| Summary: | Atlantic cod populations along the Norwegian Skagerrak coast consist of two known ecotypes. During the last decade, there has been a dramatic decline of larger cod and other piscivorous fish species in eastern Skagerrak. In my PhD-project, I set out to study the outer Oslo fjord seascape with a main focus on the Atlantic cod (Gadus morhua) populations residing in the system, with the aim to generate fundamental knowledge on ecology, and evaluate potential for restoration of local populations. We conducted studies on genetic structuring of the two known ecotypes of cod present in this area, and combined genetic and oceanographic data to reveal potential spawning sites. Moreover, the absence of top predators gave the opportunity to investigate how the ecosystem compared to a contrasting seascape harboring a system of MPAs. First, we studied the potential for natural selection to act on a specifically designed single nucleotide polymorphism (SNP) panel used to differentiate between the two ecotypes of Atlantic cod, testing if these high-graded SNPs were unreliable for discriminating populations. We found no evidence for selection and concluded that high-graded marker panels under putative natural selection indeed represent a valid tool for identifying population structure in this species. Second, we investigated the fish assemblage in outer Oslo fjord to quantify and characterize the species present and compared the observed patterns to a protected seascape in southern Norway. We found evidence of a mesopredatory release in outer Oslo fjord, likely resulting from fishing down of the larger top predator species. Average length of cod sampled in the partially protected seascape was significantly larger than for cod caught in outer Oslo fjord. Mesopredatory fish species was in general more variable in the outer Oslo fjord seascape, and more stable in the Tvedestrand MPA. Third, we used genetic data from 0-group and 1-group cod coupled with a biophysical model of ocean drift to infer likely sources of cod recruits to ...
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