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A graphical abstract of the study findings
Image Credit: Sasha Shute
Scientific Frontline: Extended "At a Glance" Summary: Coastal Saltmarsh Ecosystems
The Core Concept: Natural saltmarshes support nearly three times the density, biomass, and measurable production of fish compared to unvegetated estuarine shores.
Key Distinction/Mechanism: Unlike unvegetated shores or newly restored marshlands (managed realignments), mature natural saltmarshes act as highly productive, year-round nursery habitats, uniquely sustaining higher species richness and exclusively supporting the early life stages of various commercial and endangered fish species.
Major Frameworks/Components:
- Comparative habitat assessment evaluating fish density, biomass, and production across natural saltmarshes, managed realignments, and unvegetated shores.
- Nursery function evaluation, which determined that juvenile fish account for 83% of all individuals recorded within the marsh habitats.
- Biodiversity and species richness cataloging, identifying 21 fish species and noting the previously undocumented year-round presence of species like the Atlantic herring in these habitats.
- Ecosystem service quantification to establish baseline data for assessing coastal resilience, fishery support, and restoration efficacy.
Branch of Science: Marine Ecology, Conservation Biology, Estuarine Science.
Future Application: The data will inform future marine development policies, guide the structural design of coastal wetland restoration projects, and accelerate conservation efforts aimed at maximizing biodiversity and fishery recovery.
Why It Matters: With widespread historic loss of saltmarshes and projected future declines, proving their critical role in sustaining marine life—including commercially important fish and critically endangered species like the European eel—highlights the urgent need to protect and restore these ecosystems to maintain coastal biodiversity.
Salt marshes in South Wales support fish at nearly three times the density of unvegetated estuarine shores, according to new Swansea University research.
Published in Estuarine, Coastal and Shelf Science, the study shows the highest fish species richness recorded in a UK salt marsh and provides the first UK estimates of fish production across natural salt marshes, restored salt marshes (managed realignment), and unvegetated shores.
The research forms part of a wider evidence report commissioned by Natural Resources Wales (NRW), which examines how salt marsh habitats support fish communities and contribute to coastal wetland ecosystem services.
Researchers surveyed 14 sites across the Burry Inlet and Three Rivers estuaries between October 2023 and September 2024, recording 8,525 fish from 21 species using seine and fyke nets, 19 of which were present in salt marsh habitats.
The team found that:
- Natural salt marshes support higher fish density, biomass, and production than both unvegetated and managed realignment sites, such as Cwm Ivy in North Gower.
- Several species—including Atlantic herring, gray mullet, European flounder, lesser sand eel, three-spined stickleback, and sand smelt—were far more likely to occur in salt marshes.
- Measurable fish production occurred only in natural salt marshes.
Sasha Shute, who led the study in Swansea University’s Marine Conservation and Ecology Lab (MarCEL), said, "At the time of the study, Cwm Ivy was just 10 years old—relatively young for a managed realignment. It formed naturally after a seawall breach and wasn’t designed with fish in mind, which helps explain the lower production figures. But we were already seeing promising signs, particularly the consistently high abundance of the critically endangered European eel."
Six species were present in salt marshes throughout the year, including Atlantic herring and European flounder—neither of which had previously been documented using UK salt marshes as a year-round juvenile habitat. Lesser sand eel, an important prey species for seabirds and marine mammals, occurred exclusively in salt marshes, marking a newly reported UK association.
Dr. Nicole Esteban, associate professor in marine ecology and head of MarCEL, said, “Juvenile fish made up 83% of all individuals recorded, underlining the critical nursery role of salt marshes. Commercially important species such as Atlantic herring and gray mullet were present only at their earliest life stages. These species move between habitats as they grow, highlighting the importance of a well-connected coastal seascape.”
Angus Garbutt, principal ecologist and coastal specialist at the UK Centre for Ecology & Hydrology, added, “Salt marshes span climates from the Arctic to the tropics, yet we still know surprisingly little about how fish use them. This report makes a valuable contribution by showing that natural salt marshes support far greater fish density, biomass, and production than unvegetated shores or managed realignment sites. It reinforces the need to protect the marshes we have and to accelerate efforts to restore those we’ve lost.”
Salt marshes are a protected UK habitat under the Conservation of Habitats and Species Regulations 2017 and the Environment (Wales) Act 2016, and interest in their restoration is growing as coastal wetlands continue to decline.
The scale of loss makes restoration increasingly urgent: England has lost more than 85% of its salt marshes since the 1800s, and a further 5,262 hectares are projected to disappear across the UK by 2060 without intervention.
The new findings provide essential baseline data for assessing the ecosystem services delivered by salt marshes, from supporting fisheries to enhancing biodiversity and coastal resilience.
Ida Nielsen, specialist advisor for marine and estuarine fish at NRW, said, “This research clearly demonstrates the value of salt marshes for supporting juvenile fish and highlights the differences between mature natural marshes and newer recreated sites. The findings will inform our advice on future marine developments and help guide how we design and restore salt marshes to maximize nature recovery.”
Research material: Estimating production of finfish in saltmarshes on the South Wales coast (PDF)
Published in journal: Estuarine, Coastal and Shelf Science
Title: Fish density in NE Atlantic saltmarsh is three-fold higher than unvegetated shores
Authors: Sasha L. Shute, Lauren M. Pennack, Alex Scorey, Ida A. Nielsen, Richard K.F. Unsworth, and Nicole Esteban
Source/Credit: Swansea University | Ffion White
Reference Number: eco051326_02
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