Comparative acclimation capacity and thermal tolerance of a narrow endemic and a broadly distributed madtom catfish from the Ozark–Ouachita Mountain region, USA

Predicting the resilience of freshwater fishes to climate change requires understanding how species respond physiologically to elevated temperatures. We quantified upper thermal tolerance, as critical thermal maxima (CT_max), and acclimation capacity for two ecologically similar madtom catfishes with contrasting geographic distributions - the narrowly endemic and imperiled Caddo Madtom (Noturus taylori) and the broader distributed Black River Madtom (Noturus maydeni). Individuals of each species were acclimated to five temperatures (16, 19, 22, 25, and 28 °C) before CT_max determination. Both species exhibited significant increases in CT_max with higher acclimation temperatures, but N. taylori consistently maintained higher CT_max values (F₄ = 11.14, p < 0.001) and a greater acclimation response ratio (ARR = 0.50) compared to N. maydeni (ARR = 0.36). The thermal safety margin (TSM) declined with increasing acclimation temperature in both species (F₄ = 408.60, p < 0.001), though N. taylori retained a larger thermal buffer (F₁ = 7.21, p = 0.01). Body size and condition changed during captivity, but these factors did not influence their thermal tolerance. Results indicate N. taylori may be more resilient to acute warming events than N. maydeni, though its restricted range and habitat specialization could limit behavioral mitigation options under sustained warming. These findings underscore the importance of integrating physiological and ecological data when assessing climate vulnerability in imperiled freshwater fishes.