Black Spot Disease: What is it and what does it do to trout?

Black Spot Disease: What is it and what does it do to trout?

Over recent years, there has been concern over the amount of fish in the Lower Deschutes River that appear to have black spot disease. It’s suggested that the disease is harmless to humans. Heavily infected trout may experience inhibited vision and increased stress; although, there is no concrete evidence that suggests that black spot is fatal to trout. 

Black spot disease is caused by a type of parasitic flatworm called trematode flukes. Uvulifer ambloplitis is the specific parasite that affects fish in the Lower Deschutes. This parasite has a complicated life cycle. It starts by infecting aquatic snails and then it transfers to trout if they consume an infected snail. Trout have an immune response to the disease which creates small black raised cysts, thus the name black spot disease. The final stage in the life cycle involves a fish-eating bird or raptor, often a belted kingfisher. After a bird or raptor consumes an infected fish, the parasite lays eggs in the bird’s digestive tract. The eggs are then deposited into the river and the cycle continues.

Over recent years, black spot disease has been more prevalent in the Lower Deschutes. It’s unclear what the cause is. A 2014 macroinvertebrate study indicated a large increase in the number of aquatic snails present in the Lower Deschutes River. This could contribute to a higher population of parasites. Some say that the increase in snails is caused by an increase in algae in the river. A study performed in northern California suggested that higher water temperatures increase the abundance of aquatic snails and fish that are infected with black spot disease. There have been no studies performed on the Deschutes River.

Without concrete science, we can only wonder what is driving the increase in black spot disease. More specific research is needed to uncover what is causing the increased amount of infected trout. The good news is there is currently no evidence that the disease is significantly affecting trout or salmon populations within the river. 


Greg McMillan. (2016, March 18). Deschutes River Alliance. Black Spot Disease Seen in Lower Deschutes River Fish.

Schaaf, C. J., Kelson, S. J., Nusslé, S. C., & Carlson, S. M. (2017). Black spot infection in juvenile steelhead trout increases with stream temperature in northern California. Environmental Biology of Fishes, 100(6), 733–744.

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