Lack of relationship between morphological variance and enzyme heterozygosity in the plaice, pleuronectes platessa

The hypothesis that enzyme heterozygosity is associated with decreased morphological variance was tested using eight polymorphic loci and three meristic characters in large samples of the plaice, Pleuronectes platessa. The hypothesis was not corroborated and the conclusion drawn that such a relationship although observed by several other workers may not be a general one

A search for gametic disequilibrium in the plaice, Pleuronectes platessa

Large samples of a natural population of the plaice were examined for evidence of gametic disequilibrium (D) at five and seven polymorphic enzyme loci (younger and older fish respectively). A single pairwise locus comprison gave a statistically significant value for D: Gpdh-1 and Pgi-2 in young fish. The mean D values of the 10 pairwise comparisons of the five loci in young and old fish were low, 0.00175 and 0.00420 respectively: corresponding values of R (the correlation of gene frequencies) were 0.01473 and 0.03002. The increase in these parameters in older fish might be due either to population admixture, natural selection, a smaller sample size, or to a combination of these factors. The mean D and R values for the 21 pairwise comparisons of the seven loci typed in older fish were 0.00394 and 0.2496 respectively. Two loci catalysing adjacent steps in the glycolytic pathway, Pgm-1 and Pgi-2, showed no evidence of epistatic interactions generating disequilibrium

Body condition of predatory fishes linked to the availability of sandeels

Lesser sandeels Ammodytes marinus are eaten by a range of predatory fishes including commercially fished species, but are also exploited at large scale by industrial fisheries. Is availability of sandeels, as key prey source, linked to the body condition of predatory fishes? In the North Sea, the largest sandeel biomass is concentrated in the Dogger Bank region. Here we studied predator-sandeel interactions at two sites differing widely in sandeel abundance and local sandeel fishing effort. Surveys took place in 2004, 2005, and 2006, years when local sandeel densities observed at these sites were low, intermediate, and high, respectively. Five predator species--whiting, lesser weever, grey gurnard, plaice, and haddock--showed better body condition indices in either the years or study area (or both) characterised by higher local sandeel densities, when compared to sandeel-poorer conditions. Moreover, whiting, weever, and gurnard condition was better for those individuals actually observed to have eaten sandeels (based on stomach contents) than for those that had not. As body condition relates to growth, reproduction, and survival, predators in sandeel-rich conditions may be inferred to have a higher fitness. These links between sandeel availability, sandeel consumption, and predator condition hint that, if large-scale localised depletions of sandeels were to occur, negative indirect effects on predatory fish might become apparent, underlining the importance of considering the sandeel fishery in an ecosystem context

Critical threshold size for overwintering sandeels (Ammodytes marinus)

Several ecologically and commercially important fish species spend the winter in a state of minimum feeding activity and at lower risk of predation. To enable this overwintering behaviour, energetic reserves are generated prior to winter to support winter metabolism. Maintenance metabolism in fish scales with body size and increases with temperature, and the two factors together determine a critical threshold size for passive overwintering below which the organism is unlikely to survive without feeding. This is because the energetic cost of metabolism exceeds maximum energy reserves. In the present study, we estimated the energetic cost of overwintering from a bioenergetic model. The model was parameterised using respirometry-based measurements of standard metabolic rate in buried A. tobianus (a close relative to A. marinus) at temperatures from 5.3 to 18.3 degrees C and validated with two independent long-term overwintering experiments. Maximum attainable energy reserves were estimated from published data on A. marinus in the North Sea. The critical threshold size in terms of length (L(th)) for A. marinus in the North Sea was estimated to be 9.5 cm. We then investigated two general predictions: (1) Fish smaller than L(th) display winter feeding activity, and (2) size at maturation of iteroparous species is larger than L(th) to ensure sufficient energy reserves to accommodate both the metabolic cost of passive overwintering and reproductive investments. Both predictions were found to be consistent with data on size at maturation and total body energy in December and February