Smaller hulls thanks to cleaner water in South Wales, study finds

Hulls

Smaller cockles that don’t live as long may be the result of better water quality in Welsh waters

Better water quality has been linked to smaller cockles growing in Wales, new data has revealed.

Cockles have been harvested along the South Wales coast for centuries, with Burry Inlet and the Loughor Estuary near Swansea being the main habitats.

The Swansea University study, which looked at 50 years of data, also found a higher death rate.

Marine biologist Dr Ruth Callaway said the findings were “puzzling”.

Following her study, she argued that the change was likely a natural fit and said she thought smaller hulls might just be the price we had to pay for cleaner water at Loughor.

Between 1958 and 2009 the water quality around the South Wales coast improved significantly and before 1997 sewage effluent was discharged into the estuary from seven sewage treatment plants.

This has been modernized with two new plants using treatment processes that disinfect effluent and remove nitrogen.

This meant cleaner, healthier water for humans. Although the estuary is still subject to sewer outfalls, there seems to be less nutrients to support the hulls.

“Studies of cockles in South Wales show that they are incredibly adaptable to their environment,” she said.

“We have found healthy hulls in the most polluted waterways of the Bristol Channel, such as Port Talbot Harbour.

“However, in the Burry Inlet they are smaller and less commercially viable, that’s a question that baffles us all.”

Untreated sewage releases nitrogen into the water, promoting the growth of algae, which the cockles feed on.

As the water became cleaner, the adult cockles could not reach their maximum size, although their numbers increased.

“It’s very easy to age cockles, they grow rings like trees,” she added.

In the past, dominant adult cockles could live up to three or four years, but in cleaner water they tend to die within a year or two.

“Since the change in sewage treatment, there are more cockles, but maybe not in the size that people want to eat.”

She found that larger, older cockles were allowed to grow to large size through nutrients from pollutants.

But that meant that in some years they smothered the next generation.

“Cockle spawn are less than 1mm and must swim to shore to develop in sandbars,” Dr Callaway added.

“Adult cockles larger than 1.5 cm (0.6 in) eat these eggs before they hit the ground, which means the adults control the next generation.

“A recent French study found that smaller young cockles have shorter lives, and we’re seeing the same here in the Loughor Estuary.”

Spencer Williams of Gower Coast Seafood said he had seen the size of the cockles shrink over the years.

“It didn’t just affect the cockles, but in my experience the mussels and worms also reduced in size,” he said.

Dr Callaway added: “Perhaps we need to change our appetites to reflect this reality.”

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