Commercial fishes as predators of adult crown-of-thorns starfish, Acanthaster planci, in the southern Great Barrier Reef

Full description

Snake Reef and Gannet Cay in the Swain Reefs of the southern Great Barrier Reef, which were experiencing outbreaks of crown-of-thorns starfish, were chosen for this study and surveyed in February 1994. Areas of high starfish density were located within these reefs by systematic manta tows and snorkelling and marked with buoys.Three fishermen targeted Lethrinus miniatus, within 500 m of the starfish concentrations. The time of capture (to the nearest hour) was recorded in the majority of cases and standard length was recorded for all fish caught. The entire digestive tracts of the 98 Lethrinids caught were removed shortly after capture and preserved. In the laboratory, entire gut contents were washed out onto fine plankton mesh (17 µm), drained and weighed. Gut contents were examined under a microscope for any Acanthaster planci remains and large prey items were recorded.At Gannet Cay, starfish were located in two patches, a small one on the west side of the reef, just south of the Cay, and another larger one on the south face. Starfish were counted in a series of dives and each animal was tagged with a temporary tag once counted. Small numbers of starfish were collected for examination of their reproductive state.The outbreak at Snake Reef covered a larger area and the numbers of starfish were estimated from a series of 50 m transects. Two divers counted Acanthaster planci in a 2 m band on one side of a tape laid along the edge of the reef, giving an estimate of the length of the affected portion of reef, as well as the density of starfish. The width of the affected area was estimated by swimming transects 5 m wide and 15 m apart, starting at the sand and running at a normal to the reef edge. The width of the infestation was taken to be the distance from the sand to the starfish that was second furthest from the sand. This was to reduce bias due to outlying starfish.Lethrinids were counted in the vicinity of the starfish outbreak at Snake Reef, in twenty 50x10 m transects. The transects were placed haphazardly along the reef edge at least 20 m apart, starting at a depth of about 3 m and running down the slope. Sixteen similar transects were swum along the west face of the reef at Gannet Cay. It was not possible to count fish in the immediate vicinity of the large aggregation because of rough seas and strong currents.
This research was undertaken to: - collect a large sample of possible fish predators of Acanthaster planci close to outbreak populations of adults with developed gonads to look for evidence of predation and to estimate per capita predatory mortality- look for evidence of timing of Lethrinus miniatus feeding because Acanthaster planci, particularly smaller size classes, are active at night and cryptic during the day.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: Statement: Gut contents: Because the fish varied in size, an index of gut fullness was calculated. It was assumed that gut volume was related to the volume of the fish. For each fish, the wet weight of gut contents was divided by the standard length.Starfish counts:Initially it was planned to count the starfish at both reefs in a series of dives, with each animal being marked with a temporary tag as it was counted. These tags consisted of small (25 mm) lengths of orange flagging tape, which were attached by impaling them on one of the aboral spines and pushing them down near the base using a 10 cm length of plastic tubing (5 mm internal diameter). Each individual was marked with two tags to reduce the chance of tag loss. There was no indication of tag loss or spine necrosis over the 3-4 days of the observations. On Snake Reef, it became clear that the outbreak covered a larger area than was first thought, so tagging was discontinued.

Notes

Credit
Sweatman, Hugh PA, Dr (Principal Investigator)