It has been my experience that trout feeding on daphnia do so selectively; that is they seek out and take individuals much like they do the critters that we more tradtionally match with our flies.

In very clear water I have watch the cruising trout sipping these micro critters rather than swimming around with their mouths open - which is the behavior I would have expected if they were "filter feeding". A number of times I have sample trout that have been feeding on daphnia and consistently found that the individuals consumed were the larger ones in the population - there was a size selection in their feeding.

There are actually several "papers" that have found that in taking daphnia trout (both rainbows and browns) do so by selecting the larger (typcially those larger than something like a mm) and the more darker pigmented individuals. This size and color selection are clear indications that the trout are using visual clues in their feeding.

The good news with trout feeding daphnia and the problem of "matching" the hatch is that the heavy daphnia selection is mostly a winter/early spring issue. We are currently on the cusp of warming temperatures where the feeding trout will be forming "search imagines" for larger critters.

For the record when I found trout feeding heavily on daphnia they can still be caught (at least to some degree) on tradtional patterns. It seems that they sometimes can not resist a well presented bug for a little variety to their diet. While logic would dictate that a slow presentation for daphnia feeding trout (bobber watching) is the way to go a more successful stragety for me has to be go to the other end and fish more aggressively with streamers, attractor patterns or a stripped soft hackles. My thinking that showing my bug to more fish in a given period increased my chances of getting my bug in front of a willing biter; either some sort of reaction take or boy that looks go take.

Tight lines
Curt