Three criteria were assessed to determine the usefulness of otoliths to age individuals of Pomacentrus mollucensis collected from the lagoon of John Brewer Reef on the central Great Bamer Reef between March 1988 and February 1989. (1) Otoliths. as measured along various axes, demonstrated asymmetrical growth through time, but nevertheless grew throughout the lives of the fish. (2) A total of 18 fish recaptured after treatment with tetracycline 1 yr previously, demonstrated one thin, opaque increment in the otolith structure that had formed since treatment. (3) Marginal increment analysis, while not as definitive, strongly suggested that opaque increments were formed in the austral spring, generally between September and November These 3 results indicated that opaque increments found in the otoliths of P. mollucensis are formed annually and hence can be used to determine agespecific parameters of the populations. Von Bertalanffy growth curves and age structures were derived for 3 representative samples to illustrate this point.

ABSTRACT. The potent~al causes of the variable nature of recrultment of marlne organisms can be Inferred from the scales over which they vary Sampllng for recrults of Chaetodon ralnfordi on the Great Barrier Reef, Austral~a, was done at the end of the recruitment season In 3 yr at 3 spatial scales: at reglons separated by hundreds of km, at reefs separated by up to tens of km; and at different sites on 1 coral reef. Desplte a background of var~ability suggesting the influence of stochastlc processes, the data showed considerable pattern, implicating the involvement of determlnlstlc processes. At each spatial scale, recruitment was higher In 1985 than 1986 or 1987 Furthermore, d e s p~t e the general reduction In recrultment wlth tlme, the dlstr~bution patterns lvere consistent In each year. Southern regions always had hlgher rates of recrultment than northern onvs, some reefs always received higher rates than neighbouring ones and some sites consistently had higher rates than others only hundreds of met]-es away. Recruitment rates of C. ralnfordi were attributed to settlement rates Consequently, the observed patterns were related, at each spatial scale, to either the ava~lability of pre-settlement fish, or habitat selection actlng at the time of settlement. Thls study tvas not able to dlstlnguish between such alternative hypotheses at any scale, but lncilcated the types of studies requlred to do so

If the otoliths of fish are to be used as an ageing tool, they must fulfil three criteria: they must display an internal structure of increments, this structure must be relatable to a regular time scale, and the otoliths must grow throughout the lives of the fish at a perceptible rate. These criteria were assessed for two species of damselfish: Pomacentrus moluccensis and P. wardi from One Tree Reef on the southern Great Barrier Reef. Transverse sections of sagittae from both species contained an alternating sequence of opaque and translucent zones that formed an interpretable pattern of increments. Treatment of fish with tetracycline showed that the opaque zones were formed on an annual basis, generally between spring and early summer. The assessment of growth of otoliths from fish treated with tetracycline indicated that those as old as 10 years of age were still laying down increments of regular thickness. Consequently, the otoliths from both species successfully fulfilled the three criteria required for the direct determination of age. Furthermore, this study helped to identify two levels of variation in otolith structure, manifested in the clarity and interpretability of the increments; there was an interspecies difference and a significant intraspecies latitudinal difference.

Between November 1984 and April 1987, the butterflyfishes of One Tree Reef on the southern Great Barrier Reef were sampled on 8 occasions at 9 widely dispersed localities across the reef using a hierarchial sampling regime. Data were collected on 23 species which fell into 3 categories of abundance. Three 'abundant' species each contributed > 10 O/O of the total of all individuals. Five species were 'common', each contributing 2 to 10 "; o of the total, and 15 species were 'rare', each contributing < 1.0 % of the total. The 8 'abundant' and 'common' species were present at all localities, whilst 11 of the 'rare' species had restricted and discontinuous distributions. The former 8 demonstrated significant differences in abundance amongst localities, 5 of which also had significant differences amongst sites (within combinations of occasions and localities) Relationships between fish abundance and coral abundance were weak, and could not predict these spatial patterns. The size structures of the 3 abundant species inchcated consistent spatial differences, implying different population dynamics occurring on adjacent and local areas of reef. Five abundant and common species showed significant temporal variation, 2 species showed a seasonal pattern of variation, and 3 species showed increases in population density. Generally, however, spatial differences amongst localities were maintained through time.

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Less precise, but economic methods for estimating the age of individual fish can provide better estimates of age structure than precise, but expensive methods. The benefits of using a precise ageing method can be compromised by its cost, which may restrict the size of the sample aged. If sample size is restricted, the effect of sampling error on an age structure may be greater than the effect of ageing error from a less precise ageing method that does not restrict sample size. We used Monte Carlo simulations to assess the relative size of sampling and ageing errors when estimating the age structure of populations of Pomacentrus moluccensis from the southern Great Barrier Reef, Australia. Sampling error associated with ageing less than 200 individuals was, on average, larger than the effects of most commonly reported ageing errors. Other factors that may complicate this comparison of ageing methods involve the financial cost of different methods and the logistics of sampling more fish.

Two hypotheses regarding the origin of post-larval Sillaginodes punctata that recruit to nursery areas in South Australia were assessed. These were that post-larvae originate either hundreds of kilometres to the west and are delivered by weather-determined ocean currents or they originate from numerous spawning grounds located only

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