Developing an acoustic survey of euphausiids to understand trophic interactions in the Bering Sea ecosystem

Euphausiids (principally Thysanoessa spp.) are a key group of organisms in the Bering Sea ecosystem, linking production at lower trophic levels to top predators and important commercial fish stocks such as walleye pollock (Theragra chalcogramma). Here, we combine multifrequency acoustic survey methods, physics-based models of euphausiid backscatter, and net sampling to provide a means of monitoring the status and trends of euphausiid standing stock biomass on the Bering Sea shelf. Observations made using this approach during six summers (2004 and 2006–2010) indicate that standing stocks of euphausiids and pollock were inversely correlated over time as well as in space across the continental shelf. First-order calculations show that when pollock abundance was at its peak during these years, the pollock stock could have consumed 10–87% of the euphausiid standing stock between May and September. We hypothesize that predation by pollock is a significant top-down control on euphausiid standing stock in this system.