Renibactenum salmoninarum, the causative agent of Bacterial Kidney Disease (BKD) of salmonid fish, is a significant pathogen that has caused a great deal of concern in the fisheries and aquaculture communities worldwide since its discovery approximately sixty years ago. The bacterium is a small gram positive diplobacillus that is characterized as an extremely fastidious and often intracellular pathogen which affects feral and cultured salmonids. Both juvenile and adult salmonids are affected by high levels of mortality, producing considerable economic loss. Disease transmission occurs both horizontally and vertically, and, in adult fish, disease progression is usually chronic in nature. Prophylactic and therapeutic control measures through immunization and antibiotic administration have only been marginally successful to date. Recognition of environmental stressors which may predispose fish to infection is therefore considerably important in fish culture. Various environmental parameters, including water temperature and dietary components, commonly encountered as variables in facilities rearing salmonids were previously shown to influence BKD development in fish.

This investigation examined the effects of two commonly manipulated environmental variables which represent potential stressors of cultured salmonids: stocking density (overcrowding) and feeding rate (overfeeding). Two groups of Chinook salmon (Oncorhynchus tshawytscha) fingerlings spawned from sub clinically versus clinically BKD infected brood stock were compared as to prevalence of Renibacterium salmoninarum antigen in kidney tissue before and after exposure to different levels of stocking density and feeding rate. Three levels of each variable were included to yield nine treatment levels within each progeny group, and each treatment level was run in triplicate. Specifically, fish were maintained at densities of 1.5, 2.5, and 3.0 pounds of fish per cubic foot of water, and salmon feed was distributed at 2.8%, 5.6%, and 8.4% of body weight per day. Water quality co-variants including dissolved oxygen content and unionized ammonia concentration were measured three times weekly for all groups. Prior to distribution of the fingerlings to the various treatment groups, fifty to one hundred of these fish were sampled to determine baseline antigen prevalence and body weight values. Fingerlings were maintained at the various treatment levels for twelve weeks; fish were sub-sampled at four, eight, and twelve weeks to determine prevalence of antigen in kidney tissue. Mortalities from the experimental groups were also removed and tested. Renibacterium salmoninarum antigen prevalence was assessed by direct fluorescent antibody test (DFAT) and enzyme-linked immunosorbent assay (ELISA). Average body weight per fish was recorded at the termination of the experiment in order to evaluate the effects of the treatment levels upon growth rates.

Significantly higher antigen prevalence and mortality were observed in progeny derived from the clinically infected adults than were observed among progeny of the sub-clinically infected fish, regardless of treatment group. The DFAT and ELISA test correlation was much greater for the mortalities than for the randomly sampled fish. Survival analysis of fingerlings spawned from the clinically diseased brood stock indicated that greater survival occurred in the treatment groups exposed to the higher densities and higher feeding rates.