Status of demersal finfish stocks on the west coast of Australia

Assessments of the key indicator species for the West Coast Demersal Scalefish Resource (WCDSR; West Australian dhufish Glaucosoma hebraicum, Snapper Pagrus auratus and Baldchin groper Choerodon rubescens) in 2007 and 2009 demonstrated that the stocks were experiencing overfishing. Thus, between late 2007 and early 2010, substantial changes were made to the management of the commercial and recreational fisheries that exploit the WCDSR. These were designed to reduce catches of the entire suite of demersal scalefish species (and of each indicator species) by both the commercial and recreational sectors in the West Coast Bioregion (WCB) by at least 50 % of the 2005/06 levels (the catch benchmark), to allow stocks to recover. The current (third) stock assessment of indicator species for the demersal scalefish suite was based on age data collected between 2008/09-2010/11 for G. hebraicum and P. auratus and 2007/08-2010/11 for C. rubescens and on catch statistics for the commercial and recreational fishing sectors (including charter fishing) from 2008-2012. The assessment compared estimates of fishing mortality (F) for the most recent period with several previous time periods and against internationally accepted biological reference points to determine whether there was evidence of any stock recovery. The sampling period for this assessment included the period when major changes were being made to management in the WCB of commercial and recreational/charter fishing for demersal species. As each of the indicator species is relatively long-lived, it is expected that it will take at least 10 years from these management changes before their stocks show strong signs of recovery, and that it may take substantially longer before they fully recover (~15-20 years). Catches of the demersal suite of species in the WCB by the commercial sector and of the top 15 species taken by the recreational sector have been reduced to less than the catch benchmark. Although catches of all indicator species have been reduced, those of two indicator species, P. auratus and C. rubescens, have not been reduced below the catch benchmark by all sectors

A comparison of the stock structures of two exploited demersal teleosts, employing complementary methods of otolith element analysis

Complementary methods of otolith element analysis (IRMS of stable isotopes and laser ablation ICP-MS of trace elements) were used to elucidate the stock structures in Western Australia of the exploited demersal West Australian dhufish Glaucosoma hebraicum and Snapper Pagrus auratus. Residency of adults during their most recent part of life and the existence of important nursery locations were investigated by analysing otolith margins and cores, respectively, to evaluate the applicability of current spatial management. Stable oxygen isotope ratios (18O/16O) in both margins and cores of G. hebraicum otoliths increased with latitude and thus decreasing mean annual water temperature. Analyses of δ18O, stable isotope composition (SI; δ18O, δ13C) and trace element composition (TE; 25Mg, 88Sr, 138Ba) in otolith margins of G. hebraicum from four management areas on the west coast of Australia indicated that adults were resident in each area. Analyses of elements in otolith cores demonstrated that, by the early juvenile stage (< 1 year old), individuals will have recruited to the area where they will remain as adults. Analyses of δ18O and both SI and TE compositions in otolith margins of adult P. auratus indicated residency within each of three large bioregions (Gascoyne, GCB; West, WCB; South, SCB) along the west and south coasts of western Australia. As for G. hebraicum, the data also demonstrated residency at the finer scale, i.e. within the four management areas on the west coast. The lack of a conspicuous pattern in δ18O levels in P. auratus otolith cores implied that adults in each area recruited from a range of nurseries. Although both SI and TE composition demonstrated that there was substantial self-recruitment within the WCB and SCB and within two of the four areas of the WCB, stock(s) of P. auratus at the southern extent of the GCB are likely to be partially reliant on immigration. Elemental signatures in otoliths of G. hebraicum and P. auratus demonstrated that adults in each management area of the WCB could be treated as separate stocks. However, while recruitment of juvenile G. hebraicum to adult stocks would occur primarily from adjacent multiple nurseries within areas of the WCB, adult P. auratus in any one bioregion and at least two of the four areas within the WCB were derived from a range of nurseries, including across area and bioregion boundaries