Seasonal dynamics of small vagile predators on a marine shallow soft bottom

The seasonal on- and offshore migrations of the epibenthic community on a marine shallow soft bottom in Gullmar Fjord (Sweden) was recorded for a period of three years. Quantitative sampling was carried out in shallow water (≤ 1 m depth) using drop-nets. A new sampling device was also introduced which allowed quantitative sampling in deeper water. The brown shrimp Crangon crangon L., juvenile plaice Pleuronectes platessa L. and the gobiid fish species Pomatoschistus minutus (Pallas) and P. microps (Krøyer) constituted the dominant inhabitants of shallow water, contributing >90% of the total number. The annual production was 2.2–2.9 g m⁻² yr⁻¹ dry wt with the brown shrimp as the dominant contributor. The abundance in shallow water fluctuated strongly between the seasons, while a relatively small seasonal variation was recorded in deeper water (2.5–10 m depth). The shallow/deep water abundance ratio for the epibenthic community heavily increased during the summer months, for the brown shrimp the ratio exceeded 200 in July. However, the heavy concentration in shallow water did not correspond to the reduced density of animals in deeper water. The boardering deeper water seemed to serve only as a transition area for animals arriving from or migrating to still deeper areas. Heavy mortality in the shallow nursery due to predation by the small epibenthic carnivores may also explain the low abundance in deeper areas during the autumn migration.