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Spawning activity and reproductive biology of snapper (Pagrus auratus) in Cockburn Sound, Western Australia.

Corey Brion Wakefield, M M Gagnon and J St John

Curtin University of Technology, NORTH BEACH, AUSTRALIA

THEME: ASFB

Cockburn Sound near Perth, Western Australia, is an important spawning and nursery area for snapper (P.auratus). Although commercial fishing activity is limited, the sound supports an important recreational fishery for the metropolitan region, representing the second highest level of fishing pressure in the state. As a precautionary measure, a part seasonal closure (mid September to October) to all snapper fishing within Cockburn Sound was introduced in 2000, despite limited information on the biology and population dynamics of snapper in the area. This study investigates the spawning season of snapper in Cockburn Sound using egg surveys and reproductive biology of the adults. It has five main objectives 1. to validate the identification of snapper eggs and their ontogenic stage 2. to identify the main snapper spawning areas within Cockburn Sound 3. to estimate peak spawning activity in relation to moon phase, water temperature and time of diurnal ovulation 4. to estimate batch fecundity and gonad developmental stages using histology 5. to design a suitable adaptive sampling regime for future surveys Ichthyoplankton was surveyed from September to December 2001, using oblique tows of a 0.5 mm mesh bongo net. Snapper eggs were identified using diameter (range of 0.8 to 1.0 mm) and presence of a single oil globule (mean diameter of 0.2 mm). Eggs were categorised into 19 developmental stages. Peak egg production occurred in November 2001 proceeding the period of closure. However, by using an adaptive sampling regime for future surveys, variation of annual peak spawning between seasons can be quantified.