Rotenone is the "reset button" for fisheries, but what is it and how does F&G use it?

Unwanted fish species have shown up where they don’t belong in some of our state’s waters. Despite it being illegal for people to move live fish without a permit from Fish and Game, unwanted fish – both game and nongame fish – show up where they don’t belong and often degrade existing fish populations. And Fish and Game isn’t faultless, either. Decades ago, the agency stocked fish that harmed native fish populations, and it is working to fix those situations. Whether legally or illegally introduced, unwanted nonnative fish can harm native fish and established sport fisheries, which reduces fishing opportunities.

No easy way out

Fisheries managers often face tough choices to fix those situations. That might include adjusting fishing regulations or closing fishing seasons, netting unwanted fish, draining a waterbody, and sometimes chemical treatments that completely reset the system when necessary. The most common “reset button” is a fish pesticide called rotenone, which is a naturally occurring chemical found in the roots of certain tropical plants. 

Rotenone can be an effective way to revive fisheries

For centuries, rotenone has been used by native tribes of Central and South America to catch fish for food. Since the 1930s, it has also been used by fishery managers to control and remove unwanted fish. Rotenone blocks the exchange of oxygen across the gills of fish and essentially suffocates them, and it works in very low concentrations (as low as 50 parts per billion). When properly applied to ponds, reservoirs, and streams, rotenone is an extremely effective tool for fish control. 

While it is very effective at removing unwanted fish, it also kills desirable fish as well. But that doesn’t mean all good fish are lost. Managers often use nets or electrofishing to remove desirable fish before using rotenone so those fish can be restocked after the rotenone treatment. 

Certain aquatic organisms like zooplankton, aquatic insects, and juvenile amphibians can also be affected by rotenone, but fisheries managers can schedule a treatment during the fall and winter when those organisms are less abundant or dormant. 

Rotenone degrades quickly, and unlike many other pesticides, rotenone quickly starts to break down after it’s exposed to sunlight and organic matter. This characteristic makes rotenone very safe for human use. In fact, exposure to rotenone has never been linked to acute or chronic medical conditions in people. Nonetheless, an abundance of caution is still taken to make sure the public is not exposed to rotenone during treatments. That includes closing access to treated waterbodies until rotenone is neutralized. Applicators also use personal protective equipment and undergo training and certification through Idaho State Department of Agriculture to ensure the safe and legal use of this product.

Isn’t there a better way? 

You may be wondering why managers don’t just use other, more selective, options to remove undesirable fish. In certain cases, there are other ways. Sometimes lakes are too large to effectively use rotenone, or the presence of protected fish species, such as white sturgeon, steelhead, salmon and bull trout, makes the use of rotenone more difficult, but not impossible. In those situations, targeted removal and suppression strategies are typically the better choice to restore fishery quality. 

Unfortunately, netting or trapping rarely remove all the undesirable species. Mechanical fish removal efforts can also take years to reduce undesirable fish populations to acceptable levels. Those efforts may also have to be periodically repeated, which often makes those methods time-consuming, labor intensive and expensive. 

Rotenone is often the most economical and practical way to control unwanted fish populations, especially in ponds, small reservoirs and other smaller, manageable bodies of water. It is safe to use, relatively inexpensive, and can be applied to a variety of aquatic environments, making it a useful tool. 

There are many examples of successful rotenone treatments in Idaho. In recent years, many of these treatments have focused on small ponds to control illegally stocked nongame fish, such as projects at Castle Rocks Pond, Heagle Park, and Bruneau Dunes. After unwanted fish are removed, Fish and Game crews restock the water body and quickly rebuild the fishery.  

Sometimes, rotenone can help solve problems created decades ago. Fish and Game’s management philosophies have changed significantly over the last century to value and protect native fish, and now the negative effects of past fish introductions on native fish populations are better understood. 

Future treatments are being planned to help restore native fish populations like bull trout and cutthroat trout. These treatments are intended to eliminate nonnative brook trout in some areas that were introduced nearly 100 years ago. Using rotenone to address past stocking mistakes is a way fish managers can help native fish to thrive and improve fishing opportunity. 

More illegal stocking will mean more rotenone

As more nonnative fish continue to show up where they don’t belong, expect to see rotenone treatments become more common in Idaho. Keep in mind that illegal stocking of fish carries significant penalties (including loss of fishing privileges), and they can have a detrimental effect on your fisheries resources. Don’t move live fish, and if you know about an illegal stocking, report it. 

While rotenone is a powerful tool for fishery managers, they would prefer to avoid it in the first place. An ounce of prevention is worth a pound of cure, and also better than a barrel of rotenone.