Pesticide will be used to kill smallmouth bass in N.S. lake this weekend

Nova Scotia's Department of Fisheries and Aquaculture is going to use a pesticide on Dobsons Lake in Guysborough County, N.S., this weekend to kill invasive smallmouth bass — but it removed hundreds of native fish to safety first.

"Prior to treatment we've been actively moving brook trout and some minnow species and holding them at another location until such time as the lake begins to recover and then we will move those native fish back in," said Jason LeBlanc, director of inland fisheries division.

The department said invasive smallmouth bass were illegally introduced into the 52 hectare lake and pose a threat to native fish throughout a 5,400 hectare watershed which holds one of the best remaining wild brook trout populations in Nova Scotia. The bass eat some native species and out-compete others for food.

"It's a significant fishery there. Residents in the area really enjoy angling for brook trout in the area. By eradicating smallmouth bass in Dobsons Lake will protect those lakes further downstream in the watershed as well," LeBlanc said.

Smallmouth bass were discovered in Dobsons Lake near Canso in May 2020 and it was quickly blocked off to prevent spread.  

Attempts to remove smallmouth — including targeted angling and electrofishing — have failed.

On the weekend, 1,500 litres of a pesticide containing five per cent rotenone will be pumped into the lake. Rotenone is ingested through fish gills and works by preventing the absorption of oxygen in the cells of insects and fish.

The province says it does not pose a threat to people or animals, but public access to the lake will be restricted while the department is onsite this weekend.

In October 2020, the department used 53 litres of the same pesticide to kill off smallmouth bass introduced into Piper Lake — a shallow five-hectare lake also on the eastern mainland.

LeBlanc said it worked. The bass were eliminated, frogs and invertebrates started repopulating Piper Lake the following spring. Native fish are recolonizing the lake as well.

LeBlanc expects the same to happen at Dobsons Lake, but he cannot say when native brook trout in the area will return.

"That's going to be a bit up to them. It's a free flowing system. Once we remove the barriers that we currently have in place for containment — which we would do once we confirm that we've eradicated smallmouth bass — then native fish species can move back up into Dobsons Lake on their own," LeBlanc said.