The effect of seasonality on the immune response of rainbow trout (Oncorhynchus mykiss)

Abstract

The primary aim of the work carried out in this thesis was to determine if season influences the rainbow trout (Oncorhynchus mykiss) immune system. It is already well established that seasonality dominates the life history of fish for example in reproductive activity, food intake, locomotor activity, body weight and body condition. To accomplish this, a twelve month trial was carried out in which a variety of innate immune and haematological parameters were measured every month (Chapter 2). Several of the parameters studied exhibited seasonal patterns. Further trials were carried out to examine the proximate cues of temperature (Chapter 3) and photoperiod (Chapter 4) and their effect on a variety of innate immune responses. These studies concluded that temperature exerted a significant effect on several of the parameters studied whereas, photoperiod, was found to have little or no effect. Furthermore, winter was generally found to have an immunosuppressive effect on the immune system. A second twelve month trial was carried out, investigating possible methods of alleviating seasonal immunosuppression. The effect of melatonin on the immune system of O. mykiss was examine, firstly to determine if it acts as an immunostimulant and secondly to investigate if it improves vaccination protection. The results of this trial generally corroborated those of the Base Level Trial (Chapter 2) in that generally, parameters were suppressed in winter, with the exception of haematocrits which were suppressed in summer. To investigate if the immune system of rainbow trout exhibits a circadian rhythm a trial was carried out where a variety of innate immune and haematological parameters were measured every 6 hours over a 24 h period (Chapter 6). None of the innate immune parameters studied were observed to exhibit a circadian rhythm. It is anticipated that this research investigating the seasonal effects of natural and artificial photoperiods and temperatures on immune function will be of benefit to the aquaculture industry. It will provide information that will allow administration of commercial diets containing functional supplements to be timed effectively and will facilitate our understanding of the epidemiology of specific fish pathogens.