Dispersal and habitat use of juvenile brown trout (Salmo trutta) following egg stocking : -A case study in the river Rotälven, Sweden

Sammanfattning: Egg stocking in rivers and streams is a common measure to support and re-introduce salmonid populations. To ensure the success of egg stocking programs, knowledge of the early life stages of juveniles including dispersal ability and habitat preferences of fry is a prerequisite. This study was conducted to investigate downstream dispersal patterns and habitat use of young of the year brown trout in a mid-sized, channelized, boreal river in Sweden. Eyed trout eggs were stocked in February of 2021. During August of the same year, 2-3 months after emergence, trout were sampled by electrofishing within 1 km downstream of the stocking sites and each individual point of capture was recorded by GPS. In addition, data on riverbank slope, shoreline irregularity, stream bed substrate, water velocity and water depth were collected for each of the sampling sections. The results showed that trout fry were distributed throughout the 1 km long sampled sections and that the density of fry did not increase towards the stocking points. The observed variation in the distribution of trout fry was best explained by the riverbank slope. Approximately twice as high fry densities were found in reaches with gentle compared to medium and steep bank slopes. The differences are suggested to be an effect of 1) stream marginal water velocity differences during high flows affecting fry occupancy, and 2) that gentle slopes were often accompanied by shallow water, preferred by fry. These findings may provide helpful knowledge for fisheries managers to optimize the outcome of egg stocking and river restoration activities regarding fry recruitment.