South Wales saltmarshes vital nurseries for young fish, study finds

Nation.Cymru staff

Saltmarshes in south Wales support nearly three times as many fish as neighbouring mudflats and other unvegetated estuary shores, according to new research led by Swansea University.

The study, published in the journal Estuarine, Coastal and Shelf Science, recorded the highest fish species richness yet documented in a UK saltmarsh and found that natural saltmarshes play a crucial role in supporting young fish and wider marine ecosystems.

Researchers surveyed 14 sites across the Burry Inlet and Three Rivers estuaries between October 2023 and September 2024, recording 8,525 fish from 21 different species.

Nineteen of those species were found in saltmarsh habitats.

The study found natural saltmarshes supported significantly higher fish densities, biomass and overall fish production than both unvegetated estuary shores and restored saltmarsh sites such as Cwm Ivy in north Gower.

Several species, including Atlantic herring, grey mullet, European flounder, lesser sandeel, three-spined stickleback and sand smelt, were far more likely to be found in saltmarsh than elsewhere.

Researchers also found measurable fish production occurred only in natural saltmarsh habitats.

The findings suggest these coastal wetlands act as important nursery grounds for young fish.

Sasha Shute, who led the research in Swansea University’s Marine Conservation and Ecology Lab, 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.”

The research found juvenile fish accounted for 83% of all fish recorded during the study.

Six species were found in saltmarsh throughout the year, including Atlantic herring and European flounder, neither of which had previously been documented using UK saltmarsh as a year-round nursery habitat.

Lesser sandeel, an important food source for seabirds and marine mammals, was found exclusively in saltmarsh, representing a previously unrecorded association in the UK.

Dr Nicole Esteban, associate professor in marine ecology and head of the Marine Conservation and Ecology Lab, said: “Juvenile fish made up 83% of all individuals recorded, underlining the critical nursery role of saltmarshes.

“Commercially important species such as Atlantic herring and grey 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.”

Ecosystem services

The study forms part of a wider report commissioned by Natural Resources Wales (NRW) examining how saltmarsh habitats support fish populations and contribute to wider ecosystem services.

Interest in restoring saltmarshes has increased in recent years as coastal wetlands continue to decline.

According to the researchers, England has lost more than 85% of its saltmarsh since the 1800s, while a further 5,262 hectares are projected to disappear across the UK by 2060 without intervention.

Ida Nielsen, specialist adviser for marine and estuarine fish at NRW, said: “This research clearly demonstrates the value of saltmarsh 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 saltmarsh to maximise nature recovery.”