Saving rivers with burbot:

Can a lost species help river restoration?

By Reagan Pearce

The state of freshwaters

Pollution, habitat modification, urbanisation, and agriculture, with their collective impacts, mean that 50% of European freshwater species are threatened.¹ The UK has a long history of river modification, which can be traced to Roman times. A cumulative legacy of direct water pollution, river modification, and invasive species has resulted in a decline in freshwater biodiversity. The widespread trend of dredging and channelisation between 1950-1980, however, removed the heterogeneity of natural river channels, which steepened the decline of the remaining freshwater species.

River restoration: success or failure?

River restoration did not emerge as a practice in the UK until the 1980s. Its aim is to return natural processes to rivers, to increase habitat heterogeneity, which increases the number of niches for species to occupy, and so improves species richness and diversity in freshwater habitats.² The actions can vary in scale: adding woody debris to a reach, re-meandering a straightened channel, or removing instream barriers throughout a catchment.³

Despite steady increase in popularity and implementation over 40 years, the yielded results are mixed. For example, a study of 649 European river restoration projects found that re-meandering was a common technique applied to lowland rivers, but overall, reduced microhabitat diversity.² In fact, most river restoration projects were found to benefit terrestrial and semi-aquatic organisms most, with fish and aquatic macroinvertebrates benefitting the least.⁴ Overall, river restoration has often been inefficient at solving the problems with our rivers. Research shows we need softer re-naturalisation techniques, need to move beyond the main channel and focus on the river-floodplain continuum, and consider the impact of the whole catchment.² Species reintroductions could be the conservation tool we need to push the scope of river restoration.

Species reintroductions as conservation tools

Species reintroductions not only aim to bring back a locally or nationally extirpated species but can be used to benefit wider ecosystem functioning. These types of reintroductions come under rewilding, an increasingly popular concept that allows nature to look after itself and return habitats to a ‘natural’ or earlier state.⁵ Species reintroductions have a long history in the UK, where many larger herbivores and predators have been hunted to extinction, which has overtime, resulted in a decline in biodiversity. The first for conservation purposes was the (unsuccessful) reintroduction of the large coppers butterfly (Lycaena dispar) in 1927.⁶ The work by private landowners at Knepp Estate, Kent to reintroduce large herbivores that influence succession stages is well described in Wilding.⁷ At a national scale, however, the reintroduction of the Eurasian beaver (Castor fiber) is an excellent example of the bold action needed in UK aquatic conservation.

In the UK, we see increased winter rainfall eroding topsoil, flooding down narrow, straightened water courses to lowlands that flood annually. These rains increase nutrient and sediment loading in rivers, lowering water quality. Beavers, however, are excellent flood regulators, by building dams in the catchment, creating floodplain storage, which is slowly released overtime. These dams also provide silt filtration and act as a water purification system. Beavers introduce controlled flooding to rivers that, without human influence, would meander and shift across their floodplains, be filled with woody debris and fallen trees that would make rivers burst their banks and change course. A dynamic landscape.

Extinct since the 1500s, the first official reintroduction programme for the beaver was trialled in 2002, since then there have been several reintroduction sites across the UK.⁸ While we cannot seize developed floodplains for biodiversity, dedicating buffer land either side of rivers with beavers to allow floodplain storage is a small cost for many benefits. Flood regulation, groundwater recharge, water filtration and purification, benefits to floodplain insects and waterbirds, and to some fish. A win for both humans and biodiversity if people can adapt to this broad vision that moves beyond a single reach or meander and focuses on an entire river system.

Burbot: the next reintroduction

Another species reintroduction that has potential to help push the scope of river restoration for the benefit of freshwater biodiversity is the burbot (Lota lota). Extirpated in the 1970s,⁹ the burbot is a type of freshwater cod with one of the largest global distributions of any freshwater fish. Burbot were found in east England rivers, with evidence (both anecdotal and biological) identifying the burbot’s historical distribution between the Thames (still debated) to the Yorkshire Derwent.¹⁰ Causes of its decline have been attributed to mainly habitat modification, which resulted from a loss of access to key spawning habitats.

While burbot have a large global distribution that may make it seem adaptable, it is threatened throughout much of its range. These fish have several survival requirements, which are also key to it being a species reintroduction candidate that pushes river restoration. Essentially, each life stage requires a different type of meso-habitat.¹¹ Adults need deeper, cooler water with many shelter opportunities that can be found in woody debris, undercut banks, or boulders.¹¹ Juveniles need slow flowing water with high macrophyte density for shelter opportunities,¹¹ which can often be found in backwaters and the meandering shallows of main channels. They spawn on inundated floodplains, either open meadows or wet woodlands,¹¹ and so require connection between the main channel and floodplain. Finally, every life stage needs good water quality, particularly high dissolved oxygen.¹²

These requirements translate to a river system that possesses:

• lateral and longitudinal connectivity between the main channel and side channels for the fish to access these various habitats;
• consistent, annual connection with the floodplain;
• appropriate water quality that is suitable for the survival of fish; and
• heterogenous instream meso- and micro-habitats that are produced by variations in flow and high densities of woody debris and fallen trees.

We know these elements increase the diversity and richness of species: research shows that increased woody debris in rivers is beneficial for micro- and meso-habitat diversity,¹³ which benefits macroinvertebrates, macrophytes, fish, and aquatic mammals. Further still, we know that lateral connectivity between the river and its back and side channels, as well as the river and floodplain, benefits floodplain beetles, fish, aquatic and semi-terrestrial mammals, and increases aquatic plant diversity.² These features are overwhelmingly positive for the diversity of species across the aquatic ecosystem and have been at the core of the aims of river restoration for decades, but have not been implemented at the spatial scale needed to effect change.

An umbrella fish?

The burbot acts a kind of umbrella species. It is still protected under the Wildlife and Countryside Act 1981 and is a Biodiversity Action Plan species,¹¹ meaning once reintroduced, an active effort must be made to protect the species and its habitat. As the requirements of this fish extend beyond the main channel and into the river-floodplain continuum, so the scope of habitat protection and improvement must be increased laterally. This is similar to how the beaver needs floodable buffer land that is then utilised by other aquatic species. Further, international and national regulations must be followed for a reintroduction, so action must be taken to ensure the habitat is suitable for the burbot and causes of decline addressed, meaning river restoration must be undertaken now at the catchment-scale for a reintroduction to be successful. Finally, the goal would be to have the fish throughout its historical range, meaning this kind of catchment-scale river restoration would be replicated across England, improving river habitat for the benefit of people and wider biodiversity at a national scale.

A long time coming…

Despite being lost over 50 years ago, efforts to reintroduce the burbot to English rivers have been pushed since the 1990s, spearheaded originally by Keith Easton at the Environment Agency. Today, steps are being taken by Norfolk Rivers Trust and University College London to ensure that suitable habitat exists for the burbot at a candidate introduction site, narrowed by Thomas Worthington in his 2010 PhD thesis at the University of Southampton. This is essential work that must be undertaken before a release, and it goes beyond habitat suitability, but also species biology, locating a source population, and gauging stakeholder attitudes.  

The importance of pre-release research is overwhelming. Even though a native species, the reintroduction could have unanticipated effects. These could be positive, such as acting as a control for invasive signal crayfish (Pacifastacus leniusculus), but there could also be negatives, including disease and ecological imbalances.

Physically and ecologically, UK rivers are in a poor state. River restoration has continually tried to improve physical habitat for the benefit of aquatic species, but it has not proved wholly effective. Targeted species reintroductions that protect or enhance a variety of habitats for multiple taxa can be the answer to improving the state of our UK rivers. Research is beginning to emerge on the benefits brought by beaver reintroductions. It is now time to turn our attention to the next species, burbot. While the reintroduction process is in early stages, the requirements of this fish means that we would need to prepare or locate habitat that is longitudinally and laterally connected, provides heterogenous instream micro- and meso-habitats, that regularly floods onto the floodplain, and has good water quality. Broader river restoration can achieve these requirements, for the greater benefit of freshwater ecology and improving the state of UK rivers.

Reagan Pearce is currently a PhD student at University College London with a background in Geography and Ecology and an interest in Aquatic Ecology. 

References

Feature image: Jonny Gios, Unsplash

¹Gozlan, R. E., Karimov B. K., Zadereev, E., Kuznetsova, D. & Brucet, S. (2019). ‘Status, trends, and future dynamics of freshwater ecosystems in Europe and Central Asia’, Inland Waters, 9 (1), pp. 78-94.

²Friberg, N., Angelopoulos, N. V., Buijse, A. D., Cowx, I. G., Kail, J., Moe, T. F., Moir, H., O’Hare, M. T., Verdonschot, P. F. M. and Wolter, C. (2016). ‘Chapter Eleven – Effective River Restoration in the 21st Century: From Trial and Error to Novel Evidence-Based Approaches’. in Dumbrell, A. J., Kordas, R. L., and Woodward, G. (eds.) Advances in Ecological Research. pp. 535–611.

³Champkin, J.D., Copp, G.H., Sayer, C.D., Clilverd, H.M., George, L., Vilizzi, L., Godard, M.J., Clarke, J. and Walker, A.M., (2018). Responses of fishes and lampreys to the re‐creation of meanders in a small English chalk stream. River Research and Applications, 34(1), pp.34-43.

⁴Haase, P., Hering, D., Jähnig, S.C., Lorenz, A.W., Sundermann, A., (2013). The impact of hydromorphological restoration on river ecological status: a comparison of fish, benthic invertebrates, and macrophytes. Hydrobiologia 704, 475–488.

⁵Pearce, R. (2020). ‘Trophic Rewilding: The Science of Reintroducing Predators Into An Ecosystem’. [Online]  https://earth.org/trophic-rewilding-science-of-reintroducing-predators-into-an-ecosystem/ (Accessed: 30^th August 2022).

⁶Carter, I., Foster, J. and Lock, L. (2017). ‘The Role of Animal Translocations in Conserving British Wildlife: An Overview of Recent Work and Prospects for the Future’. EcoHealth, 14 (1), pp. 7–15.

⁷Tree, Isabella. 2019. Wilding. London, England: Picador.

⁸ Ashworth, J. (2022). ‘Beavers reintroduced to London after 400 years’. [Online]  https://www.nhm.ac.uk/discover/news/2022/  (Accessed: 30^th August 2022).

⁹Marlborough, D., (1970). ‘The status of the burbot Lota lota (L.) (Gadidae) in Britain’. Journal of Fish Biology, 2 (3), pp. 217-222.

¹⁰Worthington, T., Kemp, P., Osborne, P. E., Howes, C. and Easton, K. (2010). ‘Former distribution and decline of the burbot (Lota lota) in the UK’. Aquatic Conservation: Marine and Freshwater Ecosystems, 20 (4), pp. 371–377.

¹¹Worthington, T., Kemp, P.S., Osborne, P.E., Dillen, A., Coeck, J., Bunzel-Drüke, M., Naura, M., Gregory, J. and Easton, K., (2012). ‘A spatial analytical approach for selecting reintroduction sites for burbot in English rivers’. Freshwater Biology, 57 (3), pp. 602-611.

¹²Stapanian, M. A., Paragamian, V. L., Madenjian, C. P., Jackson, J. R., Lappalainen, J., Evenson, M. J. and Neufeld, M. D. (2010). ‘Worldwide status of burbot and conservation measures’. Fish and Fisheries, 11 (1), pp. 34–56.

¹³Thompson, M.S., Brooks, S.J., Sayer, C.D., Woodward, G., Axmacher, J.C., Perkins, D.M. and Gray, C., (2018). ‘Large woody debris “rewilding” rapidly restores biodiversity in riverine food webs’. Journal of Applied Ecology, 55 (2), pp. 895-904.

¹⁴Bunzel-Drüke, M., Scharf, M., Zimball, O. (2016) ‘Biologie der Quappe’ in Brackwehr, L., Bunzel‐Drüke, M., Detering, U., Jacobs, G., Kühlmann, M., Kuss, S., Lampert, K.P., Möhlenkamp, M., Peinert, B., Petruck, A. and Scharf, M., (2016). Die Quappe (Lota lota) im Einzugsgebiet der Lippe: Ökologie, Schutzmaßnahmen, Zucht und Wiederansiedlung. Landesfischereiverband Westfalen und Lippe e.V: Germany.