Barrier Mitigation Research Programme

Rivers function as transport routes for water, sediment, nutrients, fish and other biota. Artificial barriers that interrupt these transport routes have multiple negative impacts, inhibiting the river processes that form and sustain the habitats that support native biodiversity. The sediment that a river requires to create spawning habitat is trapped upstream of obstructions, often smothered by finer material.

Water also becomes trapped upstream of obstructions, transforming the river from a lotic (free-flowing water) to a lentic (still water) impoundment, a state preferred by many invasive species. The water held in slow-flowing impoundments also tends to become warmer compared with a flowing river. This increase in water temperatures can exacerbate the toxicity of pollution and breach the maximum thermal thresholds beyond which native fish species cannot grow and survive. This change in habitat can make impounded rivers uninhabitable for native species, especially salmonid species, which prefer to reproduce and live in colder waters.

Small (culverts and bridges) and large (weirs and dams) structures can functionally fragment river systems and prevent fish passage. Drop height, insufficient water depth and excessive lengths can prevent fish movement across instream structures. The life cycles of both resident and migratory fish species are disrupted due to the lack of access to the varied habitats necessary for feeding, spawning and refuge. This especially hinders the ability of protected diadromous species, such as Atlantic salmon, European eel, river lamprey and sea lamprey, to recruit and maintain their populations as they migrate between rivers and the sea to complete their life cycles.

Barrier Research at IFI

The Barrier Mitigation Research Programme (BMRP) aims to develop and implement a comprehensive and innovative monitoring strategy, which will examine the effects of barrier mitigation measures on river ecosystems. The programme will provide high-quality scientific data to support the management, conservation and protection of fish species and their habitats, with a focus on the impacts of barrier mitigation.

The BMRP will liaise with the National Barrier Programme (NBP) and the Hydromorphology research teams to provide support to IFI's Barrier Mitigation Division, which is developing a programme of barrier mitigation projects around the country, planned for 2024 to 2027. The objective is to improve the hydromorphology and connectivity of Irish rivers by targeting structures that impede fish passage and that cause a deterioration in river functions, flows and habitat formation.

Monitoring Design

The BMRP will create a baseline dataset prior to barrier mitigation and will map the short-term and long-term trajectory of ecosystem responses after barrier mitigation, following an adapted Before-After-Control-Impact (BACI) design. Multiple tiers of survey intensity will allow the BMRP team to efficiently employ resources to capture the optimal level of detail required for study sites. The effects of barrier removal, barrier breaching and a range of fish passage designs will be monitored.

Additionally, the mitigation measures will be investigated on rivers representing the variety of typologies within Ireland. Variables encompassing both the biotic and abiotic elements of rivers will be included.

Importance of Monitoring

A research programme to monitor the response of river ecosystems to mitigation works is vitally important for a number of reasons:

• Monitoring enables the identification of the most effective barrier mitigation measures for improving river ecosystems. This will lead to increased efficiency by improving designs and reducing the need for repeated interventions in the future. In particular, the identification of measures that provide resilience against climate change will be a key element of BMRP monitoring.
• Monitoring demonstrates the short-, medium-, and long-term responses to mitigation, which will facilitate improved planning and preparation.
• Monitoring provides a foundation of evidence on which policy makers can base future decisions.
• Monitoring can ground-truth models, providing for increased accuracy at sites where empirical surveys are not possible.
• Long-term monitoring provides an opportunity for continued stakeholder engagement.