Trophic niche variation throughout the year in the opportunistic Kelp Gull at different trophic scenarios in Patagonia, Argentina

LISNIZER, N.; GATTO, A.; YORIO, P.

Conferencia; 3rd World Seabird Conference; 2021

Changes in foraging strategies throughout the yearare key determinants of seabird population dynamics. However, due to seabirdpost-breeding dispersal, year-round trophic ecology studies rarely focus onindividuals from known colonies. Using stable isotope analysis (δ15N, δ13C)of blood and primary feathers, we tested during 2012 the trophic nichevariation throughout the year in Kelp Gulls Larus dominicanus fromthree colonies in Patagonia, Argentina, located in areas with different availabilityof anthropogenic food subsidies. Isotopic niche width, measured usingmultivariate ellipse-based metrics, was wider at all colonies during thenon-breeding season, suggesting that during this period gulls forage on agreater variety of food sources possibly related to post-breeding dispersal,seasonal changes in food resources, and/or the relaxation of central placeforaging. Individuals from the colony with year-round availability of fisheriesdiscards (Isla Vernaci) presented the lower isotopic niche width year-round comparedto those from the other two colonies, one with discards only available duringthe breeding season (Punta Tombo) and the other with only open-air refuse dumpsavailable all year-round (Punta León). At Isla Vernaci, an abrupt expansion inniche width occurred later in the non-breeding season, after the start of afisheries moratorium, suggesting that Kelp Gulls remained in nearby areas andpartly associated to trawl fisheries between breeding events. Preliminary dietresults indicate that most studied individuals from Isla Vernaci were feedingon demersal prey, only available from fisheries discards as Kelp Gulls aresurface feeders. These results suggest that the availability of discards can greatlyinfluence Kelp Gull foraging strategies and non-breeding movements, and thatchanges in niche metrics can be context dependent. Future studies shouldcomplement the assessment of niche variability with studies of diet compositionthroughout the year.