Densities of juvenile brown trout ( Salmo trutta) in two subarctic rivers: assessing the predictive capability of habitat preference indices

• Published in: Canadian Journal of Fisheries and Aquatic Sciences
• Main Authors: Mäki-Petäys, Aki, Muotka, Timo, Huusko, Ari
• Format: Article in Journal/Newspaper
• Language: English
• Published: Canadian Science Publishing 1999
• Subjects: Northern Finland; Subarctic
• Online Access: http://dx.doi.org/10.1139/f99-072; http://www.nrcresearchpress.com/doi/pdf/10.1139/f99-072

We examined the applicability of habitat preference indices for predicting density variations of age-0 brown trout (Salmo trutta) in two rivers in northern Finland. Trout densities in these rivers were monitored for 7 or 8 years. Habitat suitability for trout fry was assessed using summer and winter preference curves for water velocity, depth, and substrate. Substrate suitability indices based on summer preference curves explained 21-74% of the among-site variation in trout densities. The negative effect of high discharge on trout abundance was best predicted by the composite depth × substrate index. Sites with the highest "apparent survival" (density of age-1 trout in year t versus density of age-0 trout in year t - 1) produced high indices when winter substrate curves for age-0 trout were used, indicating high immigration rates to these sites. Our study shows that when preference indices are used for predictive purposes, the mechanisms underlying habitat "bottlenecks" must be known. In boreal areas, winter presents a bottleneck period for juvenile salmonids, and we stress the importance of using winter habitat curves when habitat hydraulic models are applied to areas with severe winter conditions.