Catch the drift: river fish and birds rely on mobile invertebrate prey
New research shows how salmon, trout and river birds in Welsh upland rivers rely on invertebrate prey with specific ‘drifting’ behaviours for their food sources.
The open-access study, led by Cardiff University, found that iconic predatory fishes, such as the brown trout and Atlantic salmon, and birds, such as the Eurasian dipper and grey wagtail, are most abundant where invertebrate prey have certain characteristics. Specifically, all the predators were most numerous where mobile ‘drifting’ prey such as mayflies occurred abundantly in fast flowing habitats. These traits also predicted predator populations more effectively than overall prey variety or abundance.
The team from Cardiff, Cambridge and Roehampton Universities, the British Trust for Ornithology and Imperial College of London assessed predator numbers along Welsh upland rivers in relation to invertebrate numbers and trait composition. Researchers were also interested in assessing whether these specific aspects of biodiversity were important elements in river ecosystem services such as economic gains from angling or cultural values.
“Our work demonstrates the importance of river invertebrates – the ‘hidden biodiversity’ that is often underappreciated by society – in sustaining well-known and iconic river predators” said Dr Cayetano Gutiérrez-Cánovas, lead author of the study. “Roughly half of European rivers currently fail against EU targets for ecological status which is likely to reduce invertebrate numbers and prey availability for freshwater predators – yet this link is seldom made or recognised by those charged with managing river ecosystems”, states Gutiérrez.
Co-author of the study, Prof Steve Ormerod added:
“We’ve known for some time that predators specialise on certain prey types – for example invertebrates of the correct size for adult birds to feed their young, or prey rich in calcium for egg formation. But this new study shows us how several river predators converge on accessible, drifting prey in faster flows. This raises intriguing questions about how they divide up these prey resources, but also how we can manage and restore rivers to ensure that these and other predators can co-exist in numbers.”
The findings of this study might go some way to explaining dramatic declines of freshwater vertebrates by showing how reliance on river invertebrates can make aquatic predators vulnerable to ongoing river degradation or the transfer of toxicants to apex predators.
Amidst growing concerns about the plight of freshwater vertebrates, the study illustrates how healthy invertebrate populations benefit predators while also helping society. Invertebrate-dependent salmonids, such as trout and salmon, are highly ranked in recreational fishing that boosts visitor numbers, employment and local economies. In addition, greater invertebrate availability also benefits bird conservation, with all its associated cultural value and opportunities for ecotourism.
This research was funded by the NERC DURESS investigation into the benefits that upland rivers provide to society as well as the EU MARS study of multiple stressors on freshwater ecosystems.
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