Natural and Artificial Trout Flies of the River Yealm

The Yealm is a relatively short (12 miles long) river representative of several rivers running off the southern slopes of the Dartmoor National Park in South Devon (UK). It carries a good head of brown trout, a run of sea trout in summer months and salmon in the autumn (November and December).

In 2011 the environmental consultancy firm APEM carried out   invertebrate surveys  at seven sites using the kick sampling procedure.

Samples were collected on 25/26th May, 2011 (Spring Survey), 19 August 2011(Summer Survey), and during the week of the 19-23rd September 2011(Autumn Survey).

This page summarizes the survey results and lists the most frequently found insects. It then discusses the implications of the survey results against the background of what we know of the diet of trout in Dartmoor rivers and the writing of local expert Mike Weaver. This may be of interest to fly fishers visiting the Upper Yealm Fishery and other similar rivers in South Devon.

The survey sites were very similar to these images of the Upper Yealm Fishery beat. The main flow types were run and riffle, with sand and pebbles/gravels dominating the substrate. The wide variety of river flies found in the River Yealm is typical of clean, fast flowing, stony rivers.

NOTE: The scientific names for insects are shown in   italics on this page. The associated link opens a new window / tabbed page showing the results of a Google search for images of that particular insect. Anglers names for insects are shown in normal typeface and the associated link also opens a new window / tabbed page showing the results of a Google search for images of the range of fly patterns used by anglers to represent that particular insect.

Invertebrate survey results: Spring 25/26th May 2011

Large Dark Olive(  Baetis rhodani )

Small Dark Olive(  Baetis scambus )

Pale watery(  Baetis fuscatus )

Angler's Curse(  Caenis rivulorum )

Midges/ Buzzers (  Chironomidae )

Cased Caddis(  Glossosoma )and Caseless Caddis(  Hydropsyche ) localized distribution

Stonefly/ Needle Fly/ Willow Fly (  Luectridae )

Olive Upright(  Rhithrogena semicolorata )- present at all sites and recorded at quite high abundances in certain locations

Blue Winged Olive(Ephemeralla /  Serratella ignita ) recorded at high abundances at all sites

Small Yellow Sally(  Siphonoperla torrentium ) - present at all sites and recorded at quite high abundances in certain locations

Invertebrate survey results: Summer 19 August 2011

Beetles were abundant.

But many of the species noted in the spring survey were either much reduced in abundance or absent.

Abundances of Olives (Baetidae) especially Large Dark Olive (B. rhodani), Olive Upright (R.semicolorata,and Blue Winged Olive (S. ignitus) and Stoneflies (L. geniculata) and Blue Winged Olive (S. ignitus)were much reduced in summer, or were absent - Small Yellow Sally(S. torrentium)- perhaps reflecting the less energetic flow environment present during the summer months

Beetles(  Elmidae ) abundance was increased in summer

Freshwater Shrimp (Gammurus pulex) abundances were much reduced in summer

a stonefly species of the highest conservation value Early Brown(  Protonemouri meyeri ) could be considered common on the Yealm

Invertebrate survey results: Autumn 19-23rd September 2011

There were high abundances of the Stonefly and Midges / Buzzers ( Chironimadae)

increase in the presence of Caddis(  Trichoptera )

high abundances of Midges/ Buzzers(  Chironimadae )

high abundances of the Stonefly(  Leuctra fusca )

high abundances of the Mayfly (  Habrophlebia fusca )

The importance of beetles in the diet of Dartmoor trout

Finding abundant numbers of  Elmis aenea beetles in summer kick samples highlights the importance of considering these - often overlooked - insects as a source of trout food that can be imitated by the angler.

The survey found  Elmis aenea a very small (2 mm) riffle beetle that is equipped with strong claws to enable it to grip in strong currents. It is a dark coloured species with very deeply ridged wing cases.It does not need to surface for air as it breathes the trapped oxygen in submerged bubbles but it does leave the water at times and can fly. If disturbed it will float to the surface.They may move downstream by drifting in the current. Here is a   large image  of Elmis aenea, useful for fly-tyers.

Of course finding an insect in a river does not mean that it is eaten by trout.

Dr JM Elliott (1967) studied the food of trout in the Walla Brook a tributory of the East Dart. His results highlighted the importance of beetles in trout diet.

Here, in order of importance, are the percentages of various food items consumed by Year 2+ (>15cm) trout over a one year period (October 1963-October 1964)

1. 50% Limnephilidae  case-constructing caddisflies
2. 16% terrestrial invertebrates e.g. helmis maugei riffle beetle. Limnius beetles, latelmis beetles
3. 5% protonemura meyeri a small to medium stonefly hatches from March to June.
4. 4% baetis known as the blue-winged olive to anglers
5. 4% Isoperla grammatica, Yellow Sally stonefly,
6. 3.5% rhyacophila caseless caddis

The importance of beetles may have been overlooked because they are relatively unimportant on chalkstreams, but "vital to summer fishing on rain-fed (freestone) rivers".

Mike Weaver fishes a beetle as a dry fly (Deerhair beetle) in broken popply water at the head of pools.

He advises using a sinking beetle (his Black Bug) especially when rivers are low and clear under low-flow summer conditions on very smooth water beneath overhanging trees. This is "usually taken with a visible swirl within a second of its hitting the waterif it is going to be taken at all," ( Weaver, 1991, 1992).

The Black Bug is dressed as follows:

Hook: 16 or 18, sometimes 20

Body: A short length of lead wie whipped to shank with bronze peacock herl over

Back: Crow or other black herl, secured at front and rear

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The importance of midges in the diet of Dartmoor trout

The Yealm survey results also show that midges (chironomids) are an abundant potential source of food throughout the year.

Elliott (1967) reported that chironomid pupae become an increasing important part of the trouts diet from May to October

There is increasing awareness of the importance of these insects to trout in rivers which is reflected by several recent excellent books and videos on this subject listed in the Further Reading section below.

Mike Weaver recommends this easy-to-tie simple Black Midge

Hook: 20 or 22

Body:Fine black fur, dubbed

Wing for visibility:White polyyarn, tied as a verical tuft about one-third back from the eye

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Further reading/viewing

1. Elliott, J. M. “The Food of Trout (Salmo Trutta) in a Dartmoor Stream.” Journal of Applied Ecology, vol. 4, no. 1, 1967, pp. 59–71. JSTOR, www.jstor.org/stable/2401409. Accessed 10 Apr. 2020.
2. Ed Engle's books on tying and fishing small flies including midges (chironomids)
3. Mike Lawson's video on midge fishing
4. Steeves, Harrison & Ed Koch (1994) Terrestrials: A Modern Approach to Fishing and Tying with Synthetic and Natural Materials. Stackpole Books
5. Holbrook, Don & Ed Koch (2001) Midge Magic. Stackpole Books
6. Weaver M.(1991). The pursuit of wild trout. Merlin Unwin Books.
7. Weaver M.(1992) Chapter 3 In Lapsley, P. (Ed) The Complete Fly Fisher . Select Editions.
8. Aquatic Ecology Surveys of the River Yealm, 2011. FINAL REPORT December 2011APEM REF: SLR 411662. APEM Ltd. Aquatic Ecology laboratories FBA East Stoke Wareham Dorset, BH20 6BB Tel: 01929 405430 Website: www.apemltd.co.uk
9. There is much food for thought in this (long) YouTube videothat highlights the importance of terrestrials as trout food; there are tying instructions after the fishing section
10. "Small but perfectly formed": Some lesser-known westcountry river flies. This page explores some of the lesser-known species of insects which can be found if you examine the stones in the rivers of Dartmoor and South Devon.

Fly tying videos

1. YouTube video by Paul Proctor tying a Straggle String Beetle
2. Tying the Black midge/gnat
3. Craig Matthews shows how to tie the Improved Baetis Sparkle Dun
4. Craig Matthews shows how to tie the X-Caddis
5. Davie McPhail tying the Cruncher
6. Detailed instructions for tying Craig Mathews's Zelon Midge