Inshore acoustic surveys in the eastern and central Gulf of Alaska

摘要

A series of daytime replicate (spring/summer/fall) acoustic surveys were conducted at 11 inshore sites along the Kodiak Island/Kenai Peninsula area and the outer coast of Southeast Alaska as part of the Gulf of Alaska Integrated Ecosystem Research Program in 2010, 2011, and 2013. A two-frequency technique was used to classify backscatter as 'fish' or'macrozooplankton' based on the observed relative frequency response, which are used as proxies for the abundance of fish with swimbladders and large-bodied zooplankton. There was a strong 'site effect'; that is, consistent differences among sites. However, acoustic backscatter classified as fish and macrozooplankton was highly variable among repeat visits. The effects of site (i.e. sampling location) were larger than those of season or year. There were no consistent differences in backscatter between sites in the Kodiak Island/Kenai Peninsula area and Southeast Alaska. The acoustic proxies for the abundance of fish and large-bodied zooplankton increased substantially with increasing bottom depth over a depth range of 5-250 m, both within and across inshore sites. Backscatter from both fish and macrozooplankton was low at water depths < 80 m. In the inshore Gulf of Alaska, water depth appears to be a key characteristic structuring pelagic communities during daytime with sound-scattering fishes and large-bodied zooplankton being scarce in relatively shallow inshore habitats compared to adjacent deeper habitats.