Fishes move to transient local refuges, not persistent landscape refuges during river drying experiment

Abstract

Anthropogenically driven flow intermittency is increasing in freshwater streams, with important implications for the management and conservation of aquatic ecosystems. Because most freshwater fishes are mobile, they are expected to emigrate from intermittent reaches, but this may not be true in streams transitioning from perennial to intermittent. Here, we attempt to determine if riverine fishes vacate drying reaches before intermittency or remain in local refuges. We implemented a controlled, in situ experimental flow reduction resulting in intermittency, reducing flows from ~1.0 to 0 m3/s over a 3‐week period. We monitored fish and fish‐habitat changes over a 5‐week period before, during and after flow reductions. During flow reductions, total wetted habitat was ultimately reduced by 91%. Habitat loss over time was not equal among habitat types: pool habitat increased slightly as run habit was lost, and backwater and isolated pool habitats were not strongly related to discharge. Likewise, water depth in run habitats decreased faster than other habitats. Only the river carpsucker Carpiodes carpio appeared to move upstream during flow recession; seven other species remained within drying sites. Habitat loss negatively affected fish populations, but at a lower rate than that at which habitat was lost, until sites dried completely. Overall, two species, red shiner Cyprinella lutrensis and western mosquitofish Gambusia affinis, had increased population growth during the study, whereas the remaining species had population declines. Fishes were able to find transient local refuges during flow reductions but did not vacate sites before intermittent conditions. Accounting for lack of emigration from anthropogenic flow‐intermittent reaches will be important for designing conservation actions for fishes threatened by increasing flow intermittency. Creation of refuge habitats may not be effective for widely dispersed species that do not actively seek refuge habitats, unless those habitats support enough individuals to maintain resilience following the intermittent conditions.