Economic values of growth and feed efficiency for fish farming in recirculating aquaculture system with density and nitrogen output limitations: a case study with African catfish (Clarias gariepinus).

In fish farming, economic values (EV) of breeding goal traits are lacking, even though they are key parameters when defining selection objectives. The aim of this study was to develop a bioeconomic model to estimate EV of 2 traits representing production performances in fish farming: the thermal growth coefficient (TGC) and the feed conversion ratio (FCR). This approach was applied to a farm producing African catfish (Clarias gariepinus) in a recirculating aquaculture system (RAS). In the RAS, 2 factors could limit production level: the nitrogen treatment capacity of the biofilter or the fish density in rearing tanks at harvest. Profit calculation includes revenue from fish sales, cost of juveniles, cost of feed, cost of waste water treatment, and fixed costs. In the reference scenario, profit was modeled to zero. EV were calculated as the difference in profit per kilogram of fish between the current population mean for both traits (µt) and the next generation of selective breeding (µt+Δt) for either TGC or FCR. EV of TGC and FCR were calculated for three generations of hypothetical selection on either TGC or FCR (respectively 6.8% and 7.6% improvement per generation). The results show that changes in TGC and FCR can affect both the number of fish that can be stocked (number of batches per year and number of fish per batch) and the factor limiting production. The EV of TGC and FCR vary and depend on the limiting factors. When dissolved NH3-N is the limiting factor for both µt and µt+Δt, increasing TGC decreases the number of fish that can be stocked but increases the number of batches that can be grown. As a result, profit remains constant and EVTGC is zero. Increasing FCR, however, increases the number of fish stocked and the ratio of fish produced per kilogram of feed consumed ("economic efficiency"). The EVFCR is 0.14 €/kg of fish, and profit per kilogram of fish increases by about 10%. When density is the limiting factor for both µt and µt+Δt, the number of fish stocked per batch is fixed; therefore, extra profit is obtained by increasing either TGC, which increases the annual number of batches, or by decreasing FCR, which decreases annual feed consumption. EVTGC is 0.03 €/kg of fish and EVFCR is 0.05-0.06 €/kg of fish. These results emphasize the importance of calculating economic values in the right context to develop efficient future breeding programs in aquaculture.

Environmental stress and life-stage dependence on the detection of heterozygosity-fitness correlations in the European eel, Anguilla anguilla.

Heterozygosity-fitness correlations (HFCs) have been reported in populations of many species, although HFCs can clearly vary across species, conspecific populations, temporal samples, and sexes. We studied (i) the temporal stability of the association between genetic variation and growth rate (length and mass increase) and (ii) the influence of genetic variability on survival in the European eel (Anguilla anguilla L). HFCs were assessed using genotypes from 10 allozyme and 6 microsatellite markers in 22-month-old experimental individuals. The results were compared with those of a previous study carried out in 12-month-old individuals, in which more heterozygous individuals showed a significantly faster growth rate. In contrast, 22-month-old individuals showed no evidence that genetic variability was correlated with growth rate. Additionally, heterozygous individuals did not show a higher survival rate compared with more homozygous individuals after either handling stress or parasite infection. The decrease in HFCs over time is consistent with the general prediction that differences in growth and survival among individuals are maximal early in life and in our case most likely due to the relaxation of environmental conditions related to population-density effects. Alternatively, the decline in HFCs could be attributed to either ontogenetic variance in gene activity between 12- and 22-month-old individuals or differential mortality leaving only the largest individuals.

Growth rate correlates to individual heterozygosity in the European eel, Anguilla anguilla L.

Heterozygosity-fitness correlations (HFCs) have been reported in populations of many species. We provide evidence for a positive correlation between genetic variability and growth rate at 12 allozyme loci in a catadromous marine fish species, the European eel (Anguilla anguilla L.). More heterozygous individuals show a significantly higher length and weight increase and an above average condition index in comparison with more homozygous individuals. To a lesser extent, six microsatellite loci show a similar pattern, with positive but not significant correlations between heterozygosity and growth rate. The HFCs observed could be explained by an effect of either direct allozyme over-dominance or associative overdominance. Selection affecting some of the allozyme loci would explain the greater strength of the HFCs found at allozymes in comparison with microsatellites and the lack of correlation between MLH at allozymes and MLH at microsatellites. Associative overdominance (where allozyme loci are merely acting as neutral markers of closely linked fitness loci) might provide an explanation for the HFCs if we consider that allozyme loci have a higher chance than microsatellites to be in linkage disequilibrium with fitness loci.