Daily variations in the occurrences of pelagic fishes and zooplankton within a coastal fishing ground of the southwestern Japan Sea

This paper analyzes daily variations in the occurrences of pelagic fishes and zooplankters on a small regional scale. The data used are based on daily observations repeated at 24h intervals during 6 days from 16 to 21, September, 1971, in coastal waters close to Esaki, a fishing port in the southwestern part of the Japan Sea (Yamaguchi Prefecture). The observations comprised acoustic surveys of the fish population, zooplankton sampling with a net, and Secchi disc readings, for evaluating relationships between the abundances of pelagic fishes and their food, as well as alternations in water masses. No clear correlation between pelagic fishes and their food, copepods, within a short time period was observed; fish population continued to decrease with eastward movements of fish schools although the abundances of copepods remained at almost the same level during the observations. Patterns of changes in both settled volumes of plankton and transparencies evidently suggested the occurrence of alternations in water masses. Pelagic fish schools proved to move in company with the transition of water masses.