Mussel populations in the river Thames in the UK have dropped by as much as 99 per cent between 1964 and 2020. Why isn鈥檛 clear, but the researchers involved suspect it is down to the effects of pollution and invasive species.
To shed light on the long-term trend, and at the University of Cambridge replicated the earliest rigorous study available on these mussel populations, a survey on the Thames in Reading, Berkshire, dating from 1963 and 1964.
In 2020, Ollard and Aldridge recreated the methods from that study, collecting samples by hand or by dredging, depending on the water depth, and taking all mussels found to a lab for measurement and classification.
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They found that the number of duck mussels (Anodonta anatina) per given area had decreased to just 1.1 per cent of 1964 levels. The painter鈥檚 mussel (Unio pictorum) had fallen to 3.2 per cent of 1964 levels, and they found no living specimens of the depressed river mussel (Pseudanodonta complanata) at all.
Mussels collected in 2020 were also smaller, reaching between 65 per cent and 90 per cent of the length they would have been in 1964 at the same age, judging by the annual growth bands on their shells.
This, coupled with lower numbers of the molluscs, means that annual biomass production by all mussels present had fallen in 2020 to just 7.5 per cent of 1964 levels.
Ollard says this will be having an impact on the ecosystem, because mussels provide important functions in freshwater environments. They stir up the riverbed, oxygenate the water and consume algae and other organic particles from the water column and the nutrients pass up the food chain when the mussels are eaten. She warns that lower numbers point to a likely 鈥渟ubstantial degradation鈥� of conditions in the Thames for other animal species.
Ollard says invasive species are a large reason for the decline of native species. While no zebra mussels (Dreissena polymorpha) and Asian clams (Corbicula fluminea) were found in 1964, she saw strong populations of both in the latest survey, but not nearly enough to compensate for the overall drop in shellfish numbers.
The zebra mussels are native to lakes in southern Russia and Ukraine and are thought to have been brought to the UK by shipping. Unlike native freshwater species, these mussels can stick to solid surfaces, so artificial structures in rivers and even the shells of native species can provide a home for them.
鈥淎 lot of those kinds of concrete-lined ditches are actually really good habitat for zebra mussels because they can settle on those hard surfaces,鈥� she says. 鈥淪o what you鈥檙e effectively doing [by building them] is replacing good habitat for native species with good habitat for the invasive species.鈥�
Ollard speculates that mussels in the Thames may be returning to earlier, slower growth rates. Less sewage has been pumped into the river in recent years, reducing the levels of nutrients found in water. But a rise in other pollutants may be to blame for reduced numbers overall, she says, which makes the case for stricter regulation and testing. More monitoring will be vital to work out why numbers are falling, and how to protect them, she says.
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