Thoughts on UK Pollution and Invertebrate Nymph Numbers

From Fishing Breaks (UK)

By Simon Cooper

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The recent publication of a paper with the headline ‘Significant improvement in freshwater invertebrate biodiversity in all types of English rivers over the past 30 years’ in the scientific journal Science of The Total Environment has thrown the pollution activist lobby into all sorts of a tizzy because, for all the obvious reasons, it throws a significant spanner into the doom narrative. 

Wildfish, the organisation previously known as the Salmon & Trout Association, now turned river environment advocates, rushed out a press release to, I think, trash or at least seriously cast doubt on the Total Environment paper. I say I think because the Wildfish release was even more baffling than the original paper and, having read both, I can tell you that took some doing. But more of that later. First let me give you my professional take which, usefully in this context, goes back 30 years.

If you were leaning over a bridge today looking down on chalkstream in a normal month of a normal year it would not look much different to 30 years ago. Indeed, I have seen colour photographs Halford took of the river at Mottisfont Abbey which you’d be hard pushed to date as 130 years ago. But appearances are deceptive. Travel to the head of the valleys, even at the height of the wettest winter, and streams have vanished in the past 30 years thanks to excessive abstraction. These will never return; we are literally losing miles of tiny, wild streams and brooks with every passing year. 

Looking over that bridge the water looks clear but we know it contains a cocktail of chemicals, agricultural, domestic and industrial that surely cannot be doing good. Indeed, you can cogently argue that the largely organic waste of Halford’s time was helpful to fish and bugs. Anyone who went sailing in the days when yachts vented waste directly into the sea will know there is nothing fish like more than s**t!

If anyone asks me what has changed most in my time on the chalkstreams it would be rising fish, or more particularly the lack thereof. Now, I cannot tell you definitively why this is. Some people point to the stocking of triploid trout, effectively a genetically modified trout. I’d go along with this except in my experience unstocked streams seem to have an equal lack of rising fish. Maybe, if the report is correct and the nymph population is on the rise, canny trout are not wasting time and effort on floating flies but rather munching away to their hearts content on the growing sub-surface population.

Part of my bafflement with regard to the lack of rising fish is the hatches. Now, it is often said we lack the hatches of old but I’ve not noticed that on a scale that would stop trout looking upwards. Often, I see a river thick with flying bugs, which supports the findings in the paper, but nary a fish moves. Are they, to repurpose that old British Rail excuse, the wrong type of flies? It could well be which brings us nicely around to the meat of the Total Environment paper.

The highlights of the paper (their words not mine) are that river macroinvertebrate richness has increased throughout England over the past 30 years with a recovery of pollution sensitive invertebrates reaching the reference condition, the improvement seen across all river types. If like me you find some of the jargon unhelpful ‘macroinvertebrate’ are bugs that can be seen with the human eye and ‘reference condition’ the expected population level in normal conditions. In short, the report is saying the assumption that pollution is causing biodiversity decline should be challenged because their data, which draws on solid monitoring going back 30 years, suggests something different.

Mayfly larvae . Image: Dr. Julian Taffner

Now this is where it gets murky, and to be fair to the authors of the report, they allude to this albeit buried in the footnotes. The question is how do you measure biodiversity or as they say in the highlights, richness? For the sake of explanation let me give you examples: your sample today shows five species compared to four at a previous sampling. That is a tick the box for increased biodiversity. However, what has not been measured, and is not part of the historic dataset, is the number of individual nymphs. 

So, quite feasibly, you might have a total population of a hundred nymphs today comprising of five species whereas in the past there were a thousand nymphs comprising of four species. But, by the metric of the Total Environment analysis, the former is considered a win whereas you and I might feel the latter is preferable. As they say of all statistics, it is often what is hidden that is more revealing than what is shown.

Frankly, I’m not sure this report is the real deal and the headline screams clickbait to me. These reports are essentially advertorial for researchers who will have paid the journal in the region £3,000 to secure publication. Whilst I am going to file this one under the question mark heading I leave you with a last thought. In the previous Newsletter I quoted Ernest Pain’s Fifty Years on the Test. In it he talked about the river as it flowed through a town where it divided into two channels. One channel was productive for fishing, whilst the other, which carried a majority of the town ‘waste’ rarely saw a rising fish. As I say, just a thought.