Adverse effects: when stream restoration improves habitat for invasive fish

Stream restoration is a commonly used tool for improving the habitat of threatened native fish, particularly salmonids, which have suffered recent declines due to human disturbances. However, as a new study shows, restoration efforts can create their own problems for native fish by unintentionally improving the habitat of invasive species.

Kai Korsu and fellow researchers used habitat-hydraulic modeling to estimate habitat quality for 4 native and 2 invasive fish in 6 boreal streams in Finland before and after restoration was conducted. Restoration efforts had included moving previously removed boulders back into the channel, adding habitat enhancement structures, and opening up previously filled side channels to form a natural meander.

These are all generally considered positive stream enhancements, and indeed the researchers found that the project led to modest improvements in habitat for two of the native fish studied, the European minnow and Atlantic salmon. However, they also found that the enhancements significantly improved habitat for the two invasive fish species, rainbow trout and brook trout, especially at low flow.

Overall, the total habitat was still much greater for the native fish in the restored streams, so the benefits to the invasive species may not be that big of a deal in this case. Nevertheless, the study findings emphasize the importance of figuring out the likely consequences of alternative restoration scenarios before the work is actually done. In this regard, the modeling employed by the researchers in this study might be very helpful to restoration planners.

The methodology involved developing habitat suitability criteria (i.e. the physical habitat requirements) for each of the species being studied.  In addition, the hyrdophysical characteristics of the streams including water column depth, velocity, substratum, and stream topography were measured before and after restoration. 

Based on this, the researchers were able to model the pre and post-restoration habitat quality for each species expressed as % of total stream area, which is usable for the fish. If planners were employing this type of modeling before a project was implemented, they would have to project the post-restoration hydrophysical characteristics.

This type of planning might help designing the project in a way that minimizes the habitat improvements for invasive fish while maximizing the benefit to native species. For example, the researchers note that while restoration efforts in Finland often have the goal of benefiting salmonid species, their results showed that the projects did not increase habitat quality (i.e. usable area) for the native brown trout. The authors speculate,

"The most probable reason for low habitat gain for brown trout from restoration procedures was too coarse material, mainly boulders and rocks, inserted in the stream in relation to substrate (gravel, cobbles, and small boulders). This indicates that species-specific information on habitat requirements should be better incorporated in restoration planning in order to achieve restoration goals"

--Reviewed by Rob Goldstein

Korsu, K., Huusko, A., Korhonen, P., & Yrjänä, T. (2010). The Potential Role of Stream Habitat Restoration in Facilitating Salmonid Invasions: A Habitat-Hydraulic Modeling Approach Restoration Ecology DOI: 10.1111/j.1526-100X.2009.00621.x