EXPERIMENTS WITH FLY BOARDS
By
Nick Sawyer
Current concern over aquatic fly abundance on the southern chalkstreams and other UK rivers has led to a resurgence in discussion on fly boards and their merits as a river and fly management technique. The theory of fly boards assumes that spinners lay their eggs on the boards in preference to, or in addition to, natural egg laying sites. The boards supposedly provide protection from predators such as caddis, and can be easily monitored by river-keepers and therefore more ’positively’ managed than the natural laying sites.
William Lunn used wooden boards tethered to bridges and river banks, Frank Sawyer used paving stones propped up on metal stakes, and others have used floating planks in various forms. The main use of fly boards historically appears to have been the transfer of eggs for the purpose of restocking fly, as Frank Sawyer discusses in ‘Nymphs and the Trout’, second edition page 253-254:
“But there are many other things to be thought of, and one of them is the occasional need for restocking with fly as well as with fish……When the late William Lunn discovered certain flies would lay eggs on the underside of boards tethered in fast running water he did us all a great service, and I have been making use of his idea for some years to transfer fly eggs from one water to another……Throughout September…...I had a number of boards tethered at an egg-laying site near my home. Here hordes of olives were in the habit of creeping down into the water to lay their eggs on the concrete apron behind Choulston Hatches. By the time I was ready to transfer them the boards were all thickly covered with many millions of eggs.”
The use of fly boards to increase fly abundance, rather than for the purpose of restocking, actually received little support, as Martin E Mosely’s letter to Frank Sawyer on 27 Sep 1944 highlights:1
“I also advocated the use of ‘hatching boards’ for the transference of eggs from one water to another. I consider they do more harm than good when moored on the surface of the stream in which position they are supposed to increase the supply of Ephemeridae. Personally I consider that they decrease this supply and that the Ephemeridae would be better left alone. I wrote an article on the subject some years ago in the Fly Fishers Club Journal in which I condemned the practice and suggested that the only purpose these boards served was to collect eggs for the purpose of stocking other streams.”
Mosely’s views on the damage fly boards do to Ephemeridae supply are interesting as they relate directly to the issue of fly abundance rather than the use of fly boards for restocking. This is of course exactly the issue we are grappling with today. Fly abundance at the time of Mosely’s letter to Frank Sawyer was four times more than today according to the Millennium fly study by Peter Hayes. This observation has altered the context of fly boards somewhat and it may be that Mosely’s view is no longer valid. This leads to a requirement for three specific pieces of experimental information in order to allow sensible judgements on the use of fly boards:
* Do fly boards in the contemporary era still provide an attractive medium to aquatic flies for the laying of eggs?
* When given the choice between fly boards, paving stones or natural sites, which do flies prefer?
* Do fly boards indeed reduce fly abundance as Mosely suggests or do they in the modern age of poor fly actually increase abundance?
In order to answer the first two questions, an experiment was developed by the Services Dry Fly Fishing Association (SDFFA) to compare the various artificial egg laying sites against natural sites. The aims of the experiment were:
* To prove that paving stones and tethered planks can provide suitable artificial egg laying sites for aquatic flies.
* To identify which artificial egg laying site is preferred by aquatic flies.
* To determine whether aquatic flies preferred artificial egg laying sites over natural ones.
* To determine whether caddis or other predation occurred on artificial egg laying sites.
Methodology
The design of the experiment was simple. A shallow section of the Salisbury Avon downstream of Choulston Bridge at Netheravon was selected as the experimental site. A number of three-foot long metal stakes were hammered into the river bed until the tops were just under the surface. Paving stones that had been ‘seasoning’ in the river for some years were then propped up on the stakes so that the downstream edge was a few inches out of the water.2
Photo 1 – Frank Sawyer’s Original Paving Stone Fly Boards.
Seasoned wooden planks were tethered to the base of the metal stakes at two locations so that the upstream edge of the plank was a few inches off the riverbed while the free-floating downstream end was a few inches out of the river. Finally a seasoned plank was tethered to a stake and allowed to float freely while another control plank that had not been seasoned was allowed to do the same.
Photo 2 – Wooden Planks Tethered to the Bottom of a Metal Stake.
Photo 3 – Floating Plank Method.
The site also had a small weir and natural woody debris and these were used as the natural sites for comparison. Regular observations were made throughout the year to measure the amount of egg-laying activity that had taken place.
Results
The boards were positioned in early May 2006 and egg laying activity commenced within days. On average, 60-70% of the underside of the paving stones at any one time were covered with eggs, with egg coverage in places approaching several millimetres thick by Autumn. Species type was overwhelmingly Baetis. Olives in the process of laying were often observed. There were two other distinct egg types but these could not be attributed to a species. There were also several hundred nymphs habiting the underside of the paving stones in the spring but numbers tailed off into summer as weed growth increased. The resident nymphs on the paving stones ranged in size from the minute to full-grown upwing nymphs. The average number of caddis cases seen on the paving stones at any one time was five. Live caddis were very rarely seen with the maximum at any one time being two. Other life on the paving stones included snails (3-4 per paving stone maximum). No weed or debris caught on the paving stones and the paving stones were not really affected by fluctuating water levels, although on one occasion in the autumn they were completely submerged.
Photo 4 – Paving Stone Showing the Results of Spring Egg Laying Activity.
On average, about 20-30% of the tethered planks were covered with eggs at any one time, although this increased to over 50% in late summer. Again eggs were predominately of the Baetis species. Like the paving stones, a large number of nymphs (300-400) were seen habiting the underside of the plank in spring and these ranged from the very small to full grown upwing nymphs. There were never any caddis present. No weed or debris caught on the planks due to them being positioned in the ‘shadow’ of the paving stones (the tethered planks shared a metal stake with the paving stones).
Photo 5 – Tethered Plank Showing the Results of Spring Egg Laying Activity.
Only one floating seasoned plank was used during the experiment. This was covered on its underside with Baetis eggs at around 40-50 % coverage by late summer, but in spring and early summer rarely exceeded 20% coverage. A similar nymph population to the bottom-tethered planks and paving stones was observed. No caddis were observed at any time. The floating plank caught a lot of weed and debris on the exposed stake and had to be cleared regularly.
Photo 6 – Floating Seasoned Plank Showing the Results of Spring Egg Laying Activity.
Egg coverage on the unseasoned plank was less than 5% throughout the year. A small number of nymphs were observed (<20) and these remained constant throughout the year. No caddis were observed at any time. You will note from the photo that this plank was a lot smoother than the seasoned ones and this may have affected the experimental results.
Photo 7 – Floating Unseasoned Plank Showing the Results of Spring Egg Laying Activity.
Three pieces of woody debris, two rocks and an old fence post trapped against the river bank were observed to provide a comparison with the fly boards. They all had eggs but only the woody debris had any coverage approaching the level seen on the fly boards (around 10-15% of available submerged surface area until late summer when it increased to 80%). There were a large number of caddis cases on all the natural items and the rocks were well covered in caddis cases. Shrimps and nymphs were present on all the natural items but nymph numbers were only a fraction (around 20%) of those seen on the paving stones.
Photo 8 – Natural Woody Debris Showing the Extent of Natural Egg Laying Activity.
Conclusions
The conclusions from the experiments are clear. Fly boards in the modern era are attractive to certain species of aquatic fly (perhaps with the exception of ‘unseasoned’ boards) and can be used to encourage concentrated egg laying. Of the three different fly board methods used, Frank Sawyer’s paving stone method is the most successful, although floating tethered planks in late summer and early Autumn appear to be just as attractive. However, without a baseline or further study, it is not clear whether fly boards actually increase fly abundance. It could be argued that if the fly boards were not present, the flies would lay the same number of eggs but in a more dispersed manner on natural sites. Whether this would lead to a greater survival rate is not certain. Finally, concern over caddis predation seems not to have been an issue at all. No evidence for caddis predation was found at any stage during the experiment.
This experiment achieved its limited aims of proving that certain species of aquatic fly will lay eggs on fly boards. Further work is now required to compare natural fly production rates against fly boards and measure scientifically whether fly boards in the contemporary era actually increase fly abundance. Work is underway within the SDFFA (advised by professional scientists) to design a longer term experiment for measuring changes in fly abundance. I hope to report on whether fly boards increase fly abundance in a couple of years!
Some of Frank Sawyer’s original work is described in ‘Keeper of the Stream’ and ‘Nymphs and the Trout’. Both books are available as special limited editions from Nick Sawyer through his website at www.SawyerNymphs.com. Also available is a paperback entitled ‘Frank Sawyer’s Nymphing Secrets’, which is made up of magazine articles and unpublished notes by Frank on how to fish the nymph.
Footnotes:
1 Martin E Mosely was nephew of FM Halford and curator of the Dept of Entomology at the British Museum. Frank Sawyer had a long and fruitful relationship with the British Museum sending them hundreds of nymph and fly samples over the years.
2 ‘Seasoning’ is the term given to the soaking of the boards and paving stones in water to dissolve all the harmful salts and other substances found in wood and slabs. There is no hard and fast rule on length of time but for this experiment the paving stones had been in the river for 50 years and the wooden boards for 3 years.