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Settlement and recruitment of Acanthaster planci on the Great Barrier Reef: questions of process and scale


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Johnson, CR 1992 , 'Settlement and recruitment of Acanthaster planci on the Great Barrier Reef: questions of process and scale' , Australian Journal of Marine and Freshwater Research, vol. 43, no. 3 , pp. 611-627 , doi: 10.1071/MF9920611.

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This paper briefly reviews information on settlement and recruitment of Acanthaster planci in the
Great Barrier Reef (GBR) system, with emphasis on the recent phenomenon of population outbreaks
of the starfish. Observations that recent outbreaks have occurred in two series of activity (1962-77 and
1979-91 +), each characterized by a southward wave of infestations in the central section of the GBR,
and that outbreaks are not preceded by observable increases in densities of juveniles, have important
implications for settlement and recruitment processes. The pattern of outbreaks indicates that primary
outbreaks occur infrequently and unpredictably in the vicinity of 16 degrees S. However, it is not possible to
assert that primary outbreaks have not occurred elsewhere in the GBR system, and isolated outbreaks
at the southern end of the GBR may be primary events. Present data are insufficient to discern whether
recruitment leading to primary outbreaks is by mass settlement of larvae or aggregation of adult starfish
of various ages. In contrast, evidence indicates strongly that the majority of outbreaks in the system
are secondary infestations as a result of water-borne transport and subsequent mass settlements of
planktonic larvae seeded by other outbreak populations (and initially by populations undergoing primary
outbreaks). The likelihood and pattern of secondary outbreaks is influenced by several parameters
that operate at vastly different scales. These include passive transport of larvae by large-scale circulation
patterns at scales of 10 to the power of 4-10 to the power of 6 m, which can largely account for the southward wave, and substratum
selectivity by larvae at small scales (0-10 to the power of -3 m). The spatial distribution of important cues for larval
settlement (coral rubble and the coralline alga Lithothamnium pseudosorum) suggests that mass
settlements are more likely to occur in deep water at the base of reefs, where they are less likely to
be observed.
Several foci are defined for future research on settlement and recruitment processes. These include
(1) identification of parameters influencing the spatial and temporal distribution of recruitment events
that initiate primary outbreaks, (2) testing of hypotheses relating to dissipation of the southward
movement of outbreaks at about 20 degrees S despite an abundance of reefs in the area supporting high coral
cover, (3) determining the period of competency of larvae for settlement, (4) elucidating the behaviour
of larvae (particularly late brachiolaria) in the water column, (5) further work to identify substrata
inductive of larval settlement and to determine the nature of inducers, the spatial distribution of
inductive substrata, and the effect of small-scale hydrodynamic processes in modifying the response
of larvae to inductive substrata, and (6) testing of the hypothesis of deep-water recruitment.

Item Type: Article
Authors/Creators:Johnson, CR
Journal or Publication Title: Australian Journal of Marine and Freshwater Research
ISSN: 0067-1940
DOI / ID Number: 10.1071/MF9920611
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