Wild trout streams and stocked sport fisheries across Colorado
- Investigate the distribution and prevalence of Renibacterium salmoninarum, the causative agent of Bacterial Kidney Disease in Colorado’s wild trout and stocked sport fisheries
Native and sport fish populations across Colorado are impacted by many factors including habitat alterations associated with changes in stream flows, temperature, and water quality, and host of less obvious biological threats from diseases and parasites. While the prevalence of many fish diseases has declined in recent years due to good management practices, cases of bacterial kidney disease (BKD) seem to be increasing. The causative agent of bacterial kidney disease is Renibacterium salmoninarum, a gram-positive intracellular parasite. The disease is characterized by the presence of gray-white necrotic abscesses in the kidney and can cause mortality in both wild and cultured salmonids. Unlike other common fish pathogens, this bacterium can be transmitted horizontally between fish through contaminated water and vertically from adult to egg. This likely plays a major role in the persistence of this bacterium in susceptible fish populations.
After going undetected in Colorado hatcheries for 18 years, Renibacterium salmoninarum has been found at four state hatcheries, one federal hatchery, and one wild broodstock lake since 2015. The objective of this study was to document the distribution and prevalence of R. salmoninarum in Colorado’s wild and stocked sport fisheries and investigate if fish stocking practices have influenced that distribution. To accomplish that, second to fifth order wild trout streams were randomly selected in each major river basin and total of 68 streams were sampled. To investigate if stocking practices have affected the distribution of R. salmoninarum, waters stocked by hatcheries with recent positive tests for were matched with nearby waters with similar management and a total of 75 different stocked sport fisheries were sampled. Kidney samples were collected from up to 60 fish from all waters and samples were tested by ELISA, real-time PCR, nested PCR, and DFAT according to standard methods.
Results indicate that the prevalence of R. salmoninarum is high in both wild trout and stocked waters by ELISA but bacteria levels are generally low and not commonly detected by standard DFAT assays. Eighty-seven percent of cases that tested positive by ELISA were below detection levels of DFAT. Only two cases of clinical disease were observed, one wild trout stream and one stocked lake. Ninety-three percent of all waters had some fish that tested positive by ELISA, 12% tested positive by both qPCR and nPCR and 13% tested positive by DFAT. All wild trout waters had some fish test positive by ELISA but there was no difference in the average R. salmoninarum antigen levels of stocked and unstocked waters. There was little evidence that stocking practices were more associated with bacteria levels than stream order or lake elevation. Stocked and unstocked waters had similar prevalence and levels of the bacteria and there was not a strong relationship between historical stocking and bacteria levels, except maybe at very high stocking densities in our largest rivers. Renibacterium salmoninarum appears common and widely distributed in Colorado’s wild and stocked trout fisheries but at low levels that rarely cause disease.