Acanthaster Planci (Echinodermata: Asteroidea) on the Great Barrier Reef
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MARINE ECOLOGY PROGRESS SERIES Published January 10 Mar. Ecol. Prog. Ser. Influence of hydrodynamics on the passive dispersal and initial recruitment of larvae of Acanthaster planci (Echinodermata: Asteroidea) on the Great Barrier Reef ' Victorian Institute of Marine Sciences, 14 Parliament Place, Melbourne, 3002 Australia Australian Institute of Marine Science, PMB 3, Townsville MC, 4810 Australia ABSTRACT: Numerical hydrodynamic and particle dispersion simulations showed that neutrally buoyant larvae are subject to a complex circulation pattern which induces a spatial variability in their relative numbers. The interaction of tidal, wind and gradient-driven flows with the reef were found to cause some areas within individual reefs to retain higher numbers of larvae than others. These locations were compared with diver observations of the abundance of Acanthasterplanci (L.) at the beginnings of outbreaks and a clear correspondence was found in all 6 simulated cases. Results suggest that hydro- dynamics have a major influence on not only the dispersal of larvae of A. planci but also perhaps the location of their initial recruitment. Surveillance of these locations may provide an 'early warning' strategy for monitoring and controlling future outbreaks of this starfish on reefs. INTRODUCTION dispersal at the scale of the reef and its surrounds. There had been no experiments, apart from some The crown-of-thorns starfish Acanthaster planci (L.) short-term localised measurements (Ludington 1981, forms large aggregations which can lead to the des- Andrews et al. 1984), which measured the long-term, truction of extensive areas of hard coral. Impacts of this reef-scale dispersal characteristics around individual type have been recorded in a number of reef systems reefs on the GBR. Over the last few years, a numerical throughout the Indo-Pacific region (Moran 1986). Two modelling investigation was established that utilised series of outbreaks have been recorded on the Great measured water velocities to reproduce flow patterns Barrier Reef (hereafter GBR) since the early 1960s, the around individual reefs (Black & Gay 1987a). Dispersal second of which is still underway (Johnson et al. 1988, characteristics were then examined numerically at the Moran et al. 1988). The cause of these outbreaks is still appropriate time scales using the velocities produced unknown (Moran & Bradbury 1989) despite a recent by the numerical hydrodynamic model (Black 1988). multidisciplinary program of research in Australia In this paper, data on the distribution and abundance (Zann & Moran 1988). The work presented here has of Acanthasterplanci and associated coral mortality are formed part of that large research effort (Baker & combined with those from numerical model investiga- Moran 1987). tions of larval dispersal. Repetitive observations of the Whilst knowledge of the early life history of Acan- former, at the beginning of outbreaks, provide useful thaster planci continues to improve (e.g. Zann et al. information about the possible location of recruitment 1987, Olson & Olson 1989) its dispersal and recruitment sites. However, the significance of this information is are still poorly understood. For example, it is still not limited when considered in isolation. The objective of known on which part of a reef the larvae normally the present study was to compare these data with those settle. Such processes are central to understanding the from numerical hydrodynamic and particle dispersion cause(s) of outbreaks (Moran 1986). sirnulations of the same reefs to gain useful insights Similarly, at the beginning of this study very little into the larval dispersal and recruitment of A. planci. was known about residence times or patterns of larval The results also have application for the management O Inter-Research/Printed in Germany 56 Mar. Ecol. Prog. Ser of coral reefs and the strategies adopted for the control searched intensively on reefs where few starfish were of the starfish. evident. The location of the start and finish of each swim were accurately recorded on an aerial photo- graph of the reef at the completion of each survey. METHODS A total of 22 outbreaking reefs were surveyed off Townsville using one or both techniques described Biological data. Surveys of the distribution and above. Overall, the data from 8 of these reefs (Bowden, abundance of Acanthaster planci were initiated on a Davies, Dip, Faraday, Glow, Helix, John Brewer, and number of reefs off Townsville in October 1982. These Wheeler) were sufficient to indicate where large popu- were conducted using the rapid reconnaisance tech- lation~of starfish were first located. The positions of nique of manta towing, which had been used in some these reefs and extent of the various surveys under- earlier surveys of the starfish (Endean & Stablum 1973). taken on each are given in Figs. 1 & 2, and details of The advantages and disadvantages of manta towing the surveys are in Table 1. have been discussed by Kenchington & Morton (1976). It should be stressed that the biological data were not Despite the fact that the data are subject to various chosen to fit the results of the simulations, as both errors and biases, the technique has proved extremely projects were carried out independently of each other. useful as a means of locating major changes in the The former were conducted from October 1982 to distribution and abundance of A. planci (Fernandes et October 1987 while the latter were conducted from al. in press). January 1987 to May 1989. The technique involves towing an observer (who Hydrodynamic models. Hydrodynamic modelling of holds onto a board) behind a small boat at a constant 6 reefs (Bowden, Davies, Glow, Helix, John Brewer and speed (generally about 1.5 knots). The observer gener- Wheeler) was undertaken with the 2- and 3-dimen- ally is towed along the surface and in areas of shallow sional numerical model 2DD (Black 1989) which has water (e.g. reef perimeter) where the reef is readily the necessary features for simulation of coral reef circu- visible. Three variables were recorded during the lation, including wetting and drying of inter-tidal manta tow surveys: the number of Acanthaster planci zones, non-linear terms and a choice of boundary pro- and the cover of live and dead coral. These variables cedures. The model was designed to be portable and were integrated over a 2 min period after which time had been applied to estuarine, ocean and continental the boat was stopped and the observer recorded a shelf environments (Black & Gay 1987b, McShane et al. value for each. In the case of live and dead coral cover 1988, Black et al. 1989). For the grids employed in this the values were given as one of 6 categories: 0 = 0 %, study, which had 4 open boundaries, it was necessary 1 = 1-10%, 2 = 11-30%, 3 = 31-50%, 4 = 51-75%, to apply analytical solutions for coastal influences out- 5 = 76-100 O/O. All manta tow surveys were conducted side the grid and the non-linear interactions of the around the perimeter of each reef. The technique is current components which occur on the GBR continen- described in more detail by Moran et al. (1989). tal shelf (Black & Gay 1987a, Black & Gay in press a). Many of the reefs off Townsville had experienced Dispersal modelling was undertaken with the fully outbreaks of Acanthasterplanci during the early 1970s Lagrangian 2- and 3-dimensional particle model 3AD and, on a number of these reefs, starfish were begin- (Black 1988). Lagrangian residuals (i.e. the distance ning to outbreak again during the present surveys. As a moved by a particle over a known time interval and consequence, the surveys were repeated on an annual starting from a known location) were used in prefer- basis, and in some instances as often as 4 times a year ence to Eulerian residuals (i.e. the average current over (see Moran et al. 1985). a time interval at a point in space) as they account for A series of underwater searches also were conducted spatial variations in currents and thus are the better on some reefs to obtain more detailed estimates of indicator of larval transport over a tidal cycle. The starfish abundance. During these surveys a team of 3 model corrected for flow curvature to second-order and divers swam over a defined section of the reef perime- eliminated the numerical dispersion characteristics of ter. They swam ca 2 to 3 m apart and covered a total Eulerian finite difference schemes (Black & Gay in search width of about 12 m. The depth at which each press b). diver swam varied according to the slope of the reef. The hydrodynamic model was calibrated using the Whilst swimming, each diver kept a record of the measurements of Pickard (1986) from Davies Reef, and number of Acanthaster planci sighted. Each starfish then verified against drogue measurements on John was marked (by slashing it with a knife) once it had Brewer Reef (Black & Gay 1987a). Bed friction was been recorded to avoid the same individual being established using wave measurements from a counted by another diver. Areas of coral in the vicinity Hawaiian coral reef (Lee & Black 1978) and from direct of feeding scars (patches of dead, white coral) were measurements of the sea bed boundary layer on the Black & Moran: Dispersal and recruitment of Acanthaster planci larva 57 Fig. 1. Location of reefs in the central Great Barrier Reef GBR. Further confirmation of the model was obtained that they are close to neutral buoyancy. Dispersion by simulating Rattray Island (Black & Gay 198713, Black associated with turbulent current oscillations and verti- 1989) where detailed flow measurements had been cal shear was modelled as a random walk, with sepa- made (Wolanski et al. 1984). rate lateral and longitudinal horizontal diffusion coeffi- Advection in the particle model was determined by cients. Because of the complex flow patterns on a coral the predicted flow patterns, and therefore was cali- reef, advective dispersion caused by horizontal velocity brated at the hydrodynamic model stage.