Gill lice on brook trout
Marcquenski, Susan V - DNR Susan.Marcquenski@wisconsin.gov
5:47 PM (2 hours ago)
to me, Van, Matthew, Lisa, Mike, Richard, Jordan, Michael, Alfred
I am the Department's fish health specialist and Mike Staggs asked me to
respond to your e-mail regarding gill lice affecting brook trout in
You are right, 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.
It would take a special study to do this, but it would be interesting to
compare the prevalence (percent of fish infected) and the intensity of
infection (number of gill lice per fish) of gill lice in brook trout
streams that have high densities with those that have low densities;
those with faster velocity vs those with slower; and slightly warmer vs
Thank you for taking the time to share your concern with us, and please let me know if you have any questions.
Susan V. Marcquenski
Fish Health Specialist
WI Department of Natural Resources
101 S. Webster St.
Madison WI 53707