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.

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5 Responses to Thoughts on UK Pollution and Invertebrate Nymph Numbers

  1. Dr Peter Trolove says:

    This article raises some fair points regarding critical evaluation of such an article.
    In NZ we have the LAWA freshwater monitoring data supplied by regional councils with “accreditation” by NIWA.
    In Central Canterbury I have done three years of my own monthly monitoring of nitrate levels, in surface water. I take a quality Nikon camera and may take note of the Ecan flow data when the rivers or streams are in flood.
    Mike Joy successfully challenged the councils for presenting their data in a misleading manner. Even now it is reported as 5 yearly means. Given that we are concerned about acute or chronic toxicity Ecan should concurrently report the range, (peaks and troughs).
    Further I have tested wells which exceed the MAV for drinking water at public sites only to be asked why I am testing as Ecan monitors some of these wells. I am unable to find the results on the LAWA website.
    In common with the head streams of the English chalk streams, there is no record of the times the lesser stream and rivers are completely dry as happens in Canterbury due to over allocation.
    Dr Mike Joy tells me the macroinvertebrate community index (MIC index) is of little use in Canterbury as nitrate sensitive species are no longer present at 18 of 20 sites monitored. Despite have support from one Ecan councillor, the wider council will not allow adequate monitoring where groundwater is most likely to show high levels of nitrate.
    It is valid to read such reports with an open though cynical mind.

  2. Jack Tuhawaiki says:

    I understood that aquatic invertebrates which we anglers call ‘nymphs’, whether abundant or non-abundant, were to be the criteria for water quality?
    Why is ECan not prepared to use the measurement of invertebrates?
    The ‘environment” in the councils name seems meaningless.
    Leaving invertebrates and fish life aside, is ECan not concerned about the human health aspect where Canterbury and South Canterbury have double the national average for bowel cancer?
    A meta-analysis, published in 2021, looked at 15 studies involving more than 2.5 million participants aged 20–85. The scientists concluded that dietary nitrate (drinking water) was linked to an increased risk of bowel cancer.

  3. J B Smith says:

    Indeed Jack! Haven’t Canterbury/South Canterbury as I understand one of the highest rates of bowel cancer in the world?

  4. Charles Baycroft says:

    Every member of every species has to eat in order to live and reproduce new members of that species.
    The prevailing sources of required food therefore influence which genetic traits will promote survival, reproduction and the preferred food sources of future generations of the species.
    Members of the trout species whose genetics influence them to eat surface “bugs” will survive and multiply if there is a greater abundance of surface bugs to feed on.
    Members of trout species whose genetics influence them to forage for bugs at other stages of their life cycles will survive and multiply if there is a lesser abundance of surface bugs.

    Members of trout species whose genetics influence them to forage on terrestrial bugs will survive and multiply if there are less aquatic insects in or on the surface of the water.

    Members of trout species whose genetics influence them to forage for other aquatic species like smaller fish will survive and multiply if there are less insects to eat.

    I, and others, have been finding it easier to catch trout using imitations of small fish than those of insects in recent years.

    This suggests to me that there has been a decline in the abundance of insects that trout feed on and therefore a decline in the abundance of trout that are genetically influenced to survive by eating bugs.

    The increasing pollution of the aquatic ecosystems with nitrates and other toxic chemicals is know to be detrimental to the survival and reproduction of aquatic insects.

    That also results in changes in the genetics of the trout that survive and successfully reproduce.

    Our trout are most likely changing from being bug eaters to eaters of smaller fish including those of their own species.

    It is most likely that we will adapt by using feathered lures and spin casting soft baits instead of fly fishing with nymphs and dry flies if we want to catch more of the remaining trout.

  5. Bud jones JonesQSM says:

    One is invited to ask looking from ourNZ precarious view,”In UK,who owns the water,as here in NZ we edge toward handing over ownership of water to the tribal elite & the fate o water quality to stone age matauranga maori ways of knowing devoid of science. Sadly we marchignorantly backwards on water facility

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