The intent of this document has been to summarize what is known about the effectiveness of habitat improvement techniques for inland fisheries and provide guidance for rehabilitation of freshwater ecosystems throughout the world. Limited information exists on the effectiveness of most techniques in restoring natural watershed processes or increasing fish and other biota abundance. However, techniques such as reconnection of isolated habitats, rehabilitation of floodplains, and placement of instream structures have proven to be effective at improving habitat and increasing local fish abundance. Techniques that restore processes such as riparian rehabilitation, sediment reduction methods (road improvements), and restoration of floods or high flows in regulated rivers also show promise. Other techniques commonly referred to as rehabilitation such as bank protection, beaver removal, and bank debrushing can produce positive effects, but more often produce negative impacts on biota or disrupt natural processes. Our review demonstrates that three key areas are lacking in rehabilitation activities: 1) adequate assessment of historic conditions, impaired ecosystem processes, and factors limiting biotic production; 2) understanding upstream or watershed-scale factors that may influence effectiveness of reach or localized rehabilitation techniques; and 3) well designed and funded monitoring and evaluation studies. These are generally the factors that consistently limit the ability of published studies to determine the success of a given technique at improving habitat conditions or fisheries resources.
Given the uncertainty in results of many habitat rehabilitation techniques and the continued threats to aquatic ecosystems, habitat protection should also be considered a part of habitat rehabilitation activities. Moreover, it is generally more cost-effective to protect existing high quality habitats and functioning ecosystems than it is to try to rehabilitate them once they have been degraded. High quality, unimpacted areas often provide important refuges for endemic species, critical information on the structure and function of natural habitats, and can serve as a reference or benchmark for rehabilitation of degraded habitats and ecosystems.
To assure that rehabilitation actions undertaken in the future are effective we suggest that the following steps be followed when planning, prioritizing, and implementing aquatic rehabilitation: 1) conduct assessments to determine historic and current conditions, disrupted processes, and rehabilitation opportunities, 2) protect high quality habitats and address water quality and quantity issues, 3) restore watershed processes and connectivity of habitats, then 4) improve reach or localized habitat conditions. Completing the first three steps will assure that habitat enhancement methods such as placement of instream structures or other in-channel activities will be successful and have the desired effects.
Commensurate with planning and implementing rehabilitation activities is the need for well designed monitoring with adequate spatial (number of sites) and temporal replication (length of monitoring). Despite the fact that we located and reviewed 334 published evaluations of rehabilitation actions, it is difficult to draw firm conclusions from many of these studies or provide concrete recommendations on which actions are successful under which conditions or for specific species. Thus well-designed monitoring and evaluation that includes specific hypotheses, adequate study design and replication in space in time (number of sites and duration of monitoring), and selection of sensitive parameters is desperately needed (Figure 16). This needs to be instituted during planning and design of the rehabilitation activities, rather than an afterthought. In addition to scientific monitoring and evaluation, detailed costs should be reported for each project. Few studies have done a cost-benefit analysis to help plan rehabilitation in part because of the lack of accurate cost data provided for various projects. Thus accurate project cost should be reported as part of comprehensive monitoring and evaluation to assist with future cost-benefit and economic analysis. Just as reporting scientific results will further restoration science, reporting cost-benefit information will assist in the socioeconomic management of habitat rehabilitation actions regionally, nationally and internationally.
