Spatial proximity of age-0 walleye pollock (Theragra chalcogramma) to zooplankton near the Pribilof Islands, Bering Sea, Alaska

Acoustic surveys, conducted in September 1994 and 1995 in the neighbourhood of the Pribilof Islands, Alaska, collected data at three frequencies, making possible the location of pollock shoals and patches of zooplankton along the survey transect. These patches were identified using threshold and morphological filters on echosounder images taken at 38 kHz (fish identification) and 120 and 200 kHz (plankton identification). We checked the morphological methods by comparing the depth distribution of acoustically determined plankton with zooplankton from net surveys and found them in general agreement. Our ability to spatially map patches of plankton and shoals of fish (mostly pollock) along the survey transects led to our examining the spatial proximity between pollock and plankton patches. Results, using both interval- and distance-based measures, suggested that fish–plankton proximity was affected by plankton biomass. When the plankton biomass was low, fish tended to remain close to existing plankton patches, while at high plankton biomass there was no consistent small-scale proximity relationship. At intermediate plankton densities there was no particular distance-based proximity of plankton patches to fish shoals. However, the interval-based fish densities tended to increase with increasing plankton density up to some plankton density threshold, above which there was no clear association between fish and plankton density. These findings suggest the existence of plankton biomass density thresholds, both overall and within plankton patches that may influence pollock feeding strategies. They also suggest a possible method for empirically estimating these thresholds using multi-frequency acoustic survey data.