Daily flow intermittence in an effluent-dependent river: Impacts of flow duration and recession rate on fish stranding

Treated wastewater, also known as effluent, is discharged into streambeds where it can augment or create aquatic habitat in arid regions. However, discharge fluctuations can result in daily stream drying and rewetting. In this study, we documented flow intermittence and resulting fish stranding and mortality over a 12-week period on an effluent-dependent reach of the lower Santa Cruz River in Tucson, Arizona, USA. We hypothesized that fish stranding would be positively related to the duration of flow prior to drying (increased recolonization potential) and flow recession rates (increased stranding likelihood). Using trail cameras, we monitored drying duration and extent along a 2.1-km reach of river known to experience flow intermittence and counted stranded fish once a week. Drying extent varied widely (range: 0–1.88 km) and averaged 0.79 km ± 0.14 SE. We observed a total of 323 fish strandings across the 12 weeks (range: 0–74 individuals per day; mean: 26.9 ± 8.3 SE). Ninety-nine percent of observed fish were Poeciliidae (63% of which were fry), including non-native western mosquitofish (Gambusia affinis). The recession rate was a positive predictor for the daily stranding count of poeciliids (p =.008). Flow duration was not significant (p =.100) but was included in the top model (R² = 0.782). As urban development continues, the discharge of effluent into rivers and streams will become more common throughout the world. Continued research is needed to understand the benefits and challenges presented by these effluent-driven flow regimes, including their impacts on aquatic taxa.