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Fausch, K. D. and M. K. Young (1995). Evolutionarily significant units and movement of resident stream fishes: A cautionary tale. Pages 360-370 in J. L. Nielsen, ed. Evolution and the Aquatic Ecosystem: Defining Unique Units in Population Conservation, Monterey, CA (USA), American Fisheries Society Symposium.

Many taxa of resident stream fishes are reported to be relatively sedentary throughout their lives. Such discrete populations would make identification and management of evolutionarily significant units (ESUs) straightforward. However, in contrast to this prevailing restricted movement paradigm, recent evidence indicates that even resident salmonids, such as interior stocks of cutthroat trout Oncorhynchus clarki living in headwater streams, move often, sometimes over relatively long distances. Resident stream fishes likely move in response to various ecological constraints, including the need to garner enough scarce resources or find a critical resource, or because the habitat they occupy becomes suboptimal or unsuitable. This emerging paradigm shift has important implications for defining and managing ESUs of resident stream fishes. For example, timing of sampling may affect which of several different "populations" mobile individuals are chosen to represent. Isolating small populations of native fishes above barriers to prevent invasion by exotic species may trade this risk for other environmental, demographic, or genetic risks caused by eliminating dispersal. Moreover, isolating small population fragments via natural or anthropogenic disturbances, or management actions, may create artificial ESUs. Biologists must understand not only the genetics and taxonomy, but also the spatial and temporal dynamics of component populations of species if they are to accurately identify and wisely manage ESUs.