The effects of stocking density on the hematology, plasma protein profile and immunoglobulin production of juvenile tambaqui (Colossoma macropomum) farmed in Brazil

Farming of Amazonian native fish species holds great promise. One species whose characteristics make it suitable for intensive farming is tambaqui (Colossoma macropomum). However, there is a lack of consensus about the most suitable stocking density (SD) for intensive farming. More information on the physiology of fish in farming systems is thus required, as this would increase our understanding of the effects of stress on these fish. The aim of this study was therefore to evaluate the effect of different SDs on the hematology and plasma protein profile of tambaqui. To this end, juvenile tambaqui were exposed to the following different SDs (kg/m3) for 96 h: SD1 = 2.11; SD2 = 8.31; SD3 = 16.34; and SD4 = 27.40. Water quality was monitored, and blood was collected for hematological analysis and electrophoresis (SDS-PAGE) to analyze plasma protein and immunoglobulin profiles. Gill filaments were removed and the number of parasites (monogenoids) was counted. Multivariate PCA analysis revealed a strong positive association between the highest SDs (SD3 and SD4) and parasite count, immune cells (eosinophils, thrombocytes, monocytes and lymphocytes) and water ammonia and nitrite levels. Conversely, the highest SDs were negatively associated with water quality variables (pH, turbidity, conductivity and O2 content) and blood hematocrit. The results show that the highest SDs contributed to a deterioration in water quality, leading to increased MetHb levels, gill parasitism, an increase in the number of defense cells, altered protein profile and stimulation of immunoglobulin production. These changes suggest that the welfare of tambaqui is adversely affected by high SDs and increase our understanding of the effects of crowding stress on the physiology of the species.