Modeling invasion: predictive tools help managers combat smallmouth bass in the Colorado River

In 2022, newly hatched smallmouth bass (Micropterus dolomieu) were spotted by National Park Service staff in the waters of the Grand Canyon for the first time. While larger individuals have been found in the Colorado River before, this capture indicated the first sign of successful reproduction within the Grand Canyon river segment, and their population has since exploded. 

This highly adaptive, non-native species, which typically dwells in warmer surface waters, has become a threat to federally protected fish populations in the Colorado River’s upper basin. Experts warn that the low water levels at Lake Powell (caused by a decline in natural runoff alongside increased water usage) and warmer temperatures of the dam releases could elevate the number of smallmouth bass passing through the dam turbines. The resulting strain on the region’s unique ecosystems, combined with the escalating costs of invasive species management and resource degradation, highlights the urgency of addressing this growing threat.

In an effort to provide usable science for management decisions, researchers at the USGS Southwest Biological Science Center, with partial funding from the Southwest CASC, recently developed applicable models to predict the behavior and spread of the invasive river predator.

One of the models, outlined in a new publication, examines the drivers (such as river hydrology, water temperature and movement rates) behind changes in smallmouth bass populations, specifically in the river’s upper basin where smallmouth bass have already been established. Using the model, researchers can estimate the strengths and uncertainties of drivers under a variety of historic climate scenarios and removal efforts. They found that the timing of spawning and conditions during hatching play an important role in smallmouth bass population dynamics, as warmer, drier years resulted in greater recruitment than in cooler years.

Simulations also showed that if managers’ removal efforts had not occurred, smallmouth bass abundance would have been greater. Managers can use the model to test other treatments, predict where smallmouth bass might establish next and learn about how best to allocate resources to stop them, considering future watershed runoff and decisions about water storage.

In another publication, researchers targeted the Colorado River of the Grand Canyon to forecast the potential for smallmouth bass invasion and establishment. Using models that connected reservoir conditions, river temperatures and fish population trends, they found that under current low water elevations and reservoir operations, the risk of increased fish passage and continued reproduction is high, leading to greater risk of establishment. Higher reservoir levels at Lake Powell could reduce smallmouth bass movement through Glen Canyon Dam turbines, cool water temperatures and make downstream environments less suitable for their continued growth.

Already in use, the models from these studies are helping shape water management strategies in the Colorado River. By highlighting the links between invasive species, reservoir operations and climate pressures, they offer managers a powerful tool for making timely, informed decisions in a changing, and invaded, river basin.

Watch this video to learn more about how USGS scientists work with partners to protect Grand Canyon ecosystems from invasive smallmouth bass. 

Read both publications, Estimating drivers and identifying uncertainties in smallmouth bass population dynamics in an invaded river network and Declining reservoir elevations following a two-decade drought increase water temperatures and non-native fish passage facilitating a downstream invasion, in the Canadian Journal of Fisheries and Aquatic Sciences.