Abstract
Fourhorn sculpin (Myoxocephalus quadricornis; Cottidae) from 17 localities in the Baltic Sea and one freshwater locality were examined for allozyme variability at 30 enzyme loci. In all, 34 samples comprising 3823 specimens from different years and geographic regions were analysed. Genetic variability, estimated as the fraction of polymorphic loci (P=9.5 %) and average heterozygosity (H=2.8 %), is slightly lower than the mean for bony fishes but well within the normal range. The largest differentiation observed was between freshwater and the brackish water populations (D=0.0067; NEI Am. Nat. 106: 283–292.1972); this does not support the contention of a preglacial ancestor and two mdependent invasions of freshwater and brackish water populations, but can equally well be explained by more recent divergence. Statistically significant spatial and temporal allele frequency heterogeneity was found among Baltic populations. The temporal allele frequency shifts were especially pronounced at highly polluted localities. and may indicate that reproductive units in the Baltic Sea are not geographically stationary.
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