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Fishery Bulletin 102(4) 



The egg and adult data were subsequently combined in 

 the DEPM model (Parker, 1985), as follows, to estimate 

 spawning biomass: 



B 



SP DEPM 



<APWk)/<SFR), 



where A = spawning area; 



P = egg production (numbers of eggs before 



losses due to mortality); 

 W = weight of adult fish; 

 k = conversion factor to bring the various units 



to a value in metric tons; 

 S = spawning fraction; the proportion of females 



that spawn per day; 

 F = fecundity; number of eggs produced by a 



female; and 

 R = ratio of females to males by weight. 



The DEPM provides a point estimate of spawning 

 biomass, with upper and lower statistical bounds. In 

 those individual surveys where all parameters could be 

 estimated, estimates of coefficient of variation (CV) for 



B, 



were undertaken by using the delta method 



to sum the CV of the component parameters (Parker, 

 1985). In turn, the CV was used to provide an estimate 

 of variability around the point estimate; specifically, we 

 used ± 1 standard deviation to indicate the upper and 

 lower bounds around the point estimates. In several 

 surveys, particularly when the DEPM was initially be- 

 ing applied in WA, few adult samples meant that val- 

 ues for adult parameters (spawning fraction, sex ratio, 

 fecundity, weight) could not be estimated and therefore 

 a CV for the final estimate of B sp DEPM could likewise 

 not be estimated. Although sex ratio and weight could 

 be reasonably estimated from the regular sampling of 

 commercial catches around the survey period and fecun- 

 dity could be estimated from a relatively small sample 

 (e.g., 70-100 fish), estimating the spawning fraction was 

 more difficult. In this latter case, the upper and lower 

 bounds for the B sp DEPM estimate were not based on a 

 statistical measure but rather on what were thought to 

 be likely low and high values of spawning fraction, re- 

 spectively, for Sardinops from other surveys in WA and 

 elsewhere (e.g., Alheit, 1993, Fletcher et al. 1996). Prior 

 knowledge of likely B sp DEPM values when applying the 

 DEPM, specifically for the purpose of providing expert 

 management advice, has recently been used successfully 

 for Sardinops in South Australia (Ward et al. 2001). 



Adult samples 



Twenty DEPM surveys were conducted between 1991 

 and 1999 to identify stocks and to estimate spawn- 

 ing biomass of Sardinops of southwestern Australia 

 (Fletcher et al., 1996a, 1996b; Fletcher et al. 3 ; senior 

 author's unpubl. data). The surveys were performed 

 during the peak spawning months for Sairlinops off the 

 west coast, Albany, Bremer Bay, and Esperance regions. 

 The timing of the DEPM survey cruises in each region 

 was based on gonadosomatic indices for samples obtained 



from commercial catches, as described in Gaughan et al. 

 (2002). The aim was to obtain samples from 35 catches 

 of adult fish, as recommended by Alheit (1993), but 

 this number was never achieved and in some cases no 

 samples were obtained (Table 1). For each catch sampled, 

 the ovaries from 15-50 females were immediately placed 

 in 10% formalin and subsequently prepared histologi- 

 cally for microscopic examination. The remainder of the 

 subsample was processed to obtain mean female weight 

 and sex ratio by weight. Mature ovaries were retained 

 for estimation of fecundity. 



Plankton sampling and estimation of egg production 



Plankton sampling extended from nearshore waters 

 to the edge of the continental shelf (Fig. 2). Sampling 

 stations were generally spaced uniformly, typically 2-4 

 nautical miles apart, along transects perpendicular to 

 the shore. Analysis of Sardi/iops egg distribution from 

 surveys conducted in the early 1990s indicated that 

 these surveys sufficiently covered the distribution of 

 the spawning stock (Fletcher and Tregonning, 1992; 

 Fletcher et al., 1994), and later geostatistical analyses 

 of Sardinops egg distribution patterns confirmed that 

 the spacing of transects and stations were adequate to 

 effectively represent the spatial distribution (Fletcher 

 and Sumner, 1999). The earlier surveys were used to 

 refine the spatial range of subsequent surveys. The 

 number of plankton samples taken in each survey has 

 generally increased since the early 1990s (Table 1). 



Sardinops eggs were collected by using vertical tows 

 that allowed the water column to be sampled from a 

 maximum depth of 70 m to the surface; Fletcher (1999) 

 showed that Sardinops eggs off southern Australia are 

 typically restricted to the upper 70 m. Bongo nets with 

 diameters of either 60 or 26 cm and constructed of 

 either 500- or 300-micron mesh were used; the change 

 to smaller nets was made to reduce sample volume and 

 hence sorting time, whereas the change to smaller mesh 

 was made to increase efficiency in capturing yolksac 

 larvae; these changes did not affect the sampling ef- 

 ficiency for Sardinops eggs. Tow speed was standard- 

 ized at 1 m/s. All samples were collected between 0630 

 and 1800 hours and immediately preserved in 5-10% 

 formalin and seawater. 



Plankton samples were examined under a dissecting 

 microscope. Sardinops eggs were identified, classified 

 into 12 developmental stages (White and Fletcher 4 ), and 



1 Fletcher W. J., B. Jones, A. F. Pearce, and W. Hosja. 1997. En- 

 vironmental and biological aspects of the mass mortality of 

 pilchards (Autumn 1995 1 in Western Australia. Fisheries 

 Research Report, Fisheries Department Western Australia 

 106, 115 p. Department of Fisheries, Government of West- 

 ern Australia, 168-170 St. Georges Tee. Perth, WA 6000. 

 Australia. 



4 White. K. V.. and W. J. Fletcher. 1998. Identifying the 

 developmental stages for eggs of the Australian pilchard. 

 Sardinops sagax. Fisheries Research Division WA. Fisher- 

 ies Research Report 103, 21 p. Department of Fisheries, 

 Government of Western Australia. 168-170 St. Georges Tee, 

 Perth, WA 6000. Australia. 



