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 a one year period. Live specimens (12.7±4.5 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 a brief laboratory acclimation, the fishes were examined for upper and lower critical thermal limits (CT_max and CT_min), critical and incipient lethal oxygen thresholds (ILOC and ILOS), routine and maximum oxygen consumption rates (MO2_rout and MO2_max), and blood haemoglobin-haematocrit. Across the seasons, mean CT_min and CT_max values ranged from ~0 to 34.7°C, suggesting a good acute thermal tolerance range for this predominantly cold-water fish. The changes in the habitat's thermal condition during winter to summer was reflected in the CT_min (~0 to 2.4°C) and CT_max (31.7 to 34.4°C) estimates, while the highest thermal scope (CT_max-CT_min; 33.2°C) was noted in autumn.

ILOC was higher in winter (0.59±0.2 mgO2/L) as compared to autumn and summer (0.19 and 0.34 mgO2/L), which is apparently due to the low oxygen carrying capacity of blood (haemoglobin-haematocrit content) during winter. Concerning metabolic oxygen consumption, the rate of routine MO₂ was found to increase in a temperature-dependent manner from 63.1±18.9 mgO₂/kg^0.8/h at 12^oC to 137.4±7.7 mgO₂/kg^0.8/h at 26^oC. Whereas, MO_2max was high in autumn (652±134 mgO₂/kg^0.8/h) and intriguingly low in spring (416±70 mgO₂/ kg^0.8/h), suggesting an inverse relationship with ambient oxygen saturation levels.

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.