Effects of stocking density and feed ration on growth and gene expression in the Senegalese sole (Solea senegalensis): potential effects on the immune response.

Stocking density and ration size are two major factors influencing aquaculture production. To evaluate their effects on growth and immune system in Senegalese sole (Solea senegalensis) juveniles, a 2 x 2 experimental design using two rations (1.0% and 0.25% of the total fish biomass) and two different initial stocking densities (7 and 30 kg m(-2)) was performed throughout a 60 days culture period. Soles fed 1.0% showed a higher specific growth rate (SGR) than those fed 0.25% (3.3-fold). No differences in SGR at 60 days were found between densities in spite of reduced values were detected at high density after 20 days (soles fed 0.25%) and 40 days (soles fed 1%) suggesting a compensatory growth. Physiologically, plasma cortisol levels were elevated in soles at high density (45-fold higher than at 7 kg m(-2)) whereas no differences associated to the feeding ration were observed. To assess the effects at a molecular level, the mRNA levels of genes involved in cellular stress (heat shock proteins HSP70 and HSP90), growth (insulin-like growth factors IGF-I, the spliced variants IGF-Ia and IGFI-b, and IGF-II) and innate immune system (g-type lysozyme and hepcidin (HAMP1)) were quantified. No differences in HSP90 expression were detected between densities or rations. In contrast, IGF-I, IGF-Ia and IGF-II showed reduced transcript levels in liver and HSP70 in liver and kidney at high density. Finally, g-type lysozyme and HAMP1 expression was greatly affected by both factors exhibiting an important reduction in the transcript levels at high density and low ration. Overall, our results show that S. senegalensis juveniles might exhibit satisfactory SGR at high density although the high plasma cortisol levels indicate a crowding stress that could negatively affect the expression levels of some of the genes studied.