Long-term effects and cost-benefit analysis of eight spawning gravel augmentations for Atlantic salmon and Brown trout in Norway

Abstract

River regulation alters flow and sediment regime, habitat availability, and ultimately the ecological functioning of rivers. Various restoration and mitigation measures have been developed to improve ecological function, and among them is the addition of gravel to enhance the reproduction of gravel bed spawning fishes. However, information on long-term efficiency, costs, and maintenance needs of gravel additions are scarce. Here, we study the functioning of gravel additions at eight sites in three rivers in western Norway for up to 18 years. Gravel was added between 2002 and 2010 to enhance spawning of Atlantic salmon and anadromous Brown trout. We monitored changes in the size of the gravel-covered areas, sediment composition, interstitial oxygen, egg survival, and juvenile fish densities. Additionally, we report monetary costs, identify potential maintenance needs, and calculate a cost-benefit ratio. Juvenile densities of Atlantic salmon and Brown trout increased significantly after the gravel augmentations. After 10–18 years, the median egg survival was still high (> 90%) and sediment conditions were still suitable for salmonid fish reproduction. The areas were, however, shrinking across time (median area reduction 26%), mostly caused by scouring of gravel in the steep, supply-limited, and partly regulated rivers. The average construction costs of spawning gravel augmentations were 11.2 NOK (1.12 €) m−2 year−1. Compared to similar measures elsewhere, the measures have had a long life span (up to 18 years) at relatively low costs. Gravel augmentation was concluded to be a successful management measure that contributed to significantly increased Atlantic salmon and Brown trout reproduction.

Predation research with electronic tagging

“…Ecological restoration frequently focuses on providing habitat and reducing density‐dependent mortality of prey whose abundance is limited by suitable refuge. For juvenile Atlantic salmon, refuge in gravel interstitia, undercut banks and woody debris is important and thought to limit predation risk and restoration efforts have shown that remediating these habitat features can increase juvenile densities (Pulg et al 2022). Direct tests have demonstrated that increased habitat complexity reduced predation (Miyamoto et al 2021).…”

The quest for successful Atlantic salmon restoration: perspectives, priorities, and maxims

“…Similarly, Hesthagen et al (2011) presented the case for liming acidified rivers to restore water quality, resulting in Level 3 success for 13 Norwegian rivers where fisheries are now active and able to draw from the harvestable surplus. Direct improvements to spawning habitat by addition of gravel were highly successful (Barlaup et al, 2008;Pulg et al, 2021), as were instream enhancements to create shelter for juveniles (MacInnis et al, 2008;Floyd et al, 2009), including de Jong and Cowx (2016) where benefits persisted for at least 20 years following addition of boulders and V-dams in Downloaded from https://academic.oup.com/icesjms/advance-article/doi/10.1093/icesjms/fsab201/6425092 by guest on 13 November 2021…”

Perspectives on the environmental implications of sustainable hydro-power: comparing countries, problems and approaches

“…Thus it is important to evaluate the efficiency and perform cost-benefit analyses of the practices. Pulg et al (2021) monitored the long-term effects of spawning gravel augmentations in three rivers in western Norway and concluded that the measures had long life span (up to 18 years) at relatively low cots (1.05 € m 2 -year -1 ), and significantly increased Atlantic salmon and brown trout (S. trutta (Linnaeus, 1758)) reproduction. The three Norwegian rivers presented in the paper covered a range of average discharges and design floods, and provided information on the lifespan of the gravel adjustments under different flow conditions.…”

“…To accomplish that, studies will increasingly need to move from evaluating the immediate presentday impacts of hydropower production, to studying effects over the longer term on populations and communities (Bradford, 2021), in part because of the lag effects associated with many of the direct effects of hydropower development. Only with the availability of good multiple endpoint data can we hope to ensure the development of appropriate adaptive and integrative resource management strategies that include effective, cost-efficient mitigation measures (Bett et al, 2021;Haraldstad et al, 2021;Pulg et al, 2021). Thus long-term data sets will become increasingly important for the sustainable management and mitigation of hydropower effects, especially in multispecies systems (Bett et al, 2021;Katopodis, 2021).…”