Ann R. Miller

Ann R. Miller

Fishing at night is much different than fishing during daylight. Sounds seem magnified in the dark, with howls, crashes, and screeches from the woods or surprise splashes from the river causing near cardiac arrest. Wading is uncertain, with each precarious step an adventure, especially when a fallen log or deep pool is suddenly encountered (and they always are). The focus of the evening is on fish, of course, but the streambed at this time is becoming quite active with invertebrates intent on finding a new home. As darkness falls, many invertebrates that have been content living on the undersides of rocks, sticks, and other substrates, will attempt to move to a new location. This may happen by crawling off the substrate or letting go and drifting with the current. The phenomenon of drift is fascinating and understanding it can improve angling skills.

Consider, for example, one of the hatches that fly anglers chase in the summer: Hexagenia limbata or the Michigan mayfly. When fishing the dun stage of the hatch, we position ourselves downstream in silty waters, waiting well into the night for bugs to pop. However, the spinner “hatch” takes place further upstream, with fertile adults gathering above gravelly riffles. Year after year, the Hexagenia larvae make their way from upstream riffles to downstream silt, often a distance of a kilometer or more. Countless other invertebrates drift downstream as well; in fact, nearly all aquatic insects begin life in shallow headwaters, emerging well downstream a year or more later as an adult. This movement is not happenstance, but an established pattern that is repeated each year.

A landmark study in the 1950’s by Karl Muller documented several patterns in stream drift. Seasonal patterns were noted (drift was found to be higher in the summer than in the winter), but more interestingly, higher densities of invertebrates were found in the water column immediately after sunset and again before sunrise. The density of some species varied dramatically, often peaking ten-fold at dusk with a smaller increase at dawn. This double peak or ‘diel periodicity’ has since been documented in streams around the world. Also noteworthy, it was found that in streams without fish, drift occurred both day and night; thus, the downstream migration occurs at dark when predation pressures are the least. Perhaps the most perplexing issue regarding stream drift is this: if insects and other invertebrates drift downstream, why don’t the headwaters become depleted?

Study after study has attempted to resolve the drift paradox, and truthfully the debate is ongoing. One of the first and foremost arguments was that progeny return upstream to recolonize once adults mature, recharging depleted headwaters. This argument seems plausible for many invertebrates, including many mayflies (especially Baetis) and caddisflies. Scuds, prominent in high numbers in the drift, have been reported swimming upstream while some winter stoneflies have even been observed walking upstream along streambanks. While this idea of drift compensation seemed like a neat and compact answer, studies showed that invertebrate populations were never severely depleted enough to require an upstream dispersal.

Nevertheless, drift happens, and in very high numbers. What causes drift? While there are no easy answers, obviously some invertebrates naturally become dislodged from substrates by predators (fish, ducks, other microfauna) or by physical disturbances (anglers, deer). Abiotic factors, including current velocity, discharge from dams, or spates also affect drift. On an ongoing basis though, it is thought that local density is most responsible for drift. As individuals grow, local resources become limited, necessitating a move to greener pastures. Thus, entering the drift is thought to be adaptive for an individual, and thereby an evolutionary stable strategy. Still, the question remains: why don’t upstream waters become exhausted?

Some scientists believe that the biomass of invertebrates in upstream riffles is so high, that it effectively “forces” invertebrates to drift, but because there are such high densities to begin with, local populations don’t become depleted. In essence, production exceeds carrying capacity. A significant number of individuals can be lost through drift, but an ample population upstream still remains. This theory again seemed neat and compact: newly hatched larvae have plenty of resources until they start to grow. Crowding becomes an issue, and drift a solution. Still, this doesn’t really answer the paradox; if it did, we would find Hexagenia adults hatching along the entire streambed.

Other studies have proposed additional models to explain drift; one recent analysis suggested that individuals should exactly compensate for drift by upstream dispersal and that overall there should be a mean net movement of zero from birth to reproduction. The critical argument here is that from an evolutionary point of view, movement needs to be analyzed from an individual perspective rather than from the population as a whole. Additional studies are sure to counter, but until then, the drift paradox remains.

From an angling point of view, things are much simpler. So often when fishing at dusk, there is a marked stop in rising fish, and everything becomes very quiet. This is when fish, especially brown trout, focus on the sudden flurry of critters drifting up from the streambed. A switch to nymphing, or better yet, to a parachute with a dropper is a great technique to pick up fish at twilight – certainly a great end to any day.

Ann has been writing a natural history column for Midwest Fly Fishing for more than 10 years. Before that she was the editor of the Great Lakes Leader for 5 years. She has contributed to many Michigan periodicals. Her essay “Under the Weather” was published in Michigan Seasons: Classic Tales of Life Outdoors in 1997. She is currently working on a hatch guide for the Midwest.