Experiments with Fly Boards and Stones

SDFFA FLY BOARD EXPERIMENT 2006

INTRODUCTION

1. General. Current concern over aquatic fly abundance has raised the subject of fly boards and their merits as a river and fly management technique. William Lunn used planks of wood attached to bridges, Frank Sawyer used paving stones propped up on metal stakes and others have used floating planks in various forms. There are few recorded observations on whether the fly boards increased fly abundance or even whether the fly boards attracted egg laying aquatic flies. The one exception is Frank Sawyer who kept a photo record of his paving stone experiments and these clearly show egg laying activity. See photo below showing eggs on a paving stone:

What is not clear is how Frank Sawyer’s paving stone technique compares to tethered and floating planks of wood or, more importantly, natural egg laying sites. An experiment was developed by SDFFA to compare the various artificial egg laying sites against natural sites. The results of the experiment would be used to suggest a technique for providing artificial egg laying sites but only if it could be shown that these were preferred by flies.

2. Aims. The aims of the experiment were:

a. To prove that paving stones and planks can provide suitable artificial egg laying sites for aquatic flies.

b. To identify which artificial egg laying site is preferred by aquatic flies.

c. To determine whether aquatic flies preferred artificial egg laying sites over natural ones.

d. To determine whether caddis predation occurred on artificial egg laying sites.

METHODOLOGY

3. Apparatus and Equipment Used.

a. 5 x seasoned paving slabs. The remnants of a previous experiment by Frank Sawyer in the 1950s were used. The paving stones measured roughly 0.75m by 1.25m and had been lying on the river bed for around 50 years.

b. 3 x seasoned planks. These were prepared from a single plank left in a pond for 3 years.

c. 1 x unseasoned plank. This was a piece of untreated wood from a hardware store.

d. 7 x three foot metal pickets. These were hammered into the river bed to provide supports for the paving slabs and a tether point for the planks.

e. Electrical wire was used to tether the planks to the stakes.

4. Experimental Technique.

a. Paving Stones. Five metal stakes were hammered into the river bed until the tops were about 5 cm beneath the river surface. The paving stones were then propped up against the stakes so that the upstream edge was on the river bed and the downstream edge just broke the river surface. This provided a dry surface on which flies could land and then crawl down the underside of the paving stone to lay eggs. The technique is illustrated below:

b. Bottom Tethered Planks. Half of the planks were tethered with wire to the bottom of a metal stake. This caused the upstream end of the plank to be fixed about 10 cm from the river bed while the other end rose out of the water a few centimetres. This provided a dry surface on which flies could land and then crawl down the underside of the plank to lay eggs. The technique is illustrated below:

c. Floating Planks. The remaining planks were tethered so they could float freely on the river surface. The technique is illustrated below:

d. Measurements. Egg laying activity was measured by regular observation of the paving stones and planks and comparison with natural egg laying sites in the vicinity.

OBSERVATIONS

5. 21 May 06. No photos were taken on this date as the observation visit was only made to check that recent heavy rain had not washed the fly boards away or caused damage. It was surprising to see evidence of egg laying activity after only 16 days in place and through a period of heavy rain and high winds. The following observations were made:

1. Paving Stones. On average, 50-60% of the underside of the paving stones were covered with eggs, with egg coverage several layers thick on one particular paving stone. Species type was overwhelmingly Baetis and several flies in the process of laying were observed. There were 2 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. These ranged in size from the minute to full-grown upwing nymphs. The average number of caddis cases on the paving stones was 5. No live caddis were seen but none of the cases were investigated internally. Other life on the paving stones included snails (3-4 per paving stone). No weed or debris had caught on the paving stones and they appeared to have been unaffected by recent heavy rains and wind. Fluctuating water levels due to rain had not caused the paving stones to become swamped.
2. Planks Wired to Riverbed. On average, about 20-30% of the plank was covered with eggs. Again these 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 and these ranged from the very small to full grown upwing nymphs. There were no caddis cases present. No weed or debris had caught on the planks.
3. Floating Seasoned Plank. Only one floating seasoned plank was used during the experiment. This was covered on its underside with Baetis eggs with around 20-30 % coverage. A similar nymph population to the bottom-tethered planks and paving stones was observed. There were no caddis cases. The floating plank had caught a lot of weed and debris on the exposed stake. This had to be cleared as the board was in danger of being swamped.
4. Floating Unseasoned Plank. Egg coverage on the unseasoned plank was less than 5%. A small number of nymphs were observed but no caddis cases.
5. Natural Sites in Locality of Fly Boards. Three pieces of woody debris, 2 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). There were a large number of caddis cases on all the natural items and the rocks were almost entirely covered in caddis cases. Shrimps and nymphs were present on all the natural items but nymph numbers were only a fraction (around 20 individual nymphs per square metre) of those seen on the fly boards.

6. 27 May 06.

a. Paving Stones. The amount of egg coverage had increased with average coverage in the region of 70-90%. Egg coverage was several layers thick in a few places on each paving stone. Species type remained overwhelmingly Baetis with many flies observed in the process of laying. There were 2 other distinct egg types but these could not be attributed to a species. Nymphs habiting the underside of the paving stones had dropped to only a couple of hundred. These again ranged in size from the minute to full-grown upwing nymphs. Caddis cases on the paving stones had increased to around 10 but an inspection of 5 cases from each paving stone revealed all were empty. Two live caddis without cases were seen on the bottom of one of the paving stones. Heavy rain had submerged one of the paving stones but the others still had a few millimetres of dry area.

b. Planks Wired to Riverbed. Egg coverage had not increased significantly although there were several layers in small areas on each plank. Eggs were predominately of the Baetis species. Like the paving stones, the number of nymphs present had dropped to only a couple of hundred. There were no caddis cases present. No weed or debris had caught on the planks.

c. Floating Seasoned Plank. Egg coverage had doubled and now approached 40-50 % with multiple layers in several places. Nymph numbers had dropped to only 40-50. There were no caddis cases.

d. Floating Unseasoned Plank. Egg coverage on the unseasoned plank remained less than 5%. Unusually there were more nymphs on the unseasoned plank than any of the other boards or stones but these were predominately gnat/chironomid type larvae.

e. Natural Sites in Locality of Fly Boards. The natural woody sites had experienced significant egg laying activity and were approaching 80% coverage with all areas covered in multiple layers. Shrimps, nymphs and caddis were observed but in much reduced numbers.

7. 03 Jun 06.

a. Paving Stones. No significant changes observed.

b. Planks Wired to Riverbed. No significant changes observed.

c. Floating Seasoned Plank. No significant changes observed.

d. Floating Unseasoned Plank. No significant changes observed.

e. Natural Sites in Locality of Fly Boards. No significant changes observed.

8. 07 Jun 06.

a. Paving Stones. Area covered had not increased but there were more areas with multiple layers. Of note, the paving stones in the shade (the week had been very sunny) had experienced far more egg laying activity than the paving stones in the sun. 20-30 flies were on the underside of the paving stones in the shade while only 3-4 were seen on the ones in the sun. Nymph numbers had dropped to less than 100 while caddis cases remained constant at ten. All were empty and no live caddis were seen.

b. Planks Wired to Riverbed. Unchanged in terms of area covered but there may have been a few areas with more layers – hard to tell. Nymph numbers much reduced to around 20-30.

c. Floating Seasoned Plank. No significant changes observed.

d. Floating Unseasoned Plank. No significant changes observed.

e. Natural Sites in Locality of Fly Boards. No significant changes observed.

9. August 06.

a. Paving Stones. No significant changes observed in egg coverage but small colonies of algae growing on the edges of the paving stones.

b. Planks Wired to Riverbed. No significant changes observed.

c. Floating Seasoned Plank. No significant changes observed.

d. Floating Unseasoned Plank. No significant changes observed.

e. Natural Sites in Locality of Fly Boards. No significant changes observed.

10. September 06.

a. Paving Stones. Increased number of different egg types but no increase in Baetis egg coverage.

b. Planks Wired to Riverbed. No significant changes observed in egg coverage but small algal growths observed on the edges of the plank.

c. Floating Seasoned Plank. Increased coverage approaching that of the bottom tethered planks.

d. Floating Unseasoned Plank. Increased number of non-Baetis eggs.

e. Natural Sites in Locality of Fly Boards. No significant changes observed but shrimp numbers increased.

ANALYSIS AND DISCUSSION

11. Comparison of Artificial Sites. The paving stones and the bottom tethered planks were overwhelmingly preferred by the flies although towards the end of the summer the floating seasoned plank was on a par. The unseasoned plank was not preferred at any stage and egg laying activity was minimal. Unseasoned planks should be discounted as a method. No caddis predation was observed on any of the paving stones or planks despite caddis larvae occasionally being present on some of the paving stones. The caddis on the paving stones were always located at the very bottom where egg laying activity was minimal. It is suspected that any caddis located further up the paving stone would have been quickly predated by fish which often used the paving stones as shelter.

a. Paving Stones.

(1) Advantages.

i. Most preferred by flies and achieved the greatest concentrations of eggs.

ii. Most number of different egg types seen.

iii. Does not present a snag for weed and debris.

iv. Cannot be easily removed by fishermen, vandals and other passers-by who may wish to move the experiment.

v. Can be used to keep water levels up in shallow areas if deployed in a weir type formation.

(2) Disadvantages.

i. Extremely difficult to move around the river due to weight and size.

ii. Do not automatically adjust to changes in water level unlike the planks which float.

b. Bottom Tethered Plank.

(1) Advantages.

i. Automatically adjusts to changing water levels.

ii. Relatively easy to move around the river.

(2) Disadvantages.

i. Metal stake used for tethering the plank catches weed and debris and has to be cleared regularly to avoid swamping the plank.

ii. Plank can get ripped away in flood conditions or if struck by a large piece of floating debris.

c. Floating Plank.

(1) Advantages.

i. Automatically adjusts to changing water levels.

ii. Very easy to set up – can be tethered to any fixed object.

(2) Disadvantages.

i. If a metal stake is used for tethering, the plank catches weed and debris and has to be cleared regularly to avoid swamping the plank.

ii. Plank can get ripped away in flood conditions or if struck by a large piece of floating debris.

iii. Least preferred by the flies although closes the gap towards the end of the summer for reasons unknown.

12. Comparison of Artificial Sites with Natural Sites. Natural sites with concentrations of eggs similar to the paving stones or planks were very rare. Those few sites suitable for concentrated egg laying that did exist never really approached the same high concentrations as found on the paving stones. What is not clear from the experiment is whether the flies laid eggs in a more dispersed fashion when there were no sites suitable for concentrated egg laying.

CONCLUSIONS

13. The following conclusions can be drawn from the experiment and should be referenced to the aims at para 2:

a. Paving stones and seasoned planks both provide suitable egg laying sites for aquatic flies.

b. Paving stones provide the best site for concentrated egg laying and surpass natural sites by an order of magnitude when it comes to concentrations of eggs.

c. Aquatic flies prefer paving stones over natural egg laying sites for concentrated egg laying.

d. Caddis predation is not a problem for any of the artificial egg laying sites.

FURTHER WORK

14. The experiment was not designed to prove whether fly boards improved fly abundance, only to assess the most preferred artificial egg laying sites for aquatic flies. An obvious area of further work is to measure fly abundance in a stretch of river before and after fly boards are used. This may provide evidence for whether fly boards increase abundance.