MEPS 514:247-261 (2014)  -  DOI: https://doi.org/10.3354/meps10980

ABSTRACT: As top predators that feed on a wide range of prey items, gull diets may serve as important biological indicators of regional prey availability and changes in marine ecosystems. We studied the diets of herring gulls Larus argentatus and great black-backed gulls L. marinus on Sable Island, Nova Scotia, Canada, a remote colony which has shown high levels of contaminants in herring gull eggs and which has experienced significant ecological and anthropogenic change in its surrounding marine region over the past 40 yr. Analysis of regurgitated pellets suggested that current gull diets have proportionally less offshore prey (e.g. fish) and terns and tern eggs, and proportionally more molluscs, rock crabs Cancer borealis, and seal Halichoerus grypus carrion than diets sampled 40 yr ago. The composition of recent diets observed from pellet analysis is supported by stable isotope mixing models of carbon (δ¹³C) and nitrogen (δ¹⁵N), which revealed that great black-backed gulls had high proportions of seals and crab in their diets, whereas herring gulls had high proportions of crab, sand lance Ammodytes sp., and terrestrial invertebrates. Isotopic analyses also identified dietary variability through seasonal, age-specific and body condition relationships for each species. Biometric-isotope relationships showed that larger great black-backed gulls fed at higher trophic levels, and that higher trophic level foraging in herring gulls was associated with better body condition. Collectively, these results indicate dietary partitioning within this community of sympatrically nesting gulls, and broad-scale dietary shifts since the early 1970s.

KEY WORDS: Bioindicator · Dietary shift · Larus · Mixing models · Scavengers · Seals · Stable isotopes