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"Gravel under their bellies"

Our July 2002 contribution from Terry Lawton

"Wild brownies like gravel under their bellies - they don't like silt," said Ron Holloway during a demonstration of using a water jet to clean potential spawning gravels. Although the spawning season is over, in the northern hemisphere at least, now that, for many, the new season is underway, it is a good time to think about helping wild trout when enthusiasm is high. Clubs or owners of rivers and streams where trout spawn, can do much to help them this coming autumn, writes Terry Lawton.

"Wild brownies like gravel under their bellies - they don't like silt," said Ron Holloway when I talked to him during a demonstration of using a water jet to clean potential spawning gravels on my local river. All though the demonstration was on a lowland stream in England, the technique of water jetting spawning gravels will be applicable to many (lowland) streams and rivers where gravels are infiltrated by silts and fines and are suffering from lime concretion as well. The object of the demonstration was to show members of the resident club, and members of other local fishing clubs, how to select and clean suitable areas of the river bed.

Ron Holloway gets the demonstration underway

Brown trout need loose clean gravels so that they can cut the redds in which they lay their eggs and the gravels needs to be clean so that water can flow through them to incubate the eggs. Excess silt in the gravel is likely to suffocate the eggs so they die before they can hatch.

Rivers have a surprising ability to heal themselves from the damage done by man - if the causes of the damage can be removed or reduced. Improving the spawning habitat will encourage fish to spawn. Ron Holloway says that if 50 pairs spawn and if just 10 pairs spawn successfully, then those fish will be well on the way to producing a self-sustaining population of wild trout.

Ron Holloway is insistent that any fishing club that wants to water jet spawning gravels does not try to do too much. He says that it is very much better to clean a few areas thoroughly - typically 1m wide by up to 15m in length - rather than try to water jet a long stretch and not do it very well. It is also very important that the work is done BEFORE fish start spawning as the work releases large quantities of silt which, if allowed to get into existing redds will do more harm than good. Obviously, the closer to spawning time that the work is done, the better because the gravels will then be at their cleanest. (For anyone (in the UK) contemplating washing gravels in areas where the Environment Agency holds sway, should be aware that the release of silts that results is classified as a pollution so permission much be sought from the EA. Also due consideration should be given to the impact on fish habitat downstream.)

Crude but effective. Tipping a load of gravel into the river.

The equipment

Water jetting involves directing a jet of water into the river bed which forces the silts and fines out of the gravel and into suspension in the water, so that they are washed away downstream. To do this you need a small, portable pump with a suitable length of inlet pipe and a longer outlet hose. The river-end of the outlet hose is fitted with a length of galvanised water pipe, with the open end flattened to produce a jet. The pump needs to have a delivery rate of about 250l per minute. Care must be taken to position the inlet pipe over an area of clean gravel so that it does not get choked by weed etc. As soon as it starts to get choked, pressure drops and the jetting is ineffective. If the angle or height of the river bank makes it difficult to position the pump, or will need a very long inlet pipe, than the pump can be put in a small rowing boat or punt which will mean that it can be moved about easily.

To do the work effectively you will need two people in the river, one with the water jet and the other with a chrome, and at least two people to replenish the gravel. (See below.) As tonnes of gravel may be required, suitable transport will be needed - a farm tractor and trailer - and some form a shute or other means to get the gravel from the trailer into the river.

Distributing the gravel with shovels was effective but hard work

The technique

You must work downstream so that the silt is washed away from where you are working so that you can see what is happening. The metal pipe can be forced into the river bed for a depth of up to about 300mm (12 inches) to help loosen compacted areas. After a while you will develop an effective technique combining jetting with "digging/levering".

It is important to make sure that you are not washing or blowing the gravel sideways, away from the strip that you are working on. It can be helpful to have an assistant with a chrome or large rake to help push gravels back into the strip.

Start at the upstream end of the area of gravel to be cleaned and create a trench of the required depth and width. Then, by jetting the downstream side of the trench, the gravel will be blown upstream into the trench and the silts washed away. It can take a considerable time to remove the silt that may have built up over many decades. Keep working from side to side, blowing the gravel back upstream and the sides of the gravel patch back into the middle.

If there is a great deal of silt to be removed, it will result in a lowering of the river bed. This may not be desirable so it is important to have a sufficient quantity of suitable replacement gravel on site to replenish low areas as well as adding to the depth of washed and cleaned gravel.

It is important to choose areas with enough gravel to make the jetting worthwhile. If a good looking patch of gravel turns out not to be big enough - more sand and silt than gravel - to make the cleaning operation worthwhile, go and look for another patch. As Ron Holloway said at the beginning of this article it is better to clean a few areas really thoroughly, rather than trying to clean the whole river. The first year's progress may be very slow. But in subsequent years, cleaning the original patches will be relatively quick and the time taken to add new areas to extend the spawning area will be less than the first year, because everyone will know what they have to do. His final hope is that more fishing clubs, syndicates and riparian owners will feel encouraged to have a go at improving the spawning potential of their own fisheries or rivers and streams.

Our thanks go to Simon Johnson a fisheries officer with the Environment Agency for arranging the demonstrations and to Ron Holloway for demonstrating the best technique. For further information: Simon Johnson, simon.johnson@environment-agency-gov.uk , Ron Holloway, ron@ronholloway.freeserve.co.uk and the Wild Trout Trust, www.wildtrout.org

Terry Lawton is a passionate nymph fisherman who caught his biggest wild brown trout (in the UK) - 4lb 2oz - on a home-tied variant of a goldhead, Sawyer-style pheasant tail nymph.