Standard length, dry mass and RNA : DNA ratio measurements of 3876 Atlantic cod Gadus morhua larvae and juveniles from 26 families of recruit (fish during their first year of spawning) and repeat spawners (fish which were in their subsequent spawning season) reared in two mesocosms (2500 and 4400 m3) under semi‐natural conditions were analysed over a period of 10 weeks using microsatellites. Larvae from recruit spawners were significantly longer and heavier at hatch and throughout the 10 weeks. RNA : DNA ratios from recruit spawner offspring were only significantly higher at week 1. The smaller (2500 m3) mesocosm was characterized by low plankton density during the transition from endogenous to exogenous feeding followed by a higher density during the metamorphosis period (weeks 4 and 5), with the reverse pattern evident in the 4400 m3 mesocosm. Patterns of larval growth followed patterns of zooplankton density. Significant differences in RNA : DNA ratios between the mesocosms at all comparable sampling dates were found and within each mesocosm individual fish exhibited a wide range of growth and condition responses under the same environmental conditions. RNA : DNA ratios as a function of size differed in the amount of variability between mesocosms, indicating that the higher food density led to a higher proportion of well‐conditioned larvae in the first 3 weeks. Food availability probably has a major role in determining offspring growth and condition, with limited effects due to maternal effects in cases where the broodstock females are approximately of similar size and condition.
Elucidating the relative roles of dispersal and retention of juvenile stages is an important issue for understanding population structure and evolution in marine organisms. We investigated the genetic population structure of juvenile Atlantic cod (Gadus morhua) within the transition zone between the North Sea and the Baltic Sea, employing nine microsatellite loci, and compared our data with adult cod data from the same area. Small but statistically significant overall differentiation (F st = 0.003) was found among juvenile samples. Samples of juveniles grouped genetically with adult samples from the same geographical regions. Individual admixture analysis of a large sample of juveniles taken within the transition zone showed that the patterns of genetic differentiation could not be explained by mixing of pure North Sea and Baltic Sea individuals. Instead, the high number of juveniles with intermediate genotypes was compatible with a scenario of exclusive local (transition zone) origin. The results support the hypothesis that population structure in marine fishes is maintained by the retention of juveniles.Résumé : L'évaluation des rôles relatifs de la dispersion et de la rétention des stades juvéniles est une question importante pour la compréhension de la structure démographique et de l'évolution des organismes marins. Nous avons examiné la structure génétique de population de jeunes morues franches (Gadus morhua) dans la zone de transition entre la mer du Nord et la Baltique par l'étude de neuf locus microsatellites et nous avons comparé les résultats à ceux obtenus chez les morues adultes de la même région. Il existe une différentiation globale faible (F st = 0,003), mais statistiquement significative, dans les échantillons de jeunes. Les échantillons de jeunes se regroupent génétiquement avec les échantillons d'adultes de la même région. Une analyse de mélange individuelle d'un grand échantillon de jeunes capturés dans la zone de transition montre que les patrons de différentiation génétique ne peuvent s'expliquer par un mélange d'individus purs de la mer du Nord et de la Baltique. Au contraire, le nombre élevé de jeunes à géno-type intermédiaire est compatible avec un scénario d'origine exclusivement locale (zone de transition). Nos résultats appuient l'hypothèse selon laquelle la structure démographique des poissons marins est maintenue par la rétention des jeunes.[Traduit par la Rédaction] Nielsen et al. 2225
Studies of the pantophysin (Pan I*) locus in Atlantic cod Gadus morhua and other marine gadoids indicate that the locus is under positive selection; in Atlantic cod, genotypic variation at this locus has been linked to differences in growth. Here, we present preliminary data comparing the growth and condition of different Atlantic cod Pan I* genotypes within families held under seminatural mesocosm conditions. Larvae from three full-sibling families carrying Pan I*bb or Pan I*ab genotypes were reared for 10 weeks in two mesocosms. Multivariate analysis of variance indicated that larvae carrying the Pan I*ab genotype exhibited significantly higher standard length, dry weight, and RNA: DNA ratio (condition factor) than did larvae that carried the Pan I*bb genotype, potentially indicating selection.
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