BIOEROSION AND SEDIMENT INGESTION BY THE CARIBBEAN PARROTFISH SCARUS-VETULA AND SPARISOMA-VIRIDE - IMPLICATIONS OF FISH SIZE, FEEDING MODE AND HABITAT USE

Erosion rates and sources of sediment ingested were quantified for the 2 most abundant parrotfish species on a leeward fringing reef of Bona ire, Netherlands Antilles: Scarus vetula and Sparisoma viride. Direct estimates of erosion by different size classes were obtained from dail y feeding rates and grazing scar frequency, scar volume and substrate density. Foraging preference and distribution of fish on the reef were used to examine patterns of bioerosion at 2 spatial scales: reef zone s and individual substrates used for grazing. Sediment mass ingested b y fish provided an independent check on erosion rates, and was partiti oned according to source. S. vetula, employing a scraping feeding mode , removed less material from grazed substrates than similar sized S. v iride, which forages by excavating the substrate. Erosion rates increa sed strongly with fish size in both species. The (indigestible) carbon ate derived from epilithic algae accounted for all sediment ingested b y juvenile fish. In adult fish, the proportion of freshly eroded carbo nate substrate ingested increased with fish size. The distribution of adults of these large scarids over different reef zones determines the rate of bioerosion on a large spatial scale. The highest bioerosional rates occur on the shallow reef (ca 7 kg m(-2) yr(-1)), and they decr ease with depth. Parrotfish foraging preferences, and the effects of f ood type and skeletal density of substrates on the size of the grazing scars, cause large differences in bioerosional rates on a small spati al scale. The highest rates of bioerosion occur on substrates infested with boring algae and of low skeletal density, while high-density sub strates and substrates covered with crustose corallines undergo lower rates. Living coral is rarely eaten by scarids, and largely escapes er osion by grazing.