A cross-shelf gradient in δ¹⁵N stable isotope values of krill and pollock indicates seabird foraging patterns in the Bering Sea

Abstract

Concurrent measurements of predator and prey δ¹⁵N isotope values demonstrated that a cross-shelf isotopic gradient can propagate through a marine food web from forage species to top-tier predators and indicate foraging areas at a scale of tens of kilometers. We measured δ¹³C and δ¹⁵N in muscle tissues of thick-billed murres (Uria lomvia) and black-legged kittiwakes (Rissa tridactyla), and in whole body tissues of walleye pollock (Gadus chalcogrammus) and krill (Thysanoessa spp), sampled across the continental shelf break in the Bering Sea in 2008 and in 2009. We found significant basin-shelf differences at fine scales (<100 km) in δ¹⁵N among murres but not kittiwakes, and no such differences in δ¹³C in either seabird species at that scale. We then quantified the multi-trophic signal and spatial structure of a basin-shelf δ¹⁵Nitrogen gradient in the central and southern Bering Sea, and used it to contrast foraging patterns of thick-billed murres and kittiwakes on the open ocean. Seabird muscle δ¹⁵N values were compared to baselines created from measurements in krill and pollock tissues sampled concurrently throughout the study area. Krill, pollock, and murre tissues from northern, shallow, shelf habitat (200 m) to the south and west. Krill δ¹⁵N baseline values predicted 35–42% of the variability in murre tissue values. Patterns between kittiwakes and prey were less coherent. The persistence of strong spatial autocorrelation among sample values, and a congruence of geospatial patterns in δ¹⁵N among murre and prey tissues, suggest that murres forage repeatedly in specific areas. Murre isotope values showed distinct geospatial stratification, coincident with the spatial distribution of three colonies: St. Paul, St. George, and Bogoslof. This suggests some degree of foraging habitat partitioning among colonies.Keywords: Isotopes, Rissa tridactyla, Food web, Shelf edge, Habitat partitioning, Kittiwake, Murre, Uria lomvi