Consumption of discards and interactions between Black-browed Albatrosses (Thalassarche melanophrys) and Kelp Gulls (Larus dominicanus) at trawl fisheries in Golfo San Jorge, Argentina

We evaluated discard consumption by Kelp Gulls (Larus dominicanus) and Black-browed Albatrosses (Thalassarche melanophrys) associated with trawlers operating at Golfo San Jorge, Argentina, and assessed differences in their feeding behaviour, prey size preferences and foraging efficiency. Observations were made onboard hake (January 2007) and shrimp vessels (November 2008). The Kelp Gull and Black-browed Albatross were the most frequent and abundant seabirds at both fisheries, while Argentine Hake (Merluccius hubbsi) dominated the composition of discards. Kelp Gulls and Black-browed Albatrosses consumed 91% of experimentally discarded hake (n = 1236). Mean sizes of prey consumed by gulls were 22.4 and 23.7 cm in the hake and shrimp fisheries, respectively, while those of prey consumed by albatrosses were 28.5 and 31.3 cm, respectively. In both fisheries, gulls selected the smaller prey available (<25 and <30 cm in hake and shrimp fisheries, respectively) while Black-browed Albatrosses selected the larger prey available (>25 and >30 cm in hake and shrimp fisheries, respectively). Intra-specific and interspecific kleptoparasitism were significantly more frequent than expected in Kelp Gulls and Black-browed Albatrosses, respectively. Robbing efficiency was clearly higher in albatrosses than gulls. In both species, sizes of prey consumed by direct capture were significantly smaller than those stolen, and the rate of kleptoparasitism increased with prey size. Gulls stole significantly smaller prey than albatrosses. Prey selection by Kelp Gulls is affected by the interaction with Black-browed Albatrosses, and the degree at which discards are used by both species appears to depend on the fishery considered, the food on offer and the selection of prey sizes according to seabird species-specific preferences.