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

摘要

Juvenile spring chinook salmon (Oncorhynchus tshawytscha) infected with R. salmoninarum (Rs) were exposed to 120% total dissolved gas (TDG) for 96 h and monitored for severity of gas bubble disease (GBD) signs in the fins and gills, Rs infection level in kidneys by ELISA, and mortality. Mortality occurred rapidly after exposure to 120% TDG, with a LT20 (time necessary to kill 20% of the population) of about 37 h, which is at a minimum about 16% earlier than other bioassays using fish that had no signs of disease. Fish that died early (from 31 to 36 h and from 49 to 52 h) had significantly higher infection levels (mean ± s.e. ELISA absorbance = 1.532 ± 0.108) than fish that survived for 96 h (mean ± s.e. ELISA absorbance = 0.828 ± 0.137). Fish thatdied early also had a significantly greater number of gill filaments occluded with bubbles than those that survived 96 h. Fish that survived for 96 h had a significantly higher median fin severity ranking than those that died early. These results indicate that fish with moderate to high levels of Rs infection are more vulnerable to the effects of dissolved gas supersaturation (DGS) and die sooner than fish with lower levels of Rs infection. However, there is a substantial amount of individual variationin susceptibility to the apparent cumulative effects of DGS and Rs infection. These findings have important implications for programmes designed to monitor the prevalence and severity of GBT in juvenile salmonids in areas like the Columbia River basin,and perhaps elsewhere.