“Quite the catch”: Removing invasive bass requires delicate balance

Protecting Piper Lake’s ecosystem sometimes takes drastic measures
  • Mar 23, 2021
  • 578 words
  • 3 minutes
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After countless smallmouth bass invaded Piper Lake, part of the St. Mary’s River in Nova Scotia, the province applied a pesticide that targets fish, monitored the results and controlled the flow of the lake with “stop logs” that prevented the pesticide from getting into the river. The pesticide, known as rotenone, killed a total of 170 bass (not counting those that were captured either by angling or stunning the fish using an electric current). 

The decision caused a divide amongst environmentalists and the province. But Steve Cooke, a professor of fish ecology at the Carleton University, says that it’s about context — taking a closer look at not only the process but the bass themselves. 

“Bass are good predators. If you put them into a system where there are small salmonids, like trout, or say, juvenile Atlantic salmon, they’re pretty darn good at feasting on them, and oftentimes will outcompete them,” Cooke says. “They eat other fish, they eat other prey and they compete for resources.”

The St. Mary’s River is an important river for these “salmonids,” specifically trout and Atlantic salmon, and Piper Lake is an important appendage for the system. When a bass enters the mainstream, they find the specific habitat they need to build a nest and lay up to 5,000 eggs. Their invasiveness can pose a bigger threat to the salmonid population and upset the ecosystem.

Kenny Silver, the vice president of the St. Mary’s River Association, says the river has been stressed by acid rain and other issues, so the bass was another factor harming the other native species. 

“Atlantic salmon and American eels are threatened in the watershed and any invasive threat like bass would further challenge their survival,” Silver says.  

A highway runs next to a lake under a grey sky Expand Image
The decision to put piscides in Piper Lake caused a divide amongst environmentalists and the province. Photo: Scott Beaver

A 2012 report from the Federal Department of Fisheries and Oceans assessed Atlantic salmon in Nova Scotia and determined that if the habitat was not improved, there was a 70 per cent chance the salmon would disappear from the river in 50 years.

Rotenone was the only solution to the problem. 

Cooke says that because the chemical is a piscicide, it offers a “clean slate” because it only harms fish species. He says this is where social acceptance issues arise, but that with careful thought, it is the only way to reclaim the lake — by entirely eliminating the population.

“It might mean going in ahead of time, and catching as many of the endangered species as possible, holding them elsewhere, nuking the system, and then reintroducing those [species],” he says.

It may seem dramatic, but it’s a careful procedure. Silver says the trout and salmon were removed from the lake before the chemical was added, and placed in cages in a nearby brook that drains into the lake. Unfortunately, some minnows, suckers and yellow perch were not removed and died as a result of the piscicide. 

He says as of now, the rotenone has dissolved in Piper Lake and the stop logs have been removed. The bass have been eradicated and the lake will be monitored for the next few months, even though, Silver says, there should be no long term impacts on the river. The salmon and trout were placed back in the lake, in the hopes that they will continue to thrive there.


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