Crowding stress induces changes in serum haemolytic and agglutinating activity in the gilthead sea bream Sparus aurata.

Sea bream Sparus aurata were subjected to crowding for 3 weeks and the levels of plasma cortisol and glucose were determined in order to ascertain the occurrence of stress. At the same time, selected indicators of the immune response were monitored through blood lymphocyte counting and selected humoral responses such as the haemagglutination activity (HA) of serum towards rabbit erythrocytes (RaRBC) and the alternative complement pathway (ACP) levels. The results show an initial moderate increase in cortisol followed by hyperglycaemia and recovery to basal levels. A pattern of immunodepression was detected afterwards as shown by decreases in ACP levels after 5 days and haemagglutination titre and circulating lymphocytes after 9 days. Nevertheless, ACP levels showed a significant increase after 9 days, and HA titres and circulating lymphocyte numbers also increased after 15 days. Normal values were recovered after 15 days for lymphocytes and ACP and 21 days for agglutination. Results are discussed in view of the incidence of stress on indicative parameters of endocrine and immune systems.