Influence of bacterial kidney disease on smoltification in salmonids: Is it a case of double jeopardy?

We investigated the effects of a chronic, progressive infection with Rediba cterium salmoninarum (Rs), the causative agent of bacterial kidney disease (BKD), on selected aspects of smoltification in yearling juvenile spring ch inook salmon (Oncorhynchus tshawytscha). After experimentally infecting fis h with Rs using an immersion challenge, we sampled them every two weeks to monitor changes in gill Na+, K+-ATPase (ATPase), cortisol, infection level, mortality, growth, and other stress-related physiological factors during t he normal time of parr-smelt transformation in fresh water (i.e., from wint er to spring). A progressively worsening infection with Rs did not alter th e normal changes in gill ATPase and condition factor associated with smolti fication in juvenile chinook salmon. The infection did, however, lead to el evated levels of plasma cortisol and lactate and depressed levels of plasma glucose, indicating that the disease is stressful during the later stages. A dramatic proliferation of BKD was associated with maximal responses of i ndicators of smoltification, suggesting that the process of smoltification itself can trigger outbreaks of disease. Our results suggest mechanisms tha t probably influence the reported inability of Rs-infected fish to successf ully adapt to sea water. (C) 1999 Elsevier Science B.V. All rights reserved .