MEPS 363:15-28 (2008)  -  DOI:

Trophic relationships and oceanography on and around a small offshore bank

Peter T. Stevick1,2,*, Lewis S. Incze1,**, Scott D. Kraus2, Shale Rosen3, Nicholas Wolff1, Adam Baukus1

1Aquatic Systems Group, University of Southern Maine, and 3Gulf of Maine Research Institute,
350 Commercial Street, Portland, Maine 04101, USA
2Edgerton Research Laboratory, New England Aquarium, Central Wharf, Boston, Massachusetts 02110, USA

ABSTRACT: Small offshore banks may be sites of intense feeding by upper trophic level predators. We studied the distribution of cetaceans, seabirds, pelagic fish, euphausiids and zooplankton over a 9 × 15 km bank to determine the conditions and processes that concentrated prey there and to examine the relative importance of bottom-up or top-down controls. Euphausiids were the primary prey during most foraging activity. While these were widespread in subsurface waters, foraging was concentrated on dense surface swarms that formed during daylight hours over 2 small crests. Internal wave passage resulted in upward movement and concentration of euphausiids in these areas through a coupling of physical processes and euphausiid behavior, resulting in surface swarms. Thus, internal waves appear to provide a critical mechanism enhancing trophic energy transfer. The formation of dense, localized and accessible prey concentrations was more important to foraging than was the overall available prey biomass. The estimated maximum daily consumption of euphausiids by cetaceans, seabirds and herring combined was <0.4% of the estimated instantaneous euphausiid biomass, and top-down control was unlikely to have substantially influenced euphausiid biomass at this site. Some predator species that do not prey extensively on euphausiids or herring were more prevalent in off-bank waters. The scales of predictability and the temporal dynamics of such features determine the manner in which populations of upper trophic level organisms utilize a variable environment.

KEY WORDS: Foraging ecology · Euphausiid · Internal wave · Consumption · Cetacean · Seabird

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Cite this article as: Stevick PT, Incze LS, Kraus SD, Rosen S, Wolff N, Baukus A (2008) Trophic relationships and oceanography on and around a small offshore bank. Mar Ecol Prog Ser 363:15-28.

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