617 



Abstract — Fishery-independent esti- 

 mates of spawning biomass (B sp ) of 

 the Pacific sardine {Sardinops sagax) 

 on the south and lower west coasts of 

 Western Australia (WA) were obtained 

 periodically between 1991 and 1999 

 by using the daily egg production 

 method (DEPMl. Ichthyoplankton 

 data collected during these surveys, 

 specifically the presence or absence 

 of S. sagax eggs, were used to investi- 

 gate trends in the spawning area of S. 

 sagax within each of four regions. The 

 expectation was that trends in B sp and 

 spawning area were positively related. 

 With the DEPM model, estimates of 

 B sp will change proportionally with 

 spawning area if all other variables 

 remain constant. The proportion of 

 positive stations (PPS), i.e., stations 

 with nonzero egg counts — an objec- 

 tive estimator of spawning area — was 

 high for all south coast regions during 

 the early 1990s (a period when the 

 estimated B SP was also high) and 

 then decreased after the mid-1990s. 

 There was a decrease in PPS from 

 the mid-1990s to 1999. The particu- 

 larly low estimates in 1999 followed a 

 severe epidemic mass mortality of S. 

 sagax throughout their range across 

 southern Australia. Deviations from 

 the expected relationship between 

 B SP and PPS were used to identify 

 uncertainty around estimates of B sp . 

 Because estimation of spawning area 

 is subject to less sampling bias than 

 estimation ofB sp , the deviation in the 

 relation between the two provides an 

 objective basis for adjusting some esti- 

 mates of the latter. Such an approach 

 is particularly useful for fisheries 

 management purposes when sampling 

 problems are suspected to be present. 

 The analysis of PPS undertaken from 

 the same set of samples from which 

 the DEPM estimate is derived will 

 help provide information for stock 

 assessments and for the management 

 of purse-seine fisheries. 



A sudden collapse in distribution of 



Pacific sardine (Sardinops sagax) 



off southwestern Australia enables 



an objective re-assessment of biomass estimates 



Daniel J. Gaughan 



Timothy I. Leary 



Ronald W. Mitchell 



Ian W. Wright 



Western Australian Marine Research Laboratories 



Department of Fisheries 



West Coast Drive 



Waterman, Western Australia 6020, Australia 



E-mail address (for D J Gaughan): dgaughan Sfish wa govau 



Manuscript submitted 5 December 2002 

 to the Scientific Editor's Office. 



Manuscript approved for publication 

 27 April 2004 by the Scientific Editor. 



Fish. Bull. 102:617-633 (2004). 



As a stock of small pelagic fish de- 

 creases, biomass assessment becomes 

 problematic because of such factors 

 as patchy distribution (Fletcher and 

 Sumner, 1999) and continuing high 

 catchability as a result of the schooling 

 behavior of some fish (Uphoff, 1993). In 

 these circumstances, ichthyoplankton 

 surveys can provide a useful means of 

 estimating spawning biomass, B sp . 

 for some pelagic fish species. Mangel 

 and Smith (1990) used a technique 

 that assessed the presence or absence 

 of sardine (Sardinops sagax) eggs in 

 a known spawning area. They found 

 that changes in adult biomass were 

 more accurately predicted by using 

 presence-absence of eggs in sampling 

 surveys than mean egg abundance 

 because of misleading results arising 

 from the spatial patchiness of eggs. In 

 their presence-absence analysis, the 

 spatial distribution of eggs is the key 

 determinant of B sp estimates and is 

 used in a model with a series of other 

 parameters to provide an estimate 

 of B sp (Mangel and Smith, 1990). 

 Although this technique provides 

 an objective indication of stock size 

 that is not subjected to the inherent 

 problems in estimating B SP with the 

 daily egg production method (DEPM, 

 e.g., Ward et al., 2001), the modeling 

 requires substantial prior knowledge 

 of adult and egg production param- 

 eters. More recently, Zenitani and 

 Yamada (2000) developed an optimal 

 relationship between B sp and spawn- 



ing area for the Japanese sardine 

 (Sardinops melanostictus) using a 

 nonlinear model that assumed patchy 

 egg distribution. In their case, bio- 

 mass was estimated by using virtual 

 population analysis with catch-at-age 

 data from the commercial fishery. 



The purse seine fishery for Sar- 

 dinops sagax in Western Australia 

 (WA) operates along the south coast 

 around the port regions of Esperance, 

 Bremer Bay, and Albany; and on the 

 lower West Coast in the regions of 

 Fremantle and Dunsborough (Fig. 1). 

 A level of spatial distinctness among 

 adult Sardinops populations neces- 

 sitates that three south coast regions 

 and the west coast region be managed 

 as separate fisheries (Gaughan et al., 

 2002). Unlike the case with Japanese 

 sardine (Zenitani and Yamada, 2000), 

 it has not been possible to estimate 

 the B sp of Sardinops in each fish- 

 ery in WA using only an age-based 

 approach. Although Gaughan et al. 

 (2002) considered the catch-at-age 

 data for the WA Sa?-dinops fisheries 

 to be reasonable, the data span a rel- 

 atively short time series, commenc- 

 ing in 1988 at Albany and Bremer 

 Bay and later at the other regions. 

 Therefore, both age-structure data 

 and estimates of spawning biomass 

 (B sp ) obtained with the DEPM have 

 provided the biological basis for 

 managing the Sardinops fisheries in 

 WA for over a decade (Fletcher, 

 1991, 1995; Fletcher et al., 1996, 



