Fly Fishers' Republic

Wandle Restoration Part VII

October 4, 2006

by Theo Pike

Theo Pike grayling fishing

Cracking the code

In days of yore, O Best Beloved, there were many ways of measuring the quality of a river. There were chemical scores, apparently, and there were aesthetic scores, and there were biotic scores aplenty.

But there still wasn’t any way of saying what really lived in a river, besides what should and what shouldn’t, and what might be able to at some point if only it got a chance.

And then there came an Idea into the heads of the Boffins, O Best Beloved.

“Let’s count the bugs”, they said to each other.

And so BMWP was born.

In case you hadn’t guessed this before, the acronym BMWP stands for Biological Monitoring Working Party. (Rather than Bug Measuring With Pride, or Precision, or Prettiness. We’ll probably never know why).

Devised by those Boffins as a means of grading watercourse health by practical common-sense observation, it’s there to complement other more complicated measurables like dissolved oxygen, biochemical oxygen demand, and ammonia.

In fact, it’s one of those ideas that’s brilliant because it’s based on a very simple truth: that a sparklingly healthy river will always be able to support more life, in greater abundance and variety, than a sick and ailing stream of pollution ever could. By extension, a clean, gravelly, oxygen-rich river will also allow its more sensitive organisms to flourish in greater numbers than in a stressed and poorly-oxygenated environment. In the latter, the very opposite is likely to happen, with a predominance of lower-order bugs that seem to positively prefer living in oozing sludge and other nasties.

When it comes to Boffins measuring aquatic macro-invertebrates, there are other technical benefits too. In their chosen habitat, bugs have reasonably long life cycles, and they don’t tend to want to move far. They respond to, and represent, the characteristics of their home stretch.

What’s more, many bugs are sensitive to infrequent pollution events that tend to be swept through a system and can’t always be picked up by chemical spot-sampling. So, over time, they provide the Environment Agency (EA) with a fairly well-integrated impression of a river’s quality.

That’s the theory, anyway. How does it work in practice?

When the scheme was first designed, it was envisaged that most rivers would be monitored on a more or less yearly basis. Since then, budget cuts have taken their toll, and from 2002 onwards each sampling site gets measured in only one out of every three years. But despite some fishermen’s misgivings, this isn’t as bad as it sounds: each site gets two visits during its every-third monitoring year, around March-April and September-October, to capture intelligence on bugs whose abundances vary from season to season.

Additionally, the system is rotationally geared so that not every site on any given river gets visited in the same year. On the Wandle, as far as I can tell, this means that only the year 2003 has seen not a single regular site monitored, and even this was probably balanced by a special three-location study at Morden Hall Park, about half-way down the river, in preparation for the EA’s water vole project there.

When the BMWP team comes to visit, the rubric is simple and standardised: three-minute bursts of kick-sampling and sweeping with fine-meshed nets across all the habitat types on the site, in proportion to how much there is of each.

Additionally, there’s a one-minute hand search of boulders and other reasonably unkickable objects. Everything gets counted – or estimated in the higher abundances. Then, back at base, the real number-crunching can begin.

From the EA’s own biological monitoring notes:

“For the BMWP score system, 85 macro-invertebrate families or taxa are listed, and each is given a score from 1-10 reflecting its tolerance to organic pollution and oxygen depletion. High-scoring families (7-10 points) are those considered sensitive to pollution and are characteristic of relatively clean, unpolluted and well-oxygenated waters. Mid-scoring groups (4-6 points) include a range of relatively tolerant taxa, many adapted to stillwater conditions and naturally low oxygen levels. These are excluded by poor water quality. Low scoring taxa (1-3 points) are most tolerant and include families that can withstand poor water quality”.

Results from this raw data, which should look something like this, can flow in several useful directions.

To start with, incremented BMWP scores will give you an idea of your site’s biological quality, a balance of variety against pollution-sensitivity which allows categorisation into strata:

Total BMWP site score Class
Over 150 A – very good biological quality
101 – 150 B – good biological quality
51 – 100 C – fair biological quality
16 – 50 D – poor biological quality
0 – 15 E – very poor biological quality

After that, dividing your site’s BMWP score by your number of scoring families of macro-invertebrates will reveal a more focused Average Score Per Taxa (ASPT): in other words, whether the bugs on your stretch of river are biased towards pollution-tolerant or -sensitive families:

Average Score Per Taxa Class
Over 5.4 Exceptional
4.81 – 5.4 Very good
4.21 – 4.8 Good
3.61 – 4.2 Fair
3.01 – 3.6 Poor
3.0 or less Very poor

Last year, here on the upper reaches of the Wandle, this ASPT calculation produced an average reading of “very good” – surprising, as far as I’m concerned, given the problems of urban run-off and the low flows that seemed to be just starting to bite. Then again, we’re well above most of the main sources of organic pollution in the river, which drop the ASPT scores down to “fair” for both the Croydon-Beddington arm and the main river by the time it reaches at Wandsworth. Still, I’ll be interested to see the numbers again, next time round.

For an even deeper understanding, BMWP and ASPT results can also be run against the Riverine Invertebrate Prediction And Classification System (RIVPACS), a computer model designed to predict the macro-invertebrate populations that should occur at your site under clean water conditions.

Your site’s percentage score against this target is termed its Ecological Quality Index (EQI), which should give a very clear picture of how the quality of your river’s water may be reinforcing or undermining your physical processes of river restoration. In turn, EQI can indicate a General Quality Assessment score, describing the current state of your river, and suggesting its potential for the future:

EQI for ASPT EQI for Taxa Corresponding GQA grade, with definition
1.00 0.85 A – very goodBetter than expected for an average and unpolluted river of this size, type and location. High diversity of taxa, and several species in each. Rare to find dominance of any one taxon.
0.90 0.70 B – goodBiology falls a little short of that expected for an unpolluted river. Small reduction in number of pollution-sensitive taxa. Moderate increase in individuals in pollution-tolerant taxa.
0.77 0.55 C – fairly goodBiology worse than expected for an unpolluted river. Many sensitive taxa absent, or number of individuals reduced. Marked rise in numbers of individuals in pollution-tolerant taxa.
0.65 0.45 D – fairSensitive taxa scarce, containing only small numbers of individuals. A range of pollution-tolerant taxa present, some with high numbers of individuals.
0.50 0.30 E – poorBiology restricted to pollution-tolerant species with some taxa dominant in numbers of individuals. Sensitive taxa rare or absent.
0.50 or less 0.30 or less F – badBiology limited to a small number of very tolerant taxa with individuals present in very high numbers. In the worst case, there may be no life present at all.

There, is that enough numbers, charts and acronyms?

Yup, I thought as much. But what can you do with all this, right now?

Firstly, you can pick up the phone to the EA and ask your area’s heroic Ecological Appraisal Officer for your own river’s BMWP scores – or indeed the scores for any other river that might interest you in England or Wales.

This service is completely free, though it usually takes a couple of weeks, and it’s one of the sources of background information you should seriously consider tapping before you go any further with your project. (I’ve never pushed my luck this far, but I’m thinking it could also be rather useful for planning a fishing trip to far-flung waters where you’ve no idea what the fish might be eating).

Secondly, once you’ve received figures from several different sites on your river, you may be able to pull these together to identify point-sources of pollution. You’ll get the idea from the stylised map at the top of this page.

Finally, when you’ve got the figures in front of you, drill down into them. Again, you’ll almost certainly find a few handy fly-tying hints, but you’ll also start to understand how, below the surface, your river’s food-chain really functions. In the Wandle’s case, I’m thinking shrimp- and caddis-factory. Was it any wonder that Halford’s trout grew up so big and strong?

In November, I’ll demonstrate how you can get back to the muddy-booted basics of our Boffins’ ecological monitoring idea, and even start your very own BMWP results service. Meanwhile, happy phoning!!

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