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Made ill United States of America

Reprinted from IBISVol. 139, No.1, January 1997

\0 1997 British Ornithologists' Union

Habitat selection by marine birds inrelation to water depth

A recent analysis of seabird abundance in relation to water depthIStone et al. 1995) showed that some species lauks and Manx

Shearwaters PlIjfillllSplIjfillllS)were restricted largely to continentalshelf waters less than 100 m in depth. while other species (Fulmars

,;

1 7 n SHORT CO~!~II"\ll'.ATIO'\S I B I S I ) Y

FlI/mnrlls g/a(jalis and Storm Petrel H!ldro/Ja1espe/ngjclIsiwere re-stricted to waters of greater depth. Fishing vessels were eliminatedas an explanation for the patterns observed in Fulmars (Stone 1'1al.199'5 i. The authors mentioned factors correlated with depth, such

as distribution of prey or water masses. as potential explanationsfor the observed association with depth.

Stone 1'/aI. (199 '51stated that the relation between seabird dis-

tribution and depth had not been quantit1ed in North Atlantic wa-

t,ers and that little work had been done elsewhere, In fact, theseassociations confirm previously quantified patterns of distributionrelative to depth in the eastern Atlantic (joiris 1978, 19831. in thewestern Atlantic (Powers 1983) and in the Bering Sea (Hunt 1'1Ill.

19811. Associations with water masses in tropical and subtropical

systems were reviewed by Schneider 1]991) and Hunt (19921. Sev-eral interesting hypotheses have been proposed to explain associa-tion with water depth, The purpose of this article is to summarize

existing knowledge of the relation of seabird distribution to waterdepth. then assess current explanations.

The association between water deptb and the abundance of sev-

eral avian species was noted (,0 years ago by Wynne-Edwards(193'51. based on repeated crossings of the North Atlantic. Thisassociation was a result largely of greater abundance in continentalshelf water relative to deeper oceanic waters. a pattern that oc-curred in the Atlantic (Baker 1947. Brown 19771. the PacificIShunto\' 1972. Wahl 1978. Harrison 19821 and the southernOceans (Shuntov 1972).

Within shelf ecosystems. Wynne-Edwards 1193'5) noted that

aerial species. such as Storm Petrels and Fulmars. which spendmuch of their time on the wing. occurred in deeper water off-

shore. More aquatic species, such as the auks, which spendmuch of their time on the water. occurred in shallower water

near shore, This interesting observation was overlooked as sub-

sequen t work focused on species distribution in relation to watermasses defined by characteristic depth ranges. In the North Sea,

loiris 11978) found that Fulmars and auks were associated. re-spectively, with inshore and offshore water masses. In the south-eastern Bering Sea, most species were associated with specit1c

depth ranges (Hunt 1'/aI, 19811, and hence were associated withdepth-related mixing regimes over the continental shelf (Coach-man 198n), Depth determines the balance between mixing andbuoyant forces and hence determines the location of transitionsfrom mixed waters to waters stratified by buoyancy forces be-

cause of heat input or reduced salinity, Depth thus determinesthe location of a succession of mixing regimes along a gradient

from shallow to deep water. For the Bering Sea, Kessel (1979)

and Iverson 1'1al. (19791 proposed the hypothesis that marinebirds are associated with mixing regimes which have character-

istic water depths, A graphic analysis of all species (Hunt 1'1aI,1981) showed that Fulmars. Fork-tailed Storm Petrels Oceall()-

droma Illrea/a and Red-legged Kittiwakes Rissa /Jrel'iros1ris wereassociated with water masses that form over the outer continen-

tal shelf at depths greater than 100 ID, Guillemots (Uria aalgeand Uria lo/lll'ial and shearwaters IShort-tailed Shearwater Pllr

Ii/Ills leJlllims/ris and Sooty Shearwater Pllffi/IlISgrisells) were as-sociated with water masses that form over the inner shelf, at

depths less than 100 m, These same data were then examinedfor differences in biomass (g/km'), occupancy (bird-days/km')

and energ\' uptake in the form of Iixed carboll II! C/km'/seasoll'jrelati\'e to mixing regime and water depth rSchneider & Hullt

19i>21, Depth-related differences in total biomass, OCC'UpallC\'and energy uptake were small, Howe\'er. surface-foraglt1l! species(Fulmars, Fork-tailed Storm Petrels and wintering gulls! ac-

counted for nearly all of the biomass and energy uptake ill watermasses of the outer shelt'. while subsurface foragers Iauks andshearwatersi accounted for most of the biomass and energ\' up-take in water masses over the inner shelt'. The obsen'ed chanl!e

from surface-foraging species in deep water to subsurface-for-aging species in shallower water (Schneider & Hunt 19i>21 wascont1rmed with new data (Schneider fill/. ]98(,1.

Water depth is thus an important habitat variable for marinebirds at the spatial scale of mixing regimes in the order of 1O()-:;O() km I\Vynne-Edwards 193'5, Weimerskirch 1'1a/. 19i>X,Dun~'19891, Why should this bee Hypotheses are numerous, These differin their degree of consistency with seabird observations accumu-

lated primarily in the last 20 years,One explanation for depth-related variation in a\'ian abundance

is that shelf waters are more productive than adjacent nonshelfwaters IShunt(1\' 1972 i. A good example comes from the northwestAtlantic. where deep waters of the Gulf of Maine support fewerbirds and lower avian biomass than shallower shell waters lying

farther offshore (Powers 1983 i. Similar differences in numbers andbiomass were observed in the Bering Sea, where the wide conti-nental shelf in the eastern Bering Sea supports greater numbers

and biomass of birds than the deeper and less-productive centralbasin IShuntov 1972. Schneider & Shunto\' 199 3i.

Depth-related differences in primary productivity have also beenoffered as explanations of differences in abundance within shelfecosystems, Robertson (19771 suggested that reduced avian num-bers in the Strait of ,;eorgia (northeast PacificI resulted from re-

duced productivity in shallDw coastal water because freshwater in-put reduced upward nutrient nux by increasing stratification,Bourne 11981, 1982) proposed the opposite efkct-greater primaryproductivity in shallow water-for the opposite reason: enhancedturbulent mixing in shallow water, Both mechanisms are plausible.but in boreal shelf systems, depth-related differences in primary

production are small (joiris 1'1ai, 19i>2, Walsh & McRoy 191\nl.asare differences in density. biomass and energy uptake b)' all birds(Schneideret1l1,198h).

Another explanation lor depth-related patterns of species dis-tribution within a continental shelf system is that breeding sea-birds compete for food, causing spatial segregation based on for-

aging distance from the colony (Cody 19 i 3), giving rise to acoincidental association with depth as this too increases withdistance from the colony,This hypothesis can be discarded be-cause birds do not segregate by species into a sequence of ringsaround colonies {Bedard 197hl. In the northern Bering Sea.

Least Auklets Aet/lia J11lSillllfed in zones of high density awayfrom colonies, but the distance to these areas depended on hy-

drographically induced variation in the abundance of prey.which consisted mainly of large cope pods (Hunt 1'1al. 19901.Marine birds do forage within characteristic ranges from ,colo-nies (Pearson 19h9. Weimerskirch 1'/ al. ] 9 88), These ranges

are relatively small for some groups (e,g, terns and cormorants),

larger for some groups (e,g, auks) and even larger for other

] 'I'll SHORT l"O~I~II':\tC,\TI()\S 1 :-,

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groups Inotably the Procellariformesi. Consequently. an asso-cation with depth can arise. being most noticeable in groupsthat forage near the coast and hence are usually found in waterless than :;0 m deep, An analysis of this effect in the Bering Sea(Schneider & Hunt] 9S41 showed that. along radii running out-ward from colonies. gradients in density of Red-faced Cormo-rants PlllIlacrocorax IIrile were strong. regardless of depth. InBlack-legged Kittiwakes Rissa rrida([yla. Red-legged Kittiwakesand guillemots, radial gradients were similar to or weaker thandepth-related gradients, In guillemots and kittiwakes in thenorthwest Atlantic. depth-related gradients within 100 km ofcolonies were stronger than radial gradients (Cairns &Schneider] 990. Schneider et al. 19901. Distance from colony cannot ex-

plain associations with depth in nonbreeding birds during theboreal winter tjoiris 1983, Stone er ai, 199:;1. nor can it explainhabitat selection by nonbreeding austral migrants in the borealsummer IHunt et al. 19811.

Depth-related differences in primary productivity in the NorthSea are small. leading joiris (1 '178. 1983) to propose that depth-

related patterns in upper trophic level organisms (including birds)result from diversion of primary production into either benthic lin-ner shell"i or pelagic (outer shell"i food webs, This explanation isconsistent with the observed predominance of surface-foragingbirds in deeper waters of the North Sea (joiris 1978. 1983). theBering Sea (Schneider & Hunt 1982) and the Gulf of Alaska (Har-

rison 1982). Two mechanisms have been proposed. Diversion ofprimary production to pelagic food webs in outer shelf water hasbeen attributed to reduced rates of microbial decomposition in deepwater (foiris 1983). Diversion has also been attributed to the pres-ence (OI'er the outer shell) of copepods large enough to consumethe spring bloom before it sinks to the bottom (Smith & Vidal 1986).In the Bering Sea, large-bodied copepods are a major source of food.largely because of consumption by auklet populations near largecolonies (Hunt et al. 1990. Schneider & Shunto\' 1993). But while

large-bodied copepods are more abundant o\'er the outer than theinner shelf (Smith & Vidal 1986). they are not the major prey ofthe outer shelf assemblage, consisting of Fulmars and Fork-tailedStorm Petrels in the summer and gulls in the winter (Schneider etai, 19861.

Another possible explanation is that outer shelf marine birds re-quire prey that in turn require large copepods. Fulmars do not ap-pear to require a specific suite of prey species, judging from thelability of diet observed in Fulmars in the Bering Sea (Schneider &Shuntov 1993) and the northeastern Atlantic (Stone et al. 1995),

Even if outer shelf marine birds do depend indirectly on large-bod-ied copepods. it remains unclear why subsurface-foraging species donot utilize this resource. either directly or indirectly. in deep waterhabitats. This line of reasoning led to an explanation based ondepth-related patterns in prey availability rather than change inprey composition or productivity across a wide continental shelf.The topographic anchoring hypothesis (Schneider et al. 1986) isbased on the observation that lIow patterns in the upper watercolumn rellect bottom topography in shallow water « 50 mI. be-come less coupled to topography at intermediate depths (50-100m) and become decoupled over the outer shelf (>100 ml, Accord-

ing to this hypothesis. surface-foraging species with low lIight costs

exploit laterally less-predictable concentrations at the sea surface in

deeper water. Similarly, subsurface feeder, rel~' Oil sill'S of predict-

able prey concentration over the inner shelf. including shallow sea

fronts t Holligan 1YS] I. This hypothesi> is collsistent with recellt

findings from the northwest Atlantic (Stolle eC/1!. ]99:;1 where sub-

surface foragers 1Manx Shearwater" guillemots and Little :\uks .~lIe

aile) were relati\'Cly more abundant in water less than] 00 m, while

surface foragers (Fulmars and Storm Petrels) were relati\'ely more

abundant in deeper waters.

These hypotheses vary in status. Coincidental associations with

depth because of distance from colony can be eliminated except in

coastal species such as terns and cormorants. Depth-related differ-

ences in primary productivity (Shunto\' 1Y7"2. Bourne 198]) ex-

plain higher total density and biomass in shelf relath'e to adjacent

nonshelf waters but do not explain depth-related differences in the

abundance of individual species within boreal shelf systems. The

remaining hypotheses, differences in food web structure (Joiris

1978. ]983) and diflerences in lateral predictability of prey

(Schneider et al. 19861. are consistent with the a\'ailable eddence.

Neither has been tested directly. Recent advances in technology

(Croxall 199:;1 now make it possible to test the topographic an-

choring hypothesis by comparing the foraging paths of gliding spe-

cies in deeper water with the loraging paths of diving species inshallower water.

DAVID C. SCHNEIDER Oceall Scienrrs Centre, Memorial Ullil'l'rsity "f

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SlIblllitlpd .27 lllllllilru 1996: Iwisioll m'CPpwl April 1996