographic Institution. Woods Hole. MA 02543 (pers. commun.) Beckett. J.S.. Fisher·ies Research Board of Canada. SI. Andrews. N.B. (pers. commun.) and the Inter·American TropicalTuna Commission, La Jolla, CA 92037.2Carrying capaci1y in metric tons.

AGE (YEARS)

Figure 7.-Age composition 01 the purse-seine catch 01 bluelln tuna from the northwestern Atlantic.

ol--L......l--l.--"--L---'-----'-..J--'--'--'-.l.-L.....J---'~

1958 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73

YEAR

---- Closs 3. _ ..... Class 6-- All Classes

l? 25zI!!' 20lL

'"~ 15o'-

'" 10g~ 5...w:l:

Figure B.-Nominal catch rates by class 01ve...1 In the purse..elne IIshery lor bluellntuna In the northweatern AtI.ntlc.

lected from the bluefin tuna purse­seine fleet since the establishment ofthe fishery in 1958. It has been pre­sented as effective effort (i.e. beingproportional to instantaneous fishingmortality) in some published accounts(e.g. Aloncle, Hamre, Rodriguez-Roda,and Tiews, 1972; Lenarz, Jones, andMather, (972). Changes within the

the J973 fleet was excessive in thatit was about 4,900 tons, bringing inonly 1,780 tons of bluefin tuna. Itappears that the recent trend has beenfor larger but fewer boats in the fisherywith a fleet carrying capacity in ex­cess of the catch.

Data on nominal fishing effort interms of fishing days have been col-

6

DISCUSSION

I L...-~-,---,--,--.J..-~~~~=~~-=1960 1965 1970 1973

YEAR

Figure 9.-Nomlnal catch ratel at 1- and 2­year old and 3- and 4-year old bluelln tunalor Cia.. 3 ve..ell In the purle-.elne nlherylor bluelln tuna In the northwe.t....n Atlantic.

ing effort of the sport fishery, whichcompetes with the seine fishery forbluefin tuna, or a combination of anyof these.

Besides the purse-seine and sportfisheries of the northwestern Atlantic,commercial fisheries in the Bay ofBiscay, off Portugal, and off northernAfrica also catch small Atlantic blue­fin tuna (50-122 cm long) during thesummer and fall. There is uncertaintyas to whether these widely separatedfisheries normally exploit fish fromseparate stocks or from a commonstock. Tagging experiments, however,have demonstrated that, at least insome years, fish of similar stocks areexploited in the eastern and westernAtlantic (FAa, 1972). Mather, Bart­lett, and Beckett (1967), for example,examined tagging data and hypothe­sized that between the summers of1965 and 1966 there was sufficientmigration of young bluefin tuna fromwest to east to cause a decline in catchfrom the northwestern Atlantic anda concomitant increase in catch fromthe Bay of Biscay. Aloncle6 concludedfrom tag returns of small bluefin tunatagged in the eastern Atlantic thatindependent American and Europeanstocks exist but that east-west migra­tion does occur. Thus, the degree oftransatlantic migration has at least insome years affected the catch from thenorthwestern Atlantic.

Tagging studies have also demon­strated that in the northwestern Atlan­tic changes in migration pattern withage of fish affect the availability ofdifferent age groups of bluefin tunato the purse-seine fishery (Mather et aI.,Footnote 5). Mather et aI., (Footnote 5)found that a higher proportion oftagged I A-year old bluefin tuna re­turned to the fishing area off theUnited States east coast the year aftertheir release than did older fish. Sincethe age groups represented in thepurse-seine catch have varied, avail­ability of the different age groups hasprobably contributed to the variationin the catch. Of course, superimposedon this effect of variable availabilityis the effect of variable year-classstrength, which would also contributeto variation in catch.

6Aloncle, H. Marquage de thons rouges dansIe Golfe de Gascogne. Manuscript, 11 p.Instilut des P&hes Maritimes, La Noe-Roulede la Janeliere, 44-Nantes.

,\

, ~p

\ .. ~J- oM 4-YEAR Ol.DS

1- GI'ICl 2-YEAR OLOS

.. "/ ""tJ,,,,,

, I

'~

5.000

<:>z~ 1,000

lL500

(f)

-~

o....I(f) '00lL~ 50

(f)0::Wm

~ '0

5·

The CPUE's for Class 3 vessels interms of numbers of fish are shown inFigure 9. Only CPUE's for age groupsI and 2 and 3 and 4 are shown. Theseage groups represent sizes of fish « 140cm) that are most sought after by thepurse-seine fishermen. The CPUE'shave not been adjusted for changes invessel efficiency. They neverthelessindicate that apparent abundance ofage groups 1-4 was at a high level andthe CPUE of 3- and 4-year olds wasless than that of the recruits, 1- and2-year-old fish, in most years.

In the 1970's, the general trend ofCPUE's for both the recruits and 3­and 4-year olds is downward, althoughthe CPUE of recruits somewhat lev­eled off and that of 3- and 4-year oldsincreased in 1973. This declining trendis probably associated with changesin survival and availability of recruitsand changes in the abundance of thespawning stock.

The bluefin tuna catch of the purse­seine fishery of the northwestern At­lantic ranged from 138 tons in 1958,the year the fishery was established,to 5,770 tons in 1964. The most recentcatch was 1,781 tons. The fluctuationsin catch are possibly due to (1) varia­tion in migration patterns (Matheret ai, Footnote 5) and location of thefishery, (2) variation in the amount ofeffective effort used to bring the catch,(3) variation' in recruitment or stockabundance, and (4) variation in fish-

3-6 years dominated the catch. Atransition phase followed in 1963 and1964 when the catch was dominatedby a combination of younger fish aswell as 3-year old to 6-year old fish.The last phase began in 1965 when theyoung age groups (1-3 years) domi­nated the catch with few older fishrepresented. In 1973 the average lengthof bluefin tuna in the catch was 89 cm,whereas it was 140 cm in 1960.

The changes in the age compositionof the catch should be recognized asinfluenced in part by the southwardexpansion of the fishing grounds. Inthe early stages of the fishery's devel­opment, the fishery was centered northof Cape Cod, where primarily largefish are available (Wilson, 1965). Asthe fishery expanded southward, be­ginning in 1962, into areas whereyounger fish are more available, moresmaller fish were caught (Sakagawaand Coan, Footnote 4; Mather et ai,Footnote 5). Finally, in the last phasethe fishery was centered off New Jer­sey-New York, where primarily youngbluefin tuna are found.

CATCH RATE

Most published catch rates (CPUE)for the northwestern Atlantic purse­seine fishery for bluefin tuna are basedon total days fishing or total nominaleffort (Fig. 8). As was stated above,nominal effort is not a reliable mea­sure of effective effort, hence fishingmortality, because of changes, partic­ularly in efficiency, of the vesselsover the years. Consequently, thenominal CPUE's in Figure 8 are like­wise unreliable for indexing abundanceunless change in vessel efficiency istaken into account.

Sakagawa and ('oan (Footnote 4)indicated that vessel efficiency, at leastfor Class 3 vessels, probably has in­creased since the mid-1960's whenvessels began relying more heavi lyonaircraft to locate schools and cooperat­ing more fully among themselves infishing operations. In Figure 8, thenominal CPUE's for Class 3 vesselssince the mid-1960's are thereforeprobably overestimated and should beadjusted downward to reflect the in­crease in vessel efficiency. The CPUE'sfor the 1970's would then indicate amore pronounced decline than de­picted in Figure 8.

7

8r.lllng bluelln luna from a pur.e .elne neloff Ihe U.S. e••' conI.

Trends in the nominal fishing effortfor the purse-seine fishery suggeststhat total effective effort has fluctuatedowing to changes in the number, size,and efficiency of vessels in the fleet.The fluctuation, of course, contributedto the variation in the purse-seinecatch.

CI ass 3 nominal catch rates for the1970's are overestimated relative tothose of earlier years because of im­proved fishing efficiency since the mid­1960·s. When improved efficiency istaken into account, a declining trendprevails in the catch rates for the1970's; the current catch rate is low,perhaps at the lowest level since theestablishment of the fishery. Thus.

the catch rates of the 1970's suggestthat the stocks fished by the fisherypossibly declined in abundance oravailability, which has contri buted tothe recent decline in the purse-seinecatch.

Finally, sport fishermen in the north­western Atlantic compete with seinefishermen for bluefin tuna. Althoughthe total sport catch of schooled blue­fin tuna is not known, rough estimates(Sakagawa and Coan, Footnote 4)suggest that the catch was largeenough in some years to have animpact on the purse-seine catch.

ACKNOWLEDGMENTS

I wish to thank the following indi­viduals for providing information

used in this report: F. J. Mather IIIand J. M. Mason, Jr. of the WoodsHole Oceanographic Institution; J. S.Beckett of the Canadian Fisheries andMarine Service; and B. G. Thompsonof the National Marine Fisheries Ser­vice. The generous cooperation of thecaptains and owners of Americantuna vessels that fish in the north­western Atlantic and tuna cannerieson the east coast of the United Statesand in Puerto Rico is acknowledged.The Inter-American Tropical TunaCommission and the Woods HoleOceanographic Institution collectedpart of the data used in this reportunder National Marine FisheriesService contracts.

LITERATURE CITEDAloncle, H., J. Hamre, J. Rodriguez-Roda,

and K. Tiews. 1972. Report from thebluefin tuna working group. Observationson the size composition of bluefin tunacatches from 1971. International Councilfor the Exploration of the Sea, C. M. 1972/J:2, 24 p.

Bigelow, H. B., and W. C. Schroeder. 1953.Fishes of the Gulf of Maine. U.S. FishWild I. Serv., Fish. Bull. 53, 576 p.

FAa. 1972. Final report of the workingparty on tuna and billfish tagging in theAtlantic and adjacent seas. FAa (FoodAgric. Organ. U.N.), Fish. Rep. 118, Suppl.1,37 p.

Fishery Statistics of the United States. 1960­69. Statistical digests 53·63. Bur. Commer.Fish., Wash., D.C.

1971-73. Statistical digests 65-67.U.S. Dep. Commer., Natl. Mar. Fish. Serv.,Wash., D.C.

Lenarz, W. H., A. C. Jones, and F. J. MatherIll. 1972. National report of the UnitedStates of America. International Commis­sion for the Conservation of AtlanticTunas, Report 1970-71, p. 198-204.

Mather, F. J. Ill, M. R. Bartlett, and J. S.Beckett. 1967. Transatlantic migrations ofyoung blue fin tuna. J. Fish. Res. BoardCan. 24:1991-1997.

Murray, J. J. 1952. Report on 1951 explora­tory bluenn-tuna fiShing in the Gulf ofof Maine. Commer. Fish. Rev. 14(3):1-19.

Squire, J. L., Jr. 1959. New England com­mercial bluefin luna purse seining- 1958season. Commer. Fish. Rev. 21(2): 1-5.

Wilson, P. C. 1965. Review of the develop-ment of the Atlantic coast tuna fishery.Commer. Fish. Rev. 27(3): 1-10.

MFR Paper 1126. From Marine Fisheries Review, Vol. 37, No.3,March 1975. Copies of this paper, in limited numbers, are availablefrom 083, Technical Information Division, Environmental ScienceInformation Center, NOAA, Washington, DC 20235.

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