14 December 2021

GWCT research finds that high levels of aquatic plant cover benefits the abundance, growth and feeding of juvenile Atlantic salmon during summer


This work was made possible with the kind support of The G & K L B Boyes Charitable Trust

In response to three decades of declining Atlantic salmon in most of their native range, scientists are looking at how the management of juvenile habitat can maximise the numbers and quality of seaward-migrating salmon smolts to increase survival at sea and the number of returning spawners.

A new study recently published in Ecological Applications, has found that high cover of aquatic plants in summer increases the abundance, growth and feeding of juvenile salmon and could reduce competition between juvenile salmon and trout.

Natural habitats in rivers provide resources for fish growth and reproduction, refuges from predators, and protection from inclement environmental conditions. In upland, fast flowing, rivers these benefits are provided by the large substrate mix of the river beds. However, lowland rivers lack these large substrates. Recently published research, which was funded by the G & K Boyes Charitable Trust and carried out by Dr Jessica Marsh at the Game & Wildlife Conservation Trust (GWCT) has uncovered the importance of instream vegetation to juvenile salmon and trout.

Dr Marsh who completed her PhD study at Queen Mary University of London, supervised by Prof John Iwan Jones, set out to test the influence of aquatic plants on juvenile salmon and brown trout in the lowland rivers, using the River Frome, in Dorset England.

By manipulating the cover of water crowfoot (Ranunculus) at nine sites and monitoring the fish, macroinvertebrate prey and physical river characteristics over a two-year period, Dr Marsh was able to investigate the effects of plant cover on abundance, growth rates, feeding opportunities and competition between salmon and trout.

Dr Marsh explained: “Populations of wild Atlantic salmon are in decline throughout most of their range, and the rivers of southern England are no exception. Brown trout are also facing pressures from predation, habitat degradation, pollution and warming water temperatures. It is important to ensure that juvenile fish have the best possible habitats in which to thrive. This research identified that instream aquatic vegetation has benefits for juvenile salmon (and to a lesser extent, trout), and demonstrates that river restoration aims for lowland rivers might be achieved through the promotion and even enhancement of naturally occurring Ranunculus beds to improve the production of both salmon and trout juveniles.”

The study found that increased Ranunculus cover supported a higher abundance of juvenile salmon in summer and autumn. Additionally, the salmon where more likely to remained within areas of increased cover and grew faster during the summer months, than salmon in areas with reduced cover.

Both salmon and trout benefited from improved feeding opportunity and less competition in the sites with increased Ranunculus cover, as the study found that they consumed larger prey and a higher biomass of prey than in low cover sites.

Dr Rasmus Lauridsen, Head of Research at GWCT and supervisor of the study said: “Wild salmon populations have declined dramatically, in some areas by as much as 70 – 80% in the last 30 years. The Game & Wildlife Conservation Trust (GWCT) has been monitoring salmon numbers in the River Frome, at our Salmon & Trout Research Centre since 1973. Creating one of the most comprehensive records of salmon movement and survival in England and Wales.

As a wildlife conservation organisation with an internationally recognised research centre for salmon and trout, we strive to support research that is both scientifically robust and has practical applications. The study carried out by Dr Marsh illustrates how habitat quality impacts production of juvenile salmon and trout and the findings have clear management applications for river restoration techniques in lowland rivers”.

Prof John Iwan Jones, Head of the River Communities Group at Queen Mary said: “This research has shown that by promoting the naturally occurring Ranunculus beds that are so characteristic of our lowland rivers we can improve conditions for other threatened species, such as salmon and trout. Lowland chalk streams suffer from many pressures: this work has implication for how best to restore these important rivers and the species that live in them”.

The paper “High summer macrophyte cover increases abundance, growth, and feeding of juvenile Atlantic salmon” is published in Ecological Applications.

Notes to Editors

1) For more information, interviews and enquiries relating to Game & Wildlife Conservation Trust (GWCT) please contact Kate Williams at kwilliams@gwct.org.uk. Tel: 01425 652381. For enquiries relating to Queen Mary University of London please contact Sophie McLachlan at sophie.mclachlan@qmul.ac.uk. Tel: 020 7882 3787.

2) Research was funded by G & K Boyes Charitable Trust. The G & K Boyes Charitable Trust supports charitable organisations with a focus on education, training, advancement of health or saving lives, arts, culture, heritage, science, environment and conservation.

3) Game and Wildlife Conservation Trust: Providing research-led conservation for a thriving countryside. The GWCT is an independent wildlife conservation charity that has carried out scientific research into Britain’s game and wildlife since the 1930s. We advise farmers and landowners on improving wildlife habitats. We employ 22 post-doctoral scientists and 50 other research staff with expertise in areas such as birds, insects, mammals, farming, fish and statistics. We undertake our own research as well as projects funded by contract and grant-aid from Government and private bodies. The Trust is also responsible for several Government Biodiversity Action Plan species and is lead partner for grey partridge and joint lead partner for brown hare and black grouse.

4) Queen Mary University of London: At Queen Mary University of London, we believe that the diversity of ideas helps us achieve the previously unthinkable. Throughout our history, we’ve fostered social justice and improved lives through academic excellence. And we continue to live and breathe this spirit today, not because it’s simply ‘the right thing to do’ but for what it helps us achieve and the intellectual brilliance it delivers. Our reformer heritage informs our conviction that great ideas can and should come from anywhere. It’s an approach that has brought results across the globe, from the communities of east London to the favelas of Rio de Janeiro. We continue to embrace diversity of thought and opinion in everything we do, in the belief that when views collide, disciplines interact, and perspectives intersect, truly original thought takes form.

…the GWCT Fisheries team work hard to undertake leading research in to the declines in salmon and trout populations. You can help them continue their vital work by supporting the GWCT Fisheries team from as little as £3. We are a small charity and every contribution, no matter how big or small, can help make a real difference. It only takes a minute and all cards, Direct Debit, Apple Pay, Google Pay and PayPal are accepted: