Abstract
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 (<3%). There was detected a highly significant differentiation among 25 out of 26 SNPs. The average FST over the loci was relatively high (0,12), demonstrating that genetically different groups of juvenile cod coexist along the Norwegian Skagerrak coast. There was a higher frequency of NS cod in more exposed outer coastal areas compared to inner sheltered fjords, but this varied both temporally and spatially along the coast and seemed to be strongly influenced by the strength of the Norwegian Coastal Current (NCC) from North Sea and into coastal Skagerrak. Also, juvenile cod assigned to NS was found to be significantly larger (~2 cm) than CC for all three years, both separately and all years pooled together. This indicates either a phenotypic response to different environmental conditions during development, or genetic differences in growth between the two groups. Nevertheless, the observed genetic variation between CC and NS imply some degree of isolation between the two groups. Spawning and retention of egg – and larvae within fjord basins that are sheltered from coastal currents, high site fidelity and natal homing of older cod may act as However, it is premature to conclude if the pattern that emerges is caused by environmental variation or by genetic differences, but the results are discussed in the light of new research findings.