Survival rates of juvenile crown-of-thorns starfish (Acanthaster planci) at Davies Reef, Great Barrier Reef

Brief description

Predator exclusion experiments, using laboratory reared juvenile crown-of-thorns starfish (Acanthaster planci), were carried out in an area of dead coral rubble on the leeward side of Davies Reef, at depths between 12 and 15 m. One month post settlement starfish were deployed in February 1991, while 4 and 16 month old starfish were deployed in May 1991 and 7 month old starfish were deployed in August 1991. Three different sized plastic boxes with tight fitting lids were prepared for the different age groups of starfish and the lids and sides of the boxes were covered with mesh. After deployment, the mesh on the boxes was cleaned every 1 to 2 days to remove silt and the adequacy of water flow through the mesh was tested periodically.Starfish were placed in boxes with either a natural rubble treatment or a control rubble treatment. The natural rubble treatment consisted of pieces of unconsolidated dead coral rubble, complete with algal and motile epifaunal assemblages, which were collected directly from the seabed at the time of deployment. The control rubble treatment was made up of sun bleached or beach collected coral rubble, which was conditioned in flow-through aquaria for several weeks to allow algal and epifaunal assemblages to develop.Ten boxes of each of the control rubble and natural rubble treatments were used for the 1, 4, and 7 month old starfish, while 5 natural rubble treatments only were used for the 16 month old starfish. In addition, 5 replicate open boxes (natural rubble treatment, without lids) were deployed with 7 month old starfish, allowing free movement of the starfish and predators in and out of the boxes. Open boxes containing 1, 7 and 16 month old starfish were deployed inside larger boxes as escape controls and on recovery the numbers of starfish which had moved to the outer boxes were counted. The number of starfish placed in each box was dependent in the age class of the starfish and the number of starfish available in each age class.The boxes were recovered after 6, 13, 16 and 13 days for the 1, 4, 7 and 16 month old starfish respectively. The contents of each box were washed into bottles and fixed in mixture of buffered 10% formalin in seawater and Rose-Bengal stain. Prior to fixing, any starfish, which could be recovered alive were removed, counted and measured (greatest diameter). The fixed material was sorted by washing the rubble over 6, 1 and 0.1 mm mesh screens and then the rubble was individually washed over the mesh. All starfish were then recovered from the mesh screens and counted and measured. Other epifauna in the 1 mm screen were also counted and categorised with the aid of a dissecting microscope. This research was undertaken to investigate mortality rates of small crown-of thorns starfish. In these field experiments, the survival of small starfish living in their natural habitat was compared with that of starfish provided with similar food resources and shelter, but in the absence of potential predators or other hazards.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: Statement: The techniques used to rear juvenile Acanthaster planci were similar to those described in:Birkeland C and Lucas JS (1990) Acanthaster planci: Major management problem of coral reefs. CRC Press, Boca Raton, 257 pp.Cages:Three sizes of commercially available plastic boxes were used. Boxes 26 cm x 18 cm x 8 cm deep were used to deploy 1 and 4 month old starfish. Larger boxes were used to deploy 16 month old starfish (35 cm x 25 cm x 12 cm) and 7 month old starfish (57 cm x 32 cm x 9 cm). The lids and sides of the two smaller box types were covered in 0.2 mm mesh and 0.5 mm mesh was used to cover all surfaces of the larger boxes. The lids of the largest of the boxes did not fit as well as those on the smaller boxes and some problems with starfish escape were subsequently encountered.Maintenance of cages:The mesh on the boxes was cleaned each day or every second day by lightly brushing the mesh to ensure water flow was not occluded by silt. To assess the adequacy of water flow, water samples were syringed from the boxes periodically over the course of one experiment and the dissolved oxygen content was measured using an ICI Dissolved Oxygen Meter. Water samples were also taken from the closed boxes (no possible water flow) and boxes within boxes (double mesh) which had been deployed with natural coral rubble. Fluorocene dye was also injected into some extra boxes to further examine the effect of the caging on water flow.Stocking density of starfish:Initial densities varied for each experiment depending on the availability of starfish at different ages. For the first experiment, 100 of the 1 month old starfish were stocked to each of the boxes (equivalent to 2137/m²), 35 per box at 4 months (748/m²), 20 per box at 7 months (110/m²) and 10 per box at 16 months (114/m²). Starfish sizes could not be measured prior to deployment.Controls for handling and sorting efficiency:To measure the efficiency of the recovery (sorting) technique, small starfish (N=50 for 1 month old and N=30 for 4 month old) were seeded into each of four natural rubble treatment boxes and the contents were fixed after a few minutes and sorted as for the experimental samples. To ensure that the degree of handling of the starfish during the counting and deployment procedure did not contribute to starfish mortality, handling controls were also carried out. These consisted of 2 or 4 replicate batches of starfish, which had been subject to the same amount of handling as those placed in the boxes were kept in dishes overnight and examined the morning following the deployment of the boxes.

Notes

Credit
Keesing, John K, Dr (Principal Investigator)