Peruvian boobies: unbiased samplers of prey availability and complementary indicators of population size structure of Peruvian anchoveta to fishery landings data

Seabird diets serve as valuable tools for monitoring forage fish populations, particularly when fishery-dependent data overlook key segments such as juvenile cohorts. In this study, we evaluated whether Peruvian booby (Sula variegata), a predator of anchoveta (Engraulis ringens), could be used as an ecological indicator of anchoveta size structures during the early industrial fishing season in northern Peru. We analysed regurgitated prey samples derived from tagged birds in combination with GPS tracking, bird-borne video footage, and official fishery landing and effort data. Anchoveta length distributions in booby diets revealed a high proportion of juveniles (< 12 cm), which were significantly underrepresented in concurrent fishery landings. Despite occasional interactions with fishing vessels, the size of the prey consumed did not differ significantly with the vessel presence status, suggesting that boobies forage largely independently of the industrial fleet. Our findings are consistent with other studies that have shown that seabirds effectively track fish recruitment dynamics through their diets. Additionally, the spatial segregation of anchoveta, with juveniles being inshore and adults being offshore, may explain the discrepancies between seabird and fishery size data. These results underscore the utility of the Peruvian booby as a sentinel species for detecting early recruitment signals in anchoveta populations. We propose that systematic seabird diet monitoring, particularly at the beginning of the fishing season, can provide complementary insights into fish population structures and strengthen ecosystem-based fishery management, especially in data-limited or logistically constrained settings.