Genetic insights into the population composition of two regional inshore mixed stocks of Atlantic cod (Gadus morhua) in West Greenland

Abstract

The recognition of biocomplexity in a species might be essential in order to achieve a more sustainable exploitation of fishery resources, especially in the light of anthropogenic pressures such as fishery exploitations and climate change. Nevertheless, the notion of a species comprising many genetically distinct locally populations are frequently neglected in fisheries management. Therefore, management strategies that are more focused on biological unit rather than area are essential for effective fisheries management and conservation. A better understanding off the genetic diversity of fished stocks is of paramount importance for creating the settings for management improvements and reliable predictions that help preserve future biodiversity and fisheries yields. The present thesis uses a catalogue of spatial population genetic approaches to explore the availability and methodology used for disentangling the genetic composition of mixed-stock samples. Primarily, the focus is to examine the spatial dynamics of Atlantic cod populations (Gadus morhua) in West Greenland. Provided contemporary samples collected by both "experts" (i.e. marine biologists) and "nonexperts" (i.e. fishermen), the spatial composition in inshore regions around Nuuk and Sisimiut was investigated. The application of a panel of 96 gene-associated Single Nucleotide Polymorphisms (SNPs) was used to assign individual fish and estimate proportional contributions to previously identified populations. Presented with different approaches for population genetics, tests of individual assignment and mixed-stock analysis showed that the suitability and application is case-specific. In particular, the evaluation of different approaches seems to be essential in the decision and selection of the most appropriate and accurate genetic tool. Here, the evaluation-process is presented and subsequently the most suitable approaches are chosen for estimating the origins at the level of individuals and populations. The individual assignment tests and mixed-stock analysis showed differences in the composition between the two regional inshore mixed stocks in West Greenland. Briefly, individuals originating from the Iceland Offshore population seemed to contribute significantly to inshore areas around Nuuk, and to a lesser degree around Sisimiut. The "West Greenland inshore" population was dominating both regions and constituted an important part of all sampled assemblages. Specifically, in the Nuuk region a small-scale genetic pattern emerged, where coastal and fjord samples showed substantially different genetic compositions. Most importantly, offshore individuals originating from both the "West Greenland Offshore" and "Iceland Offshore" populations were confirmed within the fjord system. These findings are associated with important management implications which are thoroughly discussed and recommendations are put forward. Furthermore, these studies illustrate relevant aspects that are associated with recent tagging and genetic data, but also highlight the lack of information on the poor-studied population component residing inshore. In conclusion, the results provide important insights into the genetic composition of different fished assemblages of cod in inshore waters of West Greenland and have important implications for fisheries management. These findings along with other related studies of marine fish stress the need to continuously track the genetic origin of exploited marine fish in mixed assemblages.