ASSESSING THE IMPACT OF SWIMMING EXERCISE AND THE RELATIVE SUSCEPTIBILITY OF RAINBOW TROUT Oncorhynchus mykiss AND ATLANTIC SALMON Salmo salar FOLLOWING INJECTION CHALLENGE WITH Weissella ceti  

Weissellosis is an emerging and important disease of cultured rainbow trout, and has been observed in several countries over the last decade, including the United States. Little research has been carried out assessing the susceptibility of other important commercial species to this disease. Additionally, although weissellosis is associated with cardiovascular inflammation, the potential protective effects of sustained swimming exercise against this disease have not been assessed. We therefore sought to examine whether swimming fitness was associated with reduced morbidity and mortality in both rainbow trout and Atlantic salmon. Juvenile rainbow trout and Atlantic salmon (approximately 200 g and 120 g initial weight, respectively) were maintained in small circular tanks in a flow-through system under study conditions for a period of five months. The four tank populations consisted of rainbow trout exposed to either sustained rotational water flow to provide swimming exercise (i), or minimal rotational water flow leading to non-exercise conditions (ii), and Atlantic salmon held under the same conditions, i.e. exercised (iii) or non-exercised (iv). Exercised fish swam at 2.0 body-lengths per second (BL/s), while swimming speeds in the non-exercise tanks were <0.5 BL/s. Following exercise treatment, all study fish were acclimated to a new flow-through system, before being selected for either morphometric assessments or challenged with intracoelomic injection of 9.5x106 W. ceti NC36 cells per fish. Fish were fed daily and monitored for 27 days post-challenge, mortalities were removed and recorded daily, and at 24h, 3d, 6d, 12d, and 27d post-challenge fish were sampled and assessed for splenic and brain bacterial loads. Both species demonstrated a significantly (p<0.05) greater weight gain while under exercised versus unexercised conditions; visceral index was generally lower (p=0.052) in exercised Atlantic salmon, while hepatosomatic index was generally higher (p=0.053) in unexercised rainbow trout. Atlantic salmon demonstrated significantly greater cardiosomatic indices in exercised versus unexercised groups. Post-challenge splenic bacterial load data indicated no major differences between rainbow trout and Atlantic salmon during the initial post-challenge period, although Atlantic salmon generally demonstrated less splenic bacterial loads in subsequent assessments up to 27d post-challenge. While a few individuals within the Atlantic salmon treatment groups were determined to have culturable bacteria in the brain up to 6d post-challenge, the vast majority did not, indicating that the pathogen generally did not establish the typical brain infection observed in rainbow trout. Additionally, no significant protective effect of exercise was determined, regarding splenic and brain loads, as well as overall survival, in either species. The major findings of this study are: (i) Atlantic salmon appear to be resistant to weissellosis, and (ii) sustained swimming exercise, while conferring benefits related to growth performance and physical conditioning, does not impart protection against infection and mortality in rainbow trout.