Density dependence, prey accessibility and prey depletion by fisheries drive Peruvian seabird population dynamics

In marine ecosystemstop predator populations are shaped by environmental factors affecting their prey abundance. Coupling top predators' population studies with independent records of prey abundance suggests that prey fluctuations affect fecundity parameters and abundance of their predators. However, prey may be abundant but inaccessible to their predators and a major challenge is to determine the relative importance of prey accessibility in shaping seabirdpopulations. In addition, disentangling the effects of prey abundance and accessibility from the effects of prey removal by fisheries, while accounting for density dependence, remains challenging for marine top predators. Here, we investigate how climate, population density, and the accessibility and removal of prey (the Peruvian anchovyEngraulis ringens) by fisheries influence the population dynamics of the largest sedentary seabirdcommunity (approximate to 4 million individuals belonging to guanay cormorant Phalacrocorax bougainvillii, Peruvian boobySula variegata and Peruvian pelican Pelecanus thagus) of the northern Humboldt Current System over the past half-century. Using Gompertz state-space models we found strong evidence for density dependencein abundance for the three seabirdspecies. After accounting for density dependence, sea surface temperature, prey accessibility (defined by the depth of the upper limit of the subsurface oxygen minimum zone) and prey removal by fisheries were retained as the best predictors of annual population size across species. These factors affected seabirdabundance the current year and with year lags, suggesting effects on several demographic parameters including breeding propensity and adult survival. These findings highlight the effects of prey accessibility and fishery removals on seabirdpopulations in marine ecosystems...