Seasonal variation in the vertical migration of Pontoporeia affinis (Crustacea, Amphipoda)

Abstract

Vertical migration in Pontoporeia affinis Lindstrom was studied during the years 1974-1978 and 1980 on a location north of the Hanko Peninsula, Gulf of Finland, at a depth of 30 metres. The study was carried out using traps standing on the bottom, with a funnel directed upwards to catch animals descending after excursions from the bottom. The traps were used from the beginning of May to the middle of December. With this method, it was found that vertical migration mainly occurs from the middle of July to the beginning of October. The onset corresponds to the time when cessation of growth occurs, and it may thus be related to a depletion of the locally available food supply. During the main period of vertical migration, large week-to-week fluctuations occur that appear related to outand inward movements of the water caused by changes in the sea level. In many cases, the results indicate that the whole population takes part in the migration.