Migrating haemulid fishes as a source of nutrients and organic matter on coral reefs1

Abstract

Juvenile french and white grunts (Haemulon flavolineatum and Haemulon plumieri) rest over coral colonies during the day and feed only at night in surrounding seagrass beds. We examined the amount of nitrogen, phosphorus, particulate organic carbon, and calories which these fishes deposited over the coral colonies that were their resting sites. Weight‐specific rates of nitrogen excretion by grunts decreased with increasing fish size. Rates of phosphorus excretion were not related to fish size. Excretory products were rich in nitrogen (molar N:P = 48), primarily ammonium, whereas fecal material was richer in phosphorus (N:P = 8). Feces leached over half of their phosphorus content within the first day. Half of the daily excretion and defecation occurred during the first 4 h after grunts returned to the reef, in which time they doubled the amount of NH4+ available to corals under calm conditions. Seasonal patterns of nutrient and particulate organic carbon (caloric) input to coral colonies varied with grunt biomass on the colony. The maximum input to colonies of Porites furcata from grunts occurred during August, which coincided with the time of highest coral growth rate. Grunts deposited an average of 164 and 251 mg m−2 d−1 of particulate organic carbon (feces) on the P. furcata and Acropora palmata colonies over which they rested, an energy supplement to the colonies of 0.8 and 1.2 kcal m−2 d−1. Rates of nutrient and organic matter input from grunts are comparable to or greater than rates observed in other naturally or artificially enriched ecosystems.