Interesting topic

What we know

Researchers gathered data from 21 Minnesota lakes to examine the impact zebra mussels have on the aquatic ecosystem.

They found that fish in lakes infested with invasive zebra mussels had much higher concentrations of mercury – a potent neurotoxin that threatens both aquatic life and human health.

Walleye in lakes infested with zebra mussels were shown to have 72% higher amounts of mercury compared to lakes without them.

The study also states that “average-sized walleye were more than twice as likely to exceed mercury thresholds tied to human health.”

The Minnesota Department of Health’s threshold for mercury concentrations is 0.22 parts per million, and the study notes walleye in zebra mussel-infested lakes reached that concentration at a much smaller size, specifically 14 inches instead of 18 inches.

Meanwhile, yellow perch showed a 157% higher concentration of mercury in lakes with zebra mussels, with an average-sized perch having a risk 50 times higher of exceeding the mercury level threshold considered acceptable for human health.

The study also found walleye and perch are relying more on nearshore feeding habits because of the zebra mussel invasion.

The study was done by researchers from the University of Minnesota’s College of Food, Agricultural and Natural Resource Sciences, Minnesota Aquatic Invasive Species Research Center, and the U.S. Geological Survey. It was also published in the peer-reviewed journal, Science of the Total Environment.
More zebra mussels discovered at Lake Ann in Chanhassen

Associate Professor in the Department of Fisheries, Wildlife and Conservation Biology and senior author of the study Gretchen Hansen said, “We were surprised to find such a large effect of zebra mussels in fish tissue mercury concentrations, and we are eager to conduct further research to understand exactly the mechanism behind these patterns.”

That is interesting for sure, but correlation doesnt always mean causation. To my knowledge zebra mussels need something to attach too in order to survive so like rock, rubble, etc. I would tend to believe lakes that have an abundance of this type of bottom content would be higher in mercury levels than lakes without it. I could certainly be wrong however. Regardless that is an interesting read and will be even more interesting to find out if further research is done and if there is indeed a way to counteract it. Seems costly and doubtful. I thought there were lots of treatments for zebra mussels, but you really dont hear about it often. I realize its probably a losing battle, but there was one method that was introducing a bacteria or something similar that essentially sterilized the mussels. No idea how that would affect anything else, but mussels filter the water while fish do not.

I just read that in the paper. Seems to be +/- to each side.
Couple questions –
1) Where is all the mercury coming from in those lakes? Burning fossel fuel?
2) It states “near shore feeding” do zebra mussels have a depth limit?

CaptainMusky wrote:

That is interesting for sure, but correlation doesnt always mean causation. To my knowledge zebra mussels need something to attach too in order to survive so like rock, rubble, etc. I would tend to believe lakes that have an abundance of this type of bottom content would be higher in mercury levels than lakes without it. I could certainly be wrong however. Regardless that is an interesting read and will be even more interesting to find out if further research is done and if there is indeed a way to counteract it. Seems costly and doubtful. I thought there were lots of treatments for zebra mussels, but you really dont hear about it often. I realize its probably a losing battle, but there was one method that was introducing a bacteria or something similar that essentially sterilized the mussels. No idea how that would affect anything else, but mussels filter the water while fish do not.

Bingo!

Eelpoutguy wrote:

1) Where is all the mercury coming from in those lakes?

Mercury in freshwater lakes can come from a local source but also commonly comes from acid rain; you could find high mercury concentrations in the most pristine and remote lake in the middle of the NW Ontario wilderness. So rain puts mercury in the lakes -> algae absorbs the mercury -> zooplankton eats the algae -> and it continues up the food chain from there

Hypothetically if zebra mussels are truly causing higher mercury concentrations in fish, it’s not that they are causing higher mercury levels in the lakes. They would just be acting as a catalyst to get the mercury from the water into the fish at a faster rate.

I imagine the fish are eating large numbers of zebra mussels which I believe is what TSwoboda is getting at. I cant say that I have noticed them while cleaning fish, but I havent kept many fish in recent years in lakes with zebras.

I dont think the hypothesis is that zebra mussels are causing higher mercury levels, just preventing the things that help reduce those levels from accumulating. Zebra mussles don’t care much about rubble and rocky bottoms, they do just fine in sand and gravel, they do tend to stay in areas where the light reaches them so shallower areas of lakes. My guess is that the zooplankton that they feed on is a mercury processor and as they destroy those populations the remaining fish are having increased mercury. There are no effective treatments for zebra mussels. They have tried several. I suspect that the Army Corps of Engineers used something that worked for awhile on a nearby dam, but they wont talk about it, so it probably was illegal. River Redhorse populations have grown considerably in the area, they are one of the only fish that will feed on the zebra mussels.

CaptainMusky wrote:

I imagine the fish are eating large numbers of zebra mussels which I believe is what TSwoboda is getting at.

I would be shocked that any fish can consume and digest a zebe. Those things are like sharp rocks lol

I have seen perch full of crayfish it wouldnt surprise me if fish ate zeebs but who knows. I know guys who have shot diver ducks that were full of zeebs. I realize their digestive system is drastically different, but nothing would surprise me.

This things dense, might need the laptop instead of phone

What I get so far, as super simple version, I think is:

– zebra mussels create a different type of sediment/waste
– that sediment/waste is favorable to organisms that convert inorganic mercury to methylmercury and to small prey
– increased small prey in higher methylmercury production area eventually works its way through food chain to fish

CaptainMusky wrote:

I have seen perch full of crayfish

Lots of fish eat crayfish. Stomach content studies have shown that up to 80% of a smallmouth bass’s diet consists of crayfish out of inland freshwater lakes/rivers.

In the Great Lakes, gobies are the main food source now.

I’m skeptical that any species of freshwater fish can frequently consume and digest zebra mussels though.

In North America, zebra mussels have few natural predators. Several species of fish (for example, catfish, green sunfish, freshwater drum) and ducks have been known to eat them, but these species are not an effective control.

Thanks Kraig, good info. So a few “can” consume them. Just not with any frequency.

I’ve had zeebes attach themselves to my lure ice fishing at Winnie. If enough of them they can make the water crystal clear. I could drop a lure down at Winnie and see it just past 30′. Never used to be that way. They first found zeebes in Winnie around 2012 I believe. Pretty rapid change on a fairly big body of water.

Kraig5858 wrote:

In North America, zebra mussels have few natural predators. Several species of fish (for example, catfish, green sunfish, freshwater drum) and ducks have been known to eat them, but these species are not an effective control.

Yeah I never figured fish of any type would put a damper on their numbers but just as I thought not surprised there are some fish do consume them.

Ice Cap wrote:

I’ve had zeebes attach themselves to my lure ice fishing at Winnie. If enough of them they can make the water crystal clear. I could drop a lure down at Winnie and see it just past 30′. Never used to be that way. They first found zeebes in Winnie around 2012 I believe. Pretty rapid change on a fairly big body of water.

When a lake initially gets zebras in it the population explodes exponentially at first, then it hits a carrying capacity and the numbers drop and level off. There was an extensive study done on Mille Lacs regarding this. I havent seen a recent update, but the numbers eventually fall but it first those buggers and spreading like wildfire.

Yes, that’s accurate. If I remember correctly they peaked in Mille Lacs in 2012 and then began to fall off.

CaptainMusky wrote:

When a lake initially gets zebras in it the population explodes exponentially at first, then it hits a carrying capacity and the numbers drop and level off. There was an extensive study done on Mille Lacs regarding this. I havent seen a recent update, but the numbers eventually fall but it first those buggers and spreading like wildfire.

Yup, I’ve watched our cabin lake go through this. When we bought in ’09 it was zeeb free, then they showed up in ’11 and they were everywhere within a year or two – noticeable changes in fish patterns, although water clarity seemed the same. Weeds got thicker and deeper, “green blob” filamentous algae increased in the spring to where it covered almost all the bottom near shore and then became floating bubbly, slimy mats by early summer. But in recent years, the zeebs are less prevalent. Weeds are still thicker than they used to be, but the Filamentous Algae isn’t as bad, and we’re catching fish in areas and frequency more like we did in ’09. I’ll have to look up the mercury levels but I think they’re about on-par with the average.

The hypotheis in the study is that zebra mussels change lake chemistry in a way that alters pre-existing, inert mercury into a form that can be absorbed by bugs and other organisms in the food chain. As game fish directly eat bugs and small organisms that have higher levels of mercury in their systems or indirectly by eating baitfish that preyed upon contaminated bugs and small organisms, mercury levels in game fish increase.
The higher mercury levels are not from game fish directly eating zebra mussels.

https://www.sciencedirect.com/science/article/pii/S0048969724076721?via%3Dihub

This is a link to the actual journal article. Its alot of process and description of collection, but if you want it from the horses mouth here it is.

gimruis wrote:

<div class=”d4p-bbt-quote-title”>CaptainMusky wrote:</div>
I have seen perch full of crayfish

Lots of fish eat crayfish. Stomach content studies have shown that up to 80% of a smallmouth bass’s diet consists of crayfish out of inland freshwater lakes/rivers.

In the Great Lakes, gobies are the main food source now.

I’m skeptical that any species of freshwater fish can frequently consume and digest zebra mussels though.

I was catching whitefish (not tullibees) on a north central lake and everyone of them was loaded with zebra mussels. I think eventually any fish that typically feeds on mollusks will eventually add them to their diet.

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Are they feeding on them or are they growing in them. I’ve caught fish, clams, turtles, and crayfish with zebra mussels growing into them. Pull the byssal threads of a clump of zebra mussels out of the flesh of a fish once its friggin nasty.