Techniques in Salmon Habitat Restoration

By Leah Riehl, NSEN Journalism Volunteer

Even though many species of salmon are economically, culturally, and ecologically important, North American populations have been in decline since the 1800s (Ogston et al., 2014). Salmon face many human-induced threats including overexploitation, blocked fish passage, and habitat loss due to agriculture, forestry, and urbanization (Molina-Moctezuma et al., 2021). Salmon thrive in complex habitats due to the varying resources required at different points within the salmon lifecycle (Koed et al., 2019), so habitat restoration is a key strategy for population recovery (Honea et al., 2009). Different types of habitat restoration include barrier removal, instream restoration, estuary restoration, shoreline restoration, and more.

A researcher holds an adult Atlantic salmon in a river in Nova Scotia (Palmer-Dixon, 2021).

The above diagram shows what high-quality salmon habitat looks like. Features include coarse woody debris, medium-grained sediment, variability of depth, and floodplain connectivity (NOAA Fisheries, 2022).

Barrier Removal

Structures like dams and culverts can reduce or prevent fish passage, especially for species like salmon that need to travel between fresh and salt water environments. Barriers can also degrade downstream habitat by reducing flow rates and interfering with natural transport of nutrients and sediment (Duda et al., 2020). 

Sediment transport is an important consideration in barrier removal projects. Dams and other structures can stop the natural flow of sediment down a river, causing a buildup of sediment behind a dam and a lack of sediment downstream. It is possible for tonnes of sediment to be trapped behind a dam, which can be problematic when attempting to remove the dam during restoration. If the sediment is released too quickly, it can reduce water quality, degrade downstream habitat, or even reduce fish passage (Hill et al., 2019). A solution to this problem would be to remove the dam in stages, allowing for gradual transport of sediment (Duda et al., 2020). Another solution may be to leave the dam standing and install a fish ladder, which consists of a series of ascending pools that allows fish to move over a dam. This is a good option when the dam is not degrading downstream habitat.

One possible design of a fish ladder to improve fish passage past a dam or barrier (The Constructor, 2019).

Culverts are another structure that often create a barrier to fish passage. They can become blocked by debris or may not allow enough water to pass through to maintain high enough water levels for fish to pass through (Anderson et al., 2019). Some restoration projects involve replacing small culverts with larger ones or removing culverts altogether in favour of bridges (Molina-Moctezuma et al., 2021).

Removing a barrier to fish passage can allow for immediate recolonization of upstream habitat. Salmon have been found many kilometres upstream of dam sites within days or weeks of removal (Duda et al., 2020). However, recolonization rates depend on a variety of factors, including the proximity and dispersal ability of salmon populations in the area, and recolonization of restored sites may take years despite the presence of high-quality, accessible habitat (Molina-Moctezuma et al., 2021).

Instream Restoration

Instream restoration is a method of altering stream features to increase habitat connectivity and diversity (Beechie et al., 2012). This may involve adding or removing sediment to create spawning habitat or adding boulders or logs to create pools and variation in water depth.

Sediment grain size is important for multiple salmon life stages but is especially important for adult female salmon when they are constructing redds (the depressions in which they lay their eggs) (Merz et al., 2018). A medium grain size is ideal, as smaller grains allow females to expend less energy while creating their redds, but eggs survive better in larger-grained sediment (Merz et al., 2018). Average grain size can be increased by adding gravel to streams, and it can be reduced by removing small-grained sediment, implementing erosion control measures, or introducing land management practices to reduce sediment flow into nearby water (Honea et al., 2009).

Adding large woody debris to streams is one of the most popular salmon habitat restoration techniques (Polivka & Claeson, 2020). Forestry and urban development along streams have drastically reduced the amount of woody debris naturally entering rivers and streams, so many restoration projects attempt to emulate natural addition of wood by placing logs and building jams into flowing water (Anderson et al., 2019). This can slow down the flow of the stream, create deeper pools, and increase floodplain connectivity (Bair et al., 2019).

A stream in which salmon habitat has been restored by adding logs to increase pools, reduce flow, and create habitat heterogeneity (Roni et al., 2005).

Though instream restoration is popular and does have the potential to quickly restore habitat, this method does not address the root cause of habitat degradation (Beechie et al., 2012) and reviews studying the success of this technique have been inconclusive (Koljonen et al., 2012).

Other Techniques

Another habitat restoration technique is removing invasive species. Removing aquatic invasive species can reduce competition and allow salmon greater access to resources. Removing invasive terrestrial plants can increase water levels and increase shade in some cases (Beechie et al., 2012). Project planners should consider the potential for restoration projects to unintentionally create new habitat for invasive species alongside the target species (Molina-Moctezuma et al., 2021). A plan should be in place for invasive species control or prevention prior to restoration.

Estuaries connect rivers to the ocean and are areas where freshwater meets saltwater. Estuary restoration can be very useful as estuaries are a key part of salmon habitat, especially for juveniles (Sharpe et al., 2019). Dikes are constructed banks designed to prevent flooding which are typically found in agricultural areas. Removing dikes can result in an increase in channel length, edge, depth, and connectivity as well as a decrease in water temperature, even if the estuary has been removed from tidal influence for more than 100 years (Ellings et al., 2016). Shoreline development also provides an opportunity for restoration and habitat creation. An example is the eco-engineering that was used to redesign a seawall in Seattle (Sawyer et al., 2020). The project allowed more light to penetrate the pier to the water and increased near-shore habitat for salmon in the marine habitat (Sawyer et al., 2020).

The above diagram shows the eco-engineering features of a seawall designed to improve salmon habitat (Waterfront Seattle, n.d.).

The complex life histories of salmon mean that many different methods of restoration in a variety of environments can remediate salmon habitat. All restoration projects should be site-specific in order to address the unique challenges and characteristics of the targeted stream or river. Project planners should also consider the fact that not all types of salmon habitat restoration address the source of habitat degradation, so some approaches may need to be combined with legislation changes or other efforts in order to be successful in the long term (Koed et al., 2019). Finally, though barrier removal and instream restoration are the most common techniques, creative solutions, such as eco-engineering, can play a significant role in creating salmon habitat and be just as impactful as more mainstream restoration techniques.

References

Anderson, J. H., Krueger, K. L., Kinsel, C., Quinn, T., Ehinger, W. J., & Bilby, R. (2019). Coho salmon and habitat response to restoration in a small stream. Transactions of the American Fisheries Society, 148(5), 1024–1038. https://doi.org/10.1002/tafs.10196

Bair, R. T., Segura, C., & Lorion, C. M. (2019). Quantifying the restoration success of wood introductions to increase coho salmon winter habitat. Earth Surface Dynamics, 7(3), 841–857. https://doi.org/10.5194/esurf-7-841-2019

Beechie, T., Imaki, H., Greene, J., Wade, A., Wu, H., Pess, G., Roni, P., Kimball, J., Stanford, J., Kiffney, P., & Mantua, N. (2012). Restoring salmon habitat for a changing climate. River Research and Applications, 29(8), 939–960. https://doi.org/10.1002/rra.2590

Duda, J. J., Hoy, M. S., Chase, D. M., Pess, G. R., Brenkman, S. J., McHenry, M. M., & Ostberg, C. O. (2020). Environmental DNA is an effective tool to track recolonizing migratory fish following large‐scale dam removal. Environmental DNA, 3(1), 121–141. https://doi.org/10.1002/edn3.134

Ellings, C. S., Davis, M. J., Grossman, E. E., Woo, I., Hodgson, S., Turner, K. L., Nakai, G., Takekawa, J. E., & Takekawa, J. Y. (2016). Changes in habitat availability for outmigrating juvenile salmon (oncorhynchus spp.) following estuary restoration: Post-restoration changes in salmonid habitat. Restoration Ecology, 24(3), 415–427. https://doi.org/10.1111/rec.12333

Hill, N. L., Trueman, J. R., Prévost, A. D., Fraser, D. J., Ardren, W. R., & Grant, J. W. A. (2019). Effect of dam removal on habitat use by spawning Atlantic salmon. Journal of Great Lakes Research, 45(2), 394–399. https://doi.org/10.1016/j.jglr.2019.01.002

Honea, J. M., Jorgensen, J. C., McCLURE, M. M., Cooney, T. D., Engie, K., Holzer, D. M., & Hilborn, R. (2009). Evaluating habitat effects on population status: Influence of habitat restoration on spring-run Chinook salmon. Freshwater Biology, 54(7), 1576–1592. https://doi.org/10.1111/j.1365-2427.2009.02208.x

Koed, A., Birnie‐Gauvin, K., Sivebæk, F., & Aarestrup, K. (2019). From endangered to sustainable: Multi‐faceted management in rivers and coasts improves Atlantic salmon (salmo salar) populations in Denmark. Fisheries Management and Ecology, 27(1), 64–76. https://doi.org/10.1111/fme.12385

Koljonen, S., Huusko, A., Mäki-Petäys, A., Louhi, P., & Muotka, T. (2012). Assessing habitat suitability for juvenile atlantic salmon in relation to in-stream restoration and discharge variability: Assessing salmon habitat restoration. Restoration Ecology, 21(3), 344–352. https://doi.org/10.1111/j.1526-100X.2012.00908.x

Merz, J., Caldwell, L., Beakes, M., Hammersmark, C., & Sellheim, K. (2018). Balancing competing life‐stage requirements in salmon habitat rehabilitation: Between a rock and a hard place. Restoration Ecology, 27(3), 661–671. https://doi.org/10.1111/rec.12900

Molina-Moctezuma, A., Godby, N., Kapuscinski, K. L., Roseman, E. F., Skubik, K., & Moerke, A. (2021). Response of fish assemblages to restoration of rapids habitat in a Great Lakes connecting channel. Journal of Great Lakes Research, 47(4), 1182–1191. https://doi.org/10.1016/j.jglr.2021.05.009

NOAA Fisheries. (2022). Good habitat [Illustration]. https://www.fisheries.noaa.gov/resource/educational-materials/good-salmon-habitat-bad-salmon-habitat-card-game

Ogston, L., Gidora, S., Foy, M., & Rosenfeld, J. (2014). Watershed-scale effectiveness of floodplain habitat restoration for juvenile coho salmon in the Chilliwack River, British Columbia. Canadian Journal of Fisheries and Aquatic Sciences, 72(4). https://doi.org/10.1139/cjfas-2014-0189

Palmer-Dixon, A. (2021). Researcher holding an adult salmon in a Nova Scotia river [Photo]. Accessed via personal communication.

Polivka, C. M., & Claeson, S. M. (2020). Beyond redistribution: In‐stream habitat restoration increases capacity for young‐of‐the‐year chinook salmon and steelhead in the entiat river, washington. North American Journal of Fisheries Management, 40(2), 446–458. https://doi.org/10.1002/nafm.10421

Roni, P., Hanson, K., Pess, G., Pollock, M., & Bartley, D. M. (2005). Instream habitat enhancement using a log weir [Photo]. FAO Fisheries. https://www.fao.org/3/a0039e/a0039e07.htm

Sawyer, A. C., Toft, J. D., & Cordell, J. R. (2020). Seawall as salmon habitat: Eco-engineering improves the distribution and foraging of juvenile Pacific salmon. Ecological Engineering, 151, 105856. https://doi.org/10.1016/j.ecoleng.2020.105856

Sharpe, C., Carr‐Harris, C., Arbeider, M., Wilson, S. M., & Moore, J. W. (2019). Estuary habitat associations for juvenile Pacific salmon and pelagic fish: Implications for coastal planning processes. Aquatic Conservation: Marine and Freshwater Ecosystems, 29(10), 1636–1656. https://doi.org/10.1002/aqc.3142

The Constructor. (2019). 7 types of fish ladders or fishways [Photo]. https://theconstructor.org/water-resources/types-fish-ladders-fishways/33911/

Waterfront Seattle. (n.d.) Seawall project diagram [Illustration]. https://waterfrontseattle.org/