HALIFAX — The fight against invasive smallmouth bass has moved to a lake in eastern Nova Scotia where a chemical aimed at killing the species was used this past weekend.
Jason LeBlanc, director of the inland fisheries division in the provincial Department of Fisheries and Aquaculture, said 1,500 litres of a solution containing the pesticide rotenone was pumped into Dobsons Lake near Canso, N.S.
LeBlanc said the eradication project is aimed at preserving native brook trout in the Cole Harbour watershed, into which Dobsons Lake feeds. It follows a similar fish kill at Piper Lake in Nova Scotia’s Pictou County that was carried out in October 2020.
In an interview Monday, Leblanc said what was used in the 52-hectare lake was a mixture containing five per cent of the pesticide’s active ingredient.
“It's a very small amount once it’s mixed with water that’s required to be toxic to fish,” he said. “It’s early, but so far there are no indications that there are live fish in Dobsons Lake.”
Rotenone, which has been in use since the 1950s, targets fish gills and inhibits their breathing while leaving birds and mammals unaffected. However, it does kill all fish in the area where it is used and can also affect amphibious invertebrates such as frogs and salamanders early in their life cycle, when they are still breathing through gills.
The problem with smallmouth bass in Dobsons Lake was first identified in the spring of 2020 when the department received reports that the species had been illegally transferred there.
Before resorting to rotenone, LeBlanc said the lake’s outflow was blocked off and unsuccessful attempts were made to remove the invasive species through targeted angling and electrofishing, which involves fatally shocking fish using a rod or specialized boat.
“Mechanical removals alone will not result in complete eradication,” he said. “You can certainly control populations that way, but you will never get rid of them all. The best way to approach complete eradication is using a fish toxicant.”
LeBlanc said the department also removed native brook trout and minnows from the lake and plans to reintroduce them once lake conditions improve, likely in the spring.
At Piper Lake, which at five hectares is much smaller than Dobsons, it’s believed use of the pesticide in 2020 resulted in complete eradication of smallmouth bass, he said. “There were no fish detected in followup surveys in the spring and through 2021.”
LeBlanc also said aquatic organisms such as zooplankton were found to have rebounded quickly at Piper Lake, and frogs and salamanders were detected in the area as well.
The use of the chemical in the Nova Scotia lakes has been largely supported by sport-fishing organizations such as the Atlantic Salmon Federation. In an interview last month, Raymond Plourde, senior wilderness co-ordinator for the Halifax-based Ecology Action Centre, also said that while use of rotenone is “not ideal,” it’s better than letting an invasive species forever change a particular watershed.
However, the pesticide has sparked controversy in neighbouring New Brunswick, where a group of cottagers on Miramichi Lake obtained an injunction last month against a smallmouth bass eradication project there and in the surrounding Miramichi watershed.
That court action was soon abandoned and a group of Indigenous “water protectors” then sought a judicial review of the project before the Federal Court of Canada, which is yet to make a ruling.
The group advocating the chemical’s use, the Working Group on Smallmouth Bass Eradication, has since announced that it completed the first phase of its treatment project last Thursday in Lake Brook and in about 15 kilometres of the Southwest Miramichi River.
Spokesman Neville Crabbe wouldn’t reveal Tuesday when the largest phase of the project, on Miramichi Lake and the surrounding watershed, would begin. Protesters were able to successfully stymie the use of rotenone in the area last year.
“Our intention is to complete our authorized project this year,” Crabbe said.
The group’s project received federal approval in June 2021 after smallmouth bass were first detected in the Miramichi watershed in 2008.
This report by The Canadian Press was first published Sept. 13, 2022.
Keith Doucette, The Canadian Press