Estimation of maximum densities of young of the year brown trout, Salmo trutta, with the use of environmental factors

Abstract: Brown trout, Salmo trutta, is an important species for the sportfishing tourism in Sweden and goals are set high for the possible gains to come from fishing tourism. Therefore accurate and efficient methods to estimate a streams potential production of brown trout are needed in order to apply relevant management regimes that take fish ecology into account and prevents overfishing. One measure of brown trout production is the density of young of the year fish. The usual method for estimation of young of the year brown trout density are electrofishing with three removals. In this study a method using environmental factors to estimate maximum production of young of the year brown trout, as a measure of carrying capacity, was used. Based on previous studies environmental factors important for brown trout habitat requirement were selected. Data of stream slope, water flow, annual air temperature, longest period above 0 °C, altitude, stream width and water depth were linked to data on young of the year densities in a cluster analysis. The percentage relative precision values for the estimated mean maximum densities showed a lower level of uncertainty for density estimates based on environmental factors and cluster analysis than for estimates based on electrofishing with three removals. An additional benefit of the use of environmental data for estimations of maximum young of the year brown trout density is that estimates can easier be scaled up to cover a whole stream without the use of long time series of data. As opposed to estimation of brown trout production capacity for a whole stream using electrofishing data which for reliable estimates require over time data (e.g. 10- 15 years).