Fly Fishing Devon: Instruction & Guiding on Dartmoor Rivers

Invertebrate drift: The "Hidden Harvest"

In 1963 towards the end of an exceptionally cold winter a young man - JM Elliott - set off to live on Dartmoor. He chose to camp close to Princetown, the location of a bleak prison constructed in 1806 to hold French and American POWs. He was there to collect samples of water from the Walla Brook - a tributary of the East Dart - as part of his PhD. What he discovered was to have a major impact on our understanding of the interrelationship between insect behaviour and trout feeding patterns.

Elliott anchored a fine mesh net in the Walla Brook, removed it every three hours over a 24 hour period and counted the number and types of trapped insect. He spent 18 months on Dartmoor in order to collect sufficient samples to complete this study of insect behaviour.

Elliott discovered that the numbers of insects caught in the net varied as a function of time of day. He called this phenomenon "invertebrate drift".
  • insects in the 'invertebrate drift' are drifting to locate fresh feeding grounds, avoid overcrowding or escape predators
  • the term 'invertebrate drift' does not refer to insects that are ascending to the surface prior to hatching
  • Elliott's findings generated significant research interest from freshwater ecologists but are not widely discussed in the British angling literature.

    This graph illustrates 'invertebrate drift' and is based on similar   work carried out by the Field Studies Council (FSC) in late April.
  • during the day very few invertebrates were found in the net
  • in the evening - and especially towards midnight - the numbers increase
  • this may be an  accidental consequence of insects emerging from under rocks to feed and being swept off
  • or, it may be a   deliberate attempt by insects to move to a new area in search of food or to escape from predators
  • there is evidence that drift is 'intentional' rather than 'accidental' behaviour
  • the causes of invertebrate drift are still the subject of academic debate and experimental inverstigation
  • the term   "behavioural drift" has replaced the earlier term   "invertebrate drift" to emphasise that drifting is part of the behavioural repetoire of certain insect species
  • the term   "constant or background drift" refers to random accidents, and
  • the term  "catastrophic drift" refers to insect movement that occurs when rocks move during a spate (see Gibbins et al, below)
  • Interestingly, from an angler's point of view:
  • the phenomenon of behavioural drift suggests that there will be periods during the day when increased insect food is available to trout in the absence of clearly visible surface activity. Hence my use of the term " Hidden Harvest"
  • there is a significant correlation between the tendency of an insect species to engage in behavioural drift and that species being eaten by trout (see Rader below)
  • smaller insects such as midges (Chironomidae) and blackflies (Simulidae) drift during the day
  • caseless caddis are more likely to drift than cased caddis
  • there is some evidence that younger (small) insects of a particular species drift during the day, whilst larger individuals drift at night - this may have implications for hook size
  • behavioural drift is higher during the summer than winter

  • Ephemeroptera, Simuliidae, Plecoptera, Diptera and Trichoptera engage in behavioural drift, for example:
  • blue-winged olive nymphs (Baetis)
  • caseless caddis (Rhyacophila and Hydropsyche). Stone cased caddis tend not to drift (weight?)
  • chironomids (midges) - note there may be no diurnal pattern of behavioural drift in many chironomids - it can occur throughout the day and night (see Moss, below)
  • blackfly larvae (Simulium) - but they have a sticky silk thread that allows them to return 'home' after drifting
  • freshwater shrimp (Gammarus)

  • These results beg the question "Why don't rivers become empty of insects as a result of behavioural drift?" Part of the answer may be that females tend to fly or crawl upstream before depositing their eggs. Moss (see below) offers some other explanations and discusses the possibility that behavioural drift - particularly by larger insects -may occur more often at night where there is less risk of them being eaten by trout.

    What the "L " does this mean for the angler?

    The concept of 'behavioural drift' has led me to rethink the way I imagine underwater insect activity when there is no obvious surface activity associated with a hatch. I now realise that I was concentrating too much on a straight vertical line from river bed to surface.

    I was thinking in terms of a very few dislodged nymphs when there was no visible hatch. Then the pupa ascending through the water column, breaking through the water surface, shedding its exoskeleton and emerging as a dun.
    Now my thinking is more Lshaped. I have added the horizontal component to represent behavioural drift.

    Behavioural drift explains why it is worthwhile fishing an appropriate imitation close to the bed of the river to represent a drifting insect, especially when there is little surface activity.

    This is worth doing during the day because:
  • some species - particularly smaller insects and chironomids - may drift throughout the day (Allan, 1978)
  • there may be   fewer  natural insects in the drift to compete with your fly for the trout's attention
  • most importantly, trout  expect  to find food drifting close to the bed of the river
  • Online resources and references:

    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, Accessed 9 Apr. 2020.
    2. Elliott, J. M. “Diel Periodicity in Invertebrate Drift and the Effect of Different Sampling Periods.” Oikos, vol. 20, no. 2, 1969, pp. 524–528. JSTOR, Accessed 8 Apr. 2020.
    3. JM Elliott commentary on his paper "Invertebrate drift in a Dartmoor stream"
    4. Ed Engle, Fishing Small Flies", Stackpole Books, 2005
    5. C. N. Gibbins, E. Scott, C. Soulsby & I. McEwan,The relationship between sediment mobilisation and the entry of Baetis mayflies
      into the water column in a laboratory flume, Hydrobiologia (2005) 533: 115–122. available online
    6. Moss, Ecology of Fresh Waters:Man and Medium, Past to Future, page 85, available online
    7. Ephemeroptera Galactica, Official Web Site of the Permanent Committee of the International Conferences on Ephemeroptera, available online
    8. Rader, A functional classification of the drift: traits that influence invertebrate availability to salmonids, Can. J. Fish. Aquat. Sci.  54(6): 1211–1234 (1997), Article available online
    9. Hieber, Robinson and Uehlinger, Seasonal and diel patterns of invertebrate drift in different alpine stream types. Freshwater Biology (2003) 48, 1078–1092. available online
    10. David Allan, Stream Ecology: Structure and Function of Running Waters, Kluwer Academic Publishers, 1995
    11. Siler, Current Research on Macroinvertebrate, Detritus, and Algal Drift in the Au Sable River, available online
    12. Svendsen, Quinn, and Kolbe, Review of Macroinvertebrate Drift in Lotic Ecosystems, available online
    13. Rydberg, Lecture notes on Stream Drift, available online
    14. Gary LaFontaine, The Dynamics of Nymph Fishing, Part 3 "What are the trout feeding on?", available online
    15. Allan,Trout Predation and the Size Composition of Stream Drift,Limnology and Oceanography, Vol. 23, No. 6 (Nov., 1978), pp. 1231-1237, available online