Pain in the Bass: Diet and Distribution of Invasive Smallmouth Bass (micropterus Dolomieu) in Cultus Lake, British Columbia

Invasive species are widely recognized as the second greatest threat to biodiversity loss. With rapid international trade, unpredictable human activity, and a lack of preventative action, these introduced species are spreading more each year. Invasive predatory fish can have devastating consequences on native species and are near impossible to eradicate from a water body without the use of chemicals. As a result, effective measures for fish suppression needs to be creative, sustainable, and well-planned. Since their illegal introduction in 2017, small mouth bass (Micropterus dolomieu) have proliferated in Cultus Lake, British Columbia and impacts on two species-at-risk, Cultus Sockeye salmon (Oncorhynchus nerka) and Cultus pygmy sculpin (Cottus aleuticus) are unknown. By completing a diet analysis and monitoring bass movement, we documented their impacts and laid the groundwork for a sustainable suppression plan. For the diet analysis, bass (n = 204) were sampled in spring/summer 2020-2021 in Cultus Lake. DNA barcoding (n = 145) and a visual analysis (n = 204) of diet was completed. Diet composition, factors influencing the predation of species-at-risk, and dietary shifts were analyzed using R. DNA analysis identified 32 more taxa at the family level than morphological analysis. Multiple logistic regression showed that bass were more likely to predate on salmon within the spawning grounds, and over 90% of bass had sculpinin their diets. Diet composition did not shift as bass size changed, demonstrating a strong predation on fish from 100 mm to > 300 mm total length. Then, to monitor bass movement, 43 bass were tagged with acoustic transmitters, and a receiver array was constructed in the lake. The bass were tracked for 16 months, and snorkel surveys supplemented the movement data. Bass are spawning in a 1 km stretch along the north shore of Cultus Lake, starting in early May until mid-June. They remain above the thermocline throughout the summer until water temperature drops, and they migrate offshore to deeper water for a less active winter period, eventually resurfacing in April for spawning. The ideal timing for suppression is during this congregated spawning period when adult male bass remain on the nests in 0.5 – 2.5 m of water for 6 weeks. We recommend trialing spearfishing and nest destruction in a controlled setting to suppress the population for the following reasons (1) bass are congregated in a 1 km stretch, and nests are not deeper than 2.5 m (an easy depth for snorkeling) (2) after snorkel surveys in 2021, we know that male bass guarding the nests do not move until snorkelers are closer than 1 – 2 m (3) adult spawning males are large enough for spearfishing (4) nests with eggs/fry can then be destroyed via burial, electrofishing, or natural predation from white suckers. We also recommend continuing outreach programs to aid in a preventative approach to further reduce the spread of invasive species.