Connectivity and exploitation of Acanthurus triostegus and Acanthurus leucosternon in the Indian Ocean : Application of genetics and single stock assessment to aid coral reef management

Abstract

Extrinsic/abiotic and intrinsic/biotic factors can influence the connectivity and exploitation of reef fish. Coral reef fish from the genus Acanthurus have evolved different life history characteristics that can affect their connectivity and exploitation. The aim of this thesis is to explore the population genetic structure and growth parameters of Acanthurus triostegus and Acanthurus leucosternon in the Indian Ocean, to determine the influence of biotic and abiotic factors on the connectivity and exploitation of coral reef species. First, a 491bp fragment of cytochrome b and microsatellite loci was used to show that the long pelagic larval duration of acanthurids can confer widespread genetic connectivity to A. leucosternon in the Eastern Africa region. Although the global AMOVA (Analysis of Molecular Variance) involving all A. leucosternon Eastern African population is significant, the hierarchical AMOVA and STRUCTURE does not show any genetic breaks consistent with known Eastern African oceanographic and biogeographical barriers to dispersal. Second, a mitochondrial DNA fragment spanning the ATPase8 and ATPase6 gene regions is used to demonstrate that the genetic differentiation of A. triostegus is correlated with geographic distance throughout the Indo-Pacific. In addition, this study shows that populations of A. triostegus are significantly differentiated in the Indian Ocean (Western Indian Ocean and East Indian Ocean), but not in the Pacific Ocean (West, Central, and East Pacific). Third, using syntopic sampling of the spawning aggregating A. triostegus and monogamous pairing A. leucosternon this study determined the influence of mating behaviour on the connectivity of these two Acanthurus species. Contrary to expectations, DAPC (discriminant analysis of principal components), hierarchical AMOVA, and pairwise comparisons showed that the divergent mating behaviour does not lead to differences in the connectivity patterns of A. leucosternon and A. triostegus, but the two species experienced differences in their demographic history. A detailed analysis in BEAST (Bayesian Evolutionary analysis Sampling Trees) showed iv that A. leucosternon which is often restricted to coral reef habitats had a faster and more recent demographic expansion than the habitat generalist A. triostegus. Finally, the growth parameters and mortality of A. triostegus and A. leucosternon were estimated, to determine whether differences in mating behaviour can lead to differences in exploitation rate. Consistent with expectations, the length-based stock assessment showed that the A. triostegus, the species that often forms spawning aggregation has a higher exploitation rate than the monogamous pairing A. leucosternon, supporting previous studies indicating that spawning aggregation may increase the susceptibility of coral reef fish to fishing.