Good Things Are Yet to Come

We spent this past weekend on the North Ram – the fishing was steady, pleasant, and what I would call no frills. With multiple mayfly hatches going on at the same time (mostly PMD’s, Rhithrogena, and Epeorus), the fish were decidedly non-selective. We did well with a few general dry fly patterns (mainly Parachute Adams #14, and Harrop style PMD’s #16 & #18). I did work a Drunella (Green Drake) nymph with good success here and there, but with the fish more than cooperative on top, why bother going deep.

What struck me the most however was the number of Glossosoma caddisfly larvae inhabiting the river this year. I have seen previous years where Brachycentrus caddisflies were way more prolific than usual (often after a few consecutive years of stable flows and mild runoff), and fishing larval patterns was pure dynamite. But this is the first year I have seen an “outbreak” of Glossosoma. Just about every rock had a few larval cases attached, and some rockes were virtually covered in them. They were everwhere.

We often think of aquatic insect populations as being relatively stable on any given stream – with perhaps some variation in the timing of the hatch. But in reality, population levels can vary substantially from one year to the next as overall stream conditions change (e.g. average and peak temperatures, average and peak flows, predation, parasites, etc.). This leads to years where the population of a particular species is low, and years where the population is high (think about how mosquito populations can vary depending on local conditions).  Of course when a prey source is abundant, the predators (i.e. trout) take notice.

Several dozen Glossosoma sp. larva attached to a submerged rock

Caddisflies in the genus Glossosoma are known as turtle case makers. Rather than constructing a tube like case that they can add to as they grow, Glossosoma builds a simple dome like structure out of small pebbles. They spend much of their time safely huddled under their unassuming mound of pebbles slowly grazing on the thin layer of algae that covers the rocks. As they outgrow their lair, they must abandon their old case and build a larger one. This is most often done under the cover of darkness but the process often commences just as dusk takes hold. With the added activity of thousands of larvae moving down off their grazing rocks to find suitable construction materials, many larvae find their way into the drift, and the waiting maws of hungry trout.

Close-up of Glossosoma sp. larval case attached to rock

There are four species of Glossosoma in Alberta which can emerge anywhere from June to late September. Most of the Glossosoma that I saw on the North Ram were in the later stages of larval development indicating that they will begin pupating soon, and will likely emerge some time in mid to late September (note that adults are needed to pin down the exact species).

Glossosoma sp. larva in case viewed from the bottom
(ventral) side - note the opening at either end

Glossosoma sp. larva "naked" (i.e. without its case)

If happen to find yourself on the North Ram late into the evening over the next week or so, and the mayflies have shut down due to cool evening temperatures, try drifting a simple “naked” caddis larva pattern along the bottom. Tan, cream and pale green would be the colors of choice in size #18 or #20. Later in September as the hatch gets rolling a soft hackle or sparkle emerger pattern should do the trick.

I plan to return to the North Ram in mid-September to see if the hatch materializes as I have anticipated; and perhaps collect and identify some of the adults.