March 08, 2018
By Jonathan Wright
Any tailwater trout fisherman worth this salt will tell you that midges are the go-to fly pattern -- and the dominant insect species -- in the outlet streams immediately below the dams that hold back larger reservoirs. Especially in the colder months, catching big fish on tiny flies is a seasonal discipline and an eye opening rite of passage for those new to the sport. Lake fishermen are also aware of the overall value of Chironomids (belonging to the enormous order of insects called Diptera), and when the hatch gets frustrating, dropping a tailless size #22 emerger in the film below a dry fly has saved many fishing days.
Recent scientific study is indicating that midges contribute more value to the health and vitality of lakes and tailwater streams than has been previously realized. Researchers from the Leibnitz-Institute for Freshwater Ecology and Inland Fisheries in Berlin, working in conjunction with biologists in the UK, have determined that the activity of burrowing midges and certain classes of annelid worms work to increase oxygenation of lake bottom sediments up to 300% more than had been previously assumed. The process, known as bioirrigation, increases infusion of oxygen and promotes aerobic breakdown by bacteria of the fine organic materials in sediment, providing available nutrients for species throughout associated ecological webs.
Using the bioactive tracer dye Resazurin, the researchers were able to measure the contribution of oxygen that the insects were making to the substrate they were burrowing into. Resazurin is light blue dye that turns a bright fluorescent pink after being converted via cellular metabolic processes in living organisms. Resazurin has been used in previous studies to assess the total insect biomass in stream bottoms -- measuring direct respiration of the organisms -- but in the case of the lake sediment research, the amount of converted resafurin in the treated sediment was used as an extrapolation of available oxygen for the midges to breathe in the first place.
Photo by Wikipedia
Lakes, especially deep reservoirs in colder climates, are subject to a seasonal change in biology known as "Turnover". A consequence of thermal stratification, turnover happens twice a year. In the late Fall, surface water (called the Epilimnon) that had been cooled by subfreezing air becomes denser and falls to the bottom of the lake, displacing the warmer water next to the earth at the bottom (the Hypolimnon), which then rises to the surface passing through a mixing zone known as the Thermocline. In the spring, the process repeats, but this time with melted ice cover displacing deep, warmer volumes. In both cases, the sediment at the bottom of the lake gets churned, with the entire water column becoming opaque for a period of days to weeks.
This typically includes the outflow to tailwaters. Increased bioirrigation would undoubtedly make water not only more easily turbid with thermal exchange, but also appear to contribute the nutrient health of the sediment, much as soil vitality is of crucial importance to farmers.
I have fished the famous tailwater section of the San Juan River in New Mexico below Navajo Lake at Christmas time, right after turnover. The Juan typically runs extremely clear all year, but during this trip, the water was pea green with maybe only four inches of visibility. I wasn't happy to discover these conditions after making the 400 mile drive from Denver in midwinter, but I suited up and hit the river anyway.
Much to my surprise, the nymph fishing was red hot, and in this case I'm referring the to color of the top fly of the trip. Bright red midge patterns -- as opposed to the typical chocolate and black color variations favored the rest of the year by Juan enthusiasts -- were being crushed on sight by gangs of voracious Rainbows staged in shallow lies with any slightly improved visibility.
Photo by Wikipedia
Along with dark purple chenille worms, (again, a departure from the locally favored pumpkin orange for most other months), the midges had apparently been displaced from their lake bottom dwellings into the water column, and flushing with color from the stress of the experience, were being washed out the spillway of the dam into the mouths of the thousands of trout below.
The majority of major river and stream resources in the US and abroad are now dam controlled. This is a necessary part of modern life, with larger reservoirs providing both flood control and drinking water reserves for huge populations of people. The biological health of water impoundments is thereby a priority for society, and matter of national security. If high concentrations of pesticides in lake sediments from agriculturally impacted stream inflows could affect midge biology -- driving a collapse of lake bottom insect life and potential putrefaction of lake water via stagnant sediments, then this needs to be researched more extensively in the interests of both sportsmen and the public at large.