More about Triploids

The approved investigation

Diploid trout but triploids tend to be bigger
A diploid trout caught on the dry fly in September

As far as  can be ascertained, wild breeding does not take place within our stretch, therefore a decision was taken as far back as 1892 to breed and enhance with our own fish. This practice has continued ever since.

Under the new regulations, it is possible to “enhance” the number of trout by breeding them from  fish taken from the river and returning them to the same stretch, after reaching a suitable and sustainable size.

The Club applied to the  Environment Agency to enhance the river with our own reared diploid fish. We held discussions with the representatives of the Environment Agency and when we applied, we were granted a permit to carry out an ‘Investigation of the need for enhancement’, with research over a 5 year period and involving as many members as possible. We started in March 2015 and is ongoing with over 30 members now taking part. We believe that this investigation is the only one being undertaken in a Northern spate river. One key aim is to try and protect any wild fish breeding in the river itself by having our own fish available in sufficient numbers to give a good fishing experience. All caught fish (large and small) are being recorded and also whether the large fish were caught on dry or wet fly. All large fish are marked and can be taken. All unmarked fish have to be returned.

Having diploid trout of good quality helps members to enjoy good fishing which is another key aim.

The first three years results are promising……

Fly fishing at Slee Gill where triploids are not stockedDiploids and Triploids 

Not everyone is aware of the issue of diploid and triploid trout

All wild trout are genetically diploid. With trout breeding in the wild, the ova before being fertilised,  are very sensitive and contain three sets of genes. This extra gene (from the female parent) is expelled from the egg, leaving two sets  – hence a diploid fish.

However, if the new diploid egg is physically shocked e.g. an increase in temperature, physical shaking or an increase in environmental pressure during fertilisation, then the extra set of genes are not expelled and the fish will retain the three sets of chromosomes – hence a triploid fish.

Triploids are totally infertile and so cannot cross fertilise with the wild trout in the river.

However very little information is available on the long term behaviour of triploids. They are reported to grow large and to have larger internal organs than diploid. They are able to increase in weight more readily. In addition, evidence has shown that triploids rarely rise to the dry fly after becoming stabilised in the water. This would not give a very satisfying fishing experience. The Environment Agency were recommending that triploid fish should be added to the stretches of the river where wild fish can spawn, so that these wild fish could prosper.


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