title: The Influence of Pre- and Post-Settlement Processes on the Population Dynamics of Coral Reef Damselfishes
Creators: name: Meekan, Mark Gregory
Publisher: name: Griffith University
Year: 1992
Subjects: subject: Damselfishes
subject: Coral reef fish
subject: Fish ecology
subject: Lizard Island (Qld.)
subject: Great Barrier Reef

description: Over the last decade there has been a vigorous debate among ecologists about the relative importance of pre- and post-settlement processes on the dynamics of benthic populations of coral reef fish. Advocates for the importance of pre-settlement processes claim that variabifity in the supply of new individuals from the plankton is a major determinant of the size and structure of benthic populations. This variability is thought to occur as a result of the mortality and dispersal of pelagic larvae. In contrast, those advocating the importance of post-settlement processes claim that competition (for space and/or food) and predation largely determines the distribution and abundance of benthic populations. One of the reasons that this debate remains unresolved is that there have been no complete demographic studies of reef fish. Rather, the proponents of one view or the other have tended to restrict their research to small parts of the problem. This study describes one of the first examinations of demographic processes occurring during both the pre-settlement and post-settlement life-history stages of reef fishes. In this study I documented the recruitment, larval abundance, spawning and post-settlement mortality of damselfishes in three reef habitats at Lizard Island, northern Great Barrier Reef (GBR). The recruitment patterns of three species, Pomacentrus amboinensis, P. nagasakiensis and Dischistodus perspicillatus were described by weekly collections of recruits from small, artificial patch reefs during the 1988/89 and 1989/90 summers. During the first summer, the majority of recruits of these species arrived in benthic habitats in a broad episode of settlement that encompassed a 4-week period at the beginning of the summer. This pattern was repeated only by D. perspicillatus during the following summer; P. amboinensis and P. nagasakiensis recruited during most weeks of sampling in 1989/90. Recruitment patterns were strongly correlated with the timing and magnitude of catches of these same species in light traps in the nearshore waters around Lizard Island. This suggested that recruitment patterns at this locality were largely determined by the distribution and abundance of older larval stages within the plankton. Catches of larvae by light traps often peaked over short periods of a few days. Peaks in catches occurred synchronously, but with variable magnitude among habitats. These patterns were consistent with the suggestion that physical and biological processes aggregate pre-settlement fish into large (up to km) patches within the plankton. Other evidence suggested that larval behaviour was also an important determinant of patterns in catches and recruitment. Few Pomacentrus nagasakiensis larvae were collected by light traps in the lagoon habitat during both summers. As catches of P. amboinensis and Dischistodus perspicÃœlatus did not vary in a similar fashion, differential behaviour is implied. P. nagasakiensis may either settle preferentially on the reef margins so that larval supply is exhausted by the time water enters the lagoon, or else larvae of this species may be able to detect and avoid the lagoon habitat. The relative influence of larval production and planktonic processes on recruitment patterns was examined by documenting the spawning patterns of Pomacentrus amboinensis. Males of this species guard clutches of demersally spawned eggs until hatching, and reproductive output can be estimated at daily intervals by mapping the area of eggs held within nests. Spawning occurred at lunar intervals during the 1988/89 summer, with peaks in output coinciding with the full moon. During the following season, spawning occurred in an asymmetrical semi-lunar pattern, with larger full moon peaks than new moon peaks. Significant correlations were found between temporal patterns of light trap catches, recruitment and spawning of Pomacentrus amboinensis when data sets were lagged by a period of lime equivalent to pre-settlement life (23 days). However, the magnitude of light trap catches and recruitment was only weakly correlated with the magnitude of reproduction. Peaks in recruitment were approximately three times more variable in size than peaks of spawning. This variability was attributed to the action of processes occurring within the plankton. The correlation between temporal patterns of spawning, larval abundance and recruitment contradicts prevailing views that recruitment patterns are largely determined by processes acting in the plankton. As Pomacentrus anthoinensis has a relatively short planktonic duration (19 days), this may allow little time for planktonic processes to decouple temporal patterns of spawning and recruitment. P. amboinensis larvae also have well-developed sensory and locomotory abilities at hatching, and may be more capable of influencing their fate within the plankton than the passively dispersed eggs of pelagic spawners. The cyclical pattern of reproduction of Pomacentrus amboinensis may have been determined by factors operating at both global (tidal cycles, moon phase) and local scales (predation, food availability). This combination of factors would account for the considerable variability in reproduction observed among habitats in this species. Spawning occurred in a lunar pattern in all habitats during the 1988/89 summer. However, in the 1989/90 summer, spawning occurred at semi-lunar intervals in the leeward habitat, while lunar patterns were recorded in the lagoon. Spawning occurred in an acyclic pattern in the windward habitat. During both summers, males in the lagoon produced fewer larvae than those in the windward or leeward habitats. The mortality of newly-settled reef fish was examined by comparing the numbers of recruits collected from artificial patch reefs at daily, weekly and monthly intervals. These comparisons showed that recruits experienced very high rates of mortality, probably as a result of predation, within the first week and month of settlement. Mortality varied inconsistently among species, times and habitats. Thus, patterns established at settlement may not determine the abundance of older juveniles and adults in any predictable fashion. These results suggest that predation may be an important determinant of the size and structure of benthic populations at Lizard Island. There was little evidence that the mortality of Pomacentrus amboinensis recruits was influenced by growth. Estimates of growth rates were obtained by plotting age of recruits (calculated from otolith analysis) against a measure of size (standard length). However, comparisons of growth and mortality were hampered by the migration of recruits from patch reefs and by errors in otolith analysis. There were no patterns in the growth or mortality of recruits that were consistent with any effect of density. This suggests that competition for benthic resources had little influence on patterns of abundance, at least within the first month of settlement. This study demonstrates that processes occurring during a variety of life-history stages may regulate the dynamics of populations of coral reef fish. These interactive effects must be described if predictive and general models of demographics are to be constructed. Given the increasing anthropogenic stresses being placed on stocks of reef fishes and on reef systems world-wide, the development of such models is now overdue.