The relationship of the mummichog, Fundulus heteroclitus, to its food supply was examined using 8 m x 20 m enclosures under field conditions. Density effects were examined by stocking these enclosures with fish at densities ranging from 1/8 to four times the natural population density. Mean growth rate in each pen was estimated from measurements on individually marked fish. Fish enclosed at normal density grew at the same rate as fish from the natural population, indicating that caging had minimal effects on growth rate. When enclosed below natural density, mummichogs had a growth rate that was 2-3 times higher than that of the natural population. Fish enclosed at four times normal density had a negative mean growth rate, a higher mortality rate than fish enclosed at natural density, and no egg production.In further experiments, food was added to the same type of enclosure at fish densities ranging from one-half to four times natural density. Food additions had a positive effect on growth rate at all densities. Food additions also caused egg production of fish enclosed at the highest density to increase to a rate equivalent with the natural population. We conclude that the total food supply may regulate the maximum size of the Canary Creek mummichog population by affecting the individual growth rate, mortality rate, and fecundity.

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