Scientists Find Parasites in Brook Trout on the Rise

By Jack Williams, Trout Unlimited

Gill lice in a Wisconsin brook trout.
Photo courtesy Wisconsin Trout Unlimited

In Wisconsin, Department of Natural Resources staff are finding increasing rates of gill lice parasitism in brook trout as waters warm. In North Carolina, the same gill lice have been found for the first time on brook trout from the Cullasaja River drainage. In both cases, state biologists are asking anglers for help in tracking this parasite.

Gill lice are a type of copepod parasite that attach to the gills and opercula of brook trout. The parasite, Salmincola edwardsii, only infects fish of the genus Salvelinus, such as brook trout, and not rainbow or brown trout. The parasite naturally occurs in northern states, like Wisconsin, but until recently, has not been recorded from southern states.

In Wisconsin brook trout, parasite infestations follow warm winters
Matt Mitro of Wisconsin DNR reported at the recent Wild Trout Symposium that the incidents of gill lice in some brook trout populations has risen to a 95 percent infection rate. Heavily infested brook trout may not be able obtain sufficient oxygen and often are in poorer condition going into winter. Warmer stream temperatures are implicated in recent brook trout declines in Wisconsin and increases in gill lice parasitism.

As waters warm, non-native brown trout also move in, so it’s hard to tell what is stressing the brook trout more:  warmer water, invasive brown trout or the gill lice. It’s likely all three – an unfortunate triple whammy. Regardless of the cause, the result is the same: fewer brook trout.

Wisconsin Trout Unlimited is asking anglers to check for the presence of gill lice in brook trout. Check their website for tips on identifying the white parasites on the red gills of brook trout. Remember not to touch the gills of any trout you plan to release.

In North Carolina, a disturbing discovery
Jacob Rash of North Carolina Wildlife Resources Commission recently reported on what may be the first observation of gill lice on brook trout in the South. Gill lice were found on brook trout from several headwater streams of the Cullasaja River system in Macon County.

How did the gill lice get there? Hard to tell, perhaps from some ill-conceived introduction. What are the implications of the introduction? Also hard to tell, but as gill lice stress native brook trout, the native trout become more and more susceptible to other problems, such as warming waters and invasions of non-native fishes, like brown trout and rainbow trout.

If you see any gill lice in brook trout caught from North Carolina waters, Jacob Rash would like to hear about it by email at

Invasive species and climate change
Warmer water spells trouble for native trout. Parasites and diseases can proliferate in warmer conditions. For example, as brook trout become stressed and their ability to compete successfully with browns, rainbows or warm-water fishes is diminished.

For a native trout already suffering from warming stream temperatures, with predicted further losses of habitat in the future, gill lice may pose one more twist in climate change scenario. Wisconsin Trout Unlimited is working actively with the Wisconsin Department of Natural Resources to get a grip on the problem, helping with outreach and education as well as an intensive effort to encourage anglers to report their observations of the parasite.  Check out the Wisconsin TU website for more information.

What else can anglers do to engage on the issue of climate change? Plenty. Stream temperature can be monitored with easily installed temperature loggers. Changes in the timing of aquatic insect emergence can be tracked in response to earlier runoff and warmer conditions. See our Angler Science program at TU for descriptions. Anglers can help streams prepare for climate change by replanting riparian vegetation to cool water and halt erosion, or by inserting logs or boulders that provide cold-water refuges as higher flows impact the structures and dig out pools. The key is getting involved, whether monitoring streams, restoring habitats or improving policies.

Jack Williams is TU’s senior scientist. He works from Medford, Oregon. 

11 thoughts on “Scientists Find Parasites in Brook Trout on the Rise”

  1. that whole general climate change this is just so tainted, and didn’t it used to be called global warming before all the snow and ya couldn’t sell it with that name? … when Algore was pushing his agenda and making his billions? yea…billions. the streams aren’t warming on a regular basis…all this panic is planetary weather cycles. don’t throw at me …”well the scientists are saying this…” no, real ones aren’t, the ones saying it are hired by the govt. and they like the easy work and grant money. it’s obvious author jack williams is drinking the kool-aid, just wish he drank it on his own blog and orvis stayed parched. brook trout don’t fight or move as much as the others…hence sitting in still pools with less water quality is the reason. not clibal changwarm or whatever the latest sell-able term is now.

    1. Lee –

      Actually brookies are probably the most sensitive trout species to water quality. That’s why acid rain was so devastating for Appalachian brook trout. You do believe in acid rain, right?

  2. It does seem as if the article is used as a pretext to repeat the Global Warming/Climate Change agenda: as if, “Here’s something we noticed in Brook Trout . . . and Oh, by the way, you, too, can do something about GW/CC.”

  3. I prefer to use the term climate change because some regions are experiencing record droughts and warmer temps as in the case of the western US while the eastern US has severe storms. So much of science is based on observations. The last few years we have had the warmest temperatures on record, with the northern latitudes are losing their snow and ice packs at a alarming rate. To Frank and Lee, sticking your head in the sand isn’t going to make it go away. Why do you have a problem with NOAA, NASA and Jet Propulsion Lab scientists? Do you think they are part of a conspiracy? We are fortunate to have respected scientists like Dr. Jack Williams. I was asked by some friends what is the agenda behind climate change and I was taken back by that question. I should have gotten out of my chair and picked up their little grandson, “this is my agenda, to leave the earth a better place for his life”.
    It’s not because I want to keep catching trout and salmon, it’s because these are indicator species. We need healthy streams, habitat and wildlife and if they go, we go too. Climate change awareness means changing our lifestyle and its apparent that
    some people don’t want to do that. So Frank and Lee if you have kids or grandkids maybe you should give some thought to this.

  4. It is interesting that the deniers of climate change do not state their case calmly or politely. Among climate scientists there is no doubt that greenhouse gases are increasing dramatically in concert with the increased burning of fossil fuels. CO2 levels are climbing higher and higher and part of the cause for that is man’s energy development and combustion engines. Every day our city roads clog with one passenger autos and in the larger cities this is true all day. Power plants belch greenhouse gases while keeping up with our increasing energy demands. Glaciers are receding, the ice pack is shrinking and warm loving species are being seen ever northward. So that is only part of what scientists have seen and Trout Unlimited’s Jack Williams is a scientist who has seen first hand the changes in trout streams, and works to make them more resilient. I applaud his work and that of the other scientists and hope we can begin to decrease emissions before the losses are too great.

  5. Why did this Trout Unlimited fly fisher – who kicked the climate change can down the road for decades – finally wake up? It took fish kills in North Georgia during that terrible 2006-07 drought. Fingerlings were dying in state hatcheries in superheated, 85 degree water. And trophy trout were rolling up dead in the Chattahoochee River’s famed Nagoochee Bend. Stream flows had dropped from 125 CFS (summer average) to 17 CFS – our beloved “Hootch” had run out of water. Otters and raccoons had a field day. The fish had nowhere to run and crows and buzzards were circling at the river bends.
    “Don’t attribute singular weather events to climate change, “ the scientists warn. “Focus on the trends.” But veteran guides had never seen anything like that summer. It was awful.
    Lee Garret, you don’t have to believe Al Gore. But Jack Williams I know personally. He is a good fly fisher and a dedicated TU staff scientist. As a human being, he is good as gold. It would be nice if someday you revisited climate change. If you do, be careful where you get your information! But what you can do now is give up the disrespect you show Jack and others. “It is interesting that the deniers of climate change do not state their case calmly or politely,” Larry Harris wrote. Your tone of voice is inappropriate for Orvis’s blog. Most TU fishers respect all our people, whatever their views on climate change. We focus on the fish, we work together on TU’s marvelous programs, we go down the road together and fish together. Thanks to Jack and others, we don’t let climate change come between us. It is not worth it.

  6. If you haven’t noticed weather events getting more weird, record droughts, more wild fires. super storms, an increase in invasive species in cold water streams, shortened trout fishing seasons….then I feel sorry for you and perplexed. Something is changing our weather patterns. I don’t care for Al Gore either but I do respect the considered opinion of the vast number of qualified scientists that we are changing our planet and degrading the things I love. Go fishing, you will notice changes too.

  7. Trout don’t like warm water and they sure don’t like no water. And that is what we have here in Northern California in the third year of record drought where it did not rain one day in San Francisco in January. That has never happened in recorded weather history. Our tributaries in which juvenile Steelhead and Coho Salmon must live for 1-2 years before out migrating to the Pacific are coming up dry here in March. MARCH! This isn’t late October. This is early March. There is something happening here. And what it is pretty dang clear. It is Climate Change for the worse and we need to do something about it now or we can kiss our wild Steelhead and Coho fisheries goodbye.

  8. Gill lice were found in PA in 2016 ( ) by PA Fish and Boat Commission field biologists. The link was established to connect brook trout being raised in commercial fish hatcheries that were being provided to Cooperative fish nurseries that are operated by local sportsmen’s groups to stock brook trout for fishing derbies to brookies native to those streams being stocked. Despite interest in regulating commercial hatcheries, they are still unregulated even though surrounding states have stocking permit requirements for all species that are stocked in their states. Salmincola californiensis species of gill lice infect rainbow trout and Salmincola edwardsii are specific to brook trout. Ruiz et al. further describe the commercial hatchery connection in the southeastern US in

    A simple requirement for a veterinarian certification for all brook trout and rainbow trout stocked from commercial hatcheries in PA has been delayed because of commercial hatchery cost and political concerns.

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