High gene flow in polar cod (Boreogadus saida) from West‐Svalbard and the Eurasian Basin

Abstract

The current and projected environmental change of the Arctic Ocean contrasts sharply with the limited knowledge of its genetic biodiversity. Polar cod Boreogadus saida (Lepechin, 1774) is an abundant circumpolar marine fish and ecological key species. The central role of polar cod in the Arctic marine food web warrants a better understanding of its population structure and connectivity. In this study, the genetic population structure of 171 juveniles, collected from several fjords off West-Svalbard (Billefjorden, Hornsund and Kongsfjorden), the northern Sophia Basin and the Eurasian Basin of the Arctic Ocean, was analysed using nine DNA microsatellite loci.Genetic analyses indicated moderate to high genetic diversity, but absence of spatial population structure and isolation-by-distance, suggesting ongoing gene flow between the studied sampling regions. High levels of connectivity may be key for polar cod to maintain populations across wide spatial scales. The adaptive capacity of the species will be increasingly important to face challenges such as habitat fragmentation, ocean warming and changes in prey composition. In view of a limited understanding of the population dynamics and evolution of polar cod, a valuable next step to predict future developments should be an integrated biological evaluation, including population genomics, a life-history approach, and habitat and biophysical dispersal modelling.

Distinct genetic clustering in the weakly differentiated polar cod, Boreogadus saida Lepechin, 1774 from East Siberian Sea to Svalbard

“…The suite of 116 polymorphic loci revealed a significant pattern of IBD driven by the easternmost Russian samples (East Siberian Sea and Laptev Sea), whilst no spatial population structure was detected in the region between the Kara Sea and Svalbard. This result agrees with the absence of spatial population structure and isolation by distance reported by Maes et al (2021) using nine microsatellites, which suggests ongoing gene flow in the region between the fjords off West Svalbard, the northern Sophia Basin and the Eurasian Basin of the Arctic Ocean. Likewise, previous studies showed small, but significant, differentiation along the Russian Arctic coast (Gordeeva and Mishin 2019) and lack of genetic structure within the Bering, Chukchi and Beaufort seas of Alaska, despite a consistent isolation by distance pattern (Wilson et al 2017).…”

“…Small but significant differentiation in polar cod along Russian Arctic coast (from the Kara, Laptev and East Siberian seas) was found with seven microsatellite loci (Gordeeva and Mishin 2019). Lack of spatial population structure and isolation by distance in juveniles sampled in the region between the fjords off West Svalbard, the northern Sophia Basin and the Eurasian Basin of the Arctic Ocean was also reported by Maes et al (2021) using a suite of nine microsatellites. Finally, a circumpolar study (Nelson 2020) conducted with the same set of nine microsatellite markers detected significant differentiation (F ST = 0.01, p < 0.01) with a geographically underlying basis across the species' entire range.…”

Population divergence manifested by genomic rearrangements in a keystone Arctic species with high gene flow

“…High genome-wide differentiation was detected when samples from the Barents Sea or the fjords were compared to Davis Strait, Canada (Table S1), with the highest FST estimate of 0.016 between Northwest Barents Sea summer 2018 and Davis Strait (Table S1). These results corroborate previous studies conducted on smaller sets of nuclear as well as mitochondrial markers, showing low levels of spatial population structure among polar cod sampled at a relatively large spatial scale 28,36 as well as in the Barents Sea and Arctic Ocean region 26,29 . Based on two hypothesized spawning grounds in the Barents Sea, i.e., in the northwestern Barents Sea and southeastern Barents Sea area 23–25 , our findings suggest that natal homing behavior is unlikely to be prominent, and rather indicates that there is high connectivity between different locations and/or sub-populations within the Barents Sea area.…”

“…For the Barents Sea area, previous studies have suggested that there are likely two spawning grounds, one in the Northwest of the Barents Sea and one in the southeast Barents Sea, adjacent to Novaya Zemlya [23][24][25] , Russia, suggesting that there could be at least two or more sub-populations within this region. Several studies have been carried out to characterize the population structure of polar cod throughout their circumpolar occurrence, most of which have utilized only a handful to hundreds of genetic markers, and the results have been inconsistent [26][27][28][29] . Limited population structure has been observed within the Barents Sea area [26][27][28][29] , however genetic differentiation between fjord populations on Svalbard and offshore Greenland specimens has been identified 27 .…”

“…Several studies have been carried out to characterize the population structure of polar cod throughout their circumpolar occurrence, most of which have utilized only a handful to hundreds of genetic markers, and the results have been inconsistent [26][27][28][29] . Limited population structure has been observed within the Barents Sea area [26][27][28][29] , however genetic differentiation between fjord populations on Svalbard and offshore Greenland specimens has been identified 27 . These results are indicative of possible local fjord populations, which is further supported by the evidence of local spawning populations and the differentiation of phenotypes between the western fjords of Svalbard [30][31][32][33] .…”