Combined effects of high stocking density and Piscirickettsia salmonis treatment on the immune system, metabolism and osmoregulatory responses of the Sub-Antarctic Notothenioid fish Eleginops maclovinus

The aim of this study was to evaluate immunological, metabolic and osmoregulatory secondary stress responses in Eleginops maclovinus specimens submitted to three different stocking densities: i) low (3.1kgm^-3), medium (15kgm^-3) and high (60kgm^-3) during 10 days, alone or in combination with a previous treatment of a protein extract of the pathogen Piscirickettsia salmonis (0.5μgg weight body^-1). Plasma, liver, gill and kidney samples were obtained at the end of both experiments. Plasma cortisol and amino acid levels increased, while plasma glucose, triglyceride and lactate levels decreased at higher stocking densities. However, no effects were observed on serum Immunoglobulin type M (IgM anti P.salmonis level) values. Gill Na⁺, K⁺-ATPase activity enhanced under these experimental conditions, suggesting an osmotic imbalance. Energy metabolism changes, assessed by metabolite concentrations and enzyme activities, indicated a reallocation of energetic substrates at higher stocking densities. Specimens inoculated with a protein extract of P.salmonis and maintained at different stocking densities showed primary stress response, as all groups enhanced plasma cortisol concentrations. Serum IgM levels increased after treatment with P.salmonis extract but a negative influence of high stocking density on IgM production was observed when immune system was activated. Furthermore, treatment with P.salmonis protein extract evoked deep changes in the metabolite stores in all tissues tested, indicating a mobilization of energy substrates in response to infection. The results show that stocking density induced immunological, metabolic and osmoregulatory secondary stress responses in E. maclovinus specimens and that previous treatment with P.salmonis compromise these changes.