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Gill lice threaten brookies
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In reviewing my trout fishing 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.

The most significant change in numbers that I discovered was in Crawford County. My records showed 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.

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 waterbodies to ensure that you do not spread Gill Lice.

Recently, 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 in Richland County. The significance of this discovery is that in this stream many of the eggs are extracted from the wild strain brook trout and taken to the hatchery and raised. 

The gill lice have affected the 0-1 year class of brook trout in Ash Creek. The adults are loaded with gill lice and breeding is less effective. The Ash Creek strain of 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 DNR and Trout Unlimited are asking for citizen help with reporting of gill lice.
Here is a link to help out:

Gill lice Salmincola spp. are a parasitic copepod that only infect Salvelinus species such as brook trout. Salvelinus fontinalis. Brook trout are the only salmonid native to Wisconsin streams.

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 included in 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.