The influence of density-dependent and number-dependent processes on individual demographic rates often results in ecological tradeoffs. Because such processes have important implications for individual level fitness and population regulation, they long have been an important topic in ecological research. I used Dascyllus trimaculatus, a site-attached planktivorous coral reef fish, to determine independently the effects of population density and group size on rates of individual growth and mortality. Somatic growth of D. trimaculatus was positively related to the density of D. trimaculatus outplanted to host anenomes, Heteractis magnifica, with nearly twice as much growth observed in individuals living in the highest density treatment. By contrast, survivorship of D. trimaculatus on H. magnifica exhibited a negative relationship with density. There was no significant effect of group size on either the growth or mortality rates of D. trimaculatus. These relationships suggest a tradeoff between density-dependent growth and survival in this species. My analysis of behavioral data indicated that D. trimaculatus living under conditions of higher local population densities displayed decreased rates of intraspecific aggression because of positive feedback between local population density and the foraging distance from an individual’s host anemone. In turn, lower rates of aggression and feeding higher in the water column resulted in increased somatic growth rates due to: (1) a reduced energy expenditure and (2) an increase in prey consumption. These data are part of a Master's Thesis published by ProQuest: Nielsen, Jessica Anne. Effects of density on behaviorally-mediated tradeoffs between growth and survivorship. MA Thesis, University of California Santa Barbara, 2013.