Population connectivity among migratory and stationary cod Gadus morhua in the Northeast Atlantic—A review of 80 years of study

Abstract

The population structure of Atlantic cod Gadus morhua L. in the Barents Sea and in fjords and coastal waters in Norway and northwestern Russia has been a controversial subject since the 1930s. Eight decades of scientific inquiry have compared migratory NE Arctic (NA) and stationary Norwegian and Russian coastal cod (NC). At one extreme the existence of 2 non-interbreeding groups is advocated, whereas others find support for low genetic differentiation due to substantial gene flow, with geographical distance being the limiting factor. We review studies of a wide range of phenotypic (e.g. growth, maturation, counts of vertebrae) and polymorphic genetic markers (e.g. allozymes, mtDNA, microsatellites, single nucleotide polymorphisms). Regardless of whether or not the observed differences have a genetic basis, 70% of the 54 reviewed papers conclude that NA and NC differ with respect to the characters studied. However, few papers remain after exclusion of those relying on characteristics or markers that are generally agreed to be subject to selection and therefore less suited to assessing population connectivity. The lack of studies examining the potential influence of environments on growth of annual zones in otoliths is surprising, since otoliths are used to categorize specimens in managing the NA and NC and in scientific papers. We conclude that it is still an open question whether NA and NC effectively make up 1 large population or >1 non-interbreeding 'groups'. As next-generation sequencing technology transforms 'population genetics' into 'population genomics', we will move towards a better understanding of differentiation among fish populations.

A genomic island linked to ecotype divergence in Atlantic cod

“…; Nordeide et al . ). In the eastern Atlantic, these ecotypes are well described in both Iceland and Norway.…”

High-resolution melting analysis of common and recombinant genotypes of the Atlantic cod (Gadus morhua) hemoglobin β1 gene in transatlantic populations

“…For proper management, the relationship between the multiple cod populations inhabiting the continental shelves and banks in most areas of the North Atlantic has been debated for more than 80 years (Nordeide et al 2011). Genetic studies of Atlantic cod were initiated during the 1960s after the pioneering report on the HbI-1/HbI-2 polymorphism in the oxygen-transporting hemoglobin (Sick 1961;Frydenberg et al 1965;Møller 1966).…”

Farmed cod escapees and net-pen spawning left no clear genetic footprint in the local wild cod population

“…However, the observed allele frequency differences for Gmo132 and Pan I are actually within the range of their natural temporal fluctuations in the Trondheimsfjord as reported by Karlsson & Mork (2003. It is generally accepted that Gmo132 and Pan I are under selection in Atlantic cod (reviewed in Nordeide et al 2011). In analyses of sample heterogeneity based on the full DNA loci set (exact G-tests), the prominent sources of significant allele frequency differences between Pre-farm and Post-farm samples were Tch11 and Pan I. Tch11 showed significant presence of null alleles and deviated strongly from HW equilibrium in the Pre-farm sample, signaling its unsuitability in the present analyses of introgression (Waples 2015).…”