2636 SEABIRD FORAGING ECOLOGY Ballance L.T., D.G. Ainley, and G.L. Hunt, Jr. 2001. Seabird Foraging Ecology. Pages 2636-2644 in: J.H. Steele, S.A. Thorpe and K.K. Turekian (eds.) Encyclopedia of Ocean Sciences, vol. 5. Academic Press, London. SEABIRD FORAGING ECOLOGY L. T. Balance, NOAA-NMFS, La Jolla, CA, USA Introduction D. G. Ainley, H.T. Harvey & Associates, San Jose, CA, USA Though bound to the land for reproduction, most G. L. Hunt, Jr., University of California, Irvine, seabirds spend 90% of their life at sea where they CA, USA forage over hundreds to thousands of kilometers in a matter of days, or dive to depths from the surface ^ Copyright 2001 Academic Press to several hundred meters. Although many details of doi:10.1006/rwos.2001.0233 seabird reproductive biology have been successfully SEABIRD FORAGING ECOLOGY 2637 elucidated, much of their life at sea remains a mesoscales (100}1000km, e.g. associations with mystery owing to logistical constraints placed on warm- or cold-core rings within current systems). research at sea. Even so, we now know a consider- The question of why species associate with different able amount about seabird foraging ecology in water types has not been adequately resolved. At terms of foraging habitat, behavior, and strategy, as issue are questions of whether a seabird responds well as the ways in which seabirds associate with or directly to habitat features that differ with water partition prey resources. mass (and may affect, for instance, thermoregula- tion), or directly to prey, assumed to change with Foraging Habitat water mass or current system. Environmental Gradients Seabirds predictably associate with a wide spectrum of physical marine features. Most studies Physical gradients, including boundaries between implicitly assume that these features serve to in- currents, eddies, and water masses, in both the hori- crease prey abundance or availability. In some zontal and vertical plane, are often sites of elevated cases, a physical feature is found to correlate dir- seabird abundance. Seabirds respond to the strength ectly with an increase in prey; in others, the of gradients more than the presence of them. In causal mechanisms are postulated. To date, the shelf ecosystems, e.g. the eastern Bering Sea shelf general conclusion with respect to seabird distribu- and off the California coast, cross-shelf gradients tion as related to oceanographic features is that are stronger than along-shelf gradients, and sea-bird seabirds associate with large-scale currents and re- distribution and abundance shows a corresponding gimes that affect physiological temperature limits strong gradient across, as opposed to along the and/or the general level of prey abundance (through shelf. At larger scales the same pattern is evident, primary production), and with small-scale oceano- e.g. crossing as opposed to moving parallel with graphic features that affect prey dispersion and boundary currents. availability. Physical gradients can affect prey abundance and availability to seabirds in several ways. First, they Water Masses can affect nutrient levels and therefore primary pro- In practically every ocean, a strong relation between duction, as in eastern boundary currents. Second, sea bird distribution and water masses has been they can passively concentrate prey by carrying reported, mostly identiRed through temperature planktonic organisms through upwelling, downwell- and/or salinity proRles (Figure 1). These correlations ing, and convergence. Finally, they can maintain occur at macroscales (1000}3000km, e.g. associ- property gradients (fronts, see below) to which prey ations with currents or ocean regimes), as well as actively respond. In the open ocean, where currents 30 30 Pterodroma macroptera V IS Pterodroma incerta H HS 28 Pterodroma mollis 28 S ISS 26 ISS 26 24 24 c 22 22 LTSE 20 20 18 18 Temperature (°C) Temperature 16 16 Bulweria bulwerii 1_10 _ 14 Pterodroma baraui 11 30 14 LTSW Bulweria fallax 31_100 _ 12 101 300 12 0 0 36.5 36.0 35.5 35.0 34.5 34.0 36.0 35.5 35.0 34.5 Salinity (‰) Salinity (‰) Figure 1 Distribution of gadfly petrels in the Indian Ocean corresponding to various regimes of surface temperature and salinity: (A) warm-water species, and (B) cool-water species. The relative size of symbols is proportional to the number of sightings. Symbols for water masses as follows: VHS, very high salinity; HS, high salinity; IS, intermediate salinity; ISS, intermediate salinity south; C, common water; LTSE, low temperature southeast; LTSW, low temperature southwest. (Redrawn from Pocklington R (1979) Marine Biology 51: 9}21.) 2638 SEABIRD FORAGING ECOLOGY and dynamic processes are less active, prey behavior 3000 should be the principal mechanism responsible for seabird aggregation. In these cases, locations of ag- 2500 gregations are unpredictable, and this has important ) 1 _ Sitting consequences for the adaptations necessary for 2000 seabirds to locate and exploit them. In contrast, in Flying continental shelf systems, currents impinge upon 1500 topographically Rxed features, such as reefs, creating physical gradients predictable in space and 1000 time to which seabirds can go directly. Thus, the Abundance (ind km Abundance Rrst and second mechanisms are the more important in 500 shelf systems, and aggregations are so predictable that seabirds learn where and when to be in order to eat. 0 0 4 8 12 16 20 Fronts _ Much effort has been devoted successfully to identi- 5 fying correlations between seabird abundance and _15 fronts, or those gradients exhibiting dramatic 8.8 _ 8.0 change in temperature, density, or current velocity 25 7.2 (Figure 2). Results indicate a considerable range of _35 variation in the strength of seabird responses to 6.4 Depth (m) fronts. This may be due to the fact that fronts _45 inSuence sea-bird distribution only on a small scale. _ The factors behind the range of response is of 55 interest in itself. _65 Nevertheless, fronts are important determinants 0 5 10 15 20 of prey capture. Two hypotheses have been pro- Distance (km) posed to account for this: (1) that frontal zones enhance primary production, which in turn in- Figure 2 The aggregation of foraging shearwaters at fronts, in creases prey supply, e.g. boundaries of cold- or this case the area where an elevation in bottom topography warm-core rings in the Gulf Stream; and (2) that forces a transition between stratified and well-mixed water. The bottom panel shows isotherms. (Redrawn from Hunt GL et al., frontal zones serve to concentrate prey directly into Marine Ecology Progress Series 141: 1}11.) exploitable patches, e.g. current rips among the Aleutian Island passages. general has long been related to seabird abundance Topographic Features and species composition. Depth-related differences Topographic features serve to deSect currents, in primary productivity explain large-scale patterns and can be sites of strong horizontal and vertical between shelf and oceanic waters. Within shelf sys- changes in current velocity, thus, concentrating prey tems themselves, several hypotheses explain changes through a variety of mechanisms (Figure 2). For in species composition and abundance with depth. example, seamounts are often sites of seabird ag- First, primary production may be diverted into one gregation, likely related to the fact that they are also of two food webs, benthic or pelagic, and this may sites of increased density and heightened migratory explain differences in organisms of upper trophic activity for organisms comprising the deep scatter- levels in inner versus outer shelf systems, e.g. the ing layer. A second example are topographic eastern Bering Sea. Alternatively, the fact that inter- features in relatively shallow water, including de- actions between Sow patterns in the upper water pressions in the tops of reefs and ridges across the column and bottom topography will be strong in slope of marine escarpments, which may physically inner shelf areas but decoupled in outer shelf areas, trap euphausiids as they attempt to migrate down- may result in differences in predictability of prey ward in the morning. A third example is the down- and consequently, differences in the species that stream, eddy effect of islands that occur in strong exploit them, e.g. most coastal shelf systems. Fi- current systems. nally, depending on diving ability and depth, certain Depth gradient itself is sometimes correlated with seabirds may be able to exploit bottom sub- increased abundance of seabirds, and water depth in strate, whereas others may not. SEABIRD FORAGING ECOLOGY 2639 1.0 1.0 Spring 1983 Spring 1983 0.8 Pack ice 0.8 Open water 0.6 0.6 0.4 0.4 0.2 0.2 * 0 * 0 PENA PETS PENC PRAN PTKG PENA PETS PENC PRAN PTKG FUAN PEAN PETC PEBL FUAN PEAN PETC PEBL 1.0 1.0 0.8 Winter 1988 0.8 Winter 1988 Pack ice Open water 0.6 0.6 * 0.4 0.4 0.2 0.2 0 0 PENA PETS PENC PRAN PTKG PENA PETS PENC PRAN PTKG FUAN PEAN PETC PEBL FUAN PEAN PETC PEBL Figure 3 The correspondence of various seabird species either to pack ice or open water immediately offshore of the ice; Scotia- Weddell Confluence, Southern Ocean. Abbreviations: PENA, AdeH lie penguin; FUAN, Antarctic fulmar; PETS, snow petrel; PEAN, Antarctic petrel; PENC, chinstrap penguin; PETC, pintado petrel; PRAN, Antarctic prion; PEBL, blue petrel; PTKG, Kerguelan petrel. (Redrawn from Ainley et al. (1994) Journal of Animal Ecology 63: 347}364.) Sea Ice Foraging Behavior A strong association of individual species and char- Most seabird species take prey within a half meter acteristic assemblages exists with sea ice features. of the sea surface. This they accomplish by captur- On one hand, certain species are obligate associates ing prey that either are airborne themselves (as a re- with sea ice; on the other, sea ice can limit access to sult of escaping subsurface predators, see below), or the water column, and in some cases, aggregation of are shallow enough that, to grasp prey, the bird dips seabirds near the ice margin is a simple response to its beak below the surface (dipping) or crashes into this barrier (Figure 3).