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