Abstract
Atlantic cod (Gadus morhua) is one of the most important fish species in northern Europe for several reasons including its predator status in marine ecosystems, its historical role in fisheries, its potential in aquaculture and its strong public profile. However, due to over-exploitation in the North Atlantic and changes in the ecosystem, many cod populations have been reduced in size and genetic diversity. Cod populations in the Baltic Proper, Kattegat and North Sea have been analyzed using a species specific single nucleotide polymorphism (SNP) array. Using a subset of 8,706 SNPs, moderate genetic differences were found between subdivisions in three traditionally delineated cod management stocks: Kattegat, western and eastern Baltic. However, an FST measure of population differentiation based on allele frequencies from 588 outlier loci for 2 population groups, one including 5 western and the other 4 eastern Baltic populations, indicated high genetic differentiation. In this paper, differentiation has been demonstrated not only between, but also within western and eastern Baltic cod stocks for the first time, with salinity appearing to be the most important environmental factor influencing the maintenance of cod population divergence between the western and eastern Baltic Sea.
Highlights
Sustainable exploitation of living marine resources by fishery, aquaculture and biotechnology, and monitoring and predicting the effects of climate changes require an understanding of taxonomy and population biology
Genomic rearrangements in cod populations differing in ecological behaviour, such as migration routes have been found[42], and a subset of the single nucleotide polymorphism (SNP) loci analyzed in this study has been reported as significantly correlated with temperature in North Atlantic cod populations[43,44]
Besides the important physiological differences between western and eastern Baltic cod such as haemoglobin polymorphisms[63,64], genetic structure differences were reported at the level of population markers, e.g. microsatellites, Pan I locus, mtDNA29,65–67 and using SNP analysis[62,68,69]
Summary
Sustainable exploitation of living marine resources by fishery, aquaculture and biotechnology, and monitoring and predicting the effects of climate changes require an understanding of taxonomy and population biology. A large number of SNP loci in comparison with few genetic markers are better able to report subtle differences in genomic variation and their robustness is an advantage in evolutionary and population biology studies including exploited species with high dispersal potential in the oceans[16,17,18]. The Baltic was transformed from a freshwater lake receiving melting ice waters into today’s brackish water sea body over 7000 years ago It was colonized by a variety of marine species populations[47,48], which could tolerate salinity as low as 5–7 ppt, including Atlantic cod. Significant transport of cod larvae from the North Sea to Skagerrak and Kattegat[57,58] and mixing of western and eastern stocks of Baltic cod in the Arkona Basin (ICES subdivision 24) have been reported[22,59,60,61,62]. In recognition of a serious threat to the eastern Baltic cod stock, fishing for cod in ICES subdivisions 24, 25 and 26 has been banned by the European Commission in 2019, and restricted beginning on 1st January 2020
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