Killing invasives selectively


Researchers in Montana have figured out a way to remove the invasive Brook Trout from the state’s waterways without harming the highly prized and native Westslope Cutthroat Trout.

They are doing so by electrofishing—a shocking development for the Brookies. This method doesn’t involved a Ronco product like the FishWitch. It lets scientists deal with the invasives without resorting to piscicides, toxins that affect all gill-breathing creatures, including the Cutthroat.

"Piscicides are a valuable tool to remove non-native fish," said Wildlife Conservation Society Ecologist Brad Shepard. "But where non-native and native fish co-exist in smaller streams, a potential alternative method, electrofishing, can be used to remove specific unwanted species, while reducing impacts on WCT or other native fish and macro-invertebrates."

The researchers considered how much electrofishing costs compared to piscicides as they tried to determine when and where the removal method was feasible. They used backpack electrofishing in six study streams throughout the upper Missouri River Basin, recording stream parameters such as stream size, vegetation density, substrate and more.

They used fish barriers to isolate trout populations within each treatment site. A crew member wore a backpack “shocker” and used a wand anode while dragging a cable cathode. A second crewmember followed the shocker and netted the temporarily stunned fish. They removed the non-native Brook Trout and returned the native Cutthroat.

Brook Trout were successfully eradicated over a period of 4-8 years from four of six treatment sites that together totaled a distance of 10.8 km. The number of fish removed ranged from 1,627 in Staubach Creek to 7,936 in Muskrat Creek. Two other streams, Craver and Spring creeks, contained dense willow and alder vegetation, and were excluded because of poor initial electrofishing efficiencies.

Other Findings:

• Eradication of Brook Trout using electrofishing in the two smaller streams (where channel clearing was not required) cost about $3,500 to $5,500 per kilometer (about the cost of piscicide treatment). Where extensive clearing of the stream was necessary, electrofishing costs rose to $8,000 to $9,000 per kilometer.

• Multiple removal treatments within a shorter period of time (3 or 4 years) was more effective than single annual treatments over a longer span (6 or more years).

• The scientists took advantage of how Brook Trout aggregate during winter and before and during their spawning in the fall. Focusing efforts on adult fish at these times reduced numbers of offspring in subsequent seasons.

• Because trout select beaver pond habitats (where deposited silt makes wading dangerous and turbidity makes it difficult to see stunned fish), treatment with piscicides is "probably the only viable alternative where large beaver ponds are present."

Overall, the conclusion was that electrofishing was a viable method for eradicating Brook Trout in small streams and could be done in three years if multiple removals were conducted each year. This option is attractive particularly in situations where populations of native fish live in the same location as non-native fish because "electrofishing will allow for the preservation of the native fish."

Piscicides may be the only viable alternative for larger streams (>3m width), or in streams covered by dense woody vegetation or having beaver ponds. Further studies are necessary to determine if two crews working simultaneously could get the job done in larger streams.

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(Based on materials obtained from the AAAS (Access original article >>)

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