Age-based demography and reproduction of hapuku, Polyprion oxygeneios, from the south coast of Western Australia: implications for management

Abstract

Abstract Wakefield, C. B., Newman, S. J., and Molony, B. W. 2010. Age-based demography and reproduction of hapuku, Polyprion oxygeneios, from the south coast of Western Australia: implications for management. – ICES Journal of Marine Science, 67: 1164–1174. The hapuku, Polyprion oxygeneios, inhabits deep (>100 m) continental slope waters of Western Australia. In all, 1352 P. oxygeneios were collected from the waters along the south coast of Western Australia (ca. 35°S) from 2004 to 2008. The species is gonochoristic, and spawns during the austral winter (May–September). Ages were estimated from counts of opaque zones from thin-sectioned sagittal otoliths. Classification analysis of the outer margin of sectioned otoliths indicated that a single opaque zone is deposited annually. Female P. oxygeneios (n = 630; 535–1114 mm total length, TL) ranged in age from 2 to 35 years and males (n = 691; 521–1004 mm TL) from 2 to 52 years. von Bertalanffy growth models for male and female P. oxygeneios were statistically, but not biologically, different (<5% difference in mean and estimated lengths-at-age). Estimates of the lengths and ages at which 50% of the females and males in the population reached sexual maturity were 760 and 702 mm TL and 7.1 and 6.8 years. The instantaneous rate of natural mortality (M) was estimated to be 0.09. Estimates of the instantaneous rate of fishing mortality (F) were low (0.01–0.05). Harvest rates in 2005 and 2006 were close to estimated sustainable levels. Monitoring of any future increases in catch and effort in continental slope waters in both State- and Commonwealth-managed fisheries is required in order to assess impacts to stock sustainability. Sustainable management would also benefit from improved understanding of possible pan-oceanic recruitment of the species among southern hemisphere populations.