Climate Change and Flow Regulation Alter Fish Community Structure through Species-Specific Effects on Reproductive Phenology

Arid-land rivers of the southwestern United States are experiencing significant effects of human-induced climate change. Temperatures in the Rio Grande basin have increased 1-3°C over the past century, causing reduced winter snowpack in this snowmelt-dominated system. In addition to the direct effects of climate change, flows on the Rio Grande have been extensively modified by river regulation. An important outcome of these changes is a weaker and earlier spring flood pulse. We assessed whether inter-annual variability of flow in the Rio Grande contributes to variation in reproductive phenology across the eight most abundant fish species. All species spawned earlier (e.g., 4-28 days) in years with earlier spring snowmelt runoff. However, flow variability had species-specific effects: in dry years, later spawning species advanced spawning more than early spawning species. The net result of these differential shifts was reduced temporal partitioning of reproduction across the fish community during low flows. We predict that, unless mitigation actions are taken, climate change and flow alteration will continue to contribute to increasingly frequent and more severe droughts and the Rio Grande fish community will continue to shift toward drought-tolerant species.