The impact of disease and diet on the metabolic rate, swimming performance and recovery of Atlantic salmon (Salmo salar L.)

Gill diseases represent one of the most abundant and economically important groups of diseases in fish, understanding the physiological response of fish to such diseases is crucial to the development of preventive measures. The majority of the research published on the physiological responses of the fish to changes in external stimuli, such as temperature, salinity or nutritional status have used apparently healthy fish. The study of the physiological responses of fish that are compromised with disease is inherently difficult however, due mainly to the invasive nature of the techniques used, which often result in mortality. Measuring the metabolic rate and swimming performance of a diseased organism is one of the least invasive techniques available for quantifying the impact of disease in fish Lethal sampling of the fish post-measurement can allow the researcher to determine possible reasons behind any change in metabolic performance. The nutritional status of the organism can also be a determining factor in the severity of the impact of disease. A reduction or a complete cessation of feeding can severely impact some physiological parameters that are associated with disease resistance, including a reduction in immune response, reduction in the organosomatic index of organs responsible for immune function, reduction in metabolic rate and a decrease in protein synthesis rates of some immunoregulatory organs, and therefore should always be taken into account when assessing the physiological impact of a disease. The composition of the diet can also play a significant role in physiological performance of an animal; research has suggested that the fatty acid profile of a feed can significantly affect the metabolic rate and swimming performance of Atlantic salmon. The aim of this thesis was to examine the physiological impact of two endemic gill diseases on the physiological performance of Atlantic salmon smolt, whilst simultaneously examining the possible causes of any change in physiological performance. Furthermore, the effect of nutritional status on metabolic rate, by withholding feed and by altering the fatty acid profile of the feed, was also assessed. Research was conducted to investigate the effect of Tenacibaculum maritimum, an acute necrotic bacterial gill infection, and amoebic gill disease (AGD), a proliferative gill disease, and feed deprivation on the metabolic rate of Atlantic salmon Salmo salar. For the T. maritimum infection, a significant decrease of 2.21 ± 0.97 1.1M 02 g-1 h-1 in metabolic scope was found for the fed infected group, whilst the unfed infected group had a decrease of 3.16 ± 1.29 I.LNI 02 g-lh-1, facilitated by a significant increase routine metabolic rate. Interestingly, all groups defended maximum metabolic rate despite the perceived loss of gill surface area. Increases in routine metabolic rates corresponded to a significant increase in blood plasma osmolality. A similar increase in routine metabolic rate (3.56 ± 0.62 i.tM 02 g-1 111 for fed fish, 2.94 ± 0.55 p.M 02 g-1 h-1 for unfed fish) was observed for fish affected by AGD, despite the different modes of action between the two diseases, however fish affected by AGD defended metabolic scope. Further research was conducted in order to determine whether protein synthesis was one of the possible causes of the observed increase in routine metabolic rate of fish affected by AGD. Gill protein synthesis rates were examined using the flooding dose method, AGD affected fish had an mean synthesis rate of 13.56 ± 2.05 %.day-1, which was significantly (P = 0.037) higher than that of control fish (8.05 ± 0.97 %. day-1 ).. However the increase in protein synthesis is unlikely to be the sole reason for the observed increase in routine metabolic rate, instead it is likely that a number of physiological changes within the affected fish result in the observed increase in metabolic rate. A further study was conducted to examine the effect of AGD on swimming performance. Salmon that had been inoculated via cohabitation with amoeba (Neoparamoeba spp) 14 days prior were given ramp critical swimming speed tests (Ucrit) 45 min apart. Fish affected by AGD had significantly reduced recovery ratios (RR) (0.87 ± 0.02) compared to controls (1.00 ± 0.01), even at relatively low infection levels. A second experiment was performed examining the effects a freshwater bath on the repeat swimming performance of AGD affected salmon. Freshwater bathed fish showed a significant recovery in RR compared to non-bathed controls. The results indicated that AGD can significantly affect the swimming performance of Atlantic salmon, even at low infection levels. A study was also conducted to determine the effect of replacing the fish oil component of a salmon feed with stearidonic acid (SDA) on the metabolic rate and recovery performance in Atlantic salmon. The results suggested that replacement of fish oil with SDA has little effect on metabolic rate when comparing the FA profile of the carcass of the fish and the observed metabolic rate and recovery. However, significant correlations were observed with the concentration of a number of fatty acids in the heart and the maximum metabolic rate of Atlantic salmon, suggesting that the FA profile of a diet can significantly affect the physiological performance of Atlantic salmon. In combination the results have shown that gill diseases can have a significant effect on the physiological performance of Atlantic salmon. However, the nutritional status of the fish prior to infection has little impact on the severity of the impact of the disease on metabolic rate. Furthermore, the research highlighted the link between the fatty acid profile of a diet and the physiological performance of the fish. Future studies in this area may benefit from the use of the methods outlined as a non-lethal means of quantifying the impact of disease on fish.