The effect of seasonality on normal haematological and innate immune parameters of rainbow trout Oncorhynchus mykiss L.

It is well established that seasonality dominates the life history of fish by controlling the timing of physiological events such as reproduction, food intake, locomotor activity and growth performance. Seasonal differences in immune competence and prevalence of disease have been well documented in humans. The aim of this study was to determine if season influences the immune response of rainbow trout (Oncorhynchus mykiss L.). Thus, a 12-month trial was carried out in which plasma lysozyme activity and respiratory burst of head kidney macrophages (two innate immune parameters) and total red and white blood cell counts (two haematological parameters) were monitored at monthly intervals. Since photoperiodic information is thought to be conveyed via melatonin secretion, plasma melatonin levels were also measured at four seasonal points (day and night). A general seasonal influence was observed in the parameters measured in these fish, with the exception of respiratory burst activity of head kidney macrophages, with the parameters highest in summer and lowest in winter for total white blood cell counts and lysozyme activity.