Use of winter habitat at an early age: spatial ecology and association with human activities of juvenile Olrog's Gull Larus atlanticus

Olrog's Gull Larus atlanticus is an endemic species of the Atlantic coast of southern South America and is currently listed as 'Near Threatened' on the IUCN Red List. The species breeds in coastal wetlands of Buenos Aires and Chubut provinces of Argentina and during winter migrates northwards as far as southern Brazil. During the breeding season the species is considered to have a specialised diet, foraging mostly on crabs. However, during the non-breeding season the gull presents a wider dietary spectrum (including fish) as a result of a potential association with commercial and sport fishing activities. The main goal of this study was to analyse the habitat use and overlap with natural and anthropogenic food resources of juvenile Olrog's Gull during the winter in Mar Chiquita Lagoon, Buenos Aires. Twenty-two GPS devices were deployed during the winters of 2013 and 2014. A total of 1,088 fixes was gathered by GSM (both seasons combined). Analysis showed that the individuals were (kernel 95%) distributed in a limited area along the mouth of the lagoon. The core area (kernel 50%) covered only 0.3 km(2) and was located in the south of the mouth. The overlap between juveniles and crab densities was lower than expected, while the overlap between juveniles and fishing effort was higher than expected during weekdays and lower during weekends. These results are of value for the further development of marine conservation measures for wintering areas which have so far been somewhat neglected while addressing the conservation status of the species.