VULNERABILITY TO PREDATION AND PHYSIOLOGICAL STRESS RESPONSES IN JUVENILE CHINOOK SALMON (ONCORHYNCHUS-TSHAWYTSCHA) EXPERIMENTALLY INFECTEDWITH RENIBACTERIUM-SALMONINARUM

We experimentally infected juvenile chinook salmon (Oncorhynchus tshaw yrscha) with Reinbacterium salmoninarum (Rs), the causative agent of b acterial kidney disease (BKD), to examine the vulnerability to predati on of fish with differing levels of Rs infection and assess physiologi cal change during progression of the disease. Immersion challenges con ducted during 1992 and 1994 produced fish with either a low to moderat e (1992) or high (1994) infection level of Rs during the 14-week postc hallenge rearing period. When equal numbers of treatment and unchallen ged control fish were subjected to predation by either northern squawf ish (Ptychocheilus oregonensis) or smallmouth bass (Micropterus dolomi eui), Rs-challenged fish were eaten in significantly greater numbers t han controls by nearly two to one. In 1994, we also sampled fish every 2 weeks after the challenge to determine some stressful effects of Rs infection. During disease progression in fish, plasma cortisol and la ctate increased significantly whereas glucose decreased significantly. Our results indicate the role that BKD may play in predator-prey inte ractions, thus ascribing some ecological significance to this disease beyond that of direct pathogen-related mortality. In addition, the phy siological changes observed in our fish during the chronic progression of BKD indicate that this disease is stressful, particularly during t he later stages.