Assessing methods of suppressing invasive smallmouth bass (Micropterus dolomieu) and characterizing smallmouth bass diet overlap with walleye (Sander vitreus) in Clear Lake, Manitoba

Smallmouth bass (Micropterus dolomieu) is among North America’s most popular game fish and has been introduced to freshwater systems across the continent through stocking programs, illegal introductions, and live bait release. Invasive smallmouth bass are aggressive top predators associated with changes in trophic energy flow and reduced abundance and diversity of native fish and invertebrates. In June 2021, non-native smallmouth bass were found in Clear Lake, Riding Mountain National Park. Clear Lake has important ecological, economic, social, and cultural values. Local Anishinaabe people believe its water is sacred and Keeseekoowenin Ojibway First Nation holds fish harvesting rights. The objectives of this research was to identify an effective strategy to suppress smallmouth bass and characterize the potential impacts of the species invasion in Clear Lake. Spawning males exhibit parental care, which contributes to the species’ ability to establish new populations. Nest-guarding males are aggressive and can be easier to capture; removal leaves eggs and fry vulnerable to predation. Angling males off their nests can induce nest failures but is labour-intensive and inefficient for suppressing bass invasions. Electrofishing is efficient in targeting juveniles but requires specialized equipment. Spearfishing is popular for targeting invasive fish in marine waters but has not been assessed for smallmouth bass control. In this study, we assessed the efficacy of spearfishing for invasive smallmouth bass by (1) comparing catch rates and size structure to angling and boat electrofishing, (2) evaluating spearfishers’ skills and progress, and (3) examining bycatch, costs, and safety considerations involved. Spearfishing and boat electrofishing were conducted in Clear Lake, Manitoba, while angling data was obtained from a separate study in Cultus Lake, British Columbia. This study found that catch rates for spearfishing in Clear Lake were more than triple those for angling in Cultus Lake. The length distribution among mature males was also different; spearfishers captured a wider variety of sizes compared to angling in Cultus Lake and boat electrofishing in Clear Lake. Spearfishers' skills improved as the study progressed, resulting in fewer missed shots and total shots taken. Our results suggest that spearfishing was effective for capturing mature males in Clear Lake; we recommend that regulators consider the method as part of a multi-pronged approach to managing invasive smallmouth bass. We also examined the dietary overlap between invasive smallmouth bass and walleye (Sander vitreus) in Clear Lake, Manitoba by (1) quantifying their diets, (2) assessing feeding behaviour, and (3) comparing dietary overlap. The study revealed that bony fishes were the most frequently consumed prey for both species, followed by leeches for walleye and crayfish for smallmouth bass. Schoener’s overlap index indicated there was no significant overlap (α = 0.477), suggesting that the two species might not be in direct competition for food resources. Differences in diet were primarily driven by walleye larger than 300 mm eating more leeches and fewer fish. Identifying food web impacts, studying population dynamics of crayfish, and exploring the use of non-destructive diet analysis techniques would be beneficial for guiding management decisions and supporting the long-term health of the Clear Lake ecosystem.