Hydrological Dynamics and Brown Trout Habitat Selection in the Thimphu Chhu River, Bhutan

Aims: Understanding the habitat preferences and ecohydrological factors influencing Brown trout is essential for their effective conservation and management, particularly in Himalayan River systems. However, the lack of previous studies in the region poses a challenge. Therefore, the study aimed to address the lack of research in the region by exploring these factors, with a focus on seasonal variations.

Study Design: A field-based observational study analyzing habitat parameters using statistical methods (Kruskal-Wallis H tests, Fisher's Exact Test, and Principal Component Analysis). Statistical significance was set at p< .05.

Place and Duration of Study: Thimphu Chhu River, Thimphu, Bhutan. January to December 2024.

Methodology: Stratified random sampling, dividing the study area into upper, middle, and lower river segments, was conducted across three segments, and data was collected across 57 sites using electrofishing to sample Brown trout. Habitat parameters such as depth, water velocity, and substrate type were measured during pre-monsoon, monsoon, and post-monsoon seasons. A total of 187 individuals were sampled and data were analyzed to identify statistically significant differences across seasons.

Results: Depth preference differed significantly across seasons (χ² (2) = 9.36, p= .009), with larger trout (>16 cm) occupying deeper waters during the monsoon (M = 66.0 ± 4.2 cm), while smaller individuals preferred shallower waters (<40 cm) during pre-monsoon. Water velocity also varied significantly across seasons (χ² (2) = 39.82, p< .001), with trout preferring slower velocities (<0.5 m/s) in pre-monsoon and higher velocities (>0.8 m/s) during the monsoon. Substrate preference showed a strong association with seasonality (p< .001), with large boulders (>512 mm) favoured during monsoon (72%) and post-monsoon (81%), while finer substrates were predominant in pre-monsoon (47%). Overall, seasonal variations in water velocity and substrate type were the most influential ecohydrological factors shaping Brown trout preference, particularly during the monsoon when hydrological dynamics were most pronounced.

Conclusion: Brown trout exhibit adaptive behavior in response to seasonal hydrological dynamics, highlighting the importance of season-specific conservation strategies. This study shows how Brown trout habitat preferences, such as depth, water velocity, and substrate type, vary seasonally, particularly during the monsoon and pre-monsoon. These findings inform season-specific conservation measures to preserve critical habitats and mitigate potential climate change impacts on trout populations.