Long-term shifts in anthropogenic subsidies to gulls and implications for an imperiled fish

Abstract

Over the last century, human activities have altered coastal ecosystems by fishing through the marine food web and increasing anthropogenic resources (e.g. landfills), both of which may alter native predator–prey interactions. We conducted a 100-year retrospective stable isotope analysis to investigate temporal shifts in relative resource use and individual variation of a generalist seabird (Western Gull, Larus occidentalis) and the implications of gulls' shifting resource use on one of their native prey—threatened steelhead populations (Oncorhynchus mykiss). We applied a Bayesian mixing model (MixSIAR) to historical (early 1900s) and modern (early 2000s) populations of generalist gulls and compared changes in resource use to a piscivorous seabird population (Brandt's Cormorant, Phalacrocorax penicillatus) in Monterey Bay (California, USA). δ15N significantly declined for both seabird species, suggesting shifts to lower trophic-level marine prey in the last 100years. The shift in δ15N was significantly larger for Western Gulls, suggesting a shift in gull resource use to prey not in the marine environment. Mixing models suggest anthropogenic resources (e.g., landfills) comprise the majority of modern gull diet (0.31; 0.18–0.44 95% CI), whereas it contributed relatively little to gull diet in the early 1900s (0.10; 0.01–0.26 CI). Furthermore, we found although steelhead contribute relatively less to overall modern gull diet, increasing gull populations and simultaneous steelhead population decline likely results in increased per capita predation risk to modern steelhead populations—our best estimate is that modern predation risk is ~2.4 times higher than historically, but this estimate depends on parameter values and overlaps with zero. This study highlights the impact of human activities on coastal predators and the potential consequences for native imperiled prey.