Stocked &lockedeffects of intensive aquaculture practices on immune system of gilthead seabream (sparus aurata) & european seabass (dicentrarchus labrax)

Laburpena

Storage facilities are specifically for holding fish temporarily. To keep costs down, quite high stocking densities are used. This thesis project has been focused on the hypothesis that the storage and maintenance of Mediterranean fish species such as Gilthead seabream (Sparus aurata) and European seabass (Dicentrarchus labrax) at high densities could involve immune suppression. Very often, the intensive production of fish has the following constraints: water quality and density at which the fish are kept for fattening and breeding. These two factors and environmental characteristics such as temperature, salinity, pH, or photoperiod, determine productivity and therefore profits. In this sense, the manipulation of these parameters has reached the main goal: high productivity with low costs. However the presence of pathogens is implicitly tied to high density conditions. This environment facilitates the spread of infections (particularly opportunistic pathogens) that determine the main economic losses. For instance, Nodavirus has become a serious pathogen for a wide range of cultured marine fish species. Gilthead seabream and European sea bass are species of a high economic value cultured in the Mediterranean Sea. Gilthead seabream has been initially reported as an asymptomatic carrier of the disease. However, we have previously shown that sea bream can be experimentally susceptible to Nodavirus, depending upon the temperature and route of infection. Also, Gilthead seabream is often cultured in the Mediterranean in the vicinity of European seabass and other susceptible species, raising the possibility of cross infection. Photobacteriosis, also known as fish pasteurellosis or pseudotuberculosis, is a serious bacterial disease affecting Gilthead seabream, European seabass in the Mediterranean area. The aetiological agent of this disease is Photobacterium damselae subsp. piscicida (syn. Pasteurella piscicida), a Gram-negative bipolar coccobacillus. It leads to the heaviest losses in aquaculture production of Mediterranean Aquaculture. In order to evaluate the effects on immune system under high density conditions, we compared the following situations: low-density or normal density (10 fish kg/m3) and high density (50 fish kg/m3). Moreover, it has been also studied the effect of infections either by bacteria (Photobacterium damselae subsp. piscicida) or virus (Nodavirus) in both density conditions. Thus, we tried to analyze the possible weakening of the immune response at plasma level. On the other hand we cloned and characterized the C3 protein of the Complement System. By the measure of transcript abundance of the C3 protein we indirectly measured the expression of C3 under different conditions. And in addition a transcriptomic analysis of liver was also performed in order to obtain a global interpretation of the whole hepatic response. To assess the immune system effectiveness at plasma level, we analyzed bacteriolytic activity, lysozyme activity, the haemolytic activity of the complement system (one of the most important systems of defence) and the gene expression of the key protein of this system, C3. We have found that bacteriolytic and haemolytic activities (which are mainly produced by the complement system) lysozyme activity and C3 mRNA abundance can be modified depending on the conditions of density and the presence or absence of pathogens. The immune response in terms of plasma parameters is clearly attenuated. To be able to confirm at the molecular level the previous results, we cloned and characterized for the first time the nucleotide C3 protein sequence in both species. In addition we have described two isoforms for European seabass (Dicentrarchus labrax). We have observed the same pattern, with an initial non-specific response, as a warning and a subsequent response that becomes different if we analyze the different isoforms, first described at the level of mRNA. Results are reinforced by the European seabass (Dicentrarchus labrax) hepatic transcriptomic analysis. We have corroborated a non-specific activation either in stress level or in immune level responses. Therefore and according to the results of this thesis, the density alone could not cause a significant modification of the analyzed immune parameters. The C3 haemolytic activity, lysozyme activity, bacteriolytic activity and C3 mRNA abundance did not seem to be altered due to density. However, the addition of pathogenic activity of Photobacterium damselae subsp. piscicida and Nodavirus, showed a significant attenuation of the immune responses analyzed. Thus, the potential role of density as an aggravating factor of a possible infection has been reported.