Temperature and oxygen related ecophysiological traits of snow trout (Schizothorax richardsonii) are sensitive to seasonal changes in a Himalayan stream environment.

In this study, we investigated the seasonal changes in key eco-physiological traits of a wild population of snow trout, Schizothorax richardsonii from river Gola in the Indian Himalayan region over one year. Live specimens (5.8-31.4 g) were electro-fished from their natural habitat during representative months of four seasons with notable differences in water temperature, oxygen concentration and saturation. After 24-72 h of captive-acclimation, the fishes were examined for upper and lower critical thermal limits (CT and CT), incipient lethal oxygen thresholds (ILOC and ILOS), apparent routine and maximum oxygen consumption rates (M and M), and blood haemoglobin-haematocrit. Across the seasons, mean CT and CT values ranged from ∼0 to 34.6 °C, suggesting a relatively wide acute thermal tolerance range for this predominantly cold-water fish. Changes in the habitat's thermal condition during winter to summer was reflected in the CT (∼0-2.4 °C) and CT (31.7-34.4 °C) estimates, while the highest thermal scope (CT-CT; 33.2 °C) was recorded in autumn. Concurrently, the incipient lethal hypoxia threshold observed in autumn (ILOS-2.6% and ILOC-0.19 mgO/L) was significantly lower than the other three seasons, possibly linked to warm-acclimation. The reduction in blood haemoglobin-haematocrit levels during winter could limit the oxygen carrying capacity, with possible reciprocations in thermal tolerance and aerobic metabolism. Concerning body mass corrected oxygen consumption, the apparent M was found to increase in a temperature-dependent manner from 150.3 mgO/kg/h at 12 °C to 315.2 mgO/kg/h at 26 °C, with Q ranging from 1.6 to 2.2. Whereas, changes in M was not temperature sensitive (Q of 0.7-1.3), except during spring-summer (Q-2), with lowest and highest measurements in spring and autumn (934 and 1514 mgO/kg/h), respectively. Collectively, these data form the first information report on the seasonal plasticity in thermal and respiratory physiology of a Schizothoracine fish species, bearing significance for their conservation, aquaculture and habitat monitoring.