High levels of aquatic plant cover benefits the abundance, growth and feeding of juvenile Atlantic salmon during summer
In response to three decades of declining Atlantic salmon in most of their native range, scientists at Queen Mary and the Game & Wildlife Conservation Trust (GWCT) 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 Atlantic 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 in collaboration with the GWTC, set out to test the influence of aquatic plants on juvenile Atlantic 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 Atlantic 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 Atlantic 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.