Residency or emigratory responses were triggered in bluegills 6.0-8.0 cm TL and 10.0-12.0 cm TL by availability of resources. Individuals became residents if resources were present and not at carrying capacity. Individuals in excess of carrying capacity emigrated. Carrying capacity of bluegills 6.0-8.0 cm TL increased with increased food availability. Carrying capacity also increased with increased cover. Bluegills 10.0-12.0 cm TL did not respond to changes in cover. The two sizes reacted differently when the initial number introduced into pools was varied. Single bluegills 6.0-8.0 cm TL established residency whereas bluegills 10.0-12.0 cm TL required the presence of at least one other bluegill before establishing residency. Competition between size classes were prevented by habitat segregation in which smaller fish used areas with cover and larger fish areas of open water. These results indicate that bluegills have evolved innate, stereotypic fixed action patterns for individually assessing resources by becoming residents if resources are present and numbers below carrying capacity or emigrating if resources are absent or numbers at carrying capacity. These findings suggest that the long history of overpopulation and stunting of bluegills in closed areas is due to the absence of an emigratory avenue.