MOLECULAR-GENETIC ANALYSIS OF ATLANTO-SCANDIAN HERRING (CLUPEA-HARENGUS) POPULATIONS USING ALLOZYMES AND MITOCHONDRIAL-DNA MARKERS

Allozymes and restriction fragment length polymorphisms (RFLP's) in ge nes of the NADH dehydrogenase complex (ND 5/6 genes) of mitochondrial DNA (mtDNA) were used to investigate the spatial genetic structure of Atlanto-Scandian herring (Clupea harengus). Samples (sample size=46-50 /site) comprised Icelandic summer spawners, Norwegian spring spawners, and two fjord populations, Balsfjord and Trondheimsfjord, on the Norw egian coast. Fifty enzymes were assayed, and samples were routinely ex amined at 18 enzymes comprising 28 putative allozyme loci. Significant genetic differentiation (P < 0.001) was detected between Trondheimsfj ord herring and all other samples, with apparent genetic homogeneity a mong remaining samples, including the Balsfjord; a region shown previo usly to contain an allozymically distinct population. Herring from the Balsfjord were shown allozymically to represent the Norwegian spring spawning group, suggesting the presence of two stocks, a deeper reside nt spawning (Balsfjord) and a shallower, non-native feeding population (Norwegian spring spawning). Variation at the ND5/6 genes using six r estriction enzymes revealed 96 composite haplotypes in 196 fish. Mean haplotype diversity was high (0.92), though nucleotide divergence betw een samples was small (highest=0.00029, between Trondheimsfjord and Ba lsfjord samples; lowest=0.00003, between Icelandic summer and Balsfjor d samples). Monte-Carlo chi(2) analysis of haplotype frequencies revea led no significant geographic heterogeneity among samples, thus reveal ing a discordant pattern of genetic differentiation produced by allozy mes and mtDNA markers. The detection of a genetically divergent popula tion in deeper waters of the Trondheimsfjord supports the existence of distinct deep-water resident populations in the Norwegian fjords.