The Stream Doctor

March 10th, 2003

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


Q. Is the perceived competition between rainbow and brook trout in the southern Appalachians true?

A. Well, I guess the answer to your question is yes, no, or maybe, depending on whose research you want to have the most faith in! Fish biology, especially in the Appalachians, is not my area of expertise, so I sent an e-mail message for help to several colleagues in that part of the country. I received responses with short guesses, long explanations, and several references to pertinent journal articles - some of these with full abstracts included. I suspect you may be familiar with some of these. I'll paraphrase three responses below, and list the citations.

    1. The perception is true; whether the competition is real or not I'm not convinced. I suspect that the differential enjoyed mostly by rainbows may be a result of the timing and frequency of natural events such as floods or droughts. Although it's no doubt more involved than this, here is one example. If a major flood occurs during the fall spawning season or in late winter/spring at the time of fry emergence by brookies, they may have a weak year class. If the weather then improves leading to more moderate streamflows, then rainbows may have an easier time spawning and have a good year class. The following year (and maybe more than one year) there will be more rainbows than brookies. If on the other hand the flood comes later, after the brookies have emerged and in the middle of rainbow reproduction, the situation may be reversed. Drought could do the same thing, except that disease in the redd may be the agent of mortality. Of course, competition (exploitation-type) may come into play if there are lots more rainbows than brookies so it's not straightforward. We could spend hours discussing the role of acid precipitation, other predators, quality and distribution of habitat, selection of food items, etc.

    2. Rainbows have an advantage due to faster growth and higher fecundity and will displace brook trout in our region such that brook trout are left only in the headwaters. For decades the GSMNP biologists have been attempting eliminations of rainbows to protect remnant brook trout populations, distributions of which are much reduced.

    Brook trout are most acid tolerant, but even they cannot tolerate the acid pulses in some watersheds.

    Many of the studies of brook and rainbow competition from past researchers are suspect now that new putative subspecies of southern brook trout has been confirmed. Even in Smoky Mountain NP, the brook trout populations are contaminated with northern genes.

    3. The answer, based on a series of papers in Transactions of the American Fisheries Society over the last few years, is "no." One study seems to conclude that brookies decline, but not due to direct competition, rather due to reproductive failures (lower fecundity, weaker year classes) (and what causes these failures?). The second study found no distributional changes in brookies when sympatric with rainbows. The third found brookies behaviorally dominant over rainbows. It is fairly well established that brown trout eliminate brook trout, but from the articles below, it doesn't look like rainbows are quite as successful at it.

    Here are the pertinent citations:

    Clark, M.E., and K.A. Rose. 1997. Factors affecting competitive dominance of rainbow trout over brook trout in southern Appalachian streams: implications of an individual-based model. Transactions of the American Fisheries Society, Vol. 126, No. 1, pp. 1-20. (Two people mentioned this article)

    Strange, R.J. and J.W. Habera. 1998. No net loss of brook trout distribution in areas of sympatry with rainbow trout in Tennessee streams. Transactions of the American Fisheries Society, Vol. 127, No. 3, pp. 434-440. (Two people mentioned this article)

    Magoulick, D.D., and M.A. Wilzbach. 1998. Effect of temperature and macrohabitat on interspecific aggression, foraging success, and growth of brook and rainbow trout pairs in laboratory streams. Transactions of the American Fisheries Society, Vol. 127, No. 5, pp. 708-717. (Two people mentioned this article)

    Magoulick, D.D., and M.A. Wilzbach. 1998. Are native brook charr and introduced rainbow trout differentially adapted to upstream and downstream reaches? Ecology of Freshwater Fish, Vol. 7, pp. 167-175.

    ~ 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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