Water Quality & Egg Development

Although water quality is slowly improving in our rivers ,it is a sad fact that the majority still fall a long way below the European Water Framework Directive standards. We are all aware of excess nutrients often causing algae bloom and the over abstraction ( its not all down to less rainfall ,but obviously this doesn’t help ) causing low summer flows and sedimentation drop and now the need to pump some of this water back into the river in order to avoid a public outcry at the disappearing river. A somewhat artificial river some cynics might argue

HOWEVER ,perhaps what is surprising is the results of recent experiments carried out using TAP WATER. This does nothing to excuse the above but is an interesting development in experiments recently carried out

Eggs from both the Blue Winged Olive and also the Mayfly have been kept in tap water as well as river water while in Diapause and during that state do not seem to have suffered from all the additives to the former. Once hatched the nymphs will not survive in it. It is not yet established if all tap water is safe.

Dr. Cyril Bennett has pioneered the use of tap water during his experiments with both sets of eggs and ongoing trials are taking place to establish if all tap water from the various water companies is unharmfull to the eggs. This is an important development in these tests as it shows just how impervious and resilient the egg cases are in protecting their contents

Below are photographs (courtesy of C.E. Blake ) of Mayfly eggs being successfully hatched in tap water with the plainly visible nymph inside. These eggs are only 25 days old and indicates just how short the Diapause is for this particular egg. The second shows a successful newly hatched nymph in the unnatural environment of tap water. This is significant for anyone wishing to conduct supplementation programmes as it means that visits to the river are now unnecessary in order to obtain water for top ups or change on the long time span spent in the hatching tank for the slower maturing eggs The Mayfly diapause is considerably shorter than that of the BWO and this could have a major effect on the eggs laid down in a river


BWO eggs would normally adhere to stones or rocks on the river bed after being deposited by the Sherry spinner and remain there for the long Diapause over the winter months. However,to those involve in these experiments ,it soon becomes apparent there is a lack of the necessary adhesion on the outside of the egg and these are easily washed away to be deposited on the river bed , in too many cases now ,a bed of silt and could become deeply buried in it—-not their normal habitat and maybe for a long period with disastrous results

Also of concern is that over this period of decline ,has the sherry spinner lost the capability of producing adhesive eggs ,perhaps due to some genetic alteration through poor water quality ?

Mayfly nymphs are much better adapted to live in silt than are BWO”S , this of course being their normal habitat ,and a good reason why the Mayfly thrives and is increasing in many rivers while the BWO is significantly reducing

On any such BWO programme and on past results conducted on different rivers, it seems necessary to keep the eggs out of the river and its silt and in clean water ( now possibly tap water ) until they are about to hatch , when they can then be introduced into the river , in a silt free area ( an area which would have been their normal habitat several years ago )———hopefully to survive

Until we achieve a standard of water quality which the European Framework Directive sets out ,it is difficult to see how the BWO will naturally ,if ever, return to its former glory