Influence of infection with Renibacterium salmoninarum on susceptibility of juvenile spring chinook salmon to gas bubble trauma

During experiments in our laboratory to assess the progression and severity of gas bubble trauma (GBT) in juvenile spring chinook salmon Oncorhynchus tshawytscha, we had the opportunity to assess the influence of Renibacteriu m salmoninarum (Rs), the causative agent of bacterial kidney disease, on th e susceptibility of salmon to GBT. We exposed fish with an established infe ction of Rs to 120% total dissolved gas (TDG) for 96 h and monitored severi ty of GET signs in the fins and gills, Rs infection level in kidneys by usi ng an enzyme-linked immunosorbent assay (ELISA), and mortality. Mortality o ccurred rapidly after exposure to 120% TDG, with a LT2O (time necessary to kill 20% of the population) of about 37 h, which is at a minimum about 16% earlier than other bioassays we have conducted using fish that had no appar ent signs of disease. Fish that died early (from 31 to 36 h and from 49 to 52 h) had significantly higher infection levels (mean +/- SE ELISA absorban ce = 1.532 +/- 0.108) than fish that survived for 96 h (mean +/- SE ELISA a bsorbance = 0.828 +/- 0.137). Fish that died early also had a significantly greater number of gill filaments occluded with bubbles than those that sur vived 96 h. Conversely, fish that survived for 96 h had a significantly hig her median fin severity ranking than those that died early. Our results ind icate that fish with moderate to high levels of Rs infection are more vulne rable to the effects of dissolved gas supersaturation (DGS) and die sooner than fish with lower levels of Rs infection. However, there is a substantia l amount of individual variation in susceptibility to the apparent cumulati ve effects of DGS and Rs infection. Collectively, our findings have importa nt implications to programs designed to monitor the prevalence and severity of GBT in juvenile salmonids in areas like the Columbia River basin, and p erhaps elsewhere.