Effects of genetic drift in a small population of Atlantic cod (Gadus morhua kildinensis Derjugin) landlocked in a meromictic lake: genetic variation and conservation measures

Abstract

Landlocked populations of normally salt-water, bottom-dwelling Atlantic cod exist in a few circumpolar meromictic lakes. One such population, the Kildin cod, inhabits Mogilnoe Lake (Kildin Island, the Barents Sea) that is listed among regional nature reserves. The lake has three main strata of water of about 5 m each: an upper level with nearly fresh water, a saline layer, and an anaerobic zone at the bottom. The fish lives in the saline stratum and appears in the fresh layer as well. Available food is poor, the adults heavily prey on cod juveniles. The Kildin cod has distinct morphological features, faster growth rate, earlier age of maturation, and shorter longevity that likely developed in response to the unusual lake environment. Genetic and capture-mark-recapture data show that the Kildin cod greatly differs from its marine counterparts: It has an effective population size of about one hundred and an average adult census size of about half a thousand, reproductively isolated from a parental marine population around 1800 years ago, lost a large portion of microsatellite and mitochondrial DNA variation due to long-term genetic drift, but still maintains variation at allozyme loci. DNA markers of the Kildin cod do not carry novel mutations, but their multilocus genotypes seem to be unique to the lake population. The suggested conservation strategy includes: (1) the lake and its drainage area should be strongly protected from any detrimental human activities; (2) the transplantation of any genetic material into the cod population of Mogilnoe Lake should be strongly prohibited.