Polygenic sex determination, although suspected in several species, is thought to be evolutionarily unstable and has been proven in very few cases. In the European sea bass, temperature is known to influence the sex ratio. We set up a factorial mating, producing 5.893 individuals from 253 full-sib families, all reared in a single batch to avoid any between-families environmental effects. The proportion of females in the offspring was 18.3%, with a large variation between families. Interpreting sex as a threshold trait, the heritability estimate was 0.62 6 0.12. The observed distribution of family sex ratios was in accordance with a polygenic model or with a four-sex-factors system with environmental variance and could not be explained by any genetic model without environmental variance. We showed that there was a positive genetic correlation between weight and sex (r A ¼ 0.50 6 0.09), apart from the phenotypic sex dimorphism in favor of females. This supports the hypothesis that a minimum size is required for sea bass juveniles to differentiate as females. An evolution of sex ratio by frequency-dependent selection is expected during the domestication process of Dicentrarchus labrax populations, raising concern about the release of such fish in the wild.

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253 full-sib families from 33 males and 23 females of European seabass were produced in a partly factorial mating design. All fish were reared in the same tank during 14 months, then 7000 of them were dispatched in four farms to different locations (France, Israel, Italy, Portugal) representing a wide variety of environmental conditions. Around 400 g mean weight, 1177 to 1667 fish in each site were weighed and length was measured. Condition factor (K) was calculated. Pedigrees were redrawn a posteriori using microsatellites markers: parental origin could be retraced for 99.2% of fish. Due to a high incidence of deformities, the useful sample size was reduced to 491-670 fish per site.Maternal effects were small. Using a simple animal model, heritability of weight ranged from 0.38 ± 0.14 to 0.44 ± 0.14 in the different sites. Length was highly correlated to weight, with similar heritabilities. GxE interaction, estimated through genetic correlations of weight across the different environments ranged from 0.70 ± 0.10 to 0.99 ± 0.05. Genetic correlations between weight or length and K were not similar in the different sites.

The use of selective breeding is still relatively limited in aquaculture species. Information on such activities is sparse, hindering an overall evaluation of their success. Here, we report on the results of an online survey of the major aquaculture breeding companies operating in Europe. Six main reared fish species were targeted. A total of 31 respondents contributed to the survey, representing 75 % of European breeding organizations. Family-based breeding schemes were predominant, but individual selection was more frequently applied in marine species. Artificial fertilization is the preferred means of reproduction; however, mass spawning is often used as a fallback method. The most frequently selected trait is growth performance, but the number of selected traits has been increasing over the years through the addition of traits such as disease resistance or product quality. The use of molecular tools is now common in all programs, mainly for pedigree traceability. An increasing number of programs use either genomic or marker-assisted selection. Results related to the seed production market confirmed that for Atlantic salmon there are a few dominant players at the European level, with 30-50 % market share. Only part of the European fish aquaculture industry today fully exploits selective breeding to the best advantage. A larger impact assessment still needs to be made by the remainder, particularly on the market share of fish seed (eggs, larvae or juveniles) and its consequences for hatchery stability

Malformations in farmed fish are a prime research topic as it has a major impact on fish welfare and on the economical value of the product. In this study, the genetic component of spine deformities was estimated in European sea bass, a major species of Mediterranean aquaculture, with the use of a posteriori reconstruction of pedigree with microsatellites. Our population exhibited a very high malformation rate (81%), probably due to intense swimming of fish during pre-growing. The heritability of global spine deformities was evaluated to be 0.21 ± 0.04 on the underlying scale (0.33 ± 0.06 for lordosis, 0.13 ± 0.04 for scoliosis). Deformities scored by an internal or external examination turned out to be genetically the same trait (genetic correlations > 0.9), although phenotypic correlations were moderate to high (0.45 ± 0.04 to 0.92 ± 0.01) due to difficulties in scoring the less severe deformities. Along time (from 100 to 800 g mean weight), lordosis had a constant heritability, with very high genetic correlations between stages (> 0.9). Its incidence increased from 39 to 53% indicating that the malformation was probably present from the beginning of the rearing period but that its expression increased over time. A positive linkage between spine deformities and growth rate was demonstrated both by the higher early weight of affected fish and by the positive genetic correlation between weight and malformation (from 0.21 ± 0.15 to 0.40 ± 0.14), although deformities have a negative phenotypic impact on growth. However, heritabilities of malformation traits calculated here are only estimates that may indirectly describe the sensitivity of fish to the particular pre-growing environment as a function of different body weight genotypes. A quantification of realised heritability would therefore be necessary before concluding on the existence of a genetic basis for spinal malformations such as lordosis and scoliosis. Nevertheless and by precaution, selection for fast growth rate needs careful monitoring of deformities, and selection against malformations may counteract impacts of selection for fast growth