Differential effects of a catastrophic wildfire on downstream fish assemblages in an aridland river

Abstract

Few studies have evaluated the effects of large wildfires on downstream non-salmonid fish assemblages. Using multi-year (2009–2015) data from fish assemblage surveys and high-frequency water quality monitoring, we analyzed within-site responses of a cypriniform-dominated fish assemblage at two sites located > 20 km downstream of a 633 km2 wildfire in 2011 in the Rio Grande watershed in New Mexico, USA. Season had a weak influence on fish assemblage (fish) catch rate, richness, and evenness at the upstream site, but it had a strong negative influence (during spring/winter) on fish catch rate and richness at the downstream site. Fish species richness and evenness at both sites were not strongly affected by the wildfire, despite numerous post-fire sags in dissolved oxygen (including short-lived hypoxia < 2 mg L−1and anoxia—0 mg L−1) during subsequent monsoon seasons. Although fish catch rate was negatively impacted by the wildfire at the upstream site, it was unaffected at the downstream site. Fish catch rate, richness, and evenness at the upstream site were suppressed following a major flood event that occurred 26 months post-fire, but the downstream fish assemblage was resistant to the flood disturbance. Our study provided a rare opportunity to evaluate how aridland riverine fishes respond to disturbance from wildfire and subsequent flooding. Larger and more severe wildfires are occurring due to climate warming, and responses of water quality and fish community assemblages require study and assessment.