Stocking density and sex influence individual growth of Senegalese sole (Solea senegalensis)

Growth is usually inversely correlated with stocking density of fish in culture. Senegalese sole aquaculture is affected by a high size variability and thus, this work tried to investigate the relationship of growth with density of two populations of 96 individually tagged Senegalese sole (318.7±7.9g; mean±standard error of the mean). Fish were reared at low (LD) and high (HD) density (60% and 180% of bottom coverage respectively) for 195days. After 134days (period 1), density conditions were exchanged between groups. Mean weight, standard length, maximum width and centroid size were calculated for each of the 11 census days of the experiment. White muscle biopsies were taken in 7 of the census days in order to assess the RNA/DNA ratio, as a biochemical indicator of growth. Stocking density had an important effect on growth, as fish reared under HD showed poor or no growth during a ‘lag phase’ on the first 61days of the experiment, leading to a significantly lower specific growth rate (0.23±0.014) for period 1 compared with LD fish (0.34±0.016). Fitting of linear mixed-effects (LME) models for the first 134days of experiment showed a significant effect of density and sex on all the assessed biometric parameters. These results could be attributed mainly to the first 61days of the experiment, as no differences were observed between days 61 and 134 in all the measurements, except for standard length, that showed to be lower for HD fish throughout the whole period. Fish reared under high density tended to grow slower than fish held at low density, while females showed faster growth than males, particularly in HD. Nevertheless, due to high size variability, no significant differences could be found in the mean values of weight or standard length after 134days (467.2±21.6g and 28.7±4.2cm; 502.6±22.5g and 29.7±4.5cm for HD and LD fish respectively). Size variability could be an indicator of the onset of hierarchies, being stronger and with more females as dominant individuals than males in HD. After exchanging densities, and up to day 195, a similar lagging effect could be observed in LD fish exposed to high density, suggesting that a sudden change in density, more than density itself, could be the responsible for a detrimental effect on growth. RNA/DNA ratios, were significantly lower for HD fish between days 20 and 61.