Biomedical subjects

Per Erik Jorde

Publications and source records attributed to Per Erik Jorde.

3 recordsLinked to original sources

PubMed2006

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.

Animal Migration↗

PubMed2006

Power for detecting genetic divergence: differences between statistical methods and marker loci.

Information on statistical power is critical when planning investigations and evaluating empirical data, but actual power estimates are rarely presented in population genetic studies. We used computer simulations to assess and evaluate power when testing for genetic differentiation at multiple loci through combining test statistics or P values obtained by four different statistical approaches, viz. Pearson's chi-square, the log-likelihood ratio G-test, Fisher's exact test, and an F(ST)-based permutation test. Factors considered in the comparisons include the number of samples, their size, and the number and type of genetic marker loci. It is shown that power for detecting divergence may be substantial for frequently used sample sizes and sets of markers, also at quite low levels of differentiation. The choice of statistical method may be critical, though. For multi-allelic loci such as microsatellites, combining exact P values using Fisher's method is robust and generally provides a high resolving power. In contrast, for few-allele loci (e.g. allozymes and single nucleotide polymorphisms) and when making pairwise sample comparisons, this approach may yield a remarkably low power. In such situations chi-square typically represents a better alternative. The G-test without Williams's correction frequently tends to provide an unduly high proportion of false significances, and results from this test should be interpreted with great care. Our results are not confined to population genetic analyses but applicable to contingency testing in general.

Alleles↗

PubMed2004

Transport of North Sea cod larvae into the Skagerrak coastal populations.

The Atlantic cod (Gadus morhua) is economically one of the world's most important marine species--a species presently suffering from heavy overexploitation throughout its range of distribution. Although not fully understood, the Atlantic cod is believed to be structured into populations in a rather complex manner, whereby both highly migratory and more confined ocean-spawning stocks coexist with stationary coastal populations. Owing to the complex population structure, little is presently known about how overexploitation of offshore stocks may affect other segments of the species. Here, we use microsatellite DNA analyses of coastal and offshore cod in combination with oceanographic modelling to investigate the population structure of Atlantic cod in the North Sea-Skagerrak area and evaluate the potential for larval transport into coastal populations. Our results suggest an extensive but temporally variable drift of offshore cod larvae into coastal populations. In a year (2001) with high inflow of North Sea waters into the Skagerrak we find that juvenile cod caught along the Skagerrak coast are predominantly of North Sea origin, whereas in a year (2000) with low inflow juveniles appear to be of local origin. These findings indicate that offshore cod may influence coastal cod populations over large distances.

Animal Migration↗