Ecology of a Recently Established Smallmouth Bass Population in the Flathead River, Montana

Abstract

We studied life history and ecology of small mouth bass (Micropterus dolomieu) in the Flathead River on the Flathead Indian Reservation, Montana, from 1998 through 2005. mallmouth bass are relatively newly established in the Flathead River, and thus our goals were to better understand the life history of the species and to examine their eflects on the Flathead River fish assemblage. We investigated small mouth bass movements and broad patterns of habitat use with radio telemetry, as well as spatial and temporal patterns of relative abundance, age and growth, condition factors, and food habits. We observed two broad patterns of movement, primarily related to migrations between spawning and overwintering habitats. Patterns of movement included (1) extensive (> 60 km) migrations between widespread spawning habitats in the lower river and abundant overwintering habitats in the upper river; and, (2) more restricted movements between spawning and overwintering habitats within close proximity to one another. Smallmouth bass abundance increased rapidly over our 8-year study. We documented highest relative abundances of young fish (< 180 mm TL) during autumn in low-gradient downstream nver sections adjacent to spawning habitats, and highest abundances oflarger fish (≥ 180 mm TL) in autumn in higher gradient upper reaches of the river with deep pools and abundant large substrates, e.g., boulders and fractured bedrock. Growth of small mouth bass after age 2 in our study area was relatively fast compared to other smallmouth bass populations in the Rocky Mountains West but moderate relative to growth across North America. Smallmouth bass in the Flathead River were robust, with average annual relative weights ( W,) usually > 100 in both spring and autumn. We found that smallmouth bass diets varied considerably among spring, summer, and autumn months. In early spring (Apr) and autumn (Oct), invertebrates largely comprised diets (% by weight), with aquatic insects dominating the diet in April and crayfish dominating in October. In contrast, fishes were a proportionally large dietary component (46.7%) in late spring (Jun) and were the dominant (58.2%) prey items in summer (Jul). Life history information will be used to develop and recommend options for future management of smallmouth bass on the Flathead Indian Reservation.