Genetic identification of farmed and wild Atlantic cod, Gadus morhua, in coastal Norway

Each year thousands of Atlantic cod escape from Norwegian fish farms. To investigate the potential for the genetic identification of farmed-escaped cod in the wild, three case studies were examined. Samples of farmed, recaptured farmed escapees, and wild cod were screened for ten microsatellite loci and the Pan I locus. Variable genetic differences were observed among cod sampled from different farms and cages (pairwise F-ST = 0.0-0.1), and in two of the case studies, the most likely farm(s) of origin for most of the recaptured escapees were identified. In case study 2, wild cod were genetically distinct from both farmed fish (pairwise F-ST = 0.026-0.06) and recaptured farmed-escaped cod (pairwise F-ST = 0.029 and 0.039), demonstrating the potential to detect genetic interactions in that fjord. Genetic identification of escapees was more challenging in case study 3, and some morphologically characterized wild cod were found to most likely represent farmed escapees. It is concluded that where cod are farmed in the same region as their own parents/grandparents were initially sourced, or where farmed escapees originate from multiple sources, quantifying genetic interactions with wild populations will be challenging with neutral or nearly neutral markers such as microsatellites.