Tissue condition and growth rate of corals associated with schooling fish’

Juvenile french and white grunts (Haemulorlflavolineatum and Haemulonplumieri) form diurnal resting schools over colonies of Porites furcata and Acropora palmata in St. Croix. We studied the effect of these grunts on the tissue composition and growth rate of corals. Porites furcata colonies with grunts had significantly more tissue cm*, N cm-* and zooxanthellae cm-* than colonies without grunts. Acropora palm&a colonies with grunts also showed elevated amounts of N cm-2 and P cm+. Seasonal changes were also observed in these parameters and in coral growth rates and skeletal density, apparently related to light availability. Several measures of coral growth rate were significantly greater in P.furcata colonies with grunts. Grunts were removed from two colonies, and growth rates before and after removal were compared to rates in reference colonies. Before removal most growth rate measures were greater in the colonies with grunts; after removal growth rates were lower or not significantly different. These data demonstrate that schools of resident fishes can stimulate coral growth, perhaps by providing nitrogen supplements. Abundant migrating vertebrates can be effective transporters of nutrients and organic matter in terrestrial (Hutchinson 1950), wetland (McCall and Burger 1976; Onuf et al. 1977), freshwater (Richey et al. 1975; Durbin et al. 1979), and marine ecosystems (Ganning and Wulff 1969; Bray et al. 198 1). Similarly, fishes that migrate between feeding areas away from coral reefs and resting areas on the reef provide a nitrogen and phosphorus supplement to corals (Faulkner and Chesher 1979; Meyer et al. 1983) and may also make a significant contribution of organic carbon (Ogden and Ziemann 1977; Meyer and Schultz 1985). Heterotypic schools ofjuvenile french and white grunts (Haemulon flavolineatum and Haemulon plumier) are abundant on many Caribbean reefs. During the day these fishes rest over coral colonies, and at dusk they migrate to surrounding seagrass beds where they feed on benthic invertebrates. The groups form again at dawn and return to the same coral colony over precise migration paths (Ogden and Ehrlich 1977; McFarland et al. 1979; Helfman et al. 1982). Their guts are full at dawn and empty by the time they leave the reef (McFarland and Hillis 1982; I This research was supported by NSF grant OCE 79-1046. 2 Present address: Department of Biological Sciences, University of California, Santa Barbara 93 106. Meyer pers. obs.). On patch reefs in St. Croix, where this study was done, the excretory products of resident grunts added 3.9 mmol N mm2 d-l and 0.2 mmol Pm-2 d-l to Porites furcata colonies and 9.6 mmol N rnM2 d-l and 0.62 mmol P me2 d-l to Acropora palmata colonies (Meyer et al. 1983). The nitrogen supplement is primarily NH4+, a form of nitrogen readily available to corals (Muscatine and Porter 1977; D’Elia and Webb 1977; Muscatine and D’Elia 1978). Such nutrient supplements appear to be particularly valuable in the nutrient-poor waters of a coral reef. We have previously noted accelerated skeletal growth from a limited sample of coral colonies of one species (P. furcata) in the presence of resting schools of grunts (Meyer et al. 1983). We examine here several colonies of two coral species, P. furcata and A. palmata. We present information on the impact of resident grunt schools on weight and nutrient content of coral tissue, zooxanthellae abundance, skeletal density, and coral growth rate, and on seasonal changes in these parameters. We thank the staff of the West Indies Laboratory for their assistance. G. S. Helfman provided advice and assistance in the fieldwork. R. and H. Carpenter assisted in the grunt removal experiments. We also thank G. S. Helfman, G. J. Smith, J. F. Battey, J. W. Porter, and E. H. Gladfelter for com-