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Aquatic insects in the spring, terrestrials in the Summertime

Updated: Jul 7, 2022

The post title is *greatly* modified from a great old Appalachian folk tune, Shady Grove. The song beings with a line about peaches in the summertime, apples in the fall. I know the tune from David Grisman - best known for his Bluegrass - and Jerry Garcia - of course, best known from the Grateful Dead - album, The Pizza Tapes. Linked below only because is such a great rendition of the song. Enjoy!

What the hell does any of this have to do with fly fishing you might be asking? Well, experienced trout anglers understand that aquatic insects hatches - mayflies, caddisflies, midges, and stoneflies - are concentrated in the cooler times of the year. For us in Wisconsin, hatches increase by mid-March and by the middle of June, they are mostly winding down until they ramp back up again in the fall. By the summer (Mid-June through most of August), terrestrials insects reign supreme.


An aquatic ecologist would call this a reciprocal prey subsidy, a term to describe how at different times of the year, aquatic ecosystems rely upon terrestrial inputs and terrestrial ecosystems rely upon aquatic inputs. Many of us fly anglers know this bit of ecology from our experiences. We fish the mayfly, caddisfly, midge, and stonefly hatches of the spring and fall, and in the summer, we are fishing terrestrial flies (ants, beetles, crickets, and hoppers). Conversely, terrestrial insects are sparse in the spring so birds, bats, and spiders feast on the abundant aquatic insects in their adult stages.

This image is from a paper from a 2001 Nakano and Murakami paper (https://www.pnas.org/content/98/1/166) from Japan is probably the best example of the concept of reciprocal prey subsidies. What is a bit confusing are the Y-axes - the explanatory variable - is different between in the panels. Let's walk through each panel in the figure. The great part of this paper is that it's a Proceedings of the National Academy of Sciences (PNAS) paper so it's in the public domain.


Panel A shows that stream temperatures are a lot less variable than are air temperatures. This is why there are trout streams! It takes a lot of sun energy to warm up water, particularly streams that are constantly fed by cold water. This IS NOT to say that you should not be aware of stream temperatures, they most certainly can get too warm to fish (more on that later too...).


Panel B, quickly, shows that streams get shaded by trees and get less photons of light in the summer, more in the spring and fall. Ok, that was a simple one.


In panels C and D, flux refers to prey that comes from outside of the system. Fluxes in aquatic systems are ants, beetles, crickets, and grasshoppers and fluxes to terrestrial systems are aquatic insects. What these panels show is that in the summer, most fishes are eating a lot of the "flux" - terrestrial prey items. For the fly angler, it means in March - June, aquatic insects (caddisflies, craneflies, mayflies, midges, and stoneflies) in Driftless streams are the key. Likewise, terrestrial predators are keyed in on - and dependent upon, prey produced in aquatic habitats. Again as observant anglers we know this as we see a number of warblers and full spider webs along streams in the spring and fall. By mid-summer, these birds and spiders are often more dispersed because streams are no longer providing aquatic insects to them. In the summer, mid-June through August - trout are eating more terrestrials - as are the birds, bats, spiders, and other terrestrial fauna. Aquatic in the spring, terrestrials in the summer.

Not to dwell on this but what is interesting here is that in Japan, Rainbow Trout (Oncorhynchus mykiss) - a non-native species - forages much differently than the native species. Likewise, some birds (wrens) and some fishes (White-spotted Char and Rainbow Trout) rely more heavily upon allochthounous prey (prey from "outside") the system. The sculpin, a specialized benthic (bottom dwelling) species, consumes almost no terrestrials. Conversely, the wren relies heavily upon adult aquatic insects. Most of the fishes and birds are somewhere in the middle but almost all are depend upon these fluxes.


Oh yeah, back to fly fishing, fish the aquatic insects when they are prevalent (spring, early summer, and fall) and terrestrials in the summer. But you know this, right? It is nice when data supports how we fish.

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