THE FIELD, April 1957
POPULATIONS IN A TROUT STREAM
By
Frank Sawyer
On the upper Avon, in Wiltshire, the state of the river bed has been shown to be the decisive factor in successful natural regeneration
One recent evening, having nothing much to occupy my time, it occurred to me that it would be interesting to work out some figures concerning trout population and production in the six miles of the upper Avon, in Wiltshire, which are in my charge. The outcome was surprising.
In a previous article in The Field (“Trout in a Hard-fished River,” February 7th) I described briefly the fry-stocking policy we adopted on the water in 1930, and how the fishery had depended on this method for its annual stock of trout. What I did not say was that we started this fry-stocking when it was discovered that there was little regeneration from the wild fish. Indeed, the natural trout production was so poor that some provision had to be made to ensure sport in the future. I carried out considerable research at that time and continued to do so in the years which followed. Time and again I proved that practically all the efforts of the trout to reproduce themselves in nature were wasted.
There were seasons when I estimated that 1,000 pairs of fish spawned and yet not more than 1 per cent. of the eggs hatched. In terms of ova and hatched fish this meant that of a million ova shed (allowing 1,000 for each hen) only 200 produced fish that survived the four years it takes for a trout to become of sporting size. My calculations were as follows: that of the 10,000 alevins produced, 50 per cent., were lost by the time the early-fry stage was reached; that of the balance of 5,000 fry, 75 per cent were lost by the time the yearling stage was reached; that of the balance of 1,250 yearlings, 50 per cent. were lost; that of 625 fish surviving as two-year-olds, 50 per cent were lost; and that of the 312 surviving as three-year-olds, 33 per cent. were lost, giving a balance of 208 four-year-olds.
That may seem surprising, but it will be seen that the greatest loss occurred while the eggs were still in the redds. Actually the eggs were fertilised, the mortality occurring after the first three weeks of incubation. I arrived at the figure of 99 per cent loss after thoroughly examining many redds in different parts of the fishery, and came to the conclusion that the cause was lack of oxygen. The losses afterwards, though high, are not so important, as will be seen later. The fact is that, had we been obliged to rely on naturally bred fish for sport, we would have been very lucky to get more than 100 per season – from six miles of water – so we were obliged to resort to artificial means. The post-fry losses were due, of course, mainly to predators.
The comparative calculations for hatchery-bred fish-figures based on recent observations – are as follows: that of 100,000 ova, 95 per cent. hatched; that of 95,000 alevins, 5 percent were lost; that of 90,250 fry, none was lost during its early stages, but 75 percent were lost in the fry to yearling stages; that of 22,562 yearlings, 50 percent were lost; that of 11,281 two-year-olds, 20 percent were lost, giving a final stock of 8,118 four-year-olds.
These figures are, of course, only approximate, but they are, as I have said, based on recent observations. The losses through predators are plainly much less now than when I made my first calculations, but the total of more than 8,000 four-year-old trout from 100,000 eggs proves that previous heavy losses were due to bad hatching. Now the hatch is 95 percent, and losses which follow still leave a respectable total.
But the total, though it is a lot of trout, is insignificant when compared to what could happen if nature had her chance. When river conditions are satisfactory, the natural hatch could be at least 90 per cent. In that case, assuming as before that the eggs shed by 1,000 female trout total a million, our calculations would show: that 900,000 fry would hatch; that 50 percent would be lost in the alevin/early-fry stages; that of the 45,000 fry surviving, 75 percent would be lost by the time the yearling stage was reached; that of 112,500 yearlings, 50 percent would be lost; that of 56,250 two-year-olds, 20 percent would be lost; and that of 45,000 three-year-olds, 10 percent would be lost. So the actual production of four-year-old trout would amount to the remarkable figure of 40,500.
The calculations are sufficient to prove conclusively that the real answer to trout production lies not in artificial hatching and fry-stocking, but in making the river do the work as intended by nature. That such a thing can happen has been proved here on the Avon. My calculations are based on facts. Now, instead of the 1 percent hatch we had from 1930 to 1953, we are getting at least 90 per cent. The time has come when there is no need for us to use the hatchery to maintain a stock of trout.
This successful hatching is due to the work we have carried out during the past three years. It has proved beyond question that the answer to the problem of natural trout production lies in having a clean and porous river bed, thus providing conditions whereby the eggs can have a continuous and plentiful supply of oxygen. In the past three years our six miles of the Avon have had a complete change. In many respects the river bed is now quite different from what it was. And the new environment has been accepted and approved by the trout.
The importance of an oxygenated water supply to trout eggs has yet to be understood fully. Indeed, I feel it is the solution to a problem which has puzzled me, and no doubt many others, for a considerable time. Hitherto I had thought that trout eggs received their aerated water supplies through the action of the running stream. By this I mean that oxygen was forced into the stream bed by pressure exerted by the various currents. I had assumed that the reason trout cut holes and throw up heaps of gravel was to ensure an anchorage for their eggs and to create a baffle which would cause currents to be driven to the bottom of the redd. Now I have a good reason to think otherwise. My recent observations have proved that where trout eggs hatch successfully, the water supply to the redd comes not from above, but from below.
When trout make holes in a redd, they do so not with any idea of providing a nest and an anchorage, but simply because they want to cut through the hard upper crust of the river bed to try to find an upward water supply. If they succeed in doing this, their eggs are then deposited in water which is perfectly pure and which is springing into the river course from the surrounding water table. With it comes a sufficient supply of oxygen. This may be difficult to understand, so here are the facts and the explanation as I see it.
A natural river course runs in the lowest part of a valley, but it is in fact only the visible part of a huge table of water which extends on either side. Water drains from the high lands to pass beneath the soil in the meadows, and under pressure it should enter the river course through a porous bed. If for any reason the bed of the stream has become solidified, as was the case with the upper Avon, any percolation from the water table is from the sides of the water course, from, in fact, beneath the banks where there is no access to spawning trout. I do not wish to create the impression that water gushes from the river bed in any great volume, but it does rise under pressure sufficient to counteract the pressure of the water travelling in the channel and to cause it to mingle with it, provided the natural course has not been obstructed by human agency.
This brings us to the matter of dams. Water can spring through a porous bed only if the pressure of water travelling over it is less than the pressure in the water table at either side. So the impounding of a stream prevents percolation by increasing pressure on the bed.
In our work on the upper Avon we have used bulldozers and drag-line excavators on an extensive scale, and that we have found the answer to successful trout production is due more to luck than good planning. In trying to clear the fishery of vast accumulations of mud and silt, we scraped clean and broke up all our shallows, with the result that the water from the valley table is entering the course in many new places. In some reaches the level has been reduced considerably, so that now the river is running in the water table instead of above it.
The spawning trout are now placing their eggs where they have a supply of pure water, and this regardless of the condition of the water in the river channel. Instead of the redds becoming clogged with mud and silt forced into them by stream action, the upward currents from the bed are quite sufficient to keep all undesirable spoil, of which the upper Avon has a considerable amount, in suspension, and the eggs remain clean throughout the whole period of incubation instead of being slowly suffocated and dying after three or four weeks.
Trout and Salmon, February, 1958
Observations on Spawning Trout
by
Frank Sawyer
Until one has had the opportunity to study spawning trout during at least a score of seasons, I feel it is not possible to come to any definite conclusions about all their activities.
More than forty years have passed since I first became interested, and still there is much I would like to discover. Seasons vary considerably, so much so that it is very seldom we have two years following one another closely when natural conditions are identical. However, regardless of the prevailing conditions, each season a certain number of trout will make an attempt to spawn, and of all the observations I have made, one point stands out very clearly. Each spawning season trout choose the same localities, and this I feel is a point worthy of much consideration.
With very few exceptions, to my certain knowledge, the trout in the upper Avon Valley today are using the exact places in the river and in the side-streams they used over forty years ago. Many generations of trout have been in this upper Avon fishery since those days of long ago, and many changes have taken place throughout the course of the stream. Yet the spawning locality is the same. Trout continue to spawn where I saw them as a boy, where my father saw them when he was young, and where my grandfather spent many an interesting hour when he should have been at school.
Why should trout continue to be so discriminating in the selection of spawning sites? There must be an answer, as there is to all the puzzles set by Nature. Many times I have watched pairs of trout moving up a stream to spawn when their movements have been very easy to discern. It is the hen who leads, and it is she who selects the spawning site. I have known pairs to travel along a stream for half a mile or more, passing continually over gravel which, to all outward appearance, is no different in size or in cleanliness to that where they decide to make their redds. But it has become very obvious to me that the hen fish makes a search as she journeys upstream, a search in which the cock fish takes little interest.
Cutting a redd
Pauses are made here and there, when the hen fish will lie perfectly still with her pectoral fins spread out flat upon the gravel. On occasions she may start cutting as though to commence a redd, and then move on. She may pause to feel or to cut as many as a dozen times, and then at last find a location to her satisfaction. It is obvious that instinct prompts her to search for and to find some certain things. What is it? I feel quite satisfied in my own mind that this certain thing is only present in widely separated parts of the river bed. Though necessary, clean well scoured gravel is not enough.
In the South Country streams trout spawning takes place from about the end of November until the end of February. Some seasons may be early, others late, but generally speaking, the spawning is determined by weather conditions. If we have a wet autumn and early winter, the fish will be on the redds during the former part of the main season. If rains are late, then the spawning season is late too. Trout spawning coincides with the results of wet weather, indeed with a general rising of the water tables in the valley, and here I think is the answer to my question.
With the rising of the water tables so more and more water enters the river course. Much of this comes to the course through the bed, a gradual seepage brought about by the pressure of the land at either side. But only in certain places is the river bed sufficiently porous to allow this seepage to enter. A river may run for a considerable distance where a false bottom of gravel covers a very deep formation of clay, or other watertight substance. Here no water can possibly rise into the river channel. To the casual observer, the crust of gravel is clean and of ideal size for trout spawning, yet no fish are attracted to it. But here and there along the river channel are formations of gravel which go down to a very great depth, deep enough indeed to reach well down into the water table in the valley.
These are the places used by the trout. It is not possible to pack gravel so tightly that it becomes impervious to liquid. The water when under pressure oozes up through these formations wherever the actual crust of the river bed is sufficiently loose and porous. With fins outspread, flat upon the gravel, she is feeling for this upward seepage from beneath the river bed. When she cuts the crust it is but to ascertain if her suspicions are correct, whether in fact, there is sufficient upward current for her purpose. If not, she tries elsewhere until satisfied.
However, these observations may cause certain thought in other directions. Trout spawn in various classes of rivers, and often with good results. There are the surface fed types of river where water runs in a channel cut through solid rock or other similar hard and watertight substance. Here there are no water tables at either side and therefore water cannot enter the river channel in the way I have described in South Country rivers. Yet in these types of streams, seepage of water from beneath the bed still happens, but in a different manner. In these rivers the water is forced into the bed at one point to emerge at another.
Here and there down the courses of these rocky streams are pools, and downstream of each are vast accumulations of shoal of small size washed out and deposited by the action of the water. Actually, these shoals act as dams to hold water back in the pools. The result is that considerable water pressure is brought to bear at the upstream end of the shoal and these shoals are porous. Water forced into the formation at the tail of the pool emerges again at some distance downstream, where the water thins after passing over and through to a lower level. Invariably you find trout spawning from the middle to the tail end of these shoal formations.
Locations which are similar occur in many of the South Country streams where the river has been obstructed with weirs or controls. These we call hatch or weir pools. The gravel scoured out from the pool forms a kind of lip at the tail, with the result that the water has to lift to pass over and on down the channel. Here water pressure is built up in a manner similar to the rocky types of streams. Water is forced into the loose gravel to emerge towards the tail end of the shallow. Sometimes trout will spawn on these artificial shallows, but invariably at the tail end.
Successful trout spawning and generation is what we all try to obtain in our fisheries, and I hope the brief explanations I have been able to offer may lead thoughts in the right direction.
