Edge effects on fish associated with seagrass and sand patches

Abstract

Seagrass beds form naturally patchy habitats with large areas of seagrass-sand inter- face, or edges. Fish were sampled at 3 sites in the temperate waters of Victoria, Australia, using small (0.5 m wide) push nets at 7 positions: unvegetated sand distant from the patch on the seaward side, the sand edge adjacent to the seagrass on the seaward side, the seagrass edge on the seaward side, the middle of the seagrass patch, the seagrass edge on the shore side of the patch, the sand adjacent to the seagrass on the shoreward side of the patch and unvegetated sand distant from the seagrass on the shoreward side. Samples were taken during the day and night, and seagrass variables were col- lected to describe structural complexity. As expected, more fish were caught in seagrass than over sand. Within seagrass, we found strong and consistent patterns at edges. Regardless of site, the total number of fish sampled was greater at the seaward seagrass edge (484 fish) than in the seagrass middle (231), but there was little difference between the seagrass middle and the shoreward seagrass edge (297). Two species of pipefish, Stigmatopora argus (193) and S. nigra (160), were much more abundant at the seaward seagrass edge than in the seagrass middle at all sites (54 and 46, respec- tively). The goby Nesogobius maccullochi showed a very different pattern. It was more abundant at the shoreward seagrass edge (127) than over the seagrass middle (31) at all sites, and tended to be more abundant over sand at the edge of seagrass patches than in any other sand positions. The weed- fish Cristiceps australis was significantly more abundant at the seaward seagrass edge (26) than in the middle (11), but only at night. Consistent patterns in fish distributions demonstrate clear edge effects both within and alongside seagrass at these sites in south-eastern Australia.