Individual foraging location, but not dietary, specialization: implications for rhinoceros auklets as samplers of forage fish

Abstract

Forage fish are ecologically important to the marine environment, creating a wasp-waist trophic structure whereby many piscivorous diets are composed of a low diversity of mid-trophic species. Despite the importance of these species, knowledge of their abundance, distribution, and interactions with predators is often limited. On Middleton Island, Alaska, USA, we coupled biologging (GPS-depth logging) with long-term diet observations to examine the use of rhinoceros auklets Cerorhinca monocerata chick diets as an ecologically relevant method to sample forage fish in the Gulf of Alaska. Individual rhinoceros auklets did not specialize on select prey items but did specialize on 1 of 2 foraging sites, foraging either in coastal waters or the continental shelf. The majority of forage fish in bill-loads were captured in the top 15 m of the water column. Prey composition in auklet bill-loads varied among and within sampling years (2003−2016). Capelin Mallotus villosus dominated auklet diets during the cold-water period with negative Pacific Decadal Oscillation values (2008−2013), whereas Pacific sand lance Ammodytes hexapterus were more common in warmer years. Bill-load composition changed within years, with sand lance and capelin delivered to younger chicks, and larger fish, like sablefish Anoplopoma fimbria, being introduced as the chicks aged. Auklets returned with sand lance in foraging areas where trawling detected none, implying that auklets can potentially detect fish availability at finer resolutions than conventional methods. In conclusion, auklets responded to variation in forage fish among and across years. These changes in seabird diet could expand our knowledge of changes in availability of important prey species.