1. ¹Institute of Coastal Research, Swedish Board of Fisheries, Öregrund, Sweden
2. ²Population Biology and Conservation Biology, Department of Ecology and Evolution, Evolutionary Biology Centre, Uppsala University, Uppsala, Sweden

Correspondence: Dr A-B Florin, Institute of Coastal Research, Swedish Board of Fisheries, Box 109, Öregrund SE 742 22, Sweden. E-mail: ann-britt.florin@fiskeriverket.se

Received 6 December 2006; Revised 10 January 2008; Accepted 11 March 2008; Published online 7 May 2008.

Abstract

We found significant population structure and isolation by distance among samples of flounder (Platichthys flesus) in the Baltic, Kattegat and Skagerrak seas using microsatellite genetic markers. This pattern was almost entirely due to a difference between flounder that have demersal spawning in the northern Baltic, as compared to pelagic spawners in the southern Baltic and on the west coast of Sweden. Among demersal spawners we found neither genetic differentiation nor any isolation by distance among sampling sites. We speculate that demersal flounder are descendants of a population that colonized the Baltic previous to pelagic spawners. The demersal flounder may thus have had longer time to adapt to the low salinity in the Baltic, and accordingly display egg characteristics that make it possible to reproduce at the low salinity levels in the northern Baltic. Among pelagic spawners significant isolation by distance was detected. Pelagic spawners have previously been shown to display clinal variation in egg size, which allows them to float also at the moderate salinity levels up to the region north of the island Bornholm. Management units for harvesting should ideally be based on true biological populations, and for the commercially important flounder up to 15 different management stocks in the Baltic have been suggested. We could not find a population genetic foundation for such a high number of management units, and our data suggest three management units: the northern Baltic (demersal populations), southern Baltic with the Öresund straits and the most northwestern sampling sites (Skagerrak, Kattegat and North Sea).