An evaluation of the daily egg production method to estimate spawning biomass of snapper (Pagrus auratus) in inner Shark Bay, Western Australia, following more than a decade of surveys 1997-2007

Abstract

The daily egg production method (DEPM) has been used to estimate spawning biomass of separate stocks of snapper (Pagrus auratus) in the inner gulfs of Shark Bay, Western Australia, since 1997. While these stocks have been targeted by a regionally important recreational fishery since at least the 1970s, no information on stock size was available prior to this application of the DEPM. Ichthyoplankton surveys were undertaken during the winter spawning periods standardised around the new moon and used a stratified-systematic approach combined with more intensive, fine-scale sampling in the vicinity of the known spawning locations. Samples of mature snapper were collected concurrently using rod and line fishing with the assistance of volunteer recreational fishers. Daily egg mortality and daily egg production (P) were estimated using both non-linear least squares regression and generalized linear models for comparison. While relatively imprecise, mostly due to imprecision in the estimation of P, spawning biomass estimates have confirmed that these stocks are relatively small (measured in 10-100. s of tonnes) compared with snapper stocks elsewhere in Australia and New Zealand. The DEPM-based estimates have been combined with biological (age composition) data and catch information in integrated stock assessment models that have underpinned the management of these snapper stocks and the associated recreational fishery since 2002. Under the highly conservative TAC-based management regime that has been in place since 2003, spawning stocks in the Eastern Gulf and Denham Sound have been rebuilt to the management target level while the Freycinet Estuary stock is steadily rebuilding. The utility and cost of the DEPM is briefly discussed in relation to the use of tagging as an alternate assessment method, to determine the most cost-effective and tractable approach for monitoring the status of these important snapper stocks into the future.