Determinants of critical thermal maximum (CTMax) in the Nile tilapia (Oreochromis niloticus): Effects of body mass, dissolved oxygen concentration, and thermal ramping rate

Abstract

The critical thermal maximum (CTMax) is widely used to quantify upper thermal limits, yet estimates can vary depending on intrinsic biological traits and experimental conditions. This study examined how life stage, dissolved oxygen availability, and thermal ramping rate influence CTMax in the Nile tilapia (Oreochromis niloticus). The relationship between life stage and CTMax was assessed in 123 individuals (1–66 g) categorized into fingerlings (1-10 g), juveniles (10-25 g) and adults (>25 g). Dissolved oxygen effects were tested in fish of uniform mass (20 ± 1 g) under extreme hypoxia (1 mg L⁻¹), moderate hypoxia (4 mg L⁻¹), normoxia (8 mg L⁻¹), and hyperoxia (16 mg L⁻¹). Thermal ramping rate effects were evaluated at 0.05, 0.1, 0.2, and 0.3 °C min⁻¹. Life stage had a significant negative relationship with CTMax (R² = 0.68, p < 0.0001). Fingerlings (<10 g) exhibited the highest thermal tolerance (>43.5 °C), whereas adults (>25 g), particularly those exceeding 40 g, frequently displayed CTMax values below 41.5 °C, with juveniles (10–25 g) showing intermediate thermal limits. Dissolved oxygen significantly influenced thermal tolerance (p < 0.0001), with mean CTMax values of 41.6 ± 0.7 °C and 42.1 ± 0.4 °C under extreme and moderate hypoxia, respectively, compared with 43.7 ± 0.5 °C under normoxia. Hyperoxia produced only a marginal increase (44.0 ± 0.5 °C) relative to normoxia. Thermal ramping rate also strongly affected CTMax (p < 0.0001), declining from 43.4 ± 0.2 °C at 0.05 °C min⁻¹ to 40.9 ± 0.7 °C at 0.3 °C min⁻¹. These findings emphasize the importance of standardizing protocols when assessing thermal tolerance and predicting vulnerability to warming in tropical fishes.