Seasonal Movement Patterns and Summertime Use of Thermal Refuge Areas by Muskellunge in the Nontidal Potomac River, Maryland

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.