The Stream Doctor

July 14th, 2003

Email YOUR Questions directly to the Stream Doctor. This is your opportunity to get an experts professional opinion on anything stream related.

Q. From Ann: I know that trout feed on caseless caddis fly larvae that cruise around on rocks and fall off or when they dangle by a silken thread. Have you ever heard of trout feeding on caddis while they are in their nets (i.e. Hydropsyche)?

A. This is a good question, but not one easily answered. First, a couple of clarifications are in order. Although caddisflies (note: one word, not two as you used because they are not true flies of the order Diptera) range from free-living (caseless, family Rhyacophilidae) to those building both fixed and portable cases, members of the genus Hydropsyche (family Hydropsychidae) that you cite do not build nets, per se. Instead, they build fixed retreats of small pebbles and detritus in which they live, and then construct a net across the opening to strain fine particulate matter from the water for food. Other families (e.g., Philopotamidae, Psychomyiidae) construct nets in which they live.

Now, do trout feed on caddis while they are in their nets? There is a wealth of information documenting the presence of caddisflies, including hydropsychids, in trout stomachs. The question is: Did they pick these off rocks or obtain them from drifting organisms dislodged from the bottom? I think there is no question that trout can actively pick caddisflies which are exposed, the caseless ones and ones obvious in their nets. In the case of the Hydropsychidae, I do not know of any evidence showing that trout consume them while they are still in their retreats. It is much more likely that they obtained them from the drift or when they ventured out of their retreats to actively graze on algae, both of which occur mainly at night or under low light conditions. Many trout stomachs contain small pebbles and detritus. These could be either the remains of hydropsychid retreats or items picked up accidentally in foraging. I should add that hydropsychids in their retreats would be virtually invisible to trout, although they could possibly find them from chemical stimuli.

After drafting this reply, I called several colleagues experienced in trout food habits and aquatic insect habits to see if they had any information different from the above; they didn't. If you'd like to pursue this further, contact me directly at cecushing@aol.com.
~ C. E. (Bert) Cushing, aka Streamdoctor
105 W. Cherokee Dr.
Estes Park, CO 80517
Phone: 970-577-1584
Email: streamdoctor@aol.com

The 'Stream Doctor' is a retired professional stream ecologist and author, now living in the West and spending way too much time fly-fishing. You are invited to submit questions relating to anything stream related directly to him for use in this Q & A Feature at streamdoctor@aol.com.
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