Wandle Restoration Part VIII

net

Rock-turning for experts

When you picked up a fly rod for the first time, there were probably several things you never thought you’d learn to do – and that’s if you’d even heard of them.

First, there was certainly the urgent matter of learning how to cast. (I still remember wrapping several turns of DT4 round one of my patient instructors, somewhere on the Torridge, and I’m not convinced that the years have been as kind to my loops as they have to his).

Then there was fly-tying – that obscure branch of fly-fishing history and the craft-shop movement whose main mystery is really just getting those first materials to stick to the hook.

And vaguely connected to that and pulling stuff out of urban rivers, there was Entomology, a big old chunk of Latin I thought I’d lost somewhere around the switch from O-Levels to GCSEs. Yes, if your maths is good enough, that’ll date me nicely.

But Entomology is something you tend to come back to as a fly-fisherman – mostly by default, when you’re down to that last desperate throw of “what the hell are those fish eating anyway?”

We’ve all turned over rocks to see what’s underneath, sometimes finding something to match what’s in our fly boxes, sometimes not. Before last month’s article, there’s no reason why you should have guessed that there was a professional next step after that, either. But here it is. As committed by scientists and sportsmen alike, dear readers: rock-turning for experts.

What you’ll need:

One river, stream or pond. At least to start with.

One small sheet of net curtain tied between two poles. In the photo at the top of this article I’ve used a swatch of nylon mesh about a metre square, with steel poles up the sides to make it easier to drive into the river bed if necessary. In fact, almost any piece of charity-shop net will do, suspended between two broom handles.

One or more shallow trays or bowls two or three centimetres deep, half filled with water. Preferably clear or white in colour, for sorting the macro-invertebrates you’re hoping to find.

One pair of tweezers. For picking up tiny bugs, and sorting them into the trays of water.

One pair of waders. In case you’re bothered about getting your legs wet and cold.

How to do it:

If you want to replicate the efficiency of the Environment Agency’s kick-samplers, hold your net by its handles, upright in the river and just downstream from you. Now choose a varied-featured area of river-bed, about a metre square, and spend a minute or so shuffling your feet in that area. A cloud of detritus will become waterborne, drifting downstream into the mesh of your waiting net: the particles of silt will pass through, but the bugs won’t, and they’ll show up clearly against the white mesh.

For good measure, you can also sweep your net across the tops of semi-submerged banks of sand or mud, and turn over some of the larger rocks to search them by hand, since some fast-water species are very good at sticking themselves to something heavier. Needless to say, it’s not a good idea to do any of this during spawning time for any fish, and it’s definitely illegal to disturb salmon redds if you’re reading this anywhere in Britain.

But apart from that, yes, kick-sampling really is that simple.

Now it’s sorting and counting time. Carry your sodden, invertebrate-spackled net to a convenient flat bank, and start examining what you’ve caught.

In any given year, you’ll find that your numbers of macro-invertebrates will vary from season to season. Populations of stoneflies and freshwater shrimps, for instance, may almost seem to vanish at certain times, before their next generations reappear in force, which is why the EA sampling teams re-visit their sites at each end of the summer months.

You may find it easier, as well as more revealing, to transfer your bugs into the flat trays you’ve brought with you: that way, you’ll be able to see their natural movements as well as sort them into categories and start your process of counting and identification.

You’ll find a slightly tweaked-up transcription of the same macro-invertebrate list that the Environment Agency BMWP teams use, together with the scores for each family member, right here.

At this stage, I’d better also come clean and admit that I’m still looking for a really comprehensive website with images and descriptions of all those individual bugs, so I haven’t been able to add links to all the familiar Anglo-Saxon common names. If you’ve already found a favourite solution to that problem, please let me know: meanwhile, the best general guide I’ve found so far is right here at Fly Fishers’ Republic.

Offline, I can’t do better than point you offline towards Pat O’Reilly’s “Matching the Hatch” or John Goddard’s “Trout Flies of Britain and Europe“. Both contain excellent close-up photography, and will help you identify exactly what you’ve caught with a lot more precision than I ever could.

Instead, I’ll stick to dropping a few hints about what you might want to find in your river… and what you might not be so pleased to see…

Five things you’ll want to find

Plenty of perfect 10s from the BMWP list, particularly nymphs of the mayfly and stonefly families. These are indicators of excellent water quality, and can hardly tolerate pollution at all. Stoneflies are most likely to be found in mountain and moorland streams; mayflies happen almost anywhere, except the true clichéd Ephemeridae, which need chalkstreamy silt for their nymphal burrows.

More than three species of those perfect 10s. The presence of the most sensitive species you can find will determine the ecological grading of your water. Finding more than three of those species in any one stretch should confirm your water’s biodiversity – and that the individuals you’ve found haven’t just drifted downstream to their doom, from cleaner headwaters.

Generous numbers of caddis. These may be free-swimming, stuck to rocks, or cased in mini mobile homes constructed from anything from sand to bits of leaf-litter. Most of them are sensitive to pollution, and they’re all good fodder for trout when they find their way into the drift.

Swarms of freshwater shrimp. With only a score of 6 on the BMWP scale, shrimp may seem to be dropping down the ladder of desirability. But don’t be deceived. Despite the reputation of their sea-going namesakes, most of them are fairly pollution-intolerant – and in alkaline waters, as Oliver Edwards says, they should be present in almost biblical numbers. Needless to say, this makes them steak to trout and grayling. Or should that be lobster?

An overall BMWP scoring total of more than 150, achieved by adding up the scores of all the macro-invertebrate families you find in your river: confirmation of standard good health, reasonable freedom from pollution, and rich invertebrate life. Even the top of the Wandle doesn’t achieve this (look here for last month’s summary of how those numbers work), but the pristine headwaters of the Test can still claim totals approaching 200.

And five things you won’t…

Nothing alive at all. At the very least, this should offend your fisherman’s sense of wasted space, and trigger a call to the Environment Agency.

A strong smell of oil or sewage before or after you stir the riverbed. Many of the post-industrial rivers we’re discussing were probably dangerously polluted long before you started work on them, and these pollutants may linger on in deep deposits of mud and black silt. It’s likely that there will be heavy metals and other nasties in the sludge too, and we’re still not completely sure of their health implications. Wade and handle with care.

Dead shrimp drifting in backwaters. As we’ve just seen, freshwater shrimp are good indicators of pollution, and their solid little bodies reliably preserve traces of what might have killed them. If you do detect numbers of tiny dead crustaceans, collect a round dozen adults, freeze them carefully as pathological samples, and make that call to the EA – again.

Lots of low- or non-scoring families from the BMWP list. Here at the bottom of the ratings come various macro-invertebrates that not only tolerate impurities, but can really seem to relish them. Sludge- and square-tailed worms are apparently especially fond of organic pollution – that’s misconnected drains, less-than-competent water treatment and overflowing Combined Sewage Outlets to you and me. So if you find any of these, you should have been wearing gloves and an NBC suit ten minutes ago.

An overall BMWP score of less than 50. An old fishing companion of mine once said that if a waterway scored less than this number, you might as well fish in your bathtub for all the fly-life it will hold, and he was probably right. At this level, a river will be dominated by snails, leeches and worms of various kinds. And while some of those might make good fish-food, the overall water quality is unlikely to sustain much in the way of larger life.

Then again, whatever you do find in your recovering river, you’ll know exactly the size of the problem you’re facing.

So what are we waiting for?

Grab that net curtain, jump into your waders again, and let’s get out there!!

← Read previous instalment | First instalment ↑ | Read next instalment →

Comments are closed.