Mass and energy transfer to seabirds in the southeastern Bering Sea

Abstract

It has been hypothesized that differentiation in food web structure occurs across the Bering Sea continental shelf as a result of seasonal differentiation of water masses. We tested this idea using an apex predator, pelagic birds. Seasonal abundance of birds in central Bristol Bay was estimated from counts made while underway between hydrographic stations. Prey and body mass were determined from birds collected at sea. Daily intake was estimated as an allometric function of body mass. Annual occupancy was estimated as the integral of a normal curve fit to seasonal data. Estimated carbon flux to seabirds in the middle domain was 0.12 gC m −2 y −1 in 1980, 0.18 gC m −2 y −1 in 1981. Carbon flux to seabirds in the adjacent waters of the outer shelf domain was 1.8 times higher than in the middle domain in 1980, 1.6 times higher in 1981. Carbon flux to seabirds in the inner domain was 1.2 times higher than in the middle domain in 1980, and 3.3 times higher in 1981. Carbon flux to seabirds in the outer domain was due primarily to non-diving species, principally northern fulmars ( Fulmarus glacialis) during the summer and autumn, and Larus gulls in the autumn and winter. Flux to seabirds in the inner domain was due to diving birds, principally murres ( Uria sp.) in the spring and shearwaters ( Puffinus sp.) during the summer. The euphausiid Thysanoessa raschii was the primary food source of shearwaters in shallow waters of the inner shelf domain. A more diverse set of prey, including squid, jellyfish, hyperiids, and fish, was taken by shearwaters and fulmars in the deeper waters of the outer and middle shelf domains. This result suggests that prey diversity is higher in seasonally stratified waters of outer Bristol Bay than in mixed waters of inner Bristol Bay. Greater energy flux to diving species in shallow water, and greater energy flux to non-divers in deep water may be a function of topographic control of prey patchiness.