MODELLING THE SMOLT PRODUCTIVITY OF BROWN TROUT (SALMO TRUTTA) IN HAGA Å

Abstract: The brown trout (Salmo trutta) is a facultatively anadromous species, some individuals migrate from the rearing environment in the rivers to the sea while other individuals stay in fresh water. To determine the suitability of a stream for juvenile trout the organization SGBALANST has developed what is called the "Trout habitat score" (THS) model. The THS relies on six variables; shading, slope, width, depth, water velocity and substrate, which together are summed to a point value. Depending on the point value of the stretch a THS class is assigned, from 0 to 3, which can be used for comparisons between stretches. During this study a habitat mapping was done with the THS-model in Haga å followed by an estimation of the migration mortality which was used to calculate the production of migration-ready trout, also called smolt, to evaluate how well our modelling worked. A trap at the mouth was also used which caught most smolt that migrated towards the sea. Modelling proved to work relatively well and the estimated amount of smolt that migrated was 1533,1 compared to 398 recorded in the trap in 2023 and 1072 in 2022. The difference between this year's measured productivity and the estimated productivity was found to be significant, which meant that the model couldn't estimate the smolt production accurately. The previous year's measured productivity differed significantly from the estimated productivity as well, but the difference was significantly smaller (1135.1 vs 461.1). Thus there is potential for a model that can estimate smolt production correctly but the current design of the model does not give a completely accurate estimate. The estimate was between 385 and 143% higher compared to the real productivity but this could, in many cases, be a more cost-effective and sufficiently accurate way to estimate smolt production.