Disentangling the effects of parasite infection and temperature on the aerobic swimming performance of pumpkinseed hosts

Climate change is shifting the aerobic capacity of aquatic ectotherms, affecting their ability to move efficiently through their environment. Rising temperatures also alter host-parasite interactions, yet how these stressors interact to impact locomotion remains unclear. This is especially relevant for infections that disrupt streamlining and fin function, with implications for wild fish populations, aquaculture, and fisheries. Pumpkinseed sunfish (Lepomis gibbosus Linnaeus, 1758), a popular recreational fishing species, are naturally co-infected with trematodes, forming rigid cysts on fins and body, and cestode tapeworms, infecting the liver and digestive tract. We tested whether pumpkinseed swimming performance is affected by drag from cysts by measuring critical swimming speed (Ucrit) and aerobic metabolic traits in naturally infected fish and fish treated to remove cestodes. Individuals with more cysts had lower Ucrit, maximum metabolic rate (MMR) and aerobic scope, likely due to increased drag. Next, we acclimated wild-caught, co-infected fish to 20, 25, and 30 °C and measured Ucrit and MMR. Warmer temperatures increased both metrics, and internal parasites were related to reduced MMR and Ucrit. Overall, infections can impair swimming by increasing drag and through physiological effects, but warming does not appear to exacerbate these effects in species not living near their thermal limits.