A fluke can sometimes refer to a stroke of good luck or chance, but not in the context of the animal kingdom.
The flukes that some Eel River chinook salmon experienced this fall were parasites that burrowed into their eyes and caused them to go blind, according to a preliminary report from an ongoing University of California Davis study. The blindness and other strange symptoms were first noted by divers from the local nonprofit organization, Eel River Recovery Project. The project’s Executive Director Patrick Higgins said these symptoms are likely the result of the warm, low-flow conditions that plagued the lower river this past fall.
“Once these (flukes) drill in the eye, they find a permanent medium to reproduce in,” Higgins said. “It was a pretty nasty condition.”
California Department of Fish and Wildlife environmental scientist supervisor Allan Renger said no exact cause for the eye fluke infection in the Eel River has been identified. While the fluke has been known to affect salmon, Renger said that the parasite’s presence is uncommon for adult salmon populations in California. “I don’t have any documentation of eye flukes being detected in the Eel River before,” he said.
The UC Davis study also found a currently unknown virus in the brains of the three tested salmon, which concerned Stillwater Sciences fish biologist Joshua Strange more than the flukes.
Through his studies of fish pathogens and parasites on the Klamath River, Strange said there are many types of serious viral fish diseases.
“It’s not cause for alarm until we know the identification of the virus,” Strange said.
Local kayak guide and Eel River Recovery Project board member Eric Stockwell first discovered the blind Eel River salmon in a holding pool near Fernbridge in October 2015. The pool was only 4 feet deep at the time and filled with algae — a sign of poor river flows.
As Stockwell approached some of the fish, he noticed they had clouded eyes. He also saw that many were acting lethargic and did not swim away as he approached them.
The river’s warm, low-flow conditions were also found at other holding pools along the lower portion of the river near Fortuna, which Higgins said was the result of the ongoing statewide drought.
Higgins said large amounts of algae began to accumulate at the bottom of these shallow pools as the currents weren’t strong enough to wash it away. As more and more fish crowded into the holding pools awaiting rains, Higgins said the algae would reduce the oxygen levels at night and making the water more alkaline, thus stressing the fish immune system.
At the same time, Higgins hypothesized that the algae likely attracted an intermediate host of the eye fluke parasites — snails.
The flukes have a unique life-cycle involving three or more hosts. The eye fluke reproduces in the intestines of fish-eating birds like gulls, and the eggs are deposited in the bird’s feces, Higgins said. The eggs then hatch and reproduce inside snails and are eventually grow into a new form and are released in search of a fish host. When a gull or other bird eats the infected fish, the cycle starts all over again.
While Strange reiterated that the cause of the fluke infestation is still unknown, he said the low-flow conditions may have contributed as fewer flukes were washed away.
Renger said that his department did not observe any of the symptoms on the spawning salmon in the upper reaches of the river.
Of the nearly 5,000 Chinook salmon estimated to have entered the lower Eel River before winter rains ramped up in December, Higgins estimated about 10 percent were afflicted by the flukes.
“We don’t know the degree to which it compromised their ability to reproduce, but it appears there was a substantial loss of reproductive capacity in the first wave,” Higgins said.
Higgins estimates there were about 10,000 to 15,000 Chinook salmon that entered the lower Eel River this season, which he said is lower than the historical average of around 20,000 to 50,000 salmon in the mid-20th century.
“That’s still a lot of fish,” Higgins said of this year’s run. “If they don’t have any place to sit when they come in and the rain doesn’t happen, then there could be greater consequences than the 2015 scenario.”
Higgins said one solution to the shallow pools would be to engineer a stabilized river bank near areas like the Fernbridge pool. He said this type of bank would reduce erosion of the river banks while scouring the bottom of the river bed by bolstering flows. Similar banks have been used on the Russian River, but Higgins said these proposals cause controversy as they involve modifying the natural river bank.
The lead author of the study, UC Davis associate professor of aquatic animal health Esteban Soto, said he did not want to comment on the preliminary study as the study has not been completed. However, he estimated that the study will be finished in about a month with more time needed to prepare a report for the Department of Fish and Wildlife.
Will Houston can be reached at 707-441-0504.
Article originally published by Eureka Times-Standard. Read original here.