Fish larvae settling in seagrass: Effects of leaf density and an epiphytic alga

Settlement of fish to seagrass with low leaf densities was investigated by a field experiment in Botany Bay, NSW. Artificial seagrass units (ASU) were used to simulate isolated seagrass habitats of 0, 25, 50, 100, 200 and 400 leaves m^‐2. A total of 369 fish of 21 species settled in the experimental units during a 6 week period. The labrid, Achoerodus viridis, made up almost 70% of these individuals. The abundance of recently settled A. viridis increased greatly between 0 and 25 leaves m ^‐2, At more than 25 leaves m^‐2, A. viridis settled in approximately equal numbers. Patterns of settlement for all other species combined were similar to those for A. viridis. During the experiment, an epiphytic alga. Giffordia mitchellae, colonized and grew on the ASU. The alga increased the structure associated with the experimental units. Treatment leaf densities could thus be regarded only as relative measures of shelter. At low leaf densities, fish settlement increased with increasing algal cover; at higher leaf densities, the opposite occurred. The rapid increase in abundance of recently settled fish over a relatively narrow range of leaf densities above zero implies that small increases in shelter can have large effects on settlement of larval fish.