By Greg McMillan
We started receiving reports of “black spot” disease in lower Deschutes River bull trout a few weeks ago. The first report was from Andrew Perrault from the Gorge Fly Shop in Hood River. He sent along these photos and it is pretty easy to tell these black spots are not normal on bull trout. Many of us know that bull trout are species Salvelinus and thus members of the char group. These fish don’t have black spots as normal coloration.
Since that first report we’ve heard of other observations of black spot disease on both bull trout and red band trout. The presence of black spot disease has been confirmed by sources at Oregon Department of Fish and Wildlife.
Black spot disease is caused by a flatworm (trematode) parasite known in the scientific community as Uvulifer ambloplitis, and also known as “neascus”. This parasite has a complicated life cycle that starts with eggs in water, which hatch and become juveniles known as miracidia, which in turn infect aquatic snails. In snails this form of the parasite matures into the next life form, known as cercariae. Cercariae are shed by the snails and become free swimmers, which attach to fish. Once the cercariae have attached to the flesh of fish, the fish develops an immune response that causes the dark spot.
Kingfishers are the next host, which become infected when they ingest infected fish. The cercariae develop into adult flatworms. The parasite then produces eggs, which are shed in feces by kingfishers, and deposited in water where the life cycle is reinitiated.
These flatworms do not appear to be fatal to fish, or other hosts. There are scattered reports of fish stressed from other sources dying while infected. No human infections have been reported, but there is no real surveillance mechanism to detect human infections. Although probably safe for human consumption after thorough cooking, there are no study data to confirm that.
None of us who have fished the lower Deschutes River for decades can say that we’ve seen many, if any, fish with this condition. There are reports indicating there have been infected fish in the lower Deschutes River and tributaries in the past, but they aren’t common. So what has changed? Is this random? Or linked to the ongoing ecological changes we are all seeing in the lower river?
This might be related to an increase in the snail population in the lower Deschutes River. Portland General Electric’s Year 1 Data Summary Report from their Lower Deschutes River Macroinvertebrate and Periphyton Report Study published in 2014, indicates that there has been a significant increase in snail populations in the lower Deschutes River. This increase in population in the intermediate host (snails) might be related to the increase in black spot disease noted in fish. The snail population increase is likely linked to the increase in algae in the lower river.
Is this a catastrophic occurrence? Probably not. But it could be another indication of ecological change in the lower Deschutes River.
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