Post-Herbicide-Treatment Fish Kills
Fish kills often happen after herbicide applications, especially after spiny naiad treatment. This is because the window for treatment of this invasive weed is extremely brief and in mid- to late summer when water temperatures are highest. Most victims are small yellow perch.*1 While yellow perch are generally classified as “warmwater species,” they are more dependent on cool water*2 than other warmwater species.
Are Fish Poisoned By Herbicide?
No. Fish kills are the result of oxygen depletion caused by a combination of high water temperatures and decay of plant material killed by herbicides*3 At the concentration diquat is applied for spiny naiad control (.37 parts per million*4), it is incapable of killing anything other than aquatic plants. LD50 is the standard test for toxicity of pesticides. LD50 stands for the lethal dose of nearly pure pesticide for 50 percent of test subjects. The higher the number, the less the toxicity. Diquat’s LD50 for dogs, for example, is 187 mg/kg of body weight.*5 That means that 10 ten-pound dogs would each have to drink more than two billion gallons of diquat-treated water in order for five of them to be fatally poisoned.
What Can Be Done to Prevent Post-Herbicide-Treatment Fish Kills?
Nothing. The only alternative is not to apply herbicide, in which case invasive weeds would proliferate, displacing native vegetation, degrading fish habitat and depleting the entire food chain from aquatic insects to crayfish to mussels to frogs to turtles to fish to such piscivores as loons, cormorants, mergansers, eagles, ospreys, kingfishers, herons, minks and otters. The result would devastate fish of all species, along with aquatic and terrestrial ecosystems.
What is the Ecological Significance of Post-Herbicide-Treatment Fish Kills
There is virtually no ecological significance. Again, the fish affected are mostly small yellow perch. Often, as in Big Island Pond, yellow perch are overpopulated to the point that they stunt.*6 To avoid predator fish like bass and pickerel, stunted and juvenile yellow perch seek refuge in aquatic vegetation, including the invasive weeds that are treated. When these invasive weeds decay, they deplete oxygen which, because of high water temperature, is already in short supply. In many lakes, including Big Island Pond, fish kills have happened with no herbicide treatment, solely because of warm water and low dissolved oxygen.
What Other Lakes Have Experienced Yellow Perch Kills
Yellow perch kills are common in the species’ natural and naturalized range across the entire U.S. In northern Maine, where water temperatures are significantly colder than those in Big Island Pond, yellow perch die-offs caused only by warm water have recently occurred in Long Lake, Cross Lake, Square Lake, and Madawaska Lake. David Russell, fish pathologist for the Maine Department of Inland Fisheries and Wildlife, notes that “hundreds of thousands’ of dead yellow and white perch are seen every summer in his state, usually linked to heat and oxygen depletion.*8 The New Hampshire Department of Fish and Game issues similar reports.*9
Notes:
1. Environmental Monitor | Hypoxic Conditions in Lake Erie Impact Yellow Perch Population
And:
Summer Fish Kills Occurring in New Hampshire Are Not Uncommon – EIN Presswire
3. https://www.solitudelakemanagement.com/the-guide-to-fish-kills-what-they-are-and-key-ways-to-prevent-them/
4. Task-Force-Herbicide-Comparisson-Matrix.xlsx-Sheet1-Final-1-25-21.pdf
5. EXTOXNET PIP – DIQUAT DIBROMIDE
6. chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://dnr.wisconsin.gov/sites/default/files/topic/Fishing/Species_yellowperch.pdf
7. https://www.cbsnews.com/boston/news/maine-lakes-dead-fish-kill-yellow-perch/
9. Summer Fish Kills Occurring in New Hampshire Are Not Uncommon – EIN Presswire