YEARLY IMMIGRATION OF PANDALUS-BOREALIS (KROYER) INTO PERIODICALLY ISOLATED FJORD POPULATIONS

This paper discusses the effects of deep water renewal, and stagnation on the demography of a stock of Pandalus borealis in Gullmarsfjorden, a fjord with shallow sill on the Swedish west coast. It is based on data extracted from a four year series (1984-1987) of trawl samples obtained at approximately monthly intervals. During deep water stagnation the fjord contains true, albeit shortlived (8-12 months) populations/demes of P. borealis. Differences in catch per unit effort, and length frequency distributions after deep-water renewal in the fjord indicate that new demographic patterns were created by immigration of older shrimp and by varying recruitment of new year classes. Immigration is most likely facilitated by strong advective flows over the sill area during water renewal coinciding with the winter-spring onshore migration of shrimp in the Skagerrak. The well defined nature of the stagnation populations suggests that the fjord system may be well suited for the study of population processes of this economically important and theoretically interesting species.