Subsidized Predation on Imperiled Species: Gulls, Landfills, and Salmonids

Predator prey relationships are ubiquitous in any community. The population of one often regulates the population of the other, allowing for both populations to co-exist. However, when a novel food source subsidizes the predator, this regulation can be altered, allowing for predator populations to increase beyond what the original prey species can support. Subsidized predators can potentially drive prey species to extinction and is of special concern when the prey species is imperiled. In central California, local Western Gull populations have increased almost an order of magnitude in the last 30 years, perhaps in part due to subsidies from anthropogenic food sources such as landfills. Direct observations of gulls at local creek mouths confirmed gulls are predators of out-migrating juvenile salmonids, which consists of threatened steelhead and endangered coho salmon. Our research attempts to quantify the impact of gull predation on imperiled juvenile salmonids and explore how anthropogenic sources have subsidized gull predators.

In order to estimate the impact of gull predation on salmonids, we quantified minimum gull predation rates by scanning nearby Western Gull nesting areas for salmonid PIT (passive integrated transponder) tags that were regurgitated or defecated by gulls. These PIT tags were originally inserted into juvenile salmonids in nearby watersheds as part of NOAA Fisheries’ salmonid monitoring program, such that each tag deposited on gull nesting areas resembles a salmonid that was eaten and deposited by a gull. From 2002 to 2010, the recovery of PIT tags on gull nesting areas accounted for an average of 0.84% of the tagged population of juvenile salmonids in all watersheds combined. This recapture rate, in conjunction with a large-scale experiment to calculate excretion rate by gulls, was used to determine overall consumption rates of salmonids by gulls. Results from these experiments estimate gulls consume approximately 20% of juvenile salmonids. Stable isotope analysis of historic and recent gull feathers indicate that gull diets have shifted over the last 80 years. On average, Western Gulls are now significantly less enriched in δ¹⁵N than in the early 1900’s. Furthermore, gull isotope signatures are also more variable now than historically. These data suggest anthropogenic sources may be increasing available food sources for gulls, which may translate into increased gull predation and mortality of threatened and endangered salmonid populations.