Figure 1. Map of the Skagerrak coast, with the predominant ocean current indicated by the curved arrow.
Ingrid Spies traveled to Oslo, Norway, this summer for a collaborative research project with Nils Chr. Stenseth, Per Erik Jorde, Lauren Rogers, and Halvor Knutsen at the Centre for Ecological and Evolutionary Synthesis. The project involved applying a simulation framework to answer questions about dispersal between Atlantic cod (“torsk” in Norwegian) populations along the Skagerrak coast and the North Sea ( Fig.1). Atlantic cod have been economically important in this region since 800 A.D. The North Sea stock is managed by the International Council for the Exploration of the Sea and is fished by European Union states and Norway. However, there is no management plan for cod along the Skagerrak coast. Genetic population structure has been examined between the North Sea and the Skagerrak coast, but the level of population connectivity remains uncertain.
Knowledge of connectivity between stocks is important for management, as well as for predicting how stocks will respond to higher or lower fishing rates. Migration below a certain threshold can result in observable levels of genetic differentiation among marine fish stocks, while higher levels of migration may result in populations that are not genetically distinct. Non-significant genetic results may result from relatively few migrants per generation. Such a case may be problematic for fisheries managers because non-significant genetic results can be interpreted as panmixia, but ecologically distinct populations may exist. Non-significant differentiation was found between the Skagerrak coast and the North Sea, whereas differentiation was significant between the inner coast and the North Sea.
The project incorporated both biological parameters and a multi-locus genotype for each fish in a unique genetic individual-based population dynamics model. The simulation examined dispersal between two populations by varying dispersal until genetic differentiation is similar to empirical results. Results indicate that as few as 100 age-0 migrants per year may enter the outer Skagerrak and as few as 25 per year into the inner Skagerrak. Genetic differentiation was sensitive to absolute numbers of migrants, and factors that may affect the number of migrants, namely those that increase or decrease the size of the North Sea population. Differentiation in smaller populations was also sensitive to the maturity-at-age, which affected the number of individuals that contributed to the gene pool in the next generation.
Results suggest that the inner and outer fjords may be more connected to the North Sea than expected. With this level of connectivity, the North Sea may act as a source population; simulations show that both the inner and outer coast rebounded from collapse in 10 years as a result of migration from the North Sea.
