The Stream Doctor

March 29th, 2004

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

Ladyfisher called my attention to a very interesting dialogue in the FAOL Bulletin Board pertaining to a "almost hatch" phenomena and the resulting exchange on mayfly taxonomy. She has asked me for my comments.

First, I can't comment knowingly on the "almost hatch" phenomena. It could well be related to one of the types of drift phenomena, but the brief description precludes anything definitive.

Second, I do have some comments on the related taxonomical discussion. To start, I want to emphasize that the following comments are not meant to sound elitist nor in any way denigrate the comments/opinions of those who participated in the exchange. I firmly believe that much can be learned from dedicated non-professionals and the pros can learn much from them. Having said that, however, I think some cautionary comments are warranted.

The contributors to the bulletin board dialogue spent a lot of time discussing the identification of nymphs to species. The fact of the matter is that even the pros are reluctant to do this except in certain cases. For instance, if you have a baetid nymph with two tails, it's a pretty good bet that it is Baetis bicaudatus. However, most specific identifications are based on patterns of wing veination, structure of reproductive organs, and other characteristics that manifest themselves in adults, but not immatures. Taxonomists typically raise nymphs under laboratory conditions to collect the adults and thus match them with known nymphs. Even then, they hesitate identifying immatures to the species level in the field because of (1) inherent variations in color and other characteristics that can be influenced by habitat conditions, and (2) the lack of distinctive morphological characteristics in immatures. The latter makes field identification of immatures to species particularly difficult when more than one species of the same genus is present. I know of streams where there are several species of Ephemerella present; it would be risky to try to put specific names on these unless (1) they were quite distinctive morphologically from the others, and (2) you had confirmative information from adult identifications.

To pursue this a bit further, let's look at the genus Ephemerella, the genus addressed in the bulletin board dialogue. The family Ephemerellidae contains 90 species within its 7 genera, and the genus Ephemerella contains 47 of these. All are morphologically similar, to some degree, and the other 6 genera have only recently been separated from Ephemerella. So, can a person distinguish which of the 47 species he finds? Some species, of course, can be eliminated because of distributional ranges, but some ranges overlap. What I'm trying to point out here is that it is extremely risky to put specific names on immatures unless you have a pretty damned good knowledge of what has been identified from the stream as adults.

Ecologists are sometimes successful in naming immatures to the species level when they have a limited number of species of the same genera present in their study site and they are able to collect adults for positive specific identification. In this case, it is reasonably safe to then put a specific name on the nymphs.

I guess this is why those of us with scientific backgrounds in this area and who are also fly anglers tend to be rather skeptical when we see specific names published in the popular literature. For instance, many streams have several species of Baetis present that even a taxonomist would hesitate in identifying to species as nymphs; yet the fishing literature is full of names to the specific level. I can't say they're wrong; it's only that I get skeptical when knowing how difficult it is to identify insects to the species level. Another example: Baetis and Callibaetis can be found in the same environment in places and the main morphological difference that separates the two genera are that the gills on Callibaetis have a small flap that can only be seen under a dissecting microscope! Adults are quite similar, yet writers don't hesitate in giving scientific names to the hatch they're fishing. I know some taxonomists who have confided that they wish they had as good an eye as some angler writers!

I'm very impressed by the knowledge and interest shown by all of you and certainly encourage your efforts. I guess a career as a scientist and the need for showing proof of what I say instilled a healthy dose of skepticism in my personality. So be it.

If you have a question, please feel free to contact me.
~ C. E. (Bert) Cushing, aka Streamdoctor
105 W. Cherokee Dr.
Estes Park, CO 80517
Phone: 970-577-1584

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

If you would like to comment on this or any other article please feel free to post your views on the FAOL Bulletin Board!

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