Improved water quality has been linked to the growth of smaller clams in Wales, new data has revealed.
Clams have been harvested along the South Wales coast for centuries with the Burry Inlet and the Loughor Estuary near Swansea being the primary 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 results are “staggering.”
After his study, he said the change would likely be a natural adjustment and said he believes smaller clams may just be the price we have to pay for cleaner water in Loughor.
Water quality along the South Wales coast was significantly improved between 1958 and 2009, and before 1997, sewage effluent was discharged into the estuary from seven sewage treatment plants.
This has been modernized with two new plants that use treatment processes that disinfect the effluent and remove nitrogen.
This meant cleaner and healthier water for humans. Although the estuary is still prone to sewage, there appear to be fewer nutrients to support the clams.
“Studies on clams in South Wales show that they are remarkably adaptable to their surroundings,” he said.
“We have found healthy clams in the most polluted waterways of the Bristol Channel, such as the port of Port Talbot.
“However in the Burry Inlet they were smaller and less profitable from a commercial point of view, that’s a question that baffled us all.”
Untreated wastewater releases nitrogen into the water, promoting the growth of algae, which clams feed on.
As the water became cleaner, the adult clams were unable to reach their maximum size, although their numbers increased.
“Clams are very easy to age, they grow rings like trees,” he added.
In the past, dominant adult clams could live up to three to four years, however in cleaner waters they tend to die within a year or two.
“Since the change in wastewater treatment there are more cockles, but maybe not in the size people want to eat.”
He found that the larger, older clams had been allowed to grow to large sizes thanks to nutrients from the pollutants.
But that meant that in a few years they would suffocate the next generation.
“Cockle eggs measure less than 1mm and must swim for the shore to grow into the sandbanks,” added Dr. Callaway.
“Adult clams over 1.5 cm (0.6 in) eat these eggs before they hit the ground, which means adults control the next generation.
“A recent French study found that smaller young thistles have shorter lifetimes and we are seeing the same here in the Loughor estuary.”
Spencer Williams of Gower Coast Seafood said he has seen clams shrink in size over the years.
“It not only affected the clams, but in my experience the mussels and worms also shrunk in size,” he said.
Dr Callaway added: “Maybe we need to change our appetites to reflect this reality.”