The abundance, diet and ecology of Astrostole Scabra within the East Otago Taiapure. Implications for the management of Haliotis iris populations

Abstract

In communities where commercially important species are regulated by predation, information about basic ecological dynamics is necessary to make the simple recommendations about conservation policies and resource management. The abalone Haliotis iris is a commercially important species that is recovering from the effects of over harvesting within the East Otago Taiāpure. The present study examined the ecology of a common H. iris predator, the seven armed sea star Astrostole scabra. A. scabra prey preference, handling times, isotopic turnover and the effect of water motion on predatory capability were investigated experimentally to help explain observations of wild A. scabra populations. A. scabra demonstrated no prey preference via chemoreceptive detection in turbulent water plumes but did demonstrate a rheotactic feeding strategy, moving against a current to encounter prey. A. scabra locomotion and prey capture success was negatively affected by water motion and H. iris was more likely to escape A. scabra predation in high water motion conditions. A. scabra handling time was short when consuming H. iris and the mussel Perna canaliculus but significantly greater for the Turbinid snail Cookia sulcata. Handling time information was useful for adjusting field surveys of diet to account for disproportionately greater observations of prey species with long handling times. Isotopic turnover in A. scabra was examined and found to be longer than the six month experimental period. Field surveys identified that A. scabra abundance and diet was highly variable among sites and A. scabra abundance was significantly lower at wave exposed sites. Adult (100 – 124 mm) H. iris were a major dietary component of A. scabra and constituted 55% of the observed diet. Several factors were determined to affect the distribution of A. scabra including wave exposure, substrate, salinity and prey availability. Results from isotope analysis of A. scabra diet were concurrent with field survey results and identified populations with significantly greater diets of H. iris. In conclusion, because H. iris was a major dietary component of A. scabra, if the population size of A. scabra were to increase the recovery of H. iris populations may be threatened by predation. Furthermore, the fact that adult H. iris were consumed by A. scabra, may mean that large breeding individuals would be removed and H. iris populations would suffer reduced recruitment. A. scabra abundance should be monitored in the future to detect any further increase in abundance.