Open access publications in the SLU publication database

Abstract

The construction of floatways during the 19th and 20th century profoundly changed the habitat conditions for fish and other aquatic organisms in lotic environments. Increased mortality during early life stages, reduced habitat quality and availability probably had large negative consequences for populations of salmonids. As timber floating ended during the 1970’s, restoration programs were initiated that aimed to reverse the damage caused by floatway activities and to increase the production of salmonids. We predicted that restoration would have positive effects on egg to fry survival, fry displacement, over-wintering and population density of juvenile brown trout and on fish species diversity. To test these predictions, I conducted studies in restored (treatment) and unrestored (control) reaches in tributaries to the rivers Ume-, Vindel, Pite- and Kalixälven in northern Sweden. Egg-to-fry survival was approximately six times higher in restored (10.3%) compared to unrestored (1.7%) gravel beds. Displacement of newly emerged fry was reduced from 10.1% to 2.3% and first summer recruitment increased approximately three fold (from 0.2‰ to 0.6‰) following habitat restoration. Trout density increased significantly (>360%) in restored stream reaches whereas no change was evident in unrestored control reaches during a period of eleven years. Tracking of PIT-tagged individuals revealed that brown trout managed to over-winter within a restored stream. Minimum habitat suitability index explained a large portion (66.8 %) of the variation in the proportion of individuals that over wintered within different stream reaches. Although more fish species were caught in restored reaches, restoration did not result in significantly higher fish species diversity. These results show that restoration of streams utilized for timber floating can be an efficient method to enhance and conserve populations of trout and salmon. However, success of restoration relies on good knowledge about other species occurring in the system and their ecology. For instance, using wrong substrate sizes during restoration of spawning habitat can result in increased rates of egg predation by benthic predators. As brown trout utilize a variety of different environments during their lifecycle, including streams, lakes and sea, maximal response to habitat restoration will not be achieved as long as other factors, i.e. migration barriers and over exploitation, also constrain populations.