Coexisting Atlantic Cod Ecotypes in the Barents Sea: An Issue for Managing Fisheries.

The largest remaining cod stock in the Atlantic, the Northeast Arctic cod (NEAC), is facing climate change and poor recruitment. It is currently assumed to represent a single biological population. In coastal waters, both in Northern Norway and down to mid-Norway during the spawning season, NEAC overlaps with another cod ecotype, the conspecific coastal cod (CC), which is managed as a separate stock. Here, we analysed otoliths and conducted genotyping of 3900 cod to investigate population structure and potential ecotype mixtures in the Barents Sea. The annual surveys cover both the spawning and feeding areas of the NEAC stock with the purpose of collecting data for the annual stock assessment. While classification of individual cod to population-of-origin with the genetic markers currently available remains challenging, we-for the first time-infer the presence of CC and potential NEAC × CC hybrids also far offshore in the Barents Sea. Moreover, the CC were not homogenously distributed in the Barents Sea, but were higher in the west (including Svalbard) and in the far east, as well as near the Norwegian coast. Most of these putatively identified CC fish carried NEAC-type otoliths. Thus, cod otolith type seems a poor indicator of genetic ancestry and instead reflects environmental conditions. Many questions remain on the temporal stability of the NEAC and CC mixing in the Barents Sea, and not least, the management implications of such stock mixing. More generally, further studies are needed on the connection between within-species genetic differences and the implications of such differences for stock dynamics in a management context.