Impact of the water structure in Peter the Great Bay (Japan Sea) on seasonal migrations of sandfish <i>Arctoscopus japonicus</i>

Abstract

On the background of water structure, general patterns of the sandfish Arctoscopus japonicus distribution in Peter the Great Bay are determined on the data of bottom trawl surveys in spring, summer and fall seasons. When the larval stage is finished, the underyearlings of sandfish appear in the trawl catches since July. Depth of their catches by bottom trawl in July-August is 23–60 m, under the temperature of 8–16 о С. Localization of the catches at the upper benthic front corresponds to their pelagic spreading with concentration on the seasonal thermo-pycnocline, so the belts of high catches by bottom trawl are located along the line of the thermocline contact with the bottom. They convert to bottom dwelling in fall season, being concentrated below the benthic front and shifting deeper gradually with the front. The adults dwell at the bottom, permanently in the subsurface waters. Their feeding starts near the coast in May-June when they aggregate below the upper benthic front, avoiding the remnants of the cold Bottom Shelf water mass. Under thickening of the warm surface layer in July-September, they retreat deeper and spread widely at the shelf bottom within the area occupied by the Subsurface Shelf water. Pre-spawning aggregations of adult sandfish are formed at the shelf edge and upper continental slope in October, whereas the non-spawning fish stay in this month on the shelf together with the underyearlings. The adults migrate to the coastal zone in November for spawning on seaweeds. The southern part of the bay is presumably more favorable for the spawning of sandfish, as the underyearlings are absent in its northern part in summer. The sandfish adults wintering is available outside the bay, so the southward migration is supposed in late autumn and the backward migration in spring.