The eye disease observed in an estimated 10 percent of Eel River Chinook salmon last fall was caused by eye flukes, a type of fish parasite similar to the organism that causes the allergic reaction “swimmer’s itch” in humans, a just-released study by the UC Davis School of Veterinary Medicine has found.
Large numbers of flukes infested the eyes of all three of the salmon studied, Esteban Soto, an associate professor of aquatic animal health, wrote in the four-page report, done in cooperation with the National Oceanic and Atmospheric Administration, the California Department of Fish and Wildlife and the Eel River Recovery Project, a local river advocacy organization.
Additionally, a scientist from the California Animal Health and Food Safety Laboratory System played a role in confirming the study’s findings.
The three fish, all adults taken from the Eel and euthanized for the purposes of the study because they had obvious eye problems, were also afflicted with brain and spinal cord hemorrhages. These were caused not by the flukes but by an unknown virus. Soto, who could not be reached for comment, indicated in the report that further investigation, possibly in the form of DNA analysis, would be needed to identify the virus.
Allan Renger, a fisheries biologist out of Fish and Wildlife’s Fortuna office, said last week that the eye condition found in the fish resembles cataracts. “It damages the lens of the eye and causes blurred and decreased vision down to blindness,” he explained.
“The disease is known [to occur] in fish populations [and is] sometimes detected in anadromous fish populations [such as salmon], but not commonly,” added Renger, who said that he was only able to find one other reference in the scientific literature about a local case.
It was a Humboldt State University study from the early 1980s that had to do with eye flukes in a population of juvenile Chinook salmon in the Mad River, which feeds into the ocean west of McKinleyville.
Because the disease “doesn’t manifest itself in a way that’s [easily] detected” — meaning, for example, that it doesn’t result in large fish kills that leave river banks littered with carcasses — Renger said it’s possible the condition is more prevalent than is believed.
The only reason it was detected last fall is that members of the recovery project were out on the river near Fortuna doing fish-count dives to determine the size of the run. Eric Stockwell, ERRP’s dive coordinator and the first one to notice the vision-impaired fish, recalled last week that the initial thing that stood out was odd behavior.
“You could swim right up the fish and they didn’t react” — in other words, they didn’t dart away like a normal fish would — he related, adding that he eventually noticed something else about the strangely unresponsive, or at least lethargic, fish. They had milky spots in their eyes.
It was Stockwell and Pat Higgins, a fisheries biologist with ERRP, who ended up estimating that about 10 percent of the Chinook that were observed were afflicted.
As for the flukes themselves, they have what Renger termed “a unique life history.” While he didn’t go into details, a quick Google search reveals that they start out life in the intestines of seagulls — where adult flukes sexually reproduce — and soon end up in water after the birds defecate the eggs.
They hatch, swim around for a bit and then infect freshwater snails, where they undergo asexual reproduction. After emerging from the snails — “millions … can be released over a very short period of time,” according to WikiVet. They seek out fish, first penetrating through the flank and then migrating to the eyes. “The life cycle is complete,” according to the website, “once a fish-eating bird eats the fish” and ingests the adult flukes that will give rise to the next generation.
When asked what environmental factors might have made the Chinook in the lower Eel more susceptible to the flukes, not to mention the unknown virus, Renger speculated that “because [an outbreak] has not been identified [on the Eel] in the past, it was likely due to the unique environmental conditions” that prevailed last fall.
In other words, blame the drought.
There was “decreased water quality due to low-flow drought conditions” and the eye disease was “first identified during a drought year,” Renger pointed out.
But Renger added that the low-flow drought theory is just that: a theory. “There’s nothing in the medical literature that says these are the types of conditions” — meaning low flow drought conditions — “that lead to this eye condition. So I don’t have a great answer as to why.”
Higgins said that a lack of significant rain in September and early October meant that the algae growing at the bottom of holding pools never got flushed out. Add in lower-than-normal flows and warmer-than-normal water temperatures, and you’ve got conditions in which the flukes would be “superabundant.”
“The bottom line is that there was a foot of algae at the bottom of three-to-four-foot-deep pools. That led to an abundance of [the flukes] and an infestation of the Chinook trapped there.”
“The pools were too shallow and the riffles [between the pools] were too shallow to allow for their passage,” Higgins added.
Asked whether last fall’s eye disease outbreak was a sign that more needs to be done to make the lower Eel more salmon-friendly, Renger said that his department for some time now has been “working with private landowners on voluntary restoration projects” aimed at reducing sediment inputs into the river.
He also said that “changes in management of [logging] road networks and improved environmental regulation [has led to] reductions in the amount of sediment [entering the river and its tributaries] from timberlands. So storm events don’t cause road networks to unravel.”
“We’ve had improvements in tributary streams. Cloudy, turbid water clears earlier [than it used to] following storm events,” he added.
While the upper portion of the watershed has seen improvement, Renger said that it would take longer to make things better on the lower Eel, where large quantities of sediment deposited by massive flooding events in 1955 and 1964 are still plaguing the river.
“As you move down the system, you have the accumulation of [sediment from the] tributaries for one thing. And the bottom of the watershed is flatter. So it will take a significant amount of time to mobilize that material.”
Stockwell, the diver who first spotted the blind, or going-blind, fish, argued that greater human intervention is needed on the lower Eel, perhaps by using excavators to dig out the holding pools and make them deeper.
Higgins, however, said that wouldn’t work because the holes would quickly refill. “The river is like a conveyor belt [of sediment]. If you dig it out, it will fill back in.”
Nonetheless, Higgins said other things could be done to improve the lower part of the river. Such as block water releases from the Potter Valley Project timed to benefit fall Chinook. Or bank stabilization efforts that would “prevent further erosion and create scour,” thereby deepening pools.
Such efforts, involving the placement of rock and the planting of willow trees, has proven successful elsewhere, particularly on the Russian River in Sonoma County, Higgins added.
“I’ve been raising these points since 2010. I’ve repeatedly said that habitat on the lower Eel is perilous to fall Chinook. So I’m pretty adamant.”
“No one is looking at the lower river and saying: ‘How could we resculpt it to be more appropriate for Chinook salmon?’”