Movement patterns and habitat selection of Lahontan cutthroat trout during streamflow recession in a Great Basin...

Understanding the movement of organisms is critical for species conservation in the context of changing landscapes and climate. As climatic extremes impact the United States Great Basin, quantifying the movements of native fishes like Lahontan cutthroat trout (Oncorhynchus clarkii henshawi) is vital for facilitating their persistence. These climatic extremes are projected to alter flow regimes, specifically, reducing hydrologic connectivity needed to maintain populations. By studying fish movement patterns during streamflow recession and baseflow conditions, we can identify the factors responsible for movement and habitat selection to better manage these factors in a changing world.

We radio tagged 57 stream-resident Lahontan cutthroat trout from early summer to fall in 2021 and 2022 in the Summit Lake watershed (NV, USA). The location of each fish was associated with local hydraulic, physical habitat, invertebrate drift concentration, and water quality data to assess which factors impact habitat selection, abandonment, and overall movement. Linear mixed effects models were used to assess which factors were associated with trout movement. Density estimates from electrofishing were used as a proxy to assess habitat selection using multiple linear regression modeling.

Stream-resident trout displayed little movement during streamflow recession and baseflow conditions, with median daily movements of 0.3 m/day and a median home range of 10.2 m; these results suggest even less movement than those reported in previous studies. Declines in riffle crest thalweg (RCT) depth were the primary factor associated with increases in distance traveled, yet there were only four observed movements below RCT depths of 5 cm and no observations below 4 cm. Woody debris, mean maximum stream depth, and availability of mid-channel pool habitat were the strongest predictors of habitat selection.

The findings from this study suggest that stream-resident trout movement occurs when absolutely necessary, such as escaping drying reaches or avoiding risk of predation in shallow water. We suggest that watershed managers implement low-flow hydrologic monitoring to identify vulnerable stream reaches, with an emphasis on preserving streamflow connectivity for stream-rearing salmonids. Additionally, this emphasizes the importance of tracking movements for species of interest as a strategy to identify factors potentially reducing population fitness.