River Thames mussel population under threat: is it from invasive species or change in conditions?

New research from the University of Cambridge has claimed that the native mussel population in the River Thames is being wiped out. The research, carried out by zoologist  Isobel Ollard PHD, has shown a 95 percent decline of the mussel population in the River Thames with one species, the depressed river mussel, completely vanquished.

The research, comparing the Port of London’s survey in 1964 to 2020, showed that native populations severely declined and those that remained were much smaller - reflecting slower growth.

The causes of the dwindling mussel population? A combination of invasive species brought into the River Thames by freighters since the original survey into population and, more ironically, the cleaner conditions of the River Thames.

The cleaner conditions in the River Thames has meant that the filter feeders, shellfish that consume almost anything in the water, no longer have algae to feed on. As the levels of nitrate and phosphate have fallen due to tighter regulation of sewage treatment, the algae depend on the nutrients, limiting the mussels’ food.

The research also showed that a number of species of mussels that were undocumented have caused a drop in the number of native mussels. The authors of the research believe that many of them arrived on ships calling into the Port of London, including Zebra mussels, known to smother natives to death.

Senior author of the research Professor David Aldridge said: "This dramatic decline in native mussel populations is very worrying, and we are not sure what is driving it. While this might seem like a rather parochial little study of a single site in a single river in the UK, it actually provides an important warning signal about the world’s freshwaters."

Mussels provide a good indication of the cleanliness of a water system and the River Thames has 200 tons of microplastics pulled from it each year; the most of any river in the world.