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The Gill Lice Epidemic Expands


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In reviewing my log book, I have determined that for the past three years the brook trout population in my fishing area went down.


I fish the southwestern corner of Wisconsin. The counties I frequent are Vernon, Crawford, Richland and Grant counties.

The most significant numbers change I discovered was in Crawford County. It had a 65 percent decline in brook trout numbers in the past three years.

I contacted the Department of Natural Resources and asked if they had seen a similar decline. Officials there acknowledged there has been a decline, attributing it to the recent drought and to a parasite called gill lice.

DNR officials also said some brook trout streams may have been taken over by brown trout, with which they compete for the same waterways.

Matthew Mitro, a coldwater fisheries research scientist for the DNR, said the fall shocking crews are out and about and will report on any declines in populations -- and also the water levels -- they might find.

I found it odd, however, that my log books indicated a significant increase in brown trout populations over the past three years.

Gill lice (a parasitic copepod called Salmincola edwarsii) can cause significant physical trauma to the gill filaments, causing deformities which affect respiration and the efficient uptake of oxygen and the release of carbon dioxide, ammonia and other metabolites. Fish that are heavily infected cannot obtain sufficient oxygen when they are exercised, such as when they are caught by angling.

Gill lice have a direct life cycle- when the egg sacs release nauplii, they immediately molt and become the first copepodid (larval) stage and they have about 24 hours in which to find a new host and anchor onto the gills and continue their development. After several molts, the copepods reach maturity and remain permanently anchored in the gill tissue. This is a significant stress especially when more than one parasite is attached to a gill arch.

In streams with dense brook trout populations, the success rate for the larvae to attach to gills increases due to the greater chance of contacting a fish within the 24 hour “post hatch” period. Streams with faster water flow (velocity) can make it harder for the larvae to successfully attach. So fish density and water velocity are two factors that affect the prevalence and intensity of infection by Salmincola edwardsii in a stream. A third factor that may play a greater role in the future is temperature trends. Gill lice are invertebrates and therefore their development is proportional to the water temperature of the stream. If water temperatures increase, the parasites will develop to maturity faster and will then be able to reproduce one or more extra “generations” each year. Because the copepods remain on the fish, the affect of more generations of parasites is cumulative and we may see far higher numbers of gill lice on individual fish in the future.

So rather than not fish the streams where gill lice are present, I would encourage people to fish and take fish home (reduce the density of the fish) as long as the fishing regulations allow this. Anything that can be done to keep water moving (faster velocity) may also help reduce the probability of larvae to successfully attach to fish.

Always carry clean tap water, a bleach solution and a scrub brush with you when you go fishing. Disinfect your gear away from the water before moving between waterways to ensure that you do not spread Gill Lice.

Just yesterday I was in contact with Matt Mitro again and he informed me that Gill Lice had been found in the vast majority of the adult brook trout in Ash Creek. This stream is in Richland County but the significance is this stream has many of the eggs extracted from wild strain brook trout in Ash Creek and taken to the hatchery and raised. The Gill lice have effected the adult brook trout in Ash Creek. The adults are loaded with gill lice and breeding is less effective. The 0-1 year class of brook trout are down significantly. The Ash Creek strain brook trout are used throughout the state for stocking of wild strain brook trout. The finding of Gill Lice in Ash Creek is very significant.

The only way to rid the brook trout of gill lice is to individually dunk each trout in a solution. The trout is let back go and can be re-infected if it runs in to infected fish. You can not scrape off gill lice because they are attached to the gills and it will kill the trout.

The WDNR and Trout Unlimited are asking for citizen help with reporting of gill lice.

Here is a link to help out:


more information on Gill Lice

Gill lice Salmincola spp. are a parasitic copepod that only infect Salvelinus species such as brook trout Salvelinus fontinalis, which is the only salmonid native to Wisconsin streams. The gill lice life cycle begins when egg sacs release nauplii, which immediately molt into the larval first copepodid stage during which they have about 24 hours to find a host. The larval copepods anchor onto the gills of their host and continue development. After several molts, the copepods attain maturity and remain permanently attached to a gill arch.

Gill lice can cause significant physical trauma to the gill filaments, causing deformities that may affect respiration and efficient uptake of oxygen and release of carbon dioxide, ammonia, and other metabolites. Heavily infected brook trout cannot obtain sufficient oxygen when they are exercised, such as when caught by angling. Respiration may be particularly difficult for infected fish during times of high water temperatures and low dissolved oxygen levels. High rates of infection may slow the physiological processes of growth and sexual maturation, which in turn may negatively affect brook trout population growth rates. Gill lice are a parasite specific to brook trout and are not known to infect brown trout.

All information on this article was gleaned from correspondence with Susan V. Marcquenski

Fish Health Specialist for the WDNR and Matt Mitro Coldwater Fisheries Research Scientist for the WDNR.

Red writing above is direct quote from Susan Marcquenski.

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This spring it will be 12 years since I began seeing gill lice in my streams.

I have talked in person with Mike Staggs one time about it. About 9 years

ago. I spoke with him on the phone 4 times in the last 5 years and also contacted The WI TU president and all the fish managers from the southern part of the state.

Now a study is being done.

The really sad part about the WDNR can only advise anglers to harvest more brookies and to disinfect their gear when leaving a stream. They can remove beaver dams....but the only way to get rid of it is to individually dunk each brookie in a solution and.....when they go back to their homes they can be reinfected.

I fear this parasite caused the demise of the brookies 40 years ago around here and it is happening again.

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Thought you would find this interesting Len. Doesn't look like it copied too well though.


(Salmincola sp.) Biosheet

Carolyn Gunn, DVM

Aquatic Veterinarian

CPW Aquatic Animal Health Lab


Salmincola is a crustacean in the Subclass Copepoda which parasitizes salmonids in both free-ranging and hatchery populations in Colorado. They are often called by the common name of gill lice. Salmincola is an obligate parasite of fish with no intermediate hosts, but with several stages of development occurring off of the host. The species found in Colorado has not been delineated, with the exception of the population at Crystal River State Fish Hatchery, Poudre River State Fish Hatchery, and Catamount Reservoir (S. californiensis). We are currently pursuing identification of species found within the state through the use of DNA studies.

S. californiensis is native to the western United States, but has spread via fish transfers as far east as New Jersey. It has been documented to infest rainbow trout (Oncorhynchus mykiss), Chinook salmon (O. tshawytscha), lake trout (Salvelinus namaycush), Kokanee salmon (O. nerka), and cutthroat trout (O. clarki) (Hoffman 1999).

Salmincola edwardsii is holarctic in distribution and affects primarily fish in the genus Salvelinus, but can infect and has been reported from O. clarki, O. mykiss, O. nerka, mountain whitefish (Prosopium williamsoni), arctic char (Salvelinus alpinus), Dolly Varden (S. malma), lake trout, and arctic grayling (Thymallus arcticus) (Hoffman 1999).

Salmincola sp. infections of fish are not to be confused with Nanophytes salmincola. The latter is a parasitic fluke with a snail being the first intermediate host, salmonid fish being the second intermediate host, and canids, felids, mustelids, bear, and humans as the definitive or final host.

Range in Colorado: Within the CPW State Hatchery system, gill lice were found at Chalk Cliffs State Fish Hatchery in the late 1990s and at the Poudre State Fish Hatchery in 1995 and 2007, and in the Crystal River Hatchery in 2009 and 2012. It has also been found in broodstock fish at several private aquaculture facilities in the state. In free-ranging fish, it has been documented in the Blue River (between Dillon dam and Green Mountain Reservoir, and in the Blue to its confluence with the Colorado River, and downstream to Radium), Rio Grande, Yampa, and Arkansas Rivers; in the South Platte drainages near Jefferson and Evergreen; and the North Fork of the Cache la Poudre River (Walker, 1995). It has been found in the North Fork of the South Platte River and in Eleven Mile, Green Mountain and Gross Reservoirs (Walker, pers. comm.). It is known to have occurred in Woody Creek and the Upper Roaring Fork for 20 years (Walker, pers. comm.). Samples were recently collected from the Colorado River at New Castle. It has also been found in Cheesman Reservoir in kokanee during the spawn (Johnson, pers. comm.), Pinewood and Flatiron Reservoirs (Ingram, pers. comm.) and Williams Fork Reservoir (Ewert, pers. comm.). It was identified in Catamount Reservoir in and in Spinney Mountain Reservoir in 2010 and Stagecoach Reservoir in 2012 (Atkinson, pers. comm.). Specimens collected in 2012 from the Big Thompson River between Estes Park and the base of the canyon in Loveland were heavily infested (Swiggle and Walker, pers. comm.).

Known hosts in Colorado:

In Colorado, Salmincola sp. has been found on kokanee salmon and rainbow trout. Although not reported, other fish within the state are susceptible, including those listed above that occur in the state.

Life Cycle:

See attached illustration for S. californiensis. Historically, the infective swimming stages were thought to die after about two days without finding a host, but for S. edwardsii it was discovered that swimming/infectivity was temperature dependent: at 8°C (46.4°F), the copepodids stopped swimming at day 17; for 12, 16 and 20°C (53.6, 60.8, and 68.0°F), maximum swimming durations were 12, 8 and 5 days, respectively. Onset of egg sac hatching was directly related to increasing water temperatures (Conley and Curtis 1993). It is unknown whether this is true for S. californiensis or not.


The females, which are very evident to the naked eye, bear two clutches of eggs from each of two egg sacs, and then die (Kabata and Cousens 1973). The entire life cycle can take about 2.5 months, depending on various factors such as water temperature.

Presence and activity of the parasites on gill tissues cause thickening of the upper quarter of the primary lamellae, reduction in length of secondary lamellae, and deformation of the primary lamellar support cartilages (Roberts et al. 2004). In response to the infestation, heavily infected fish develop a tenacious mucoid mass of fibrin and cellular debris on the gill lamellae. In combination with parasite tissues over the gill surface, respiratory function is severely reduced (Roberts 2001) by inhibiting oxygen uptake and gas exchange at the gill lamellae/water surface interface. The problems are exacerbated by high water temperatures and low dissolved oxygen levels. Damaged tissue at the attachment site is also susceptible to invasion by opportunistic bacteria and fungi and fish weakened by heavy infestations may be more likely to succumb to environmental, nutritional or pathogenic diseases.

In the literature, it is often stated that larger, older fish are capable of having heavy infections, possibly due to their continued exposure to the copepods for a longer period of time than younger fish. In some studies, size, rather than age, has been an indicator for heavy infestations. Environmental conditions, such as low flows and high water temperatures have also been implicated in heavy infestations (Horton and Staigmiller 2005). Habitat use patterns have also been implicated in differing infection rates among fish species. The infective stage of one species of Salmincola was shown to swim up into the water column and attach to a host when stimulated by shadows or shock waves from passing objects, thereby causing species that use upper portions of the water column to develop heavier infections (Barndt and Stone 2003).

Prevention, control and treatment:

The adult females have an impermeable cuticle rendering them quite resistant to common chemical agents, but larval stages can be killed with some chemical treatments. However, since infective stages continue to be produced, repeated and frequent chemical treatments would be needed to control numbers of infective larvae. Severity of infestation may be decreased by this method.

Prevention: Secure water sources, prevention of introduction of infected fish, and eliminating bird and animal vectors are all first-line prevention measures.

Control: Increased flows to flush free-swimming stages from the fish’s environment are often recommended for partial control. Free-swimming stages can be killed by using 166-250mg/L formalin for one hour (Kabata 1970), or by using sodium chloride at 20,000mg/L (duration not given) (Hoffman and Meyer 1974), or with magnesium sulfate immersion at 30,000mg/L plus 7,000mg/L sodium chloride for 5-10 minutes (CVM 2007) Since the adult copepod may remain alive on a fish for 2 months or more, producing two broods/female during that time, treatments to kill free swimming stages are variably effective. Biological control by holding brook trout upstream of rainbow trout effectively removed copepodid larvae from the water and reduced S. californiensis infestation in rainbow trout by more than 89% (Modin and Veek 2002). Anecdotal evidence exists that holding Snake River cutthroats upstream of fish achieves the same result, but this has not been proven. A hatchery within the California Department of Fish and Game was treated with 2% NaCl for 24 hours, and they claimed this treatment was successful (Cox, pers. comm.).

Treatment: Currently in the United States there are no Food and Drug Administration-approved drugs available for eliminating Salmincola sp. in food fish. Emamectin benzoate (SLICE™) is a pesticide for control of parasitic copepods, but has only been approved in countries other than the US. In 2010, treatment authority was granted to the Investigational New Animal Drug (INAD) Program of the Federal Drug Administration’s Center for Veterinary Medicine for use of SLICE™ to control mortality caused by external parasites (copepods) in a variety of freshwater fish species and has been used successfully at one state hatchery. Emamectin benzoate belongs to the avermectin class of chemicals (ivermectins), which are derived from the bacterium Streptomyces avermitilis. It kills by interfering with the animal’s nervous system.

A 2% NaCl bath for 24 hours used at a production facility was deemed a successful treatment of Salmincola (W. T. Cox, California Department of Game and Fish, personal communication).

Of the parasitic copepods Argulus, Lernaea, Ergasilus, and Salmincola, the latter are reportedly most resistant to chemical treatment. Organophosphates, such as trichlorfon (Dylox® and Masoten®) and growth regulators such as diflubenzuron (Dimilin®) may be effective, but are not approved for use in fish.

Field collection: If a fish is suspected of having gill lice in the mouth, opercular cavity, on the gills, or on other parts of the body such as fin bases, an attempt should be made to collect the entire organism for identification. The adult female attaches to underlying cartilaginous or bony tissue via an attachment organ (bulla), and this structure is important in identifying the parasite species. The bulla is easily destroyed or left behind in the tissues if the gill lice are pulled from the tissues. Therefore, collection of a whole gill arch containing gill lice or removal of a segment of tissue at the attachment point and placing in 70-90% ethanol or isopropyl alcohol will preserve structures for identification. Submit samples to Carolyn Gunn, DVM, P.O. Box 791, 205 South 7th Street, Dolores, CO 81323.

Last updated December 6, 2012

References cited:

Barndt, S. and J. Stone. 2003. Infestation of Salmincola californiensis (Copepoda:Lernaeopodidae) in Wild Coho Salmon, Steelhead, and Coastal Cutthroat Trout Juveniles in a Small Columbia River Tributary. Transactions of the American Fisheries Society 132:1027-1032.

Conley, D. C, and M. A. Curtis. 1993. Effects of temperature and photoperiod on the duration of hatching, swimming, and copepodid survival of the parasitic copepod Salmincola edwardsii. Canadian Zoological Journal 71:972-976.

Center for Veterinary Medicine. 2007. Program Policy and Procedures Manual 1240.4200: Enforcement priorities for drug use in aquaculture. Accessed 11 February 2010 at http://www.fda.gov/downloads/AnimalVeter...l/UCM046931.pdf

Hoffman, G. L. and F. P. Meyer. 1974. Parasites of freshwater fishes. T.F.H. Publications, Inc., Neptune City, NJ.

Hoffman, G. L. 1999. Parasites of North American Freshwater Fishes, 2nd edition. Comstock Publishing Associates, Ithaca, New York and London.

Horton, T. and Staigmiller, K. 2005. Environmental and Biological Factors Contributing to Salmincola sp. Infections in Missouri River Rainbow Trout Oncorhynchus mykiss. Abstract from the annual meeting of the Montana Fish, Wildlife and Parks.

Kabata, Z. 1970. Diseases of Fishes. Book I. Crustacea as enemies of fish. T.F.H. Publications, Inc., Neptune City, NJ.

Kabata, Z. and B. Cousens. 1973. Life cycle of Salmincola californiensis (Dana, 1852) (Copepoda:Lernaeopodidae). Journal of the Fisheries Research Board of Canada. 30:881-903.

Modin, J. C. and T. M Veek. 2002. Biological Control of the Parasitic Copepod Salmincola californiensis in a Commercial Trout Hatchery on the Lower Merced River, California. North American Journal of Aquaculture 64:122-128.

Roberts, R. J. editor. 2001. Fish Pathology, 3rd edition. W. B. Saunders, New York.

Roberts, R. J, K. A. Johnson and M. T. Casten. 2004. Control of Salmincola californiensis (Copepoda:Lernaeapodidae) in rainbow trout, Oncorhynchus mykiss (Walbaum): a clinical and histopathological study. Journal of Fish Diseases 27:73-79.

Walker, P. G. 1995. The “Trout Louse,” Salmincola sp., a Parasitic Copepod. The Fishline, Colorado Aquaculture Association, Vol. 7, No. 4, pp. 1-5.

LIFE CYCLE OF SALMINCOLA CALIFORNIENSIS Eggs hatch after 28-32 days ↗ ↘ Adult female fused to fish for Copepodid stage – active swimmers. duration of parasite’s life. Grows Move over surface of fish seeking and may have male attached. suitable implantation site and then Each of two vaginal pores receives anchors to host. Molts. sperm packet from male. She can produce two broods. ↑ ↓ 4th Chalimus female stage. May last 1st chalimus stage – lasts about 12 2 weeks. Finds a final attachment site. hours ↖ ↓ ↖ ↖ 2nd chalimus stage – lasts about 12 hours ↖ ↖ ↓ ↖ ↖ Adult male – detaches from ↖ 3rd chalimus stage – point at which males host and wanders in quest of a ↖ and females become differentiated. Male female. Becomes attached to stage lasts about 24 hours; female stage lasts genital region of female about 48 hours ↖ ↙ 4th chalimus male stage – lasts about 40 hours; molts into young adult male

Adult female and attachment to gill filament – C. Gunn photo

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"I fear this parasite caused the demise of the brookies 40 years ago around here and it is happening again."

On top of poor land management, poor harvest management plans, and lack of understanding of environment.

It's good to be concerned about gill lice,however there is no real quantitative data to substantiate this as being a real problem-yet. Given that the brook trout are native species to many of the streams, it's safe to assume that gill lice is a parasite that's been a part of the eco infrastructure as well.

According to Jordan Weeks in a recent interview I had with him, that the rise of brown trout is parallel to the rise in stream temperatures. He also recognizes brown trout as a pioneer species, an invasive species that gains foothold in an ecosystem where the native species displays a reduction in population; that as the brown trout become more prevalent, they push out and predate on brook trout.

Another point Jordan pointed out to me is how often the words pandamic and epidemic are misused.

The two questions I'd like to see answered by this study:

1) is the gill lice really a pandemic? Is the gill lice a naturally occurring cycle for the brooks, and we are just creating an understanding of the creature in its natural setting.

2) Is our introduction of the brown trout coupled with global warming trends affecting the brook trout streams and we are failing to see the line due to human ignorance out of love for the bigger more aggressive brown trout.

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So rather than not fish the streams where gill lice are present, I would encourage people to fish and take fish home (reduce the density of the fish) as long as the fishing regulations allow this. Anything that can be done to keep water moving (faster velocity) may also help reduce the probability of larvae to successfully attach to fish.

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streams with gill lice in them here will not be stocked until the gill lice problem is under control. It makes zero sense to stock clean fish in the streams and just be infected and die.

Because there is no cure for Gill Lice this means the streams that have gill lice will be barren of brook trout before stocking continues.

Prepare for the near extinction of Brook Trout again like 40 years ago. I guess the question of what was the cause of that is quite clear now.

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email to Matt Mitro TODAY:

I am wondering about stocking brook trout in streams with know Gill Lice infections?

Would it not only make sense to NOT put healthy brook trout in infected streams?

Any stocking should be done in streams with NO reported infections?

Maybe brown trout should be stocked again?

Len Harris


Matt's response


These are all important questions. I don't think we have the data yet, however, to adequately answer all of these questions right now.

Host density (brook trout in this case) is a significant factor in whether or not gill lice become epizootic (infection rates rising to epidemic proportions). But it is likely not the only factor. We are trying to figure out what those other factors are.

Brown trout co-occurring with brook trout may be an important factor. One hypothesis is that brown trout may be effecting locally-high densities in brook trout. That is, brown trout may be pushing brook trout into locally-higher densities than may have occurred otherwise had brown trout not been present. If that is the case, then that would be one more reason not to stock brown trout on top of brook trout if we desire to keep brook trout in the stream.

The questions you raise are another reason why we need to compile data on gill lice presence and absence. Until we have those data, we cannot adequately implement such a stocking strategy.


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Like Matt said, there is no sufficient data to substantiate the claim that gill lice are exclusive to the decline in brook trout; and there is no data to support the claim that gill lice had an impact on brook trout populations 40 years ago.

Man played a huge role in the decline of water quality, which brook trout need to thrive. We had poor land management skills and poor harvesting policies.

On top of it all, we introduced brown trout to the eco system, clearly a decision that is being reflected upon in today's fishery as a mistake.

I agree with ichey on this one, is the gill lice a result of a naturally occurring cycle, one we have little to no knowledge of or is it an alien (invasive) conductive to the dense populations of brook trout.

It does appear to me to be a native creature because it fails to attach itself to the invasive brown trout. Now to await further data from the DNR experts studying this phenomenon.

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On top of it all, we introduced brown trout to the eco system, clearly a decision that is being reflected upon in today's fishery as a mistake.

I'm pretty sure they were introduced by Germans. They had barrels full of eggs when they came over and someone had too much to drink, kicked over a couple and the rest is history....

But I digress............

Gill lice are not exclusive to the Driftless or Brookies. Here is a link to some info from CO.

Gill Lice in CO

Here's some info from Maryland.

Gill Lice in Maryland

Some more from CO

CO Gill Lice

It appears many factors are involved with Gill Lice including water temperature and water flow. Maybe we should smoke all the brookies in SE MN and WI and just start from scratch..........

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