Fish life history trends indicate increasing flow stochasticity in an unregulated river

Life history theory provides a framework to understand environmental change based on species traits that capitalize on stable, predictable, or stochastic environmental conditions. We evaluated life history strategies and temporal trends in abundance from 1975 to 2017 for 28 fish species within the Potomac River of eastern North America. Multivariate analysis identified life history strategies defined by small‐bodied species with extended spawning seasons and early maturation (opportunistic strategists), large‐bodied species with delayed maturation and long lifespans (periodic strategists), and intermediate‐sized species exhibiting parental care (equilibrium strategists). Linear mixed models detected temporal trends in abundance for 13 species (46%), of which nine increased and four decreased over time. Increasing species were characterized by opportunistic life history strategies (e.g., banded killifish, Fundulus diaphanous; mosquito fish, Gambusia holbrooki), whereas decreasing species were characterized by periodic or equilibrium strategies (e.g., smallmouth bass, Micropterus dolomieu; river chub, Nocomis micropogon). Recent introductions can account for temporal increases in some cases, but most increasing species are native to the study area. Observed increases in opportunistic native species and decreases in periodic and equilibrium species indicate that environmental conditions have become less stable and less predictable over time, consistent with observed increases in spring peak flows in the study area. Our study indicates the importance of environmental stochasticity for fish community responses to land use and climate change and demonstrates the utility of life history theory in this regard.