Trout have been artificially stocked in chalk streams, or assisted in their reproduction, for many, many years. A desire for big ‘easy’ fish is one reason, but mainly it is the natural river’s inability to produce enough quality trout for the increasing number of anglers who fish the water. Other inabilities of the natural river have also been noted over the last decade or so. The chalk stream malaise is a widely reported phenomenon and studies have shown a decline in fly abundance over the last few decades of about two-thirds throughout many Wessex Rivers. But it goes further than this. Snails, a staple of chalk stream trout, seem to have been substantially reduced in number, and the crayfish plague decimated the native white clawed variety (another staple of wild fish) in the 1980s. Whatever the reasons, and many theories have been proposed, it would appear that some rivers, particularly chalk streams, are no longer able to naturally and regularly support the large numbers of fly, snail and crayfish that used to be common. This presents a serious problem. Every dry fly fisherman craves large hatches that induce surface feeding, and every keeper dreams of abundant natural food supplies to fatten up both the wild and stocked fish.
Artificial stocking, or assisted reproduction, of these important food sources is not an activity that is immediately associated with fisheries, but the idea is not new. Historically, flies have always been viewed with importance on chalk streams. Years ago it was routine in some areas for keepers to swap mayfly in order to introduce new genes onto each river. Frank Sawyer built a number of experimental fly hatcheries when he was river keeper on the Upper Avon and used paving stones and egg boards to assist the reproduction of certain species of fly. A few keepers still use egg boards today but generally this and other historical techniques have been abandoned on the majority of rivers.
Perhaps the first attempt at assisting the reproduction of fly was the use of egg boards. Egg boards (an idea originally conceived by William Lunn) were traditionally used on many rivers throughout the country in order to encourage egg laying flies to enter the water and lay their eggs on boards rather than natural or permanent river features. These boards were then transported to stews and hatcheries to provide food, in the form of the hatching larvae, for young trout. The following is an extract from Sawyer’s report on the 1936 season:
“We also have in hand the further construction of fly boards which, next season will be moored near hatches and bridges, and, when covered with eggs, moved down to the weed-clad shallows.
We have found that the wooden hatches are thickly covered with eggs, also the stone work; so this indicates that the above policy should have a good chance of success.”
Sawyer found later on in his career that paving stones were easier to use in many locations, as they didn’t have to be secured. He also considered them to be, on occasion, more effective than egg boards. Paving stones were propped up with stakes in shallow water so that the bottom edge lay on the river bed while the top edge was just out of the water. This allowed egg laying spinners to land on the dry top edge and crawl down the paving stone to lay their eggs underwater, usually on the underside of the paving stone. The technique clearly worked as large numbers of eggs could be found on all the paving stones. It is less clear whether this method resulted in more nymphs and flies, or whether the flies just laid their eggs on the paving stones instead of somewhere else. By the late 1940s, Sawyer himself was not at all convinced that this method increased the number of flies. A point noted by Martin Mosely (nephew of F M Halford) of the Entomology Department of the British Museum in a letter to Frank Sawyer in September 1944.
“I also advocated the use of hatching boards for the transference of eggs from one water to another. I consider they do more harm than good when moored on the surface of the stream in which position they are supposed to increase the supply of Ephemeridae. Personally I consider that they decrease this supply and that the Ephemeridae would be better left alone. I wrote an article on the subject some years ago….in which I condemned the practice and suggested that the only use these boards served was to collect eggs for the purpose of stocking other streams. When you transfer your boards to your fry ponds, they should be moored on the bed of the stream, not on the surface and the boards should be placed eggs downwards with battens nailed to the boards to keep the eggs just clear of the bottom. Then the young nymphs will be able to find the shelter of the gravel without the risk of being carried down by the stream.”
Other methods were developed for assisting fly reproduction. The experimental development of fly hatcheries was an attempt to make entomological studies of river fly life easier. Sawyer’s original hatchery was a small box affair and is described in detail in ‘Nymphs and the Trout’. Many years later he built a much larger concrete hatchery in a secluded spot upstream from Choulston Bridge. Although the concrete hatchery produced a large number of flies and was a great success from an entomological point of view, it could not hope to match a healthy river for fly production. Before their disappearance, water meadows were an excellent form of large scale fly hatchery. Unfortunately they had been in decline for some time when Sawyer took over as river keeper in 1928, hence his experiments to try and reproduce some of their fly hatchery effects.
An indirect method of assisting fly reproduction was the old tradition of swapping mayfly with other keepers during the mayfly hatch. Frank Sawyer considered this fly swapping to be extremely important, although he was not the first to suggest that the artificial introduction of fresh genes helped to strengthen the various strains. Sawyer developed this fly swapping technique further by exchanging fly eggs. This enabled gene swapping to take place for other species of fly, notably the Baetis types with spinners that crawl underwater to lay their eggs. It was also easier to transport eggs on egg boards than live adult flies. For some reason the practise died out. Perhaps the abundance of fly life was not a cause for concern, or maybe subsequent keepers thought it unnecessary. More likely it was the development of artificial fish food for young fry that removed the need for transporting fly eggs to hatcheries and stews.
This gene transference is perhaps of more relevance today than it was in Sawyer’s days as a keeper. Before his death in 1980, Sawyer hypothesised that pesticides, roads and urban areas prevented flies from travelling between different rivers and other stretches of the same river. This in effect led to isolated populations of fly. Last year over 35,000 tonnes of pesticide were sprayed onto crops and fields. The chances of any fly crossing fields sprayed with pesticides are much reduced. Without the tradition of swapping flies and eggs there is little chance of fresh genes being introduced into the various fly populations. Isolated populations can lead to inbreeding and an exponential decline in numbers. Sterility, predominance of a single sex, deformities and many other abnormalities are all symptoms caused by inbreeding and are well documented in biological studies. Is it coincidence that some of these indicators appear to be present among many chalk stream fly species?
It would be very cheap and easy to reintroduce the tradition of swapping flies and eggs, although a degree of caution would have to be applied. Disease and parasites could easily be spread by adult flies or eggs. The last thing any fisherman wants is the decimation of the remaining fly life by foreign pathogens. Modern DNA sampling may be able to determine whether inbreeding is a problem or if any interchange of genes is occurring naturally between isolated populations. If the evidence for inbreeding was clear, then perhaps the most favourable option would be gene transference along traditional lines.
The ideal solution to the decline in fly abundance and snail populations would be a simple fast acting remedy. In 1965, at a river keeper’s seminar, Sawyer gave a brief overview of what he believed to be the most promising technique for increasing fly abundance and snail populations.
“We had plenty of fly last year when by all accounts the hatches were very poor on most rivers. Indeed the fly hatches only a few miles downstream on the Avon at Woodford were poor too. This was of special interest to me. On several occasions I left our water at Netheravon when good hatches of fly were on, to go to Woodford, it only takes about quarter of an hour by car, and then on arrival to find nothing doing. I have returned at once to find good hatches still on, and fish rising well. I am well aware there can be local appearances of insects on any river, and indeed on very short stretches, but the difference between these two fisheries was too consistent to be ignored. It is too early yet for me to make a positive statement. This I can say however, for the past two years our upper Avon water has had calcium (chalk) added to it in fair quantity and I feel this is going to be the answer where insect population is poor.”
The following is an extract from Sawyer’s handwritten field notes on his chalking experiments and the effect on snails.
“[Chalk] mixes quickly with water and a fall out occurs very rapidly even in fast water. Action on bed astonishing. Rapid cleaning of all algal matter in one week. Vast snail production in one month.”
Frank Sawyer went on to carry out a large amount of research into the addition of chalk to rivers and still waters. Some of his results were published in ‘Trout and Salmon’ and other journals. In France the experiments were repeated with similar results and published in ‘La Peche’ by André Gagniard. Experiments on the vast lakes and ponds of the North Landes were carried out by Paul Capdevielle in 1965. Again these correlated with Sawyer and Gagniard’s work. A company in France called Omya produced a product specifically for the chalking of rivers which was sold under the brand name Nautex. Omya produced accompanying technical notes describing how to best apply the product. These notes were based on Sawyer’s findings. Sydney Vines collated many of these notes and articles in ‘Man of the Riverside.’
No further large scale chalking work was carried out by Sawyer after his experiments due the expense and large amount of labour required. Five tons of chalk per kilometre was the dosage recommended by Omya for Nautex, with annual treatment in the spring. Sawyer did however retain a keen interest in the subject and corresponded with many authorities. John & E. Sturge Limited and Omya both provided Sawyer with scientific theories on why the chalking of rivers was so beneficial. The phenomena is certainly worth investigation and perhaps a repeat of the chalking carried out in the early 1960s could halt or reverse the devastating decline in fly abundance.
Whatever the cause of the chalk stream malaise, some sort of remedial action will be required. Pollution, urban expansion, human population growth, disease and climate change may all be contributory factors and these are unlikely to be reversed in anything less than a timescale based on many decades. A great deal of scientific work is required to produce a firm theory on the causes, but while we await a scientific theory, and a proposal for rectifying action, we could perhaps provide assistance to the existing fly populations by readopting some of the old techniques and chalking traditions.