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Where Do Coral Reef Fishes Come From?Knowing where coral reef fishes originate is important if they are to be properly managed. The dogma is that coral reefs are open systems where currents and winds bring larvae to a particular reef from multiple source reefs. However, recent studies at Lizard Island and elsewhere suggest that reefs rely heavily on locally-retained larvae to seed populations (self-recruitment).
Heather Patterson of Sydney University (soon of James Cook University) will examine the question of self-recruitment in reef fishes by utilising trace element signatures in otoliths (ear bones) as natural tags of natal reefs. Otoliths are commonly used to age reef fishes because bands are laid down as the bone grows, similar to rings in tree trunks. Heather will be looking at the chemical composition of otoliths at different distances from the centre and relating this to the chemical composition of the environment at the time the bone increment was laid down. She will be funded from 2001 for two years of research at Lizard Island by the Lizard Island Reef Research Foundation under the Lizard Island Doctoral Fellowships program. Heather will use specimens in early life stages from two species of reef fish (Acanthochromis polyacanthus and Pomacentrus coelestis) in her study. A. polyacanthus is the only species of pomacentrid fish that does not have a pelagic larval phase, so all larval A. polyacanthus that are found on a particular reef have never lived on any other reef. In contrast, P. coelestis is like most reef fishes because it has a pelagic larval phase that allows dispersal to other reefs. To find out where the parents of young P. coelestis live, Heather's research uses the novel approach of ground-truthing the chemical "signatures" of individual reefs by examining otoliths from A. polyacanthus, then comparing these signatures with those found in otoliths of P. coelestis. Sampling the core region of the otolith of this species should provide a chemical signature from its reef of origin. In addition, a chemical profile of the fish can be determined by sampling from the core outward to the edge of the otolith. This should allow Heather to determine life history features of the fish such as its residence time near a particular reef. It is possible that chemical signatures within otoliths vary with time, between nearby reefs, and of course between widely spaced reefs. To determine the stability of chemical signatures in otoliths, Heather will collect otolith samples twice during each recruitment season over two consecutive years at Lizard Island and on several reefs nearby. A similar sampling routine will occur at One Tree Island on the southern part of the Great Barrier Reef for comparison of chemical signatures in reefs several thousand kilometres apart. By combining chemical information derived from otoliths with data on ocean currents, Heather will be able to determine if retention mechanisms aid in the self-recruitment of fish larvae. Her research is timely given the current interest in marine protected areas since patterns of reef connectivity will be important in determining the appropriate design for marine reserves.
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