Using settlement collectors to measure settlement intensity of echinoderms and other taxa at Davies Reef, Great Barrier Reef

Net bags containing 100 plastic biospheres were deployed at 6 sites, 300 to 500 m apart, on both the front (windward) and back (leeward) sides of Davies Reef. At each site, two pairs of bags, each 10 to 20m apart were deployed. Within each pair, the bags were placed 1 to 2 m apart.

Samplers were deployed prior to the summer spawning season of Acanthaster planci and many other coral reef echinoderms. On recovery, each net bag was placed in a separate large plastic bag by divers. At the surface, water within each plastic bag was passed through a 0.1 mm sieve. Material from the sieve and the net bag of biospheres were frozen until sorting occurred.

Three methods of sorting were trialled to determine which gave the best recovery of echinoderms for the least effort. The bucket technique, was to thaw the frozen samplers overnight in a 10 litre bucket of 99% alcohol. The bag was then shaken vigorously in the alcohol to dislodge any organisms. The contents of the bucket were strained through a 0.1 mm mesh screen and the material retained was then resieved through 3.5, 1 and 0.5 mm mesh screens. The three fractions were sorted under a stereo-microscope. Echinoderms retained on all three sieves were identified and counted. For other taxa, only organisms retained on the 3.5 mm sieve were identified and counted. In the rinse technique, alcohol preserved sampling bags were cut open to release the biospheres into a box with a 0.1 mm mesh base. The spheres were rinsed with a hose and the material left on the mesh was resieved and sorted as above. In the individual washing technique, spheres underwent the rinse technique and then each individual sphere was squirted with a hose to dislodge as much remaining material as possible. Material left on the 0.1 mm mesh was resieved and sorted as above.

Only 10 randomly selected samplers were used for the comparison of sorting methods. All other samplers were subjected to the individual washing technique.

Samples of rubble were collected from the sea bed during the same week that the settlement samplers were collected to examine naturally occurring levels of abundance of juvenile echinoderms in the benthic substrate. Collections were made at 4 sites on the front reef, reef flat and back reef. At each site, three boxes (26 x 18 x 8 cm) of rubble were collected and the contents later fixed in 4% formalin and sorted using the individual washing technique.