Why does the government deliberately poison lakes?

You occasionally hear about the government deliberately poisoning lakes for certain reasons, like to count the fish. To me, this sounds like about the stupidest thing I’ve ever heard of. Why is this done, and is this really a good thing to do?

Can you point to an instance of this happening? I can understand poisoning a lake to kill an invasive species, but I’ve never heard of it happening to count fish.

Have you stopped beating your wife yet?

They do it in lakes near the Great Lakes, to see if Asian carp have gotten there.

if there is an undesirable species in a small lake then you could poison the lake and reintroduce desirable species.

small lakes are not self sustaining. game fish are often stocked into small and medium lakes.

I’ve heard of poisoning lakes in attempt to limit the carp population. Apparently carp have a habit of wallowing in the bottom mud, and this is blamed for the severe algae problems in some lakes. Though others have said that this is like blaming roaches for your kitchen being dirty.

I looked on google for a bit, and couldn’t find any instance of it being done just to count fish. I did find several instances where it was done to kill off an invasive species, as Telemark mentioned. Most recently, they’ve done this in the area around the Great Lakes where they are trying to prevent the Asian Carp from invading. The Asian Carp would wreak havoc on the local ecosystem and could devastate the local fishing industry (I see on preview that Anne Neville scooped me on this one). It was also done last year in Lake Davis in California where they are fighting off the invading Northern Pike.

So to the OP, it’s not done just to count fish, and it’s not the stupidest thing we’ve ever heard of, and it is a good thing to do. It stops entire ecosystems from being destroyed.

Hoo boy. The Asian carp thing was not in a lake. It was in the Chicago Ship and Sanitary Canal. In this case about 5 miles of canal were poisoned using rotenone to keep any carp which might be close to an electric barrier from passing through the barrier while it was turned off for maintenance. A second barrier is under construction , that will be right next to the first one, so that they can turn them off one at atime in the future, and avoid poison application. But it is not completed yet. It should be completed this year - lets hope anyway.

In that case only the canal was poisoned. The canal is not a natural waterway, and it is already so polluted that almost no native fish live there, besides a few bluegills and bullheads. There were a lot of common carp (also invasive) killed. There is no fishery in the canal. The canal is an unnatural connection between the Great Lakes and the Mississippi River basin, which has been invaded by Asian carps. The Great Lakes fishery is valued in the billions. They don’t want Asian carp there, because the carp may damage that fishery. (notably, the Great Lakes fishery is to a large part dominated by other non-native, but desirable fish, like salmon and trout.)

The poison used was rotenone, a product made from the roots of south American plants, and used by native americans for thousands of years to catch fish to eat. The poison is shortlived and only effects things with gills, for the most part. (The connection to Parkinsons even at high and continuous doses is very tenuous, and certainly not real at the concentrations that people associated with fish removal would ingest or inhale, much less anyone downstream.) Furthermore, rotenone can be detoxed by the addition of an oxidizing agent, and this was done at the dam below the 5-mile poisoned stretch.

They usually electroshock lakes if they want to count fish. As far as I know, this only stuns the fish temporarily.

Invasive species can play havoc with an ecosystem. Rabbits cause a lot of damage to crops and native species in Australia. Gypsy moths defoliate and sometimes kill trees in North America.

and leaves them pining for the fjords.

Rotenone sometimes is used to count fish, usually only over a very small part of a lake or stream. In lakes, it is usually done as “cove rotenoning”. This is because no fishing technique is very good at catching all kinds of fish. If you really need to know what is there, under some conditions cove rotenoning makes sense. Basically, you net off a small area, poison, detox, count the fish. It doesn’t hurt anything outside the cove (so the extent of the damage is small), it does not hurt birds or mammals, and it allows you to count nearly everything with fins. You could do a lot more damage sampling if you were looking for presence/absence of one difficult-to-catch and uncommon species, and just went netting with enough nets to be sure you would eventually catch one. Plus that would just be very expensive. Also, if you need to know the ratio between species, there probably is no netting technique that would be equivalently successful between species, or if there is, you could never be sure that it was. So it can make sense.

Rotenone and other piscicides like antimycin are also sometimes used to “renovate” a pond. For example, if you have a farm pond, and some moron used common carp for bait in your pond and they escaped and turned your pond into a mud hole with no gamefish, or if some well-meaning idiot stocked crappies in your pond and they overpopulated to the point where there was only tiny crappies and no bass and bluegill, you’d probably want to kill all the fish and start over, because catching all the carp or crappies out would be impossible without draining and drying the pond. The poison only lasts a little while, and you can go back and restock with no ill effects on the new fish.

Common carp (not usually bighead and silver carp - the Asian carps of most concern in Chicago and the Great Lakes) do a tremendous amount of damage to some types of environment. There is plent of published research on this. One of the things they do is uproot vegetation during their feeding. It is not so much wallowing as their feeding methods include pumping bottom muds through their gill rakers to filter out the invertebrates within. And in so doing, they also uproot vegetation, which in some cases keeps the lake bottom and margins from being overly agitated by wind. Keeping carp out will increase harvest of desirable fishes in midwestern shallow lakes. They may have less effect in some other environments, but no one with a clue thinks they are very positive in their effects on desirable species anywhere.


True - but electrofishing is also very selective of different sizes and species of fishes. So if you want to get the ones hard to get by electrofishing and netting, or to know the relative number of fish species, sometimes limited cove rotenoning can be useful.

There’s always the option of dynamite. :smiley:

A page on Sea Lamprey which ruined fishing in the Great Lakes.


Currently, the primary method to control sea lampreys utilizes a lampricide, called TFM, that kills sea lamprey larvae in streams with little or no impact on other fish. After extensive testing — which began in the 1950’s — scientists determined that TFM is nontoxic or has minimal effects on aquatic plants, invertebrates, fish, and wildlife.

There are different ways of counting fish.
The most common one is creel counting. A DNR representative will check fisherman and sometimes take a scale from fish, then the elecro-shock counting system is used, most fish survive this count.
Our MN DNR also uses hoop nets and gill nets to survey the lake,
Note, fish very rarely survive a gill net count. It is not unusual to find hundreds of dead fish on shore after a survey.
I have used gill nets to harvest white fish and tulipies(fresh water herring kind of fish)
Also many of our designated trout lakes are stocked with hybrid fish like splake trout,

These lakes are killed off and restocked on a regular schedule.

I’ve come up with a novel solution to the Asian Carp problem. It’s a rather crude and brute-force-ish method, but what the hell? Any port in a storm and all that.

Since the carp panic and jump out of the water when a boat comes near, simply outfit boats with giant nets. Launch a few hundred of them a few miles downstream of Chicago and slowly head south. When your net gets full, pull out. Rinse and repeat.

I have also heard of some small northern lakes in Canada being killed off and restocked with trout; usually also a weir or filter at the outlet to stop the bigger native species from coming back in.

Really? Are these lakes privately owned?