Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Biological Sciences

First Advisor

J. Michael Fitzsimons


The instream distribution of the tetra, Astyanax fasciatus, was investigated through approximately 700 hours of field observations in a small Costa Rican stream on the southwestern Pacific coast near Golfito. Individuals tended to remain in particular pools where they subsisted primarily on a diet of terrestrial insects. These fish were able to migrate, and only as many individuals stayed in any pool as could be sustained there by available food resources. Because the water intercepted insects fairly uniformly over its surface, the biomass of fish occupying any particular pool was proportional to its surface area (r$\sp2$ = 0.652). The amount of insects available to fish was sometimes augmented inadvertently by the activity of lizards, Basiliscus vittatus. These lizards regularly climbed the nearly vertical stream banks and caused avalanches of soil containing small insects to fall into the water. Fish biomass was also related to the amount of surface area of streamside walls (r$\sp2$ = 0.687). When the effective surface area of such walls was experimentally reduced, fish populations in adjacent pools decreased through emigration. Reduction in wall surface available for input of food caused changes in rates of foraging that led to delays in the daily pattern of behaviors associated with feeding and satiety. In this pattern of feeding behavior, larger individuals tended to feed first with a progression of smaller, less dominant fish having an opportunity at incoming food as dominant fish became sated. Because these fish came to the surface to feed, their foraging behavior was influenced by risk of avian predation. Schools tended to remain at greater depths in areas with actively feeding kingfishers (Chloroceryle americana)(t = 8.349; df 19; p = 0.0001) Astyanax distinguished between models of piscivorous and non-piscivorous species of birds. The fish responded to a model of a predatory species by moving deeper in the water, by tightening the school, and by moving to areas with rippled surfaces. The study, showed that Astyanax distribution within this stream was influenced by availability of the resources of both space and food, by intraspecific competition, and by risk of predation.