In-stream restoration in forestry impacted catchments: benefits to stream habitats, brown trout populations and society

Abstract

Forestry and wood industry have formed a foundation for Finland’s national economy, yet inevitably at the expense of aquatic environments. From 1940s to 1970s, to provide energy resources for the industry sector, majority of large rivers were dammed for hydropower, blocking off their longitudinal connectivity. Concurrently, majority of the stream channels were dredged to facilitate timber transportation from headwaters to downriver factories. While most large rivers remain blocked, the timber floating ceased in the 1980s, and legislation imposing restoration as well as intensive restoration programs have been established ever since. Habitat restoration has mostly been motivated by the enhancement of recreational fisheries through the provision of better living conditions for salmonids. The measures have included construction of in-stream structures, such as weirs, flow deflectors, boulder dams and gravel beds, and recently also installation of large wood. Based on before-after-restoration measurements of physical variables and habitat-hydraulic modeling, the measures have been effective in enhancing complexity of stream beds and hydraulic conditions, thus potentially increasing the availability of suitable habitat for juvenile brown trout. The increase in habitat heterogeneity has been persistent over time, and has also shown positive signs in functional processes of stream ecosystem. Nevertheless, overall substrate variability in restored streams has remained lower than in natural streams, especially with a shortage of gravel beds for brown trout spawning. Sound ecological monitoring of restoration projects has been limited in Finland. A few long term before-after-restoration monitoring of brown trout densities, together with a meta-analytical synthesis of all good-quality monitoring data, have indicated an overall positive effect on brown trout parr densities. Yet, the absolute juvenile brown trout density in post-management streams has mainly remained lower than in natural reference streams. In addition, brown trout responses have varied strongly between restored streams. The strong context-dependency in restoration outcomes is explained mainly by catchment scale (e.g. river basin size, dominant geology) and local (potential interspecific competition, fisheries management) factors. Among the local fisheries management measures, loosely restricted fishing has diluted the positive effects of restoration, and stocking by eggs and parr have either been ineffective or affected negatively brown trout’s response to restoration. Stream-specific differences have also been observed in the delivery of ecosystem services, mainly reflecting stakeholder perceptions of landscape values and fish provisioning. Yet, stream restoration in its present form has a strong public acceptance in Finland, mainly because it is considered beneficial for recreationally important fish, ecotourism, and the well-being of local people.