Is the European eel slipping away towards extinction? A review of research and management challenges

Marine organisms experience a broad range of intrinsic and extrinsic influences during their lives, which impact their population dynamics and genetic structure. Subtle interpopulation differences reflect the continuity of the marine environment, but also pose challenges to those wishing to define management units. The catadromous European eel (Anguilla anguilla) is no exception. Its spawning habitat in the Sargasso Sea and long migration across the North Atlantic qualify it as marine. However, the synergy between hydrographic variability, changing climate, and the impacts of habitat degradation and overfishing in continental waters has negatively affected stock sizes. Its protracted spawning period, variance in age-at-maturity, parental contribution and reproductive success, and the difficulty in sampling the spawning region together may mask a weak geographical genetic differentiation. Recent genetic data report evidence for spatial as well as temporal differences between populations, with the temporal heterogeneity between intra-annual recruitment and annual cohorts exceeding the spatial differences. Despite its common name of ‘fresh-water eel’, the European eel should really be managed on a North Atlantic scale. The fishery may have to be curtailed, migration routes kept open and water quality restored if it is to survive. Eel aquaculture has to focus on efficient rearing in the short term and controlled breeding in the long term. Future research on eel genetics should focus on (i) sampling and analysing spawning populations and recruitment waves to detect spatio-temporally discrete groups, and establishing a biological baseline from pre-decline historical collections for critical long-term monitoring and modelling of its genetic composition; (ii) the analysis of adaptive genetic polymorphism (genes under selection) to detect adaptive divergence between populations, perhaps requiring separate management strategies; and (iii) improving artificial reproduction to protect natural stocks from heavy exploitation, especially now the species has been categorized as endangered (Maes and Volckaert, 2007)

Evolutionary and population genetics of Siluroidei

The genetic characterization of catfishes by means of phenotypic markers, karyotyping, protein and DNA polymorphisms contributes to or forms an integral part of the disciplines of systematics, population genetics, quantitative genetics, biochemistry, molecular biology and aquaculture. Judged from the literature, the general approach to research is pragmatic; the Siluroidei do not include model species for fundamental genetic research. The Clariidae and the Ictaluridae represent the best studied families. The systematic status of a number of species and families has been either elucidated or confirmed by genetic approaches. Duplication of ancestral genes occurred in catfishes just as in other vertebrates. The genetic structure of and gene flow among natural populations have heen documented in relatively few cases, while the evaluation of strains for aquaculture (especially Ictaluridae and Clariidae) is in progress. The mapping of genetic markers has started in Ictalurus. It appears that a more detailed knowledge of catfish populations is required from two perspectives. First, natural populations which are threatened by habitat loss and interfluvial or intercontinental transfers are poorly characterized at the genetic level. Secondly, the selection of suitable strains for aquaculture should be encouraged. Implementation should pose no problems given the present powerful means, such as DNA characterization combined with protein polymorphisms and phenotyping, to solve the above-mentioned issues

The implications of heterozygosity in the scallop <i>Placopecten magellanicus</i> (Gmelin)

In many instances, variation in phenotypic traits has been correlated with electrophoretic heterozygosity of plants, vertebrates and invertebrates. Enhanced growth of selected heterozygous molluscs has been attributed to lower energetic requirements for basal metabolism. I investigated the correlation between single- and multiple-locus heterozygosity and phenotype of the juvenile scallop Placopecten magellanicus (Gmelin). Two questions were being asked: do heterozygous scallops grow faster and which metabolic processes are affected by the degree of heterozygosity? No significant correlation was observed between genotype (scored at 6 polymorphic loci) and growth in six different samples of juvenile scallops, all belonging to the same time series. This result and published records indicate that the allozyme heterozygosity of pectinids does not influence growth to the same degree as in the bivalves Mytilus edulis, Crassostrea virginica and Mulinia lateralis. A decrease in heterozygote deficiency with age suggests selective mortality in scallops between the ages of 2 months and 13 months. The metabolism of 13 month old scallops switched from the catabolite carbohydrate (i.e. glycogen) to protein when they were starved for 4 weeks. Allozyme genotype had no measurable effect on oxygen uptake, excretion rate, O:N ratio and carbohydrate content, either under routine or basal metabolic conditions. Two hours after induction of muscle contractions in 20 month old scallops, the end-product octopine reached a concentration of 4 µmol.(g wet muscle weight)-1. Multiple-locus heterozygosity was positively correlated with octopine accumulation in the phasic part of the adductor muscle. Octopine dehydrogenase activity was on average higher among ODH homozygotes than among heterozygotes. In summary, multiple-locus heterozygosity is correlated with selected traits related to functional anaerobiosis. A model integrates the above mentioned results with the hypotheses of ''associative overdominance" , of a "balanced energy pathway" and of "energetic efficiency". I argue that energy savings due to heterozygosity are used for enhanced "activity" (such as feeding and swimming) in freely moving molluscs and for enhanced "growth" (such as somatic growth in juveniles, gonadal growth in adults and resistance to starvation) in sessile molluscs

Small-scale clinal variation, genetic diversity and environmental heterogeneity in the marine gobies <i>Pomatoschistus minutus</i> and <i>P. lozanoi</i> (Gobiidae, Teleostei)

Genetic variation was assayed at 14 allozyme loci in estuarine, coastal and offshore samples of Lozano's goby, Pomatoschistus lozanoi and the sand goby, P. minutus. Samples were taken from locations on the Belgian Continental Shelf and in the Schelde estuary with a range of environmental heterogeneity. We evaluate whether any differences in (1) the degree of genetic variation and (2) allele frequencies at the various loci exist within samples occurring in various habitats on the BCS and in the Schelde estuary. No significant differences in levels of genetic diversity were recorded between estuarine, coastal and offshore samples in either species. A temporally stable clinal gradient in allele frequencies at the two-allele locus GPI-A* was observed in P. lozanoi, differentiating the samples in an estuarine, coastal and offshore group. We suggest that these differences might be maintained by balancing selection at locus GPI-A*

Elemental composition of sole otoliths as a population discrimination tool

Otolith microchemistry, the study of the minor and trace elemental composition of otoliths (earbones), has been developing rapidly with a wide range of applications in fishery science. This is because otoliths (1) grow continuously, resulting in daily and seasonal structures, (2) have an elemental composition that reflects the environmental concentrations and conditions, (3) are metabolically inert. All of these characteristics make trace element uptake useful for reconstructing environmental histories. The elemental composition is therefore a powerful tool in stock To study connectivity among North Sea sole (Solea solea) populations and to improve our understanding of the relationship between its spawning grounds and nursery areas, genetic markers and otolith microchemistry will be used as complementary discrimination tools. We performed a pilot study to test for differences in sole otolith elemental fingerprints among three sampling locations, using LA-ICPMS. This technique makes it possible to determine composition at discrete points across the otolith (corresponding with different events in the fish’s lifetime). A hierarchical design (Basin, Sea, within Sea) enables us to assess the power of this method for the simultaneous large and small scale discrimination of populations.Preliminary results of this study will be shown and discussed in the light of current European wide research objectives. size and increasing the number of sampling locations

Seasonality of the pelagic ichthyofauna in the Lazarev Sea

The Southern Ocean is characterised by an extreme seasonality which is best observed by the huge variation in sea ice extent. It expands from a minimum extent of 4 million square kilometres in February to a maximum of 19 million square kilometres in September and influences the physics and chemistry of the surface layers and the corresponding life histories of organisms. Reduced light condition and sea ice cover have a pronounced impact on primary production and predation risk in the pelagic zone. Here we present comparison of the distribution of fish in the Lazarev Sea correlated to biotic an abiotic features. Samples were collected in the framework of three LAKRIS (Lazarev Krill Study) campaigns onboard RV Polarstern during summer, fall and winter. The main focus was on factors affecting krill distribution. As such a wide variety of biotic and abiotic measures were made. Fish and fish larvae were collected from trawls from the upper 200m during both seasons. Both during summer and winter larval composition was dominated by pre-metamorphic Electrona antarctica and Notolepis coatsi. Post-metamorphic E. antarctica showed a pronounced seasonal difference in occurrence

Exploring glacial and present evolutionary patterns of a marine goby, <i>Pomatoschistus minutus</i>

A classical understanding of marine fishes is that they have high effective population sizes and high levels of dispersal due to an apparent lack of barriers in the marine environment. The realization of a genetic population structure is therefore thought to be a slow process. To gain insights in this process, it is a challenge to disentangle the interaction between selection, gene flow, population history and genetic drift. The sand goby (Pomatoschistus minutus), which is an important ecological but noncommercial species, was chosen to distinguish between natural and anthropologically induced processes. A spatio-temporal analysis with samples from different localities throughout the species distribution and with two types of genetic markers was performed to assess the neutral genetic population structure. Phylogeographical patterns were studied by sequence cytochrome b (mtDNA). Nine new nuclear microsatellites were developed and used to describe the current genetic diversity and population structure. The genetic structure of the sand goby is best explained by a combination of present and historic factors. Due to its high potential for dispersion and high effective population size, it shows the typical features of a marine fish with a high level of diversity and a limited degree of genetic differentiation. The large genetic distance between the Venetian and all other samples shows that the sand gobies from the Adriatic Sea should be considered as a distinct cryptic species of the genus Pomatoschistus. Low but significant differentiation is observed between Atlantic and western Mediterranean P. minutus. In the Atlantic and Baltic basins, there is evidence for a postglacial range expansion and a weak pattern of isolation-by-distance. Furthermore the results support the hypothesis of a glacial refugium and a fine-scale genetic structure in the southern North Sea. The neutral genetic pattern will be compared with putatively adaptive loci in order to study the genomic characteristics of local adaptation in the marine environment. This should provide a better understanding of how fish respond to changes in the environment