In this study, we aimed to delineate evolutionarily significant units ( ESUs) and management units ( MUs) for the European bullhead in Flanders ( Belgium). Therefore, we determined the genetic interrelationships between 11 bullhead populations, using length variation at 7 polymorphic microsatellite loci and sequence variation in the d- loop of the mitochondrial DNA ( mtDNA). Despite the relatively small geographical scale of our study, the analysis of the d- loop sequences shows that the Flemish bullhead populations contain 3 haplotype groups, which can be assigned to 3 previously described European mtDNA clades. Because of the important differences between these clades, they may be defined as evolutionarily significant units, which should be managed separately. Analysis of microsatellite data reveals very high degrees of isolation between populations, with the exception of 3 pairwise comparisons which involved adjacent populations. Our data suggest that the 3 haplotype groups probably qualify as ESUs, as they show phylogeographic differentiation for mtDNA variants as well as significant divergence of allele frequencies at nuclear loci. However, one of these units, limited to a single population, may be of Central European origin. All populations of the Scheldt basin meet the criteria for MU recognition, since significantly different microsatellite allele frequencies as well as private alleles are found. In contrast, genetic differentiation among the 3 populations of the Meuse basin is very low.
