Pilot study on electrophoretic variation and stock structure in the mackerel icefish, Champsocephalus gunnari, South Georgia waters

The use of allozymes as a tool in stock structure analysis was examined in the commercially-exploited mackerel icefish, Champsocephalus gunnari, together with a preliminary assessment of stock separation among samples in South Georgia waters.
Four samples (116 individuals) of icefish were collected from South Georgia waters, incorporating Shag Rocks, during January 1990, and subjected to starch gel electrophoresis. Thirty-one putative enzyme-coding loci were screened for enzyme polymorphisms. Clearly resolved enzyme phenotypes were obtained at 18 loci, of which 7 exhibited polymorphism at the 0.95 criterion ( α-glycerophosphate dehydrogenase, α-GPDH 1.1.1.8; hexokinase, HK 2.7.1.1; isocitrate dehydrogenase, ICD 1.1.1.42; malate dehydrogenase, MDH-III 1.1.1.37; mannose phosphate isomerase, MPI 5.3.1.8; phosphoglucose mutase, PGM-II 2.7.5.1; pyruvate kinase, PK 2.7.1.40). Routine scoring of 116 fish at the 18 loci revealed average levels of genetic variability, with the proportion of polymorphic loci, P = 0.278, mean heterozygosities per locus, HL = 0.089, and the effective number of alleles, Ne = 1.05 to 4.72.
Allozymic analysis of icefish samples (N = 18 to 42 per sample) indicated stock separation on a local scale. Genetic differentiation of samples was supported by the markedly non-random genotypic distributions (Wright’s fixation index, FIS) detected at most loci which were due mainly to heterozygote deficiencies. There was a positive association between the extent of geographic separation and genetic divergence as estimated by Nei’s mean genetic distances (D = .007 - .057) and identities (I = 0.993 - 0.945). Individuals collected from Shag Rocks were most genetically distinct, providing preliminary evidence of stock separation from South Georgia-shelf fish. The degree of reproductive isolation among samples is as yet uncertain, and requires analysis of larger sample sizes with more comprehensive electrophoretic screening.
Genetic data indicate that localised stocks of C. gunnari exist in South Georgia waters. The questions posed by the non-equilibrium genotypic distributions and genetic differentiation on a local scale require urgent attention if the species is to be managed effectively. Proposals for an electrophoretic and biological analysis of stock structure are presented.