Wednesday 23 October 2013

Floods and Fish

The 26th of November 2012 and the floods warnings were both numerous and scattered across the UK. There was widespread devastation and those unfortunate enough, or perhaps daft enough, to live on the flood plains were again suffering, wading through their possessions in the lounge, climbing over ineffective sandbags to get out of their doors, and worrying that during the following year, they would no longer be able to get any flood insurance. But this year, 2013, after the inconvenience, they will have replacement sofas, new carpets and a well watered garden. And as I write, October 2013, my usual rivers are again pretty well unfishable hell and high water.

The news reels tell little of the trials undergone by the wildlife at these times.  Birds, especially ducks, probably see few if any problems, although during the breeding seasons birds that nest in or near the river banks might well suffer badly.  Kingfishers, dippers, and wagtails come to mind, and are unlikely to be able to fall back on the terms of their insurance contracts to reinstate their homes, or indeed their families.  Most wild animals swim well, and are likely to find safety.  Domesticated farm animals might be less able and so suffer.  Bred for their food value, cows and sheep are far less fit than the average fox or badger, and being possessed of  very low IQs, will often drown.

 Seeing a river in full flood is impressive.   I live near the Upper Mersey, a spate river.  3 or 4 years ago a
Receding, but Still Over Ten Feet of Floodwater, Well up the Bank.
major downpour raised the river level to an astonishing height. At one point where there is often less than a foot of water, flowing over a gravel bed, I observed over 13 feet of water. That 13 feet also widened the river at the spot by some 50%, and the speed of flow increased enormously.  I calculated that the river flow was increased by about 100 fold over the normal summer flow. You would NOT want to fall into the Upper Mersey when it is in this sort of mood.  It had become the colour of Cadbury's drinking chocolate, very turbid, with turbulence to match.  A veritable maelstrom, awesome in its power.  Full sized trees could occasionally be seen floating downstream at speeds approaching 15 or maybe 20 mph.  Smaller trees, and broken branches were more common.  
Entirely unseen is the mass transport of sand and gravel within such flows.   On the Goyt, the main tributary of the Mersey you can sometimes see, post flood,  whole areas, some dozens if not hundreds of yards long, ten or more metres wide, that have been covered by new sandbanks.   Whole areas of river change during a
"Sediment" Transported During a Flood"
flood.  One weir pool, some 30 metres square, 6 or 7 feet deep, developed a gravel island during the course of one flood, an island that occupied much of its previous surface area, I can only guess at how many tons of gravel had been deposited, and indeed, how many more tons were swept on downstream. The gravel deposition in each flood is very different, and depends not only upon the maximum flow achieved, and the "start condition" of the river, but also how long it takes for the river to rise and fall.   As each depth/time graph is different, so will the final outcome be different.  Think how hard it can be to walk against a strong wind, and then imagine trying to walk against 6 feet of fast flowing water, laden with sand and gravel. The pressure of water in heavy water conditions is such that some very large rocks can be moved away downstream.  Prediction of gravel deposition is very difficult, but it does not stop idiots building small scale Hydro-electric schemes.

Up at New Mills on the Goyt lives the Torrs Hydro-electric scheme.  A flagship project, and one that in its 4 or 5 years of operation has yet to make any profit.  But it is green, and allows people and companies to square back their shoulders and claim they are doing so much for the environment.  In reality the negative effects of such small scale hydro schemes far outweigh any advantages.  This is a lose/lose situation. Only the contractors and directors profit.  Of particular note is that the inlets to the Archimedes screws can get massively blocked by sediment transport during major floods.  In 2011 the Torrs scheme was inoperative for several months, from mid February until mid September, the inlet being completely blocked by silt and stone, and requiring extensive and expensive work to re-instate the flow.
Other post flood effects can be seen in the trees and other vegetation that line the banks. The local rivers have CSOs, combined sewage outflows.   Under flood conditions the rainwater mixes with the sewage, and the volume becomes such that the sewage farms cannot cope.   They just let it all straight through, raw sewage mixed with heavy rainwater run off from field, roads and roofs.  After the flood recedes, the highwater mark can be seen in the trees, marked by sanitary towels and other rubbish caught up in the branches.  Most unpleasant, but unless the money to deal with it becomes available, it will be a long time before this one is resolved.  Some river areas choose, or have chosen in the past, to dredge areas in which the river is prone to flooding.  This may well alleviate the flooding of properties nearby, but in reality it channels the water downstream far quicker, and there the floods will become worse.  It merely shifts the problem downstream.  Unfortunately such canalisation, confining the river between uniform high banks destroys the diversity of the river itself.  The uniform, evenly flowing channel is able to hold far less life, less in numbers, less variation of species.   

But what of the fish ( and other water living invertebrates)? How do they cope with a flood?  There has been very little actual research done on this. Understandable, due to the extreme conditions under which the observations would have to be made.  Most research has centred on how many tagged fish remain once river levels subside.  Much else is speculation, and that has to apply to what I will now write.

But the first thing to remember is that floods are not all bad news.  The scouring action, and the disturbance effect on the gravel means that large areas of stones will be thoroughly cleansed, very important to some of our rivers that carry input from sewage farms.  Such sewage outlets, added to sunshine and shallow water encourage the growth of some quite horrible brown algae. It coats the gravel, and often even your fishing line. To have a flood occasionally blast it all out to sea is very good for the river. Species that spawn in gravel benefit greatly from the cleansing of the gravel, which opens up the spaces between individual stones and allows ova and fry to shelter safely, and to be away from contaminants.  But a heavy flood at the wrong time of year can effectively wipe out all those ova and fry, resulting in very low recruitment to some year classes of fish. The flood washes away the eggs and tiny fish, in exactly the same way it cleans the rocks of algae. In the UK we are probably lucky, in that our floods and the dates that they occur, vary wildly year on year.  The young and eggs of coarse fish and winter spawning game fish will be differently affected by any particular flood, with some species possibly missing the negative effects that might impact another species. 

Have a look in the grass at the edge of the floodwater when a river is in spate.  Maybe you could use a child's fishing net.   At, or near the high water mark, you will almost certainly find lots of small fish, an inch or so long.   They have ridden up the edge of the river as the water rises.   The grass provides them with large enough areas of slacker water in which to avoid the full force of the flood.  In these areas they seem to feed as if nothing is different.  Floods seem to have absolutely no effect on minnow numbers. And so it is with the juveniles of other species.
Many larger fish will lie very close in to the banks of the river.  The edge effect will always result in a narrow band of  slower water, and by hugging what would normally be grass covered banks, some fish will be able to swim or take refuge in far less violent flows.     Most rivers will also have many natural eddies, backwaters, and areas where smaller tributaries enter the main streams.    All these provide slower flows.   Even a large rock on the river bed will provide areas of slacker water above and below it.  Such rocks might even carve out local depressions in the river bed.      There are always slacker areas at any point on the river, no matter what the height of the river.  Whether any specific slack is slack enough to hold fish in a flood, is another matter entirely.
As the river rises higher, bankside trees will be subsumed by the flow.   As they gradually submerge, so they create areas of slack water in their wake.    More refuges for the fish.   As the river rises further still, it might even break its banks and overflow.    Occasionally fish will venture out, possibly intentionally, perhaps not, into flooded fields, where they will be assured of a quieter life, if briefly.   For when the water recedes they will then have to find their way back into the main river.   Sometimes they may not make it, as this BBC link from May 2012 demonstrates. 

There are species that seem to be totally at home under flood conditions.   Ignoring the obvious: the salmon, fish like barbel and the humble bullhead are brilliantly adapted to live in fast flows.    Watch a mixed shoal of chub and barbel in clear fast water.  The chub are constantly working at maintaining their position.   Many of
Large Pectoral Fins Help a Barbel Hold Position Effortlessly
the barbel will be wholly stationery, scarcely flicking a fin.     Their body and fin shapes enable them to appear  glued to the river bed.   Such fish are far better able to cope with, if not actually to ignore a major flood, and may even be encouraged to feed more readily on anglers' baits.   Floods wash in food from the land, and disturb other food items that might have been hidden in the gravel, unavailable to the fish.  Any fish that can take advantage of such freebies will benefit.
But there are dangers for any fish that remains active in the floods, or is unable to find an adequate sheltered spot.  Shortly after heavy floods have receded, I have caught fish, usually larger individuals, that show signs of having been battered by the heavy water.  For instance, pike with red marks on their sides.    Visibility is restricted for fish in flood conditions, their eyes become useless to them,  unfortunate; for it is under those conditions that the most debris is being pushed downstream.   I believe the red marks to be where some object, a tree branch, a rock or perhaps just as often these days, a shopping trolley, has hit them as it passed by.   And the marks show.  A fish in the wrong lie in a flood is somewhat akin to being in one of those computer games where you have to dodge objects coming rapidly towards you.  Except that the fish are playing it blindfolded.
Fish Are Surviving in This:  Photo by Len Emory

But we can conclude one thing from all of this, a conclusion that has to surprise all those that have seen a river at its magnificent raging best.   And that conclusion is that the fish are well able to cope with such conditions.   Because, quite simply, after the waters have receded, most of the fish will still be there.

P.S.   I was reading "Mary Barton" by Elizabeth Gaskell this week.   A book published way back in 1848, a tale about Manchester life.  I never expected to find it containing an interesting fish fact, but it does.   Remember the Exocet anti-ship missiles that the French sold to the Argentinians?  And which they used to sink HMS Sheffield during the Falklands war?     Well, whilst reading it suddenly clicked with me that the missile is named after a fish.    The book tells of a dried fish brought back by a sailor, a flying fish, and lists its Linnaean name as being one of the Exocetus,  although the name seems to be more commonly spelled as Exocoetus.  Apparently there are about 70 species of flying fish, some of which are called  Hirundichthys,  which looks as it it might translate into something like swallow fish. ( Hirundinidae are  birds: swallows and martins, and anything itchy seems to be a fish).  Anyway, when the question next comes up in Trivial Pursuits,  or the Weakest Link,  I am now well prepared.  

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