Response of a Brown Trout Population to Anthropic and Environmental Factors Over 25 Years in the Boiron of Morges, Switzerland

ABSTRACT Stream restoration measures in Swiss rivers have included fish passage construction, creation of shelters and water quality enhancement. Nevertheless, the effectiveness of these actions on brown trout populations has not been elucidated through long‐term studies. The initiation of stream restoration measures in the Boiron of Morges, a tributary of Lake Geneva, Switzerland, provided an opportunity to (1) determine the effectiveness of stocking, (2) assess the impact of pollution, and (3) evaluate the influence of stream restoration measures on a potamodromous brown trout ( Salmo trutta ) population, which was suspected to increase in population density. Electrofishing has been used to survey the population biannually since 1997. Surveys revealed that stocking was ineffective, whereas recolonisation after pollution was rapid in sections accessible by potamodromous spawners migrating from the lake, but was delayed by 2 years in sections upstream of an impassable barrier. Overall, water quality improvement and fish migration were the main restoration actions essential for enhancing brown trout population viability.