Comparative foraging ecology of planktivorous auklets in relation to ocean physics and prey availability

Gl Hunt,Ko Coyle,Rw Russell,T Weingartner

doi:10.3354/meps167241

Abstract

We tested the hypothesis that the spatial distributions of foraging least, crested and parakeet auklets (Aethia pusilla, A. cristatella and A. psittacula, respectively) in the shallow passes of the Aleutian Islands would be determined by physical mechanisms that control near-surface prey concentrations. We recorded currents using an Acoustic Doppler Current Profiler, volume scattering using 200 and 420 kHz scientific echosounders, and the numbers of foraging birds. Zooplankton were sampled using a multiple opening/closing net and environmental sampling system (MOCNESS). Prey choice of birds was ascertained by collecting foraging birds and examining their stomach contents. Most sampling occurred between 8 July and 6 August 1993, when we conducted 50 passages along a transect that crossed a sill between Unalga and Kavalga Islands, western Aleutian Islands, thereby sampling the foraging activity of auklets at a variety of times of day and tidal phases. We found that the abundance of foraging individuals of each of the 3 auklet species was a function of tidal speed. Auklet species were selective about the species of prey taken. Regardless of tidal direction, crested auklets foraged on euphausiids upwelled on the upstream side of the pass, whereas least auklets consumed copepods concentrated in near-surface convergences on the downstream side. Parakeet auklets foraged over the top of the pass and took fish and invertebrates. Tidal speed and direction influenced the distance between the peak numbers of some, but not all, species of auklets. Auklet prey preferences dictated where they foraged in the pass and the physical mechanisms exploited for successful foraging. Thus, in this instance, resource partitioning by these closely related planktivores was enhanced by a spatial segregation forced by the physical processes that enhanced the availability of prey. Our findings emphasize the important role of physical processes in the structuring of marine communities.

Highlights

In the Northern Hemisphere, continental shelf regions support the majority of resident marine birds
We report on the foraging ecology of least (Aethia pusilla), crested (A.cristatella) and parakeet auklets (A. psittacula) and the physical mechanisms that influenced the availability of their zooplankton prey in an Aleutian Islands pass
The format~onof zooplankton patches occurs at the scale of fronts, tidal processes a n d river plumes and is dominated by physical processes (Pingree et al 1974, Herman et al 1981, Gagnon & Lacroix 1982, Franks 1997).Tidal currents flowing through passes can upwell zooplankton (Thompson & Golding 1981, Thompson & Wolanski 1984) and can subsequently concentrate them in vortices and secondary circulation cells (Wolanski & Hamner 1988, Wolanski et al 1988)

Summary

Introduction

In the Northern Hemisphere, continental shelf regions support the majority of resident marine birds. O Inter-Research 1998 Resale of full article not permitted by the interaction of currents with bathymetry which can produce spatially and temporally predictable concentrations of prey. We report on the foraging ecology of least (Aethia pusilla), crested (A.cristatella) and parakeet auklets (A. psittacula) and the physical mechanisms that influenced the availability of their zooplankton prey in an Aleutian Islands pass. The format~onof zooplankton patches occurs at the scale of fronts, tidal processes a n d river plumes and is dominated by physical processes (Pingree et al 1974, Herman et al 1981, Gagnon & Lacroix 1982, Franks 1997).Tidal currents flowing through passes can upwell zooplankton (Thompson & Golding 1981, Thompson & Wolanski 1984) and can subsequently concentrate them in vortices and secondary circulation cells (Wolanski & Hamner 1988, Wolanski et al 1988). Species of vertically migrating zooplankton may become concentrated when they swim against currents (e.g. Simard et al 1986, Coyle et al 1992),or they may be advected into shallow regions and become trapped against the bottom (Genin et al 1988).in coastal regions, zooplankton patchiness is llkely to be the result of interacting biological and physical factors (Haury et al 1978, Haury & Pieper 1987), and the strength of the physical process is likely to determine the extent to which it forces zooplankton patchiness

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Conclusion