Impact of Different Aquaculture Densities on the Growth Performance and Intestinal Health of Triploid Rainbow Trout (Oncorhynchus mykiss) Fry in High-altitude Environments.

The high-altitude regions of the Tibet Autonomous Region possess abundant cold-water resources, with annual average water temperatures suitable for culturing triploid rainbow trout. However, environmental challenges-including low atmospheric pressure, hypoxic water conditions, and significant diurnal temperature fluctuations-necessitate precise optimization of stocking density. Inadequate densities result in suboptimal resource utilization, whereas excessive densities induce chronic stress, leading to suppressed growth, reduced survival, and intestinal microbiota dysbiosis. Currently, research on appropriate stocking densities under these specific conditions remains limited. This study investigated the effects of stocking density on growth performance, intestinal microbiota, and tissue health of triploid rainbow trout, to identify the optimal density to support sustainable aquaculture in high-altitude plateau areas. Three stocking densities were tested over a 60-day culture period in 0.25 m3 cylindrical tanks (radius 0.45 m, water depth 0.45 m, adjusted for internal volume): Low-Density (LD, 100 fish/barrel), Medium-Density (MD, 200 fish/barrel), and High-Density (HD, 300 fish/barrel). Results demonstrated that the final body weight (Wt) and specific growth rate (SGR) in the LD treatment were significantly higher than those in the HD group (Wt: P ≤ 0.009; SGR: P ≤ 0.019). Survival rate was also significantly greater in the LD treatment compared to HD (P < 0.036), with values of 84.67%, 80.83%, and 72.67% for LD, MD, and HD, respectively. Alpha diversity of both water and gut microbial communities varied with stocking density. Principal component analysis (PCA) revealed differentiated clustering of microbial communities in water and the intestine across density treatments (Water: P = 0.35; intestinal microbiota: P = 0.7). The dominant phyla in aquatic and intestinal microbiomes were Proteobacteria, Firmicutes, and Bacteroidetes. In intestinal samples, the genus Pseudomonas was significantly more abundant in the HD and MD treatments than in the LD treatment. Co-occurrence network analysis revealed a higher average degree in LD and MD treatments, suggesting enhanced stability of microbial ecosystems in both the intestine and water under these conditions. In conclusion, low and medium stocking densities are more suitable for cultivating triploid rainbow trout in high-altitude plateau environments. These findings provide a scientific basis for ecologically sound, efficient, and healthy aquaculture practices for this species in alpine regions.