B.W.O Supplementation Programme – Update



It would appear that  the habits of BWO”S are changing  , from what might have been expected some years ago.

The appearance of Sherry Spinners and consequent  egg stripping  began this year in  very early June , actually during the Mayfly season .

Very recently (late September) I had the opportunity to witness a large column of sherry spinners flying upstream ,many with attached egg balls, and later the same evening , a hatch of Duns take place.

I have also been informed by very good authority, that a similar occurrence took place in early October on a different Southern stream, where  eggs at that location, were also collected for the laboratory.

As the BWO has always been associated with mid summer evenings  (mainly July and August), for both  egg laying and hatching, it seems that this extended behavioural pattern could provide some indication of its changing life style   due to climate change.

How long these flights will continue into the Autumn is now being observed  and noted  and  how widespread it is on different Wessex rivers.

Possibly the flight of sherry spinners during the late evening,which once common, could be changing to include other times of the day (or night) when anglers are not  by the water to observe such activity.

Movements of this fly have  also been seen early in the morning.


Eggs laid down in the river from June onwards have suffered a depletion in numbers.

Initially thought to be Crayfish attack, which may have accounted for some of the losses, other reasons are now suspected and  investigations into the cause/causes ongoing.

A two year programme is the minimum  needed to establish the best and easiest method of success.

Unfortunately there are no manuals or reference works  to consult.

Some of the  remaining slides have now been transfered to a new board /cage which will stop any further losses by predation but may not protect against other causes .A few of the  slides have been returned to the laboratory after spending 3 or 4 months in the river, so that  comparisons can be made to those left instream.

Water temperature here , unlike the river ,can be varied.

A minority of the “Farmed Eggs” in the river have possibly hatched already but the majority are now in Diapause and will stay in that condition until next   Spring and rising river temperatures.

The water temperature is  now decreasing  significantly even with the combination of low level and “ below normal “ flow rates for September.

( Figures  just released by the E.A.  for the River Piddle)

At present ,the nymphs are becoming increasingly difficult to locate on the river bed.

The two photographs are of BWO eggs  and magnified 100 times

The first ( courtesy, C.Blake), shows eggs taken recently from one of the local project slides . They are in Diapause and appear in form as we might have expected at this stage

The second ( courtesy, Dr. C Bennett ), illustrates eggs taken from his slides and shows other stages of development

A  presentation regarding the  “Farming” of BWO eggs will probably take place at the next  RIVERFLY  PARTNERSHIP conference in London next March

Hopefully this will encourage further interest and development in this field and invite greater participation

The organisation ( RFP ) needs support to flourish and is crucial to the “well being”  of our rivers and their fly life

As most Fly fishermen know, particularly those  in Southern England, the latter is decreasing at an alarming rate, and any help to reverse this trend must be welcomed

Could we be entering a new era where the phrase “ captive breeding of endangered species “ is taking on a whole new meaning in world of fishing ???

Perhaps  in the next few years we will know

Rod Crane

October 28, 2010 by Filed under: B.W.O Supplementation Programme 

B.W.O Supplementation Programme

B.W.O Supplementation Programme

Introduction to the Programme

Any fisherman who has fished the evening rise over several years cannot have failed to notice the steady decline in Serratella lgnita (Blue Winged Olives) , particularly the column of “Sherry Spinners ” flying upstream. Concerned by this, several angler/entomologists have decided to see if anything can be done to temporarily assist nature There maybe one , but more likely a combination of several reasons for the present situation ranging from low water flows,rising temperatures, agriculture practices and several more.
Re introduction of Ephemeroptera Danica (Mayfly) has been successfully achieved at one or more sites but the decline in these cases was due to different causes than that of the BWO.
However the Mayfly and the BWO have similar egg laying habits, so at three sites in Southern England ( Surrey,Wiltshire and Dorset) attempts are being made to instigate a supplementation programme until hopefully conclusions have been reached enabling a halt to this steady decline.
Rivers in the North of the country seem to have suffered far less than those in the South This project involves identifying an egg laying site, catching the “Sherry spinner”, removing her eggs and attaching them to a slide under controlled conditions.
The slides ( in Dorset, holding approximately two thirds of a million eggs) are then placed in the river and left for nature to take her natural course . Fly “egg farming” is in its infancy with no guaranteed results of success

Why bother ??
It has been estimated that under normal egg laying conditions the failure rate is extremely high (as with most things in nature extremely high numbers are produced to compensate for very high natural losses) The aim of this project is to try and ensure that losses are kept to a minimum with hopefully a success rate being increased by eighty per cent plus.
Obviously it will be some time before any results become known What improvements can be made to BWO habitat ? A question which at present we are searching for. There are many things that are not understood regarding fly life. Habitat may be one of the reasons for its overall decline Other ” Grey areas” include diapause in egg development towards the end of the hatching period. This is a natural process and eggs lay in diapause during the winter months in cold water emerging from this condition in March Water temperature can have a dramatic effect on fly eggs from perhaps a small rise in temperature causing a faster development of the eggs but possibly a smaller percentage successfully developing, There appears to be only a small window in water temperature where maximum success occurs.
After hatching, natural phenomina such as ” Natural Nymph Drift” may come into the equation Do Baetis nymphs and Serratella nymphs behave in the same manner ? What determines this behaviour ?are there other factors to take into account such as day or night drifts , diseased nymphs etc. etc. the questions seem endless !

Dr. Cyril Bennett of the Riverfly Partnership has been extremely helpful with both information and advice concerning the the Dorset project now underway.My thanks go to him. Any person concerned with Flylife and water quality in our rivers should be very gratefull for the existance of such an organisation and I hope to be able to provide them with relevant and useful information gained during this experiment in return for their support




August 12, 2010 by Filed under: B.W.O Supplementation Programme