Cattle and the freshwater pearl mussel

It was Sunday 22nd July around 19:30. I was sitting down to play some FIFA 12 on my flatmates Xbox 360 when my friend Cherith text me. “I’m watching Countryfile and they say that cattle are killing freshwater mussels in the River Clun”. By the time I’d paused the game and switched over to BBC1, Matt Baker was saying something insightful about gliders “They’re very white!” Yes Matt, yes they are.

A cow ramp

The freshwater pearl mussel (Margaritifera margaritifera) is a pretty extraordinary mollusc. You can read all about it on Wikipedia but I’ll pull out the key facts:

Now my research into the effects of cattle grazing upon chalk streams hasn’t looked at the pressures influencing populations of these protected bivalves, so I thought I’d do a quick literature search to see what the academic community says about it. A recent paper by Addy et al. (2012) suggests that in the River Moriston in Scotland, the main reasons for a declining mussel population are historic, intensive fishing, the prevenation of fish migration, and river bed compaction. In Spain, Outeiro et al. (2008) have shown that tree cover is an important control on the distribution and abundance of freshwater pearl mussels (more is better). Giest and Auerswald (2007)  argue that in German rivers, fine sediment is a major risk to mussels, whilst in Sweden, management practices dating back several decades are accountable for a lack of juvenile mussels, according to Englund et al. (2007).

Maybe Countryfile is right, albeit inadvertently: these mussels are like pandas; an evolutionary dead-end. The slightest perturbation in water quality or habitat availability and they stop reproducing.

But can we say, with confidence, that the principle reason for failing freshwater pearl mussel populations in the River Clun (or any other European river) is cattle grazing? It is irrefutable that cattle have the potential to negatively influence the key controls on mussel populations, but where do a few cattle drinks fit into the big picture of change over the last century? Intensive arable agriculture, increased surface runoff due to urbanisation, inputs from sewage works, historic management practices and overfishing. What of these?

And the major problem with this isn’t one of poor or inaccurate communication. It’s not even that the scientific basis for these assertions is absent.  The big issue is that thousands of pounds and more of taxpayers money is being spent on excluding cattle from rivers when, and I would put my house on this (if I could get on the market), there will be no change in the fortunes of the freshwater pearl mussel.

I do like the idea of the WWF adopting the mussel as their logo though. “The freshwater pearl mussel made me do it”. Rolls off the tongue.

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