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
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
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
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
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)
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
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
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
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.
Ed Engle's books on tying and fishing small flies including midges (chironomids)