Seasonal and geographic variation in juvenile pink salmon diets in the northern Gulf of Alaska and Prince William Sound

Fish survival is often determined early in life and can be affected by diet and prey availability. To understand how climate changes affect food web dynamics and salmon survival, it is important to know the types of prey that salmon consume. We describe geographic and seasonal differences in the diets of juvenile pink salmon Oncorhynchus gorbuscha during 3 months of early ocean residence in Prince William Sound (PWS) and the Gulf of Alaska (GOA). Fish were sampled in PWS in July and in the GOA in July, August, and October. The diets and standard lengths of pink salmon varied significantly among stations within each sampling period. There was no discernable geographic pattern in fish length variability. Pink salmon in PWS (July) generally consumed small prey items, such as gastropods, cladocerans, small calanoid copepods, and bivalves, along with some large prey items, such as large calanoid copepods and larvaceans. In August, juvenile pink salmon sampled in the GOA consumed fewer small prey items, and most of their prey biomass consisted of pteropods Limacina spp., larvaceans, hyperiid amphipods, and euphausiids. Prey items consumed by fish sampled in October in the GOA were larger. The prey items that comprised the largest biomass were large pteropods Clio spp., large hyperiid amphipods, euphausiids, crab megalopae, and fish. Prey size increased over the three sampling periods. Fish shorter than 150 mm consumed prey smaller than 3 mg in weight, whereas larger fish tended to consume larger prey weighing up to 16 mg. Pink salmon diets differed among stations as little as 20 km apart.