Are diseases playing a role in salmon decline?
Chinook, coho and steelhead populations in Puget Sound have declined dramatically over the past 30 years. In some cases, counts of fish returning to the rivers are just a tenth what they were in the 1980s. While many possible causes of this decline are under consideration, some researchers are focusing on the combined effects of predators and disease. This article continues our coverage of the ecological impacts of disease in Puget Sound.
Studying disease among migrating salmon is no easy task, said Michael Schmidt of Long Live the Kings, explaining why so few studies outside of hatcheries have focused on salmon diseases.
In hatcheries, people notice when fish are dying. They can send the dead fish to a pathology lab and identify the disease agent. Consequently, hatchery managers keep a list of diseases to watch out for and protect against.
These same infectious agents can affect wild fish, but it is much harder to track the spread of disease in open waters. As far as anyone can tell, the fish there just seem to disappear, although researchers have been able to show that 80 percent of juvenile steelhead in Puget Sound die before they reach the ocean. In Hood Canal, this disappearing act seems to take place around the Hood Canal bridge, which suggests that predators may be picking them off there.
Long Live the Kings is one of many participants in a $20-million, five-year study examining the fate of salmon and steelhead in marine waters on both sides of the U.S.-Canada border. On the U.S, side, one intriguing study is looking at the effects of the parasite Nanophyetus salmincola, which seems to be infecting steelhead from streams in southern Puget Sound at a much higher rate than those from streams up north.
Nanophyetus is a parasitic flatworm with a complex life cycle that involves three hosts, a snail and a warm-blooded animal, as well as the fish. “Salmon poisoning,” a potentially deadly disease in dogs, can occur when a dog eats raw fish infected with the Nanophyetus parasite. The parasite itself is relatively harmless to dogs, but it can carry a parasite of its own, Neorickettsia helminthoecad, which kills 90 percent of dogs that become infected — unless they are treated quickly. Salmon poisoning affects only the biological family Canidae, including dogs, wolves, foxes and related animals.
In a stream, the Nanophyetus parasite gets into freshwater snails via the feces of mammals and birds that have eaten the infected salmon. The parasite then leaves the snail, goes back into the water and burrows under the skin of a fish.
In the Nisqually River between Tacoma and Olympia, researchers have found that virtually all of the juvenile steelhead leaving the river are heavily infected with the parasite.
“The parasite loads we see in these fish is really high,” said Paul Hershberger, whose Marrowstone Island lab is involved in the research. “Looking under a microscope, there is no kidney left in some of these fish. It looks really alarming, but whether it is killing the fish we don’t know.”
Compared to Nisqually, the percentage of steelhead infected is somewhat lower for other South Puget Sound streams, while in northern Puget Sound relatively few of the steelhead are infected. The cause of the variation may be related to the abundance of snails, but the reasons for more snails in South Puget Sound is unknown. Some say it could be related to temperature, streamflow or type of sediment.
Laboratory studies suggest that fish burdened with the parasites may be unable to swim as fast or as long as healthy fish. If that’s the case for steelhead in Puget Sound, they could be easy targets for predators, such as harbor seals. An upcoming experiment will test that idea by releasing an equal number of diseased and healthy steelhead migrants into South Puget Sound. Acoustic tags implanted in the fish will send out signals to determine how far the fish make it and possibly reveal their ultimate fate.
If Nanophyetus turns out to be a significant problem for steelhead and possibly other salmonid species, experts could look for ways to disrupt the life cycle of this parasite and reduce at least one obstacle to the long-term recovery of fish in Puget Sound.