Cleaner water linked with smaller cockles that die younger

Cockles have been harvested along the South Wales coast for centuries. Burry Inlet and Loughor Estuary near Swansea are important habitats for the popular and widespread Cockle (Cerastoderma edule).

Like other mussel species, cockle populations are susceptible to change, either due to sudden or gradual changes in the natural environment, or due to human activities.

The new research provides more insight into this variation and into long-term trends that will aid in the conservation and management of cockle stocks.

The research was led by bioscientist Dr. Ruth Callaway from Swansea University. She examined 64 cockle monitoring reports from Burry Inlet that were produced between 1958 and 2009. She used these to analyze trends in total cockle numbers, their size and mortality rate against a backdrop of changes in weather, climate and wastewater treatment.

The overall message of the data varied profoundly over the 50-year period from 1958 to 2009.

Concrete results of the survey were:

• Both birth and mortality rates were high in the first and last decades of the study, and the variation was related to the total number of cockles in the population

• The larger the cockle population, the smaller the cockles – there was a significant relationship between the number of cockles and the average size of the specimens

• Cockle size declined in the late 1990s and modernization of wastewater treatment was significantly associated with this downward trend, suggesting that the altered nutrient regime in the estuary may have resulted in a reduced food supply for cockles

• The smaller the cockles, the higher their mortality rate: the average size of young cockles was significantly related to their longevity

• Environmental factors such as temperature were not significantly associated with decreasing cockle sizes

• Fishable cockle stocks declined in the final years of the study.

The study did not assess the impact of harvesting on cockle populations as monitoring reports did not provide data on this, although fishing cannot be ruled out as a contributing factor to cockle population variability.

However, changes in water treatment emerge from the data as a significant factor. Before 1997, wastewater from seven sewage treatment plants was discharged into the estuary. This was modernized with two new plants with treatment processes that disinfected the waste water and removed the nitrogen. This meant cleaner, healthier water for humans, but fewer nutrients for cockles.

dr Swansea University’s Ruth Callaway, the life scientist who conducted the research, says that “the data provide further evidence that changing sewage treatment in Burry Inlet in 1997 reduced food availability for cockles, resulting in smaller average cockle sizes , which in turn shortened their lifespans.”

“High water quality standards are essential. The challenge is finding ways to keep our water clean while keeping our cockle population strong.”

“In order to find a solution to any problem with cockle stocks, we first need as complete a picture as possible of its history and background variation. The new research results can help here. They deepen our understanding of the relationship between wastewater management and mussels, giving us more insight into variations and long-term trends that will aid in the conservation and management of estuarine and cockle populations.”

Spencer Williams of Gower Coast Seafood says: “I have personally observed cockles getting smaller over the years. It not only affected cockles, but in my experience, mussels and lugworms were also getting smaller, which I think is related to the change in wastewater treatment.”

Byron Davies, Baron Davies of Gower says: ‘I have personal memories of the cockle hunters as a young boy in Gower, going into the sands of North Gower and returning with the shellfish. The cockle fishery creates a lasting legacy in coastal communities.”

“As an MP, I have regularly negotiated between fishermen, government organizations and researchers, and Ruth and I have discussed changes in cockle populations for years.”

“These long-term data highlight the profound changes in fishable cockle populations over the past few decades, which may be natural or influenced by human activity. The findings are crucial for the management of the fisheries, which must be supported in the long term. “

Andrea Winterton (Natural Resources Wales—Marine Services Manager), responsible for day-to-day cockle management at Burry Inlet, says: “Our goal is to develop a thriving fishery that supports, protects and enhances the needs of the community and that environment on which it depends.”

“This is a landmark paper that provides evidence to help us better understand long-term trends in cockle populations over a 50-year period up to 2009, when a number of factors affected the population.”

“While challenges remain, fisheries management has improved in recent years, including better survey methods and a new management plan. This provides much better information about population distribution and population trends.”

“To build on this important paper, we hope to publish an overview of the latest survey data and evidence from 2010 to fully update this story.”