Biodiversity net gain and fish passage – a lost opportunity?

Our Principal Hydro-ecologist David Mould, reflects on his initial experiences of BNG and the role fish passage can play in BNG calculations.

Written by David Mould | Principal Hydro-ecologist

Biodiversity net gain

The aim of BNG is to encourage developers to provide an increase in appropriate natural habitat and ecological features over and above that affected by a given development. This involves metrics to determine a baseline score and calculating how much new or restored habitat is required to deliver net gain.

When we started to use the various metrics in our project work, we noticed that there is no way to explicitly include for the benefits of improved upstream and downstream movement of fish in the calculations.

When working on large structures such as navigation weirs or other weirs, the inclusion or improvement of fish passage facilities could lead to large benefits for the fish population of the river in terms of biodiversity, and genetic diversity.

A missed opportunity

There is also a clear Water Framework Directive (WFD) benefit for the watercourse.

In these multidisciplinary projects, fish passage improvements often have to compete for budget and priority. There is a risk that this could lead to lost opportunities for our rivers, as the developers/structure owners may prefer to spend money on habitat improvements more easily quantified in the metrics.

This feels like a huge missed opportunity for BNG to drive improvements to longitudinal connectivity in our rivers.

Or is it?

The most effective way for a river to be longitudinally connected is by weir removal, as this will lead to the most fish passage improvements, as well as reconnect sediment transport in the reach. Often this is not feasible due to many constraints – at a navigation for example. But sometimes a technical fish pass is preferred over weir removal. Due to uncertainties and risks, often over the behaviour of the river upstream.  A weir removal will lead to greater variety of in-channel habitat than a technical pass, which will retain the current water levels. And so would generate new habitat which could be included in the BNG metrics.

Will this encourage developers and structure owners to be braver at the options assessment stage to obtain the BNG points, rather than taking a more risk averse approach? It could be argued that an as-nature bypass channel, assuming it creates suitable habitat will also provide quantifiable (BNG) habitat improvements. 

A need to improve connectivity at structures

Will we now see a preference for more natural fish passage solutions? We will see, as BNG becomes further embedded in the planning process, there is a lot to get to grips with but our breadth of experience is developing rapidly!

For clarity, I’m not saying that fish passage improvement works are a silver bullet or the only form of improving our rivers, but often there is a clear need to improve connectivity at structures, and I am pulling on that thread. I think the above analysis could apply to the connectivity of terrestrial habitats, in that BNG metrics and their focus on habitat presence may be reducing the breadth of opportunities to improve our rivers in the next few years.

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Want to know more?

Contact David Mould for more information about our hydroecology and fisheries management services.


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