Seasonal and annual variation of coral-reef fishes on the upper reef slope at Guam
Molina ME. 1983. Seasonal and annual variation of coral-reef fishes on the upper reef slope at Guam. University of Guam
Coral-reef fishes were monitored monthly at four upper reef-slope depths (5, 9, 18 and 30 m) at two locations at Guam between September 1979 and November 1980. Overall fish density increased markedly at all depths during the spring and summer months, corresponding to the onset of the rainy season and the diminishing of the tradewinds. Maximum abundances were recorded between May and July. A less pronounced increase in fish abundance occurred in the fall. Most of the observed seasonal variation in fish abundance resulted from juvenile recruitment and the movements of subadults and adults of a relatively small group of abundant species at each depth. Planktivores, piscivores and benthic-invertebrate feeders, primarily in deeper water, were largely responsible for the spring/summer peak, while the fall increase was significantly influenced by herbivorous fishes at shallower depths. Fluctuations of fish abundance may be related to variations in the availability of food resources. Climatological and oceanographical phenomena may have favorably influenced food resource availability as well as reproductive success during certain months. Estimates of site-and depth-related annual variation in fish abundance and species composition of 35 ubiquitous fish species indicated relative constancy over extensive areas of reef. Fish species richness was found to be greatest at 18 m. An explanation for this trend in species richness based on the "intermediate disturbance hypothesis" is offered.