Spatial assemblage and interference competition of introduced Brown Trout (Salmo trutta) in a Himalayan river network: Implications for native fish conservation

Often regarded as a potential threat to the native fish fauna worldwide, the Brown Trout (Salmo trutta), has successfully established its population in the majority of the Himalayan rivers post its introduction dating back to the eighteenth century. Over the years, the species has gained infamy as a sport fish and is considered a profitable source of income to the locals ensuing a heightened propagule pressure due to lack of appropriate management actions. No comprehensive study has been conducted to date in order to understand the mechanism by which the Brown Trout poses threat to the native fish populations. Through the present study, we could assess its competition with the native Snow Trout (Schizothorax richardsonii) to understand the spatial assemblage of both the species across space in Tirthan, a pristine high-altitude river of the western Himalaya. River Tirthan is one of the major tributaries of River Beas traversing for most of its stretch within the protected boundaries of the Great Himalayan National Park Conservation Area. A total of 108 sampling points were chosen from confluence to origin of rivers/streams, ranging from 989 to 3677msl. A total of 28 explanatory variables were recorded at each point. Overall, the Brown Trout adults were found to be greater in relative abundance (66.1%) than the Snow Trout adults (33.9%). The fingerlings of Snow Trout on the other hand, were distinctively high in relative abundance (61.9%) than those of the invasive Brown Trout (38.1%). Non-native trout showed higher abundance in the higher stream orders i.e. in the main streams while natives mostly restricted themselves to the lower order streams. Redundancy analysis (RDA) for species and environmental covariates resulted in 40.75% of constrained variance with higher eigen values for Redundancy analysis1 and Redundancy analysis2. Ward's minimum variance clustering of Hellinger transformed data revealed sites agglomerating into six reasonable distinct subgroups with respect to species abundances. Immature individuals of non-native and native trout used similar habitat conditions, but they differed in using habitats at adult stage. Our results show a competitive dominance of Brown Trout in terms of higher abundance and maximum space utilization that highlight an urgent action for preventing its introductions to new areas. We recommend a national policy of ‘The Indian Invasive Species Act’ and management level interventions to control overstocking in the areas of established population.