Influence of bacterial kidney disease on smoltification in salmonids: is it a case of double jeopardy?
The effects of a chronic, progressive infection with Renibacterium salmoninarum, the causative agent of bacterial kidney disease (BKD), on selected aspects of smoltification in yearling juvenile spring chinook salmon (Oncorhynchus tshawytscha), were investigated in 1993 and 1994 in Washington, USA. After experimentally infecting fish with R. salmoninarum using an immersion challenge, fish were sampled every 2 weeks to monitor changes in gill Na+, K+-ATPase (ATPase), cortisol, infection level, mortality, growth, and other stress-related physiological factors during the normal time of parr-smolt transformation in fresh water (i.e., from winter to spring). A progressively worsening infection with R. salmoninarum did not alter the normal changes in gill ATPase and condition factor associated with smoltification in the salmon. The infection did lead to elevated levels of plasma cortisol and lactate and depressed levels of plasma glucose. A dramatic proliferation of BKD was associated with maximal responses of indicators of smoltification, suggesting that the process of smoltification itself can trigger outbreaks of disease. The results suggest mechanisms that probably influence the reported inability of R. salmoninarum-infected fish to successfully adapt to sea water.