MEPS 303:113-121 (2005)  -  doi:10.3354/meps303113

ABSTRACT: Habitat heterogeneity, particularly patchiness in seagrass cover, greatly affects the community dynamics of seagrass ecosystems. Heterogeneity in seagrass habitats can also be caused by unvegetated patches within the otherwise continuous seagrass cover. Blowouts are bare areas with an eroding edge that forms a vertical wall, often with overhanging seagrass roots and rhizomes. Fishes, including parrotfishes, aggregate in these blowouts. These aggregations of herbivorous fishes might influence the structure of the adjacent seagrass habitat. We compared parrotfish Sparisoma spp. grazing patterns at different distances from the edge of blowouts in seagrass beds (mainly Thalassia testudinum) at St. Ann’s Bay, Jamaica, by counting and measuring the number of bite marks on the seagrass blades. The percentage of grazed blades, number of bites per blade, and total area of seagrass removed by grazing all increased with increasing distance from the edge of the blowout. However, the size of the bite marks, which is indicative of the size of the fish that made them, was significantly larger near the blowout than at further distances. Few large bites were found at distances greater than 4 m from the blowout, while small bites were more abundant at distances further from the edge. These data suggest that larger fishes feed in closer proximity to the shelter of the blowouts, while smaller fishes avoid the blowout edge. Both seagrass morphology and parrotfish grazing characteristics were significantly different among the blowouts used in the study, indicating that blowouts in and of themselves increase the structural heterogeneity of seagrass beds. Thus, blowouts affect the structure of seagrass ecosystems directly and indirectly (via their effects on the grazing behavior of parrotfishes).

KEY WORDS: Habitat heterogeneity · Grazing · Blowouts · Parrotfishes · Thalassia testudinum · Seagrass