Age-based demographic assessment of fished stocks of Lethrinus nebulosus in the Gascoyne Bioregion of Western Australia R. J. MARRIOTT, D. J. ADAMS, N. D. C. JARVIS, M. J. MORAN, S. J. NEWMAN & M. CRAINE Western Australian Fisheries and Marine Research Laboratories, Department of Fisheries, North Beach, WA, Australia Abstract Age-based demographic analyses were undertaken to assess the current status of fished stocks of spangled emperor, Lethrinus nebulosus (Forsska˚ l) in the Gascoyne Bioregion of Western Australia. Differences in age and growth characteristics were detected for samples collected from different assessment zones, with North Gascoyne fish observed to grow faster and reach a shorter average maximum length and younger average age than South Gascoyne fish. A significant difference in North Gascoyne catch-at-age data from different time periods demonstrated historical effects of fishing on population age structure. Instantaneous rates of fishing mortality (F) from catch-curve analyses of age–frequency data sampled for the North Gascoyne stock from recreational fishing catches from April 2007 to March 2008 were beyond the limit reference point compared with estimated instan- taneous rates of natural mortality (M) (i.e. F > 1.5M), indicating that there is currently a risk to the sustainability of that stock. KEYWORDS: catch-curve analysis, performance indicator, recreational catch, stock assessment, uncertainty. Introduction A major challenge facing the sustainable management of fish stocks is to ensure that stock assessments are based on accurate information and identify relevant sources of quantitative and qualitative uncertainty (Hilborn & Walters 1992; Charles 1998). This chal- lenge is often made increasingly difficult for fisheries where there are limited data available on target species for assessments, as historically has been the case for many tropical fisheries (Medley et al. 1993). Further, with the advancement of age estimation techniques (Fowler 1995; Choat & Robertson 2002) some tropical species have been found to be relatively long-lived, slow-growing and late-maturing (e.g. Wilson & Nie- land 2001; Newman & Dunk 2003; Marriott et al. 2007); characteristics that typically afford a higher susceptibility to over-exploitation (Adams 1980; Kirk- wood et al. 1993; Parent & Schriml 1995). Therefore, the biology of many exploited tropical teleosts is being revisited, and the inherent vulnerabilities of many stocks to over-fishing are being reassessed. The Gascoyne Bioregion is a fishery management zone of Western Australia (WA) extending from south of Shark Bay (27°00¢ S) to east of Exmouth Gulf (114°50¢ E), including the Ningaloo Marine Park (Fig. 1). Within this Bioregion commercial, recrea- tional and indigenous fishers use a variety of methods to catch scaled fish (teleosts) from a wide range of tropical and sub-tropical habitats. Data collection streams are limited for fisheries in this Bioregion because WA-licensed commercial fishing vessels have been prohibited from fishing in state waters within a large area (between Point Maud and Tantabiddi; Fig. 1) since the early 1970s and recreational fishing surveys have seldom been undertaken (1998/1999 and 2007/2008) because of the large coastline and associ- ated high cost. Despite this limited monitoring, there are growing concerns over the sustainability of fish stocks as a result of increasing levels of fishing arising from increases in the residential population, increasing levels of domestic and international tourism and the sustained popularity of recreational fishing in the Bioregion and Marine Park. Correspondence: Ross J. Marriott, Western Australian Fisheries and Marine Research Laboratories, Department of Fisheries, Government of Western Australia, P.O. Box 20, North Beach, WA 6920, Australia (e-mail: ross.marriott@fish.wa.gov.au) Fisheries Management and Ecology, 2010 Ó 2010 Department of Fisheries, Government of Western Australia 1