Spatial and temporal variance of walleye pollock (Theragra chalcogramma) in the eastern Bering Sea

Abstract

Mobile acoustic surveys attempt to map and count aquatic organisms without biasing abundance estimates. Horizontal and vertical movements by target species may influence density measurements and net samples during acoustic surveys. To investigate the influence of fish movement on density data, we compared temporal and spatial variability of walleye pollock (Theragra chalcogramma) in three sets (2 night, 1 day) of 14.8-km transects in the eastern Bering Sea. Walleye pollock density distributions were also compared with those in the five nearest daytime survey transects. We found that horizontal density distributions did not change at temporal scales ≤4 h and that spatial variance remained consistent at scales ≤2.5 km. Spatial variance density patterns were similar in transects sampled during the day compared with those sampled at night and were also similar in along-shore compared with cross-shore transects. Transects that contained two biological scattering layers could be vertically separated into zooplankton and fish. Spatial variance patterns in the upper zooplankton layer mimicked those of passive tracers, while patterns in the lower layer were consistent with those previously observed for mobile nekton. Current sampling resolution of acoustic surveys adequately captures horizontal spatial variance of walleye pollock in the Bering Sea.