A critique on the relationship of surface area of live coral with total number of fishes as well as the biomass of fish in a coexisting system of Chromis caeruleus and Dascyllus aruanus (Pomacentridae) at Minicoy atoll

Abstract

The pomacentrid fishes Chromis caeruleus (Cuvier) and Dascyllus aruanus (Linn.) coexist on ramose live corals without apparently displaying aggressive reactions. The population in a coral colony is of many size groups, an obvious result of continual s ettlement of postlarvae of fishes from plankton, probably a strategy in nature to prevent in breeding in resident fishes. The relationship between the total number of fishes residing on a coral and the upper surface area of the habitat (coral colony) can be expressed as : Y - 0.2117 — 0.000064 X, where Y is the number of fishes per unit area of the coral and X the upper surface area of the coral sampled ; the correlation coefficient (r) being — 0.486. The relationship between the fish biomass and the upper surface area of the habitat can be expressed as : Y = 0.07888 - 0.00005225 X, where Y is the wei^t of fish per unit area of the coral sampled and X is the upper surface area of the coral; the correlation coefficient (r) being —0.99. As indicated by these correlation coefficients, there exists a closer relationship (inverse) between the biomass of fish and the surface area of the microhabitat (live isolate coral) than between the total number of fish and the surface area at a time in a coexisting system of resident reef fishes. The ratio of the fish biomass and the upper surface area of the coral (an index of density of fish) is also found to vary in different samples, within a range of 380.3 to 934.25 cm' of the coral and a fish biomass of 22.42 to 30.87 gm weight of fish sampled. The total length of the fishes ranged from 7 mm to 40 mm in the samples. The ptesratt study also indicates that neither a numerical nor biomass consistency of fishes can be anticipated in a microhabitat over a prolonged time. Both these factors are ever changing as also the area of the living habitat viz. the live coral colony. This is due to the interplay of many natural and artificial factors, such as, continual recruitment of postlarvae, growth of fish at sites, migration and mortality of fishes as well as the partial death or growth of corals that cause a dwindling or expansion of living space for resident fishes. The above factors in nature control the density of fish population in a coral live isolate. The maximum carrying capacity of the habitat perhaps is never allowed to reach so that coexistence is made possible without the fishes displaying apparent agonistic reaction.. However, the present data did not clearly indicate the maximum carrying capacity of unit area of Acropora corymbosa sampled; the systems analysed were in different stages of density level.