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 Wisconsin streams.
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 colder streams.
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