Ecological and genetic impact of Atlantic cod larval drift in the Skagerrak

We evaluate the hypothesis that Atlantic cod larvae are passively transported by sea currents from off-shore spawning areas to settle in coastal waters, a hypothesis which has recently gained support from genetic analysis of cod in the North Sea-Skagerrak area. Such larval transport has been suggested to be an important mechanism behind the commonly observed low spatial genetic differentiation in many marine organisms. Here, we apply an ARMAX(2,2) model for juvenile abundance and use long-term monitoring data from the Skagerrak coast, constituting 54 continuous annual series from 1945 to 1997. Analysing the model, we find that the product of the size of the North Sea breeding stock and the strength of the net inflow of North Sea waters had a significant, positive effect on the abundance of coastal juvenile cod. The peak effect occurs during the month of March, just after spawning, when eggs and larvae remain pelagic and sensitive to currents. In contrast, we find no evidence of any direct effect of the North Sea spawning stock alone. Our analyses indicate that 15-20 000 0-group larvae from the North Sea reach each fjord per year, on average. This corresponds to about 1-10% of the total 0-group population in each fjord on average. These findings clearly demonstrate a direct link between larval drift and gene flow in the marine environment.