Potential for resource competition between juvenile groundfishes and salmon in the eastern Gulf of Alaska

Abstract Ecologically important juvenile groundfishes and salmon co-occur in the upper water column of the eastern Gulf of Alaska during the summer, a period when growth is critical to their survival. We quantified fine-scale spatial and trophic overlap of juvenile groundfishes (arrowtooth flounder, Pacific cod, walleye pollock, and rockfish) and salmon (piscivorous coho and Chinook as well as planktivorous pink, chum, and sockeye) to examine trophic structuring and potential survival bottlenecks for these fishes in the Gulf of Alaska between 2010 and 2014. In general, juvenile groundfish catches had low coherence with station environmental variables with the exception that across the years, rockfish were correlated with deeper station bottom depths. Juvenile salmon catches were correlated with station environmental variables such as shallower station depth and where the pycnocline was closer to the surface, higher chlorophyll levels, and lower salinities. The overall fish community in 2011 was disorganized compared to 2010 and 2012–2014 based on higher ordination stress and had poorer environmental correlations. Fine-scale spatial overlap among juvenile groundfishes and salmon populations was highest in 2011 and 2012 and lowest in 2013 and 2014. Juvenile rockfish had the least spatial overlap with the juvenile salmon due to their offshore distribution. Fine-scale diet overlap between juvenile groundfishes and planktivorous juvenile salmon species ranged from 0% to 78%, was highest in 2012, and was lowest in 2011. Fine-scale diet overlap among juvenile groundfishes and piscivorous juvenile salmon occurred but was typically lower than that with planktivorous juvenile salmon. Additionally, juvenile groundfishes were directly consumed by juvenile salmon. Neither the abundance nor stomach fullness of the juvenile planktivorous groundfishes or salmon correlated with station-level zooplankton biomass in 2012–13, suggesting a lack of a resource bottleneck for these planktivores in these 2 years. Juvenile groundfishes were less frequently caught at stations where the highest catches of juvenile piscivorous salmon occurred, which could be due to predation. Overall, during years when juvenile groundfishes survival was high, juvenile salmon survival was also high, suggesting sufficient food resources in the GOA. Alternatively, when food resources are low in the GOA, as seen in 2011, competition for resources by groundfish and salmon was likely, and both appeared to be negatively impacted.