Spatial and temporal distribution of two groups of juvenile Atlantic cod (Gads morgue) at the Norwegian Skagerrak Coast

Spatial and temporal scales of phenotypic and genotypic variation in marine systems are fundamental to understand the ecological and evolutionary processes influencing the system. Until recently, local adaptation in marine fishes have been believed to occur on broad geographic scales due to lack of obvious physical barriers, mobile life stages and high dispersal ability compared to terrestrial and freshwater systems. However, recent studies have challenged these beliefs since strong evidence of geographically structured local populations have been found. Here, I study the spatial and temporal variation in two groups of juvenile Atlantic cod (Gadus morhua), coastal cod (CC) and North Sea cod (NS), both situated along the Norwegian Skagerrak coast. A total of 2113 juvenile cod were sampled in three different years (2000, 2001, and 2003) along the Skagerrak coast during an annual beach-seine monitoring program conducted by the Institute of Marine Research at Flødevigen Research station. By using genetic markers (Single Nucleotide Polymorphisms, or SNPs) that were designed to segregate well between the two groups, I was able to assign the individuals to either North Sea (NS) or coastal origin (CC) based upon adult reference samples from the two areas with a relatively high precision rate (