The inshore demersal fish community on the Swedish Skagerrak coast: regulation by recruitment from offshore sources

Abstract

To elucidate the species composition, size structure and abundance of inshore demersal fish community on the Swedish Skagerrak coast relative to the offshore community, a series of seven trawl surveys have been made in 2000-2001 and the results have been compared with scattered information from historic sources. The results show that abundance of fish >30 cm is presently extremely low for most long-lived species compared to historical records from the 1920s to 1970s. Cod, haddock, whiting, plaice, dab, long rough dab all showed a marked shift of the size spectra to the smaller sizes. Flounder was a notable exception: the size distribution had even widened and included more smaller as well as larger fish than in the historical records. Because flounder is the only long-lived species that is coast-bound and stationary, local fishing activity is apparently not responsible for the observed shift in the less stationary species. Also, fishing in the coastal zone has presumably been reduced because of the absence of the size range suitable for consumption. Overall, the demersal fish catches were dominated by immature fish that disappear when they grow older and most likely migrate offshore. The persistently high abundance of juvenile fish and the absence of adult fish suggest that the inshore demersal fish populations are presently regulated by recruitment from offshore sources, while historical information indicate that spawning aggregations of several of these species were common in these areas. It was hence hypothesized that a major change of the inshore demersal community has taken place during the last twenty years: local (sub-)populations of demersal fish have been eradicated, and the inshore area has become more and more dependent on transport of recruits from offshore spawning areas.