Density dependence via intercohort competition in a coral-reef fish

Demographic density dependence is essential for regulating population dynamics. For populations with multiple age or size cohorts, interactions between cohorts may be an important source of density dependence. Between-cohort interactions may be especially intense for many marine fish populations because of the potential disparity in competitive ability between small new recruits and relatively large resident conspecifics. Indeed, for marine fishes, density dependence in local populations appears to be widespread, particularly during a crucial time period following settlement of larvae into juvenile and adult habitats. I tested whether the demography of newly settled juveniles of the planktivorous fairy basslet (Gramma loreto) was affected by the density of adult conspecifics. Using spatially isolated local populations on coral reefs in the Bahamas, I experimentally transplanted a cohort of juveniles to local populations with three treatments of adult density: adults absent, average adult density, and high adult density (twice average). By following the fate of the transplanted juveniles, I determined that juvenile mortality increased as a function of adult density. Juveniles did not experience significantly higher rates of aggression or lower growth at higher densities of adults. However, in the presence of adults, juveniles generally fed at the back of aggregations, where feeding rates tend to be lower. Automated time-lapse video revealed that predators were found most frequently toward the back of aggregations. Thus competition with adults for prime feeding positions may increase predation risk for juveniles by forcing them to overlap spatially with predators. Consequently, interactions between juveniles and adults could influence the demography of subsequent generations in ways that promote local population regulation.