Linking physics and biology - Structuring of cod-populations in the North Sea/Skagerrak water-system

We propose to study the physical and biological mechanisms that generate and maintain a highly genetically structured population of cod in the Skagerrak water system. We hypothesize that a combination of oceanographic features (i.e., within fjords water c irculation), spawning behavior (i.e. strong phylopatry), and ecological traits (i.e. lower mortality of locally retained early-life stages) contribute to maintain the observed genetic differentiation of the returning adults. It is thus proposed to (1) stu dy the inflow of North-Sea larvae to Norwegian Skagerrak fjords with focus on linking physics with population structure and dynamics, and (2) to study the maintenance of costal cod populations within fjords focusing on linking physics, behavior and biolog y. Issue (1) involves improving models of the linkages between North Sea spawning cod and local fjord populations of cod in the Skagerrak, explicitly considering the sub-structuring of the North Sea cod population and the variability of the oceanic dynami cs. Issue (2) involves a detailed field study of spawning site selection, egg retention, larval drift, and current patterns in two representative fjords, to understand the mechanisms for maintaining local populations. Furthermore issue (2) requires develo ping and analyzing evolutionary and physical models to assess the degree to which mature coastal cod spawn at optimal sites so as to maximize the probability of the spawning products staying within the fjord given the dynamics of the local water masses. O ur proposed work, besides being highly relevant to manage an economically important Norwegian resource, also has broader ecological relevance, in that it addresses an issue at the core of evolutionary biology: the generation and maintenance of genetic str ucture in marine fish populations.