Observations on Stream Minnows in a Patchy Environment: A Test of a Theory of Habitat Distribution

The seasonal habitat use and movement patterns of juvenile and adult Semotilus atromaculatus and Rhinichthys atratulus, two stream—dwelling cyprinids, were studied for a period of 2 yr in a small headwater stream. The responses of the fish to the patchy habitat were used to test a prediction of the Fretwell—Lucas Theory of Habitat Distribution, namely that populations tend to become more uniformly distributed over habitat patches as population size increases. Marked individuals executed complex movement patterns which were species, age, and season dependent. Although all age classes of both species were able to move among all habitat patches, adult S. atromaculatus were clumped at high densities, while adults of R. atratulus and juveniles of both species conformed to the prediction of the theory. The clumping behavior observed at low population densities is discussed as a possible adaptation to ambush predation.