I posted the first portion of a conversation between myself and a fisheries biologist that works for the MN DNR here. After reading some information put out by the WI DNR and Wisconsin Trout Unlimited I became more concerned about this issue. I sent the fisheries biologist a follow up email asking…

There seems to be quite a bit of concern in Wisconsin regarding this issue, are you not seeing much of it in Minnesota? What can anglers do to prevent the spread of it? Do I need to disinfect my waders or is letting them completely dry out enough? The WI stuff I’ve been reading has me more concerned that you seem to be, not that I should be. Just trying to wrap my head around this, if there is one thing I love it’s SE Minnesota brook trout.

His reply is as follows:

Inflammation of Gill Filaments
Inflammation of Gill Filaments

I’m not sure where the concern comes from in Wisconsin.  Gill lice (Salmincola edwardsii) are native to Minnesota and Wisconsin and have co-adapted with brook trout for centuries.  In Southeast Minnesota, we began to note the presence of gill lice in our brook trout populations starting in the 1970’s.  That is not to say that gill lice weren’t present before then, but that’s when we started to record their presence in our stream files.  I did a survey in 2006-2008 to document what percentage of our brook trout streams had gill lice, and found that 41% of Driftless Area brook trout streams had some level of infestation at that time.  It’s important to note that brown trout are not a host to gill lice, so brown trout only streams were not included in that evaluation.  I have also seen the infestation level vary across years within a brook trout population.  One year the infestation rates are extremely high, and then a few years later, the gill lice are absent or at a very low level.   As I said earlier, we have never noted an effect on growth or condition of brook trout infected with gill lice.  However, we have never done a controlled experiment where we have measured individual growth of both infected and non-infected brook trout in the same environment. 

As far as what anglers can do to prevent the spread of gill lice, some common sense things come to mind.  1) drying and cleaning off your waders before going to another stream.  This is something folks should do anyway to prevent terrestrial invasives like garlic mustard and wild parsnip.  2)  Don’t move fish from stream to stream.  This is illegal and I realize it’s not something you would do anyway.  Other than that, there is not much you can do.  We have documented stream to stream movement of brook trout in the Southeast, meaning that an infected brook trout can spread gill lice from an infected stream to a previously uninfected stream.  We have more streams with brook trout today than we did in the 1970’s thanks to increased groundwater flow and management efforts (we’ve reintroduced brook trout to many streams).  This makes it easier for gill lice to spread across streams.  The more brook trout you have, the more gill lice will spread.

I started to dabble in some gill lice research back in 2008, but at that time there did not seem to be much public interest in that topic so I focused on brook trout/brown trout interactions instead.  If you think anglers would be interested in knowing more about gill lice impacts in Minnesota, let me know.  I’m always looking for suggestions on future research.

I’ve attached some photos of some gill lice from Maple Creek in Fillmore County.  One photo is a close up of the gill (you can see how it causes inflammation of the gill filaments).  The other is of a female gill louse with egg sacs.

Female Gill Louse with Egg Sac
Female Gill Louse with Egg Sac

So I’m now left wondering about the current situation and the future. If Gill Lice are native and they’ve been here since the 70’s they were likely here long before that. If that’s the case it’s hard to argue they are having much of a negative impact on our brook trout populations but I also don’t have hard numbers on brook trout populations throughout SE Minnesota. I’m half tempted to send an email to some of the WI DNR staff I see listed on some of the research I’ve been looking into simply to try and understand why there is such a difference in perception between MN and WI DNR when it comes to this issue or non-issue depending on your view point (and apparently which side of the Mississippi you happen to be on).

I’m also wondering if I shouldn’t be pushing to see more comprehensive research done on this topic. As the biologist says they’ve never noted an effect on growth but they also haven’t done side by side comparisons of trout with and without Gill Lice infestations. If you feel more research should be done on this leave a comment here and I’ll pass things along to this biologist. The folks in the MN DNR have always been good sources of information and I know they do there best so I have no reason not to trust their logic. I believe that public opinion does drive a lot of what our government agencies focus on and if until now there hasn’t been much said on the subject of Brook Trout and Gill Lice infestations I can see why the MN DNR hasn’t poured valuable resources into studying it further. At the same time I wonder if perhaps taking climate change/watershed stressors into consideration if it isn’t time to re-evaluate and try to better determine if other factors in combination with Gill Lice will put added stress on our brook trout and thus potentially result in a weakened population.

Going forward I will be documenting brook trout streams and locations on those creeks when I find a fish infested with this. If anything just to see for myself what kind of numbers I find. I’ll be passing that information along to DNR officials as I see fit. It might be helpful if you did the same, remember there is only one Conservation Officer for all of Winona County and that includes our side of the Mississippi. It is the job of every user of our resource to remain vigilant for abuses, violators and negative impacts. Note the time/day and location then do something with that information, send an email. It doesn’t take long. Keep this conversation going.


  1. I think further study on ability to survive stress (temperature extremes, changes in diet, etc) would be very helpful even if done in tanks (a controlled environment). I have to go back and search my photos but I know I’ve seen evidence of gill lice here in Virginia. One thought about increasing gill lice populations, if the Brook trout population is increasing because of stream restoration efforts etc I would think we would also see an increase of gill lice population because the population density has increased enough for them to more easily spread. Kinda like endemic spread of disease in overpopulated cities. So, though the gill lice may or may not be a harmful, worrisome thing, I could also see how their increased presence could be a sign of an otherwise healthy population. Really interesting justin. I need to look at the Virginia info!

    1. Hey D! It’s good to hear from you. At first glance I thought this was the first time I’d seen it here in Minnesota but thinking about it more I’m sure I’ve come across it before I just didn’t know it, didn’t know what to look for and probably chalked it up to a random deformity. Your note re: increasing population equating to increasing infestation is basically exactly what the DNR here is thinking. Hope all is well down there! Take care man.

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