There are many things that get the focus or attention of fly anglers that get the attention owed to them….we all love hatches and the great dry fly action that comes with them. Some love the aggressive eats of a pre or post spawn trout , be it a brook trout, rainbow trout, or my personal favorite…brown trout. Some like low water terrestrial fishing in summer…others high water nymphing in early spring. But there are some things that have a profound effect on fishing, and are worthy of the same attention of the things I mentioned above. Yet few know about this one.
I’d say if you lined up 10 experienced anglers up in a line, almost no one would know anything about this. I’d say the same for flyfishing guides. Yes, fly fishing guides. There I said it. So what is this “thing” I’m talking about? It is called Behavioral drift.
Basically, it is when insects leave the substrate /bottom and enter the water column and drift with the current. This activity occurs daily, seasonally, annually, and so forth. But the fact that it occurs goes largely unnoticed by the angling community. There’s a reason our nymph fishing on trout waters is really good during our early season. One, the bottom is more densely populated with insect fauna right now than at any time of the year. Most if not all insects who hatched last year and successfully laid eggs those eggs are now fully developed or nearly mature insects. Those are ready or will be ready to hatch. And they drift in large numbers.
There are several categories of drift that entomologists are familiar with: Catastrophic drift, where high water or floods displace numbers of insects; Behavioral drift as a result of activity, be it feeding activity, moving from one spot to the next when food supply dwindles, avoiding predators (other animals and insects), and so forth. There’s also distributional drift, which is thought of as a method of dispersal. That is once eggs are laid in the riffle areas of a stream the eggs hatch and then the nymphs as they develop and feed they will enter the drift occasionally and move to other areas of the stream thereby preventing the depletion of a food supply in the place they came from. To sum it up, this is all drifting activity not associated with an insect emergence.
It has been my experience in the early spring to mid spring time frame that this drift activity, collectively, has a great effect on fishing. Fishing is excellent, and the fish are well attuned to this activity that most folks aren’t even aware is occuring. They just notice the nymph fishing is really good, but they don’t know why.
Drift occurs daily, and there are, from what entomologists often tell us, and the ones I know reiterate this, some definite times of day that this activity is more prevalent. Insects drift on and off throughout the day but a large part of this occurs during the low light hours and overnight. Often significant activity occurs very early in the day, waning a bit during the daylight hours, then picking up again nearing sunset, once again reaching significance in terms of impact. Light often retards some of the activity, and even moonlight can suppress some of it.
Fish do notice this, and will often feed heavily on the drifting nymphs, and often in areas in the shallows where you wouldn’t normally expect to find large fish, particularly gravel strewn edges with weed patches, shallow tailouts of large, flat , deep pools, and so forth. To the angler who knows this is occurring they can catch some tremendous fish in very shallow water, and often sight fish to large fish who are feeding in response to this. I have personally witnessed this phenomenon on the South Holston River in East Tennessee, where large fish gorged themselves on the drifting ephemerella invaria (sulphur) nymphs that were visible in huge numbers in the water column in the shallow tailout of a pool I was sight fishing to fish in. I had actually heard of drift from a friend, professor, fellow angler and gentleman I once worked for at the Fly Line Fly Shop, Dr Jim Sellers. Jim was a master angler, to this day the best I have ever known, and who I name as a mentor having more influence on my early fly fishing endeavors than anyone. Jim is still the only person to this day that I have ever heard mention it.
Certainly a fascinating thing drift is, and its probably the reason you have enjoyed great early season nymphing, and particularly early in the day. A lot of research has been done on drift. There’s a great paper or research piece that explains a bit more about it, and its an article called “Invertebrate Drift-A Review”, which was published in Hydrobiologia, January 1988 and revised by Dr JE Brittain in 2017, University of Oslo…a joint paper written by John E. Brittain and Tor Jan Eikeland, Zoological Museum, University of Oslo, Norway. This is now located at the Murray-Darling Freshwater Research Centre, Albury, NSW 2640 , Australia. If you desire to look into it further, you can find this article online. You can also download it for FREE.