Biometric analysis of Oncorhynchus mykiss (Walbaum, 1792) and Salmo trutta, Linnaeus, 1758 from the different habitat in Himachal Pradesh

Biometric analysis offered important information on growth patterns, population differentiation, and habitat effects in trout. The present study aimed to evaluate variations among diverse ecological conditions and growth patterns across three farm sites of rainbow trout (Oncorhynchus mykiss) and three river sites of brown trout (Salmo trutta) in Himachal Pradesh. Length weight relationship (LWR) was estimated via a log-transformed linear model (LM), unweighted nonlinear least squares (NLS), and weighted NLS (wNLS). Analysis of covariance (ANCOVA) was used to test differences among rivers and farms. Morphometric and meristic variables were measured, and principal component analysis (PCA) was used to assess shape variation among sites. Rainbow trout specimens demonstrated isometric and positive allometric growth across all samples, with 'b' values ranging from 2.992 to 3.15. The brown trout population exhibited more variable growth patterns; S6 showed negative allometric growth (b = 2.822-2.847), while others approached isometric growth (b = 2.936-2.993), which reflect differences in body form. PCA revealed that size-related traits explain most of the variation for PC1 (principal component 1), while shape and cranial traits variation contribute to PC2, distinguishing populations among different habitats. Length-length relationships (LLRs) also showed a strong correlation between total length and standard length (r² ≥ 0.90). Meristic features varied slightly between the populations. These findings underscore the phenotypic plasticity of both species and highlight the significance of localized habitat conditions. The study provides baseline data that can inform future biological assessments, conservation planning, and the sustainable development of coldwater aquaculture.