Black Spot Disease in the Lower Deschutes

For anyone who has fished the lower Deschutes River this year, it is not news that many of the fish being caught have Black Spot Disease (BSD). How many fish? We’ve received reports of as many as 100% of 30 fish caught over a three-day trip between Trout Creek and Harpham Flat. Most reports are that 60 to 80% of landed trout have obvious evidence of BSD.

Lower Deschutes River bull trout showing obvious Black Spot Disease. Photo courtesy of Nick Wheeler.

We, along with several of our supporters, have contacted representatives of the Oregon Department of Fish and Wildlife about this issue, and have been told there is nothing to be alarmed about. One of our supporters received an email from ODFW that included the following:

“ODFW has done some research on the effects of blackspot [sic] on spring chinook [sic] smolts in the John Day River and found that the parasite had no adverse effects on condition or survival, even fish that were severely infected performed the same as uninfected fish. Our pathologists also have stated that blackspot [sic] is not categorized as a disease, meaning that it does not appear to effect the host. It is also important to note that blackspot [sic] is very cyclical, and most often comes and goes through time.”

We’ve not seen any research reports from ODFW regarding BSD, although it’s not unusual for these reports to not be advertised or be made readily available. What is unusual is that anglers who fish the bodies of water mentioned by ODFW do not report seeing BSD. This is not to say that BSD isn’t present on the John Day and other rivers, but it’s clearly not present right now to the same extent as in the lower Deschutes.

According to the statement from ODFW, BSD “is not categorized as a disease.” This is a curious claim. Why is it called Black Spot Disease? In all of the scientific literature that we searched, it is always referred to as a disease. This is because infection with BSD results in both systemic inflammation and tissue changes in fish. Inflammation is evidenced by increased cortisol (a hormone associated with stress and inflammation) levels. The skin and scale changes seen on fish with BSD are not caused by trauma. So we have a transmissible infective organism causing inflammation and tissue changes. That meets the definition of a disease.

The fish ODFW representatives have observed with BSD are noted to be in good condition. Yes they are, when they are caught. But no one is performing long-term observation to see what the consequences of chronic infection might be. We are now in the third year of BSD being observed in lower Deschutes River fish, so it’s obvious that more fish are being infected for longer periods of time. None of the studies on BSD to date look at longer-term infections, so those consequences are unknown.

What is known is that fish do die of BSD. According to reports, once fish are infected in the eyes or mouth, survival is limited. And fish with high parasite loads tend to be of lower weight.

The ventral surface of a redband trout with Black Spot disease, caught in the lower Deschutes River in late April 2017. Photo by Jamey Mitchell.

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 a fish, the fish develops an immune response that causes the dark spot.

Fish-eating birds are the next host, which become infected when they ingest infected fish.  The cercariae develop into adult flatworms, which means that fish-eating birds are internally infected with the parasite.  The parasite then produces eggs, which are shed in feces by fish-eating birds, and deposited in water where the life cycle is reinitiated.

This summer, many have observed decreases in fish-eating birds in the lowest forty miles of the Deschutes. Kingfishers are rarely seen now in that reach of river (they were previously seen in pairs occupying nearly every reach of river), and merganser populations in the lower forty miles have declined. Are these birds becoming infected with neascus and dying? Or is something else going on? Unfortunately, no one seems to be investigating this phenomenon.

Increases in BSD are associated with increased water temperature and increased aquatic snail populations—both conditions that Selective Water Withdrawal Tower operations have created in the lower Deschutes River. Further, research has demonstrated that rather than being “cyclic,” BSD is linked to sustained elevated water temperatures and algae growth.

The likely solution to reducing BSD is a return to cooler water temperatures and less nutrient loading in the lower Deschutes River. This would require that the SWW tower draw more water from the bottom of Lake Billy Chinook before discharging downstream.

Sources

Schaaf, Cody J, Suzanne J. Kelson, Sébastien C. Nussle, & Stephanie Carlson . Black spot infection in juvenile steelhead trout increases with stream temperature in northern California. Environmental Biology of Fish,; April, 2017.

McAllister, CT, R. Tumlison, H.W. Robison, and S.E. Trauth. An Initial Survey on Black-Spot Disease (Digenea: Strigeoidea: Diplostomidae) in Select Arkansas Fishes. Journal of the Arkansas Academy of Science, Vol. 67, 2013

Schaaf, Cody J. Environmental Factors in Trematode Parasite Dynamics: Water Temperature, Snail Density and Black Spot Disease Parasitism in California Steelhead (Oncorhynchus mykiss). Submitted to University of California Berkley for Masters Thesis, May, 2015.


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