Dietary and spatial overlap among jellyfish and small pelagic fish in the eastern Bering Sea during recent marine heatwave conditions Marine Ecology Progress Series

Populations of scyphozoan jellyfish in the eastern Bering Sea (EBS) can grow rapidly within a single season and have fluctuated widely over recent decades. Understanding the role of jellyfish in the EBS ecosystem is required for fishery and ecosystem management; however, we lack direct measurements of the impact that changes in jellyfish biomass have had upon this ecosystem and its fish populations. We examined the role of jellyfish as competitors of juvenile forage fishes (herring and gadids) and juvenile salmonids by (1) examining the diets of the dominant scyphozoan jellyfish in the region, Chrysaora melanaster, and (2) estimating the dietary and spatial overlaps between jellyfish and major planktivorous pelagic fish taxa. Ocean sampling for diet analyses occurred in the summers of 2 contrasting years: 2014 (high jellyfish biomass, but only slightly above-average temperatures) and 2016 (low jellyfish biomass, anomalously high temperatures associated with a marine heatwave). Jellyfish diets primarily contained small copepods and pteropods and were generally more diverse, showing no significant overlap with the diet of small pelagic forage fishes, which consumed mainly euphausiids and other small fishes. Generally, jellyfish and small pelagic fishes showed low spatial overlap, but there was high spatial overlap between the small pelagic fish and salmonids, particularly in 2016. Spatial overlap and trophic interactions were not consistent throughout the EBS nor across years, although high overlap did occur between the species examined. These temporal variations in the degree of spatial overlap among zooplanktivores could result in resource competition in years or areas with low productivity or high jellyfish biomass.