Bacterial coldwater disease threatens wild and hatchery-raised salmonid fishes around the world, as well as the economic impacts that they offer. The disease is unfortunately spread through both contact with other fishes as well as through sexual reproduction.
But a new study led by researchers at Michigan State University has uncovered some hope for fighting the fatal sickness, at least in hatcheries.
Through the research, scientists at the university have found that a number of trout and salmon fishes seem to be less likely to contract the disease if they’re reared at fish hatcheries. This is in spite of a notion that fish in captivity are more likely to catch the sickness because they are living in such close proximity to others and in artificial conditions.
To make that find, researchers worked over a course of five years to sample fish during spawning runs, as well as many that were living in state fish hatcheries. Fish that they considered in their analyses include lake, brook, brown and rainbow trout. On the salmon side, Coho, Atlantic, Chinook and steelhead salmon were sampled. All of the fish were from different watersheds around Lake Huron, Lake Michigan and Lake Superior.
Scientists relied on polymerase chain reaction confirmation, a type of DNA analysis, to check for the disease in fish. The results they tabulated were then also run through a logistic regression model to account for the many variables at play.
This helped to show that bacterial coldwater disease was present in all broodstocks except for captive lake trout and brook trout. Among the wild fish they considered, Chinook salmon from Lake Michigan had the highest prevalence of the disease. And of those fish held in captivity, the Gilchrist Creek strain of brown trout had the highest prevalence of infection.
But overall, their numbers showed that captive broodstocks seem to do much better than wild fish at avoiding the spread of the disease.
Researchers note that, in spite of the fact that the bacterium causing the disease is found in many hatchery fish, the disease was rarely detected in their offspring. There were still some outbreaks that researchers observed in fish hatcheries, and those had predictably very bad effects.
Taking it all into account, scientists write in the study that their findings confirm that the disease still continues to threaten Great Lakes fish species. As for why the prevalence of infection seems to be a little less morbid in hatcheries, it’s likely due to the extra care that the fish receive from those working at the facilities.
Things like administering vaccines, controlled diet and good breeding management all help to protect hatchery fish. Likewise, hatchery fish get around-the-clock attention from managers who are trained to recognize if something is amiss.
Full results of the study are published online in the Journal of Aquatic Animal Health.