SHOOTING TIMES & COUNTRY MAGAZINE – MARCH 2, 1974
BACKGROUND TO SPORT BY FRANK SAWYER
Hatchery Experiment
You can never be sure about anything where Nature is concerned and though you may plan to do this or that, often such plans are upset. Such has been the case this year with our experiment in stripping the big brood brown trout in our nurseries. Everything went well with the taking and fertilising of the eggs but we had to improvise the incubation. For the first time for 40 years the springs which supply the water for our hatchery failed and the whole unit became completely dry. We had to resort to another means of accommodating the big batches of eggs. Instead of laying these down under cover in the clear running water of the hatchery, we had to make a series of rafts into which our hatching trays could fit and float, and then put these into one of our open stew ponds, fill the trays with eggs and hope for the best.
It was not an ideal start to an experiment for it meant that the water temperature, instead of holding a steady reading of around 45 degrees F as it normally does in the hatchery, fluctuated considerably, from days when it was only just above freezing point, to others when it reached 48 degrees. Variable temperatures such as these are not good for trout eggs and I was not at all surprised to find, after a month or so, that we had far more than our usual average of casualties. As well as the variable temperature we had dirty water to contend with as our stew ponds are fed directly from the main river which, after such a long period of drought, coloured badly after every heavy storm or rainy day.
Extraction of dead eggs is always a tedious and back-aching job even in our hatchery, when one can get around the troughs easily, but here in the stew pond it meant wading into a foot of water to get alongside the rafts. This was no pleasant task on a wet, or cold frosty morning.
In the cold water the development of the embryos was slow – so slow in fact that at one time I began to wonder if any of the eggs had lasted through the first week. Then about five weeks after stripping, I was able to assure Alan, our keeper, that his daily and patient work in extracting dead eggs was almost over. Eyes were then starting to show plainly. Normally the full period of incubation in our hatchery has been between 50 and 60 days but I knew that these floating batches would take much longer – and so it proved.
The heavy rains about Christmas-time and in early January were enough to start the springs running and in due course the reservoir for the hatchery filled to a level where it could feed the troughs. I was more than pleased to see this, and the water supply came at a time when the eggs had reached a stage when they could be moved in safety. We chose a morning when air temperature and the temperature of hatchery and stew pond corresponded, and then transferred all our trays of eggs. It was a relief to get these under cover and in the clear and running water where I knew there would be ample oxygen when the alevins appeared. Here there would be no danger of frost and no further trouble with flood water.
As though to make up for lost time, the embryos developed rapidly, but it was exactly 70 days before the first eggs began to hatch and life began to show in the trays. It was encouraging. Despite the adversities our total losses had not exceeded ten per cent and the hatched alevins appeared to be very agile and healthy. One thing had been proved: the slower development during the early stage of incubation had not weakened the end product. These alevins were quite as good as any I had previously produced.
If you go on in the same way year after year you learn nothing. Crude though they were, our floating rafts did serve their purpose, and we know now that if ever again there was such a necessity, we could revert to the same principle with a certain amount of confidence. Perhaps the results we obtained with the improved incubation layout were in some way due to the big eggs, and the healthy condition of them at the time of stripping, but of this we can never be certain.
A ten per cent loss from 50,000 might seem a lot but it is far below anything which happens naturally, for in most localities a ten per cent survival rate is extremely high. Further losses take place in Nature even after the alevin stage has passed successfully, for the most vulnerable period is just when the little trout are in need of their first food. And so, taking all the stages through to the time when a young trout is well on the feed, the losses from the time of egg-laying can be enormous. Were it not for this high mortality there would be no need to resort to artificial methods.
As it is, we bridge a very high danger period during the incubation and alevin stages and, by artificially feeding the young fish at the time of their greatest need, the number which are reared can be at least 50 per cent more than would occur in natural conditions.
By the time this is read, the great majority of our fry will have been planted out into the shallows of the main river and adjacent side-streams where they will have to rely entirely on Nature for a food supply. But we are keeping some 10,000 to feed artificially in the hatchery troughs and then, after a month or so, these will be transferred to the nurseries where they will remain until yearlings. We have had big parent fish, big eggs and alevins. The fry are healthy and strong. It remains to be seen if the yearlings will also be larger than the average we have had through the years.
