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Comparing herring population structure markers

Atlantic herring (Clupea harengus) has traditionally been divided into different stocks based on spawning behaviour and meristic or morphological characters. It is unclear, however, if these stocks represent reproductively distinct populations. Recently, the application of new molecular methods has proven useful to distinguish between populations of marine fish. Here, we compare the ability of four different genetic markers, allozymes, mitochondrial DNA, nuclear microsatellite DNA and immune defence genes (MHC) to discriminate between stocks of herring. MtDNA and microsatellites are supposedly selectively neutral, whereas allozymes and especially MHC may be under natural selection. Using the same individual fish for all genetic and morphological analyses, we compare samples (ca75 fish) collected at five different sites in two subsequent years in the Baltic, Skagerrak and the North Sea.

Microsatellites and MtDNA were the most powerful markers to detect differentiation among samples. For allozymes only one single sample was distinct from the rest, Tjome 2002, whereas MHC analysis indicated that the Rugen samples were differentiated from the rest. Microsatellites and to some extent MHC showed higher differentiation among localities than between temporally spaced samples indicating temporal stability of the population structure. Concordance among marker types appeared to be fairly low. This observation could be due to several factors that differ between marker types, such as levels of heterozygosity, allellic richness, mutation rates and susceptability to selective forces. The two marker types that potentially could be under selection, MHC and allozyme loci, showed distinct differentiation for certain samples. However, if this pattern could be linked to specific environmental conditions is presently unknown.

The population structure suggested by microsatellite data coincides with the geographical separation of Baltic, Skagerrak and North Sea locations. This finding together with the fact that microsatellite population structure was highly significant statistically, suggests that microsatellites are the most powerful genetic marker, among the ones investigated here, to distinguish between different stocks of herring.

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