So the photo’s from last week’s Brook trout adventure turned out to be evidence of Salmincola edwardsii also known as Gill Lice as Cam noted on my last post. This parasite is Salvenius specific and does not affect Rainbow or Brown trout. I emailed staff at the SE Minnesota Fisheries office in Lanesboro and Lake City and the reply I got was as follows…

What you are seeing is a heavy infestation of gill lice.  When gill lice reach a high enough density on the gills, they tend to deform the gill cover.  Surprisingly, we haven’t noticed a decrease in growth or survival during these conditions.  It certainly is ugly looking though!


The Gill Issue...Being an inquisitive person I started researching Gill Lice to find several articles and postings regarding this problem on Wisconsin streams. Wisconsin Trout Unlimited appears to be fairly concerned about this issue. WI TU has also put out an informational .pdf that can be found here. According to the information I read once the infestation becomes severe enough it can affect respiration causing the trout to be more susceptible to issues especially once caught and stressed through the activity of angling. This seems to be in direct conflict with what the fisheries biologists that I’ve been corresponding with in Minnesota. I have sent a follow-up email asking for a bit more clarification. In a short research article released by the MIDNR looking at Brook trout and gill lice in lakes they had this to say.

This parasite was considered a possible cause of natural mortality of fish in these lakes.

I should note it appears that this was released in 1969. I can only assume that newer research is available on this subject.

Gill lice grow until they release larva which must swim to another host within 24hrs or they will die. One suggestion I’ve read in the WI literature is that anglers keep the infested trout to prevent the lice on those fish from being able to spread to other fish and to also “thin the herd” thus making transmission more difficult because they have to swim further to find another fish. Other factors can be water flow and temperature. The faster the flow the more difficult time the larva will have finding another suitable host. The water The Gills....temperature may affect the cycle of hatching and transmission, typically this takes place in 30-40days.

WITU with the River Alliance of Wisconsin and the WIDNR conducted a survey in 2013 asking anglers to report instances of gill lice infestations and out of a total of 540 brook trout caught and observed 256, nearly half were infested. According to the survey map the infestations are basically state wide. WI Trout Unlimited has released a brochure regarding proper cleaning and disinfecting your waders and gear after fishing the creeks. That document suggests using a tablespoon of chlorine bleach/gallon of water to disinfect waders/boots and other gear that contacts the water. It also suggests you submerge your gear in this solution for 10minutes and dry the gear completely whenever possible. I wonder if just drying your gear is enough to prevent gill lice from spreading?

The Bottom Line:

Brook Trout FinsI love our brookies and I’m concerned. In the last few years I’ve felt like I catch fewer and fewer brook trout wither this is accurate or not may or maynot be the case. I might just be fishing different locations or not catching as many brookies and not that their numbers are infact diminishing. What I do know is that Brown trout have a leg up on the Brookies, they can survive under harsher conditions, they typically spawn later than brookies resulting in lost eggs that get rooted out and they tend to grow larger than brookies resulting in lunch for larger brown trout. The last thing that our Brook trout population in SE Minnesota needs is another challenge and gill lice appears to be just that. I will be inspecting every brookie I catch for this and noting it accordingly for the rest of my angling career and more importantly I will be passing it along to our DNR staff regardless if this is a concern or not. It’s possible it’s not a concern because newer information gives our DNR biologists reason to believe it shouldn’t be but until my follow-up conversation confirms this I will make sure to document where I find it. I also plan on learning best practices for disinfecting my gear, I really hope I don’t have to travel with a bleach solution everytime I go fishing.

More to come: check the comments section on this post for follow-up dialogue with MN DNR Fisheries biologists.


    1. With regards to the specific stream I noted no I didn’t get any numbers on previous infestations or amounts on that creek. He didn’t seem too surprised to hear me bring it up though so I would imagine he has some information from previous surveys that indicated gill lice were present in some quantity before.

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