In Upper Klamath Lake, low survival of juvenile endangered Lost River and Shortnose Suckers is resulting in fewer and fewer fish reaching reproductive age. This is putting these fish populations at risk of extinction. The survival challenge faced by juvenile suckers has been attributed to a combination of stressors, likely including water quality, habitat suitability, and disease. However, pinpointing the exact causes and developing effective management strategies to address them has been challenging.

Parasitic disease has been implicated as a potential contributor to poor survival of juvenile Lost River and Shortnose Suckers in Upper Klamath Lake. Fish health monitoring efforts in Upper Klamath Lake have revealed a high prevalence of warmwater pathogens, including two types of parasitic worms that affect suckers: nematodes that infect the heart and trematodes (eye flukes) that infect the eye. 

Eye flukes have a complex three-host life cycle involving birds, snails, and fish. The parasite travels through each host at different stages of maturity, from bird to snail when birds poop, from snail to fish when the snails shed the parasite, and from fish back to bird when the fish are blinded by the eye fluke and eaten by water-faring birds. 

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Parasites can increase in numbers if the environment changes in their favor. For example, with eye flukes, highly nutrient rich waters could mean a greater abundance of snails. More snails means more eye flukes, which when shed at high levels could outright kill suckers. Alternatively, more transparent waters or more birds could make for higher predation of suckers. 

Further examination of the relationship between ecosystem changes, these parasitic worms, and our endangered suckers is needed to determine whether they are playing an outsized role in the death of juveniles suckers in Upper Klamath Lake. This summer, scientists from the USGS Western Fisheries Research Center (WFRC), led by Jan Lovy, are investigating the parasitic eye fluke through a new study. 

Our partners at the Bureau of Reclamation and WFRC scientists from Seattle, Washington and Klamath Falls, Oregon are working together to collect snails and the associated prevalence of eye flukes. The snails are collected from Upper Klamath Lake, then brought to the laboratory where they are screened for shedding parasites. The eye flukes are then identified and counted to see how many are there.   

Jan Lovy will then conduct infection challenges in WFRC’s Seattle Laboratory using the larval eye flukes collected from snails. A small research population of laboratory suckers, that were provided from the US Fish and Wildlife Service sucker hatchery, is used for these experiments. The goal is to understand how sensitive juvenile suckers are to the parasites by evaluating the concentration of eye flukes that can directly kill them or make them very susceptible to being eaten by birds. 

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This research comes at a critical time as fewer and fewer suckers reach maturity. Actions can be taken if eye flukes are found to be a major contributor to the death of juvenile suckers. For example, snail and bird colonies could be managed and/or water quality issues addressed.