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.

The nontidal Potomac River is Maryland's most popular freshwater fishery, largely due to the multiple angling opportunities and accessibility. Increasing popularity of the Muskellunge Esox masquinongy fishery has prompted further investigation of Muskellunge seasonal movement patterns and summer habitat use during thermally stressful periods. Radiotelemetry was used to investigate seasonal movement and summertime habitat use by Muskellunge in the Potomac River from 2017 to 2020. Movement was greatest during spring, with reduced movement during summer, fall, and winter. Potomac River main‐stem temperatures during summer often exceeded levels considered stressful to Muskellunge. During thermally stressful periods, Muskellunge sought thermal refugia and movements ceased. A generalized linear mixed model predicted that (1) when daily average temperatures reached 24.0°C, over 50% of the Muskellunge habitually occupied thermal refugia and (2) at 26.0°C, greater than 90% of tagged Muskellunge were located in refuge sites. Aggregation of Muskellunge in thermal refugia during summer has implications for increased angling vulnerability and fishing mortality in a warming climate, which could potentially limit the quality of these sport fisheries in southern river systems.

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Angling for Muskellunge Esox masquinongy has become widespread and increasingly popular. Management strategies for Muskellunge include high minimum harvest lengths (>1016mm), closed seasons, and catch‐and‐release regulations. Due to these strategies and angler behaviors, up to 97% of Muskellunge caught are released, and anglers assume these fish survive to be caught again in the future. Previous research on catch‐and‐release mortality for Muskellunge has suggested relatively low mortality rates (0%‐5%). However, these studies were all conducted within the range of water temperatures that are thermally optimal for Muskellunge and generally at water temperatures <25°C. Muskellunge populations in some latitudes routinely experience temperatures >25°C during the summer months. Fisheries managers and anglers have expressed concerns regarding warm water angling mortality, representing a need to evaluate mortality rates at various water temperatures and multiple latitudes. Our objective was to quantify warm‐water (>25°C) catch‐and‐release mortality rates in Muskellunge (>760mm) and identify factors influencing mortality using experimental ponds. Adult Muskellunge (n=102) were stocked into eight earthen or plastic lined flow‐through ponds (0.06‐0.71 ha) at densities of <16 fish/ha. Muskellunge (n=50) were angled utilizing specialized Muskellunge fishing gear at water temperatures of 19.6–32.6°C, with 32 fish being caught at temperatures >25°C. Fish were closely monitored for 2 weeks after being angled to assess mortality, and fish that remained uncaught during the experiment were used as controls (n=53). Mortality was greater for angled (30.0%) compared to control fish (11.3%). Differences in catch‐and‐release mortality were compared across a range of temperature regimes using firth's logistic regression. Five‐day cumulative temperature and net time were positively related to the probability of mortality, but size and sex were unrelated to mortality. Elevated catch‐and‐release mortality rates at warm water temperatures warrant investigations into the population level effects at varying levels of exploitation.