Smallmouth bass were introduced to many streams and lakes in the Pacific Northwest in the early 20th century. Famous for their fight on the line and gorgeous color, “smallies” have a lot of sport fishing fans in the region. Although providing wonderful fishing opportunities, smallmouth bass are also top predators that can have negative impacts on valued Pacific salmon species. In 2012, Dave Lawrence, a graduate of the Olden lab, discovered that smallmouth bass had expanded their range well upstream in the John Day River (a Columbia River tributary), presenting an unanticipated threat to naïve juvenile salmonids that rear instream for a year.
To figure out what might be driving smallmouth to expand their range and how well they were succeeding, I (and a small army of volunteers) spent 2 years doing extensive and intensive field surveys in the headwaters of the John Day River, Oregon. We snorkeled hundreds of kilometers of river, surgically implanted fish with radio tracking devices, and dragged seines across rocky bottoms to study smallmouth bass reproduction, movement, and recruitment along the range boundary.
As it turns out, nonnative smallmouth bass have diverse individual responses to changing seasonal and inter-annual conditions at its range boundary. Some portion of the population seems unperturbed by the changes, taking residence in a specific location and remaining there to reproduce and forage. Others, however, make enormous (for a smallmouth bass) journeys to explore previously unoccupied habitats. Some adults swim up to 50 km upstream in the spring to reproduce, forage for the summer, and later journey back downstream for the winter – unexpected behavior for a supposedly sedentary fish.
So what’s driving them to do this? It appears that competition for spawning habitat downstream and a search for more optimal temperatures could be a primary driver of these seasonal movements. However, the ability for young bass to survive over the winter is still the dominant determinant of the range boundary of smallmouth bass. So even though some smallmouth bass colonize new upstream habitat each spring, the severity of the winter often drives the range boundary back downstream.
For more information about my work on range dynamics of smallmouth bass, check out a recent publication in the journal Oecologia.