300,000 diseased cutthroat trout have to be destroyed
A stubborn disease at the Lahontan National Fish Hatchery resulted in a fish biologist’s nightmare this week when more than 300,000 fingerling Lahontan cutthroat trout had to be destroyed.
The disease, called furunculosis, is caused by bacteria occurring naturally in the wild waterways in Northern Nevada, but in the closed environment of the fish hatchery, it proved to be impossible to eradicate.
Biologists first discovered infected fish in late November and had been using several antibiotics to treat it, with assistance from California/Nevada Fish Health biologists, according to hatchery supervisor Larry Marchant.
“We routinely keep track of the mortality and typically we’ll lose one to two fish per raceway per day,” he said. “But at the end of November, first part of December, things started to break a little bit, and when we got to a mortality of five or six fish per raceway, we knew something was wrong.”
The fish were treated with an antibiotic for furunculosis, which did not knock it out as hoped, Marchant said. A second antibiotic was administered, with disappointing results. Finally, a third antibiotic gave the results they’d been waiting for.
“At the worst, our mortality was 3,500 fish per day, but with this third antibiotic, after 10 days we were down to 100 per day,” he said. “It was very encouraging.”
Last weekend, the mortality went back up and Marchant said the mood of everyone who takes care of the fish again plummeted.
“That was so discouraging, because we’d been having good results and then it just dropped off,” he said. “It was obvious they were being reinfected, and at that point we knew we had no choice but to remove enough fish so we could go on to fresh water and save the rest.”
n Threatened species. The Lahontan cutthroat trout is listed as a threatened species under the Endangered Species Act. Hundreds of thousands are raised at the Lahontan National Fish Hatchery – usually around 650,000 per year – for release primarily in Pyramid Lake and Walker Lake.
Right now, the fish are fingerlings, just under 1-year-old and around 5 inches long, spawned last April, May and June from eggs gathered at Pyramid Lake. Because the sick hatchery fish were discovered to have a resistance to one or more of the antibiotics used in December, none of the remaining 200,000 fish will be released into Pyramid Lake this year since it is important to not contaminate the breeding stock with a resistant bacteria at Pyramid, Marchant said.
Although the lake usually gets around 100,000 fish per year from the Carson Valley hatchery, a spokesman for the Pyramid Lake Paiute tribe’s hatchery said the impact of not getting the fish this year will be minimal.
n Need new wells. Pyramid Lake had its own outbreak of furunculosis, but was able to successfully eradicate it with antibiotics because it is a fresh water facility and not a re-use system, Marchant said. Out of 19 working hatchery facilities in the West, the Lahontan National Fish Hatchery is one of only three to reuse water facilities, recycling fresh water through the system, with fresh water always being added. The other 16 Western facilities use fresh water that flushes through the system one time and then back into the environment.
This is the first significant outbreak of disease at the Lahontan Fish Hatchery in 30 years. Whirling disease equally vexed the experts in 1970. One of the reasons for the failure of antibiotics to kill the bacteria is the reuse system, Marchant said. Fisheries biologists will want to eventually convert the Carson Valley hatchery to a fresh water, non-reuse facility, especially after this year’s disaster. Three additional wells will be needed, bringing the total on site to seven wells. One well can cost up to $750,000, Marchant said.
n What the disease does. Furunculosis affects the internal organs of the fish, shutting down the kidneys, Marchant said. In its worst expression, sores erupt on the skin.
Wild fish can contract furunculosis, but because of the open waters and much less dense populations, it isn’t considered a big health threat. The bacteria dies quickly without a host or when exposed to high temperatures, and isn’t considered a threat to humans, Marchant said.
The source of the bacteria in the water could have been from birds that frequent the rivers and then visit the hatchery, said Randi Thompson, a government public affairs spokesperson. There is also a possibility that a compromised water pipe, perhaps a result of the 1997 New Year’s flood repairs, could be letting in the bacteria deep underground.
To test this, fish will be placed in live cages in the fresh water intake area in coming weeks, Marchant said.
More than 80,000 fingerlings died before officials decided to take action Thursday by removing and destroying the sick fish, bringing the total loss to 380,000 fingerlings.
“It’s tough on us because we’ve been with these fish for eight months or nine months and there’s nothing worse for a fisheries biologist than to have to euthanize fish,” Marchant said.
n Humane method. Batches of fish were placed into a large tanker truck and carbon dioxide gas was added to the closed environment.
“This is actually what we use to anesthetize them, so it is like they just go to sleep,” Marchant said. “It’s the least painful way to do it, we believe.”
The cost of the treatment and euthanization is minimal – perhaps a couple of thousand dollars, said Thompson. The dead fish were to be converted to fertilizer or feed by Reno Rendering, she said.
Following a complete sterilization of the tanks and runways, the remaining 200,000 fish will be divided among 21 runways and be medicated immediately, Marchant said. Hatchery biologists will know within one week if those fish are healthy, and if so, all are scheduled to be released into Walker Lake around the first part of March. When the hatchery is empty, Marchant said, the facility will be sterilized again, a procedure performed every year.
“This year, we’ll probably use extra hot water,” he said. “We don’t want to go through this again.”
The cycle will begin again, he said, with eggs being gathered at Pyramid Lake beginning in April, and hopefully these fish will thrive to be returned there next spring.
n Not to worry. A strain of genetically pure original Lahontan cutthroat trout, transplanted to a river in Utah many years ago, the “Pilot Peak” fish, have been kept separate in their own circular tanks and are unaffected by the outbreak, Marchant said.
“They’re in their own separate units – we have 80 in one circular tank and 100 in another, so they’re just fine,” Marchant said. “We’re going to Utah to collect more eggs this spring.”
Fish will still be available for this year’s popular annual Carson Valley Kids Fishing Derby, according to Thompson.
“They use rainbow trout for the fishing derby and they are OK,” she said. “I know lots of people will be concerned about the derby. We’ll have help from other hatcheries, too.”