Native mussel numbers down nearly 95% since 1960s, Thames survey finds | marine life

A mussel survey has revealed an alarming deterioration in the Thames ecosystem since the 1960s, a study has found.

Scientists attempting to replicate a 1964 study of freshwater bivalves in a section of the River Thames near Reading found striking results, as native mussel populations had fallen by almost 95%. A native species, the depressed river mussel, had completely disappeared, and the remaining species were much smaller for their age, reflecting slower growth.

“Mussels are an excellent indicator of the health of the river ecosystem,” said Isobel Ollard, a PhD student in the University of Cambridge’s Department of Zoology and first author of a paper published in the Journal of Animal Ecology.

“Such a drastic drop in mussel biomass is likely to be an indicator of environmental deterioration,” she said. “It is also likely to have a ripple effect on other species, reducing overall biodiversity.”

One adult mussel can filter up to 40 liters of water a day, Ollard said, removing large amounts of algae and keeping rivers clean.

The original 1964 survey is still cited by scientists as providing evidence of the great contribution of mussels to river ecosystems.

The research, carried out at a site near Reading, sends an important warning about the world’s freshwaters, according to the authors of the paper.

Professor David Aldridge, from the University of Cambridge and lead author of the report, described the collapse in the population of native species as “very worrying”.

“While this may seem like a rather parochial small study of a single site in a single river in the UK, it actually provides an important warning signal about the world’s freshwaters.”

The survey revealed a large number of invasive species, the non-native zebra mussel and Asiatic clam, which were absent from the 1964 survey. The species likely hitched a ride on boats, dropping as was traveling up the Thames, the scientists said.

The zebra mussel, known to push on native species, choking them to death, could be responsible for the decline, they said. Other possible causes could be changes in land use along the river or changes in fish populations that mussels depend on as part of their life cycle.

The duck mussel population had declined to just 1.1% of 1964 levels, with the painter’s mussel at 3.2% of 1964 levels.

Scientists believe the reduced growth rates of mussels may reflect the river’s return to a natural state following tighter regulation of sewage treatment since the 1960s. They looked at the nutrients measured by the Agency for environment and found that nitrate and phosphate levels had dropped. A reduction in these nutrients would reduce algae growth, limiting the food available to mussels.