Spring temperature predicts timing of seasonal upstream migration of invasive Sacramento pikeminnow (Ptychocheilus grandis) in a salmon-bearing river

Philip Georgakakos, Integrative Biology, University of California, Berkeley, Berkeley, CA, David Dralle, USDA Forest Service, Sacramento, CA and Mary Power, Department of Integrative Biology, University of California, Berkeley, Berkeley, CA

Background/Question/Methods
Rapid climate change and invasive species introductions are major threats to ecological communities across the globe, and freshwaters are particularly vulnerable and impacted. We document the first instance of an invasive piscine predator, the Sacramento pikeminnow (Ptychocheilus grandis), migrating within its introduced range, the South Fork Eel River, in Northern California. Through a combination of snorkel surveys, temperature monitoring, and statistical modeling we report on what motivates pikeminnow migration, and what environmental conditions might exacerbate their impacts on native animals.
Results/Conclusions
In 2015-2019 we show upstream migration by pikeminnow during spring and early summer occurred earlier in years when river water was warmer. Pikeminnow were more likely to occur in pools where their primary prey, North Coast Range Roach (Hesperoleucus venustus) were more abundant and denser, suggesting that feeding motivates seasonal pikeminnow migration. We developed a statistical temperature model to forecast the timing and extent of upstream migration by pikeminnow under combinations of discharge and air temperature that were not observed. This model was calibrated with our field observations and showed that river temperature decreased with river flow and increased downstream, and with air temperature. In years with low flow and high air temperature, we predict pikeminnow will move upstream earlier and overlap there with native fishes for a longer period of time. Pikeminnow consume or compete with all the native fishes of the South Fork Eel River, including culturally and economically important salmonids. Understanding the conditions which limit overlap between pikeminnow and threatened salmonids in important refuge habitat can direct habitat restoration efforts and aid the recovery of these native fishes. Additionally, insight into the phenology of life history events, like migration, exposes invasive pikeminnow to potential control. We suggest that capturing individuals as they move upstream or downstream or decreasing water withdrawals to keep river temperatures cool to limit co-occurrence of pikeminnow and rearing salmonids could minimize the negative impact of pikeminnow on native fishes. Invasive pikeminnow will likely have larger impacts in the South Fork Eel River with global warming and increasing drought severity. As with other invasive organisms, understanding pikeminnow life history and phenology can focus control efforts to take advantage of vulnerable life stages and seasonal time windows to benefit native species.