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Fishery Bulletin 100(4) 



J FMAMJ JASOND 

 Month 



Figure 9 



Monthly percentage of macroscopic ovar- 

 ian stages for adult Thyrsites atun (>75 cm) 

 sampled (A) offshore (f!=369) and (B) inshore 

 (n = 1172). 



men throughout the year, and that the seasonal availabil- 

 ity of adults on the trawl grounds results from an offshore 

 spawning migration rather than southward movement 

 from Namibian waters. Based on these results, niter alia. 

 separate nursery areas and egg and lai-val distributions 

 north and south of the cold upwelling cell (25-27°S) off 

 southern Namibia, Griffiths (in press) concluded that Ben- 

 guela snoek exist as two subpopulations, and have limited 

 exchange. 



The life history of South African T. atun is summarized 

 by the conceptual model presented in Figure 15. Movement 

 patterns were inferred largely from spatiotemporal trends 

 in fishery-dependant catch and effort data. Considering 

 that snoek is the most important line fish on the South 

 African western seaboard and that it is a bycatch in the 

 trawl fishery, catch trends are unlikely to have been biased 

 by switches in targeting. Moreover, even though line-fish 

 catches are under-reported (Sauer et al., 1997), they have 

 been demonstrated to accurately reflect seasonal trends in 

 the abundance of migratory species (Griffiths and Hecht, 

 1995). Given the offshore movement of adult fish during 

 the spawning season and the associated ovarian condition 



(greater GSls and the presence of hydrated oocytes), it 

 is concluded that South African snoek spawn offshore 

 between the 150- and 400-m isobaths. Although most of the 

 inshore catch was line-caught, the absence of stage-4 fe- 

 males from trawls made shallower than 150 m during the 

 spawning season (n=46, including 32 ripe |stage-31 speci- 

 mens) indicates that this observation is not an artifact of 

 gear selectivity. Spatial patterns in trawl CPUE from both 

 fishery-independent (Fig. 2) and fishery-dependent (Fig. 11) 

 data, collected during winter-spring, suggest an extensive 

 spawning ground that encompasses the western edge of the 

 Agulhas Bank and most of the South African west coast, 

 to a point just north of Hondeklip Bay. Presence of snoek 

 eggs and larvae (off South Africa) from the southern tip 

 of the Agulhas Bank to Hondeklip Bay and their great 

 abundance between the 200-m and 600-m isobaths (Olivar 

 and Fortuiio, 1991; Olivar and Shelton, 1993) corroborate 

 offshore spawning and an extensive spawning ground. The 

 occurrence of snoek preflexion larvae in region 6 (Wood, 

 1998 ) indicates that spawning does occur farther to the east, 

 but the relatively low abundance of adult fish and spawning 

 products suggests it is not an important spawning area. 



The sex ratio of adults (>75 cm) on the west coast 

 (regions 0-3) was skewed towards males on the spawning 

 grounds (2M;1F), and towards females further inshore 

 (2.9F:1M), particularly during the spawning season. Based 

 on the differential loss of intestinal fat, it is evident that 

 the energetic demands of spawning are higher in females 

 than in males. The higher rate of prey consumption by 

 females during winter-spring (i.e. spawning season) than 

 during summer-autumn and the greater prey consumption 

 by females than by males during the spawning season 

 indicate that they are able to enhance their spawning 

 effort with exogenous energy. It is therefore postulated that 

 because snoek are indeterminate serial spawners, females 

 on the west coast move inshore between spawning events, 

 where their principal prey — Sardinops sajax and Engraulis 

 japonicus — are more abundant. 



Females spawning off the western Agulhas Bank do not 

 appear to move inshore to feed between spawning events, 

 according to near equal sex ratios and low inshore catch 

 rates in winter-spring (region 5). Although line catches in 

 region 4 were highest during the spawning season, which 

 at first glance may appear to be contradictory, CPUE was 

 an order of magnitude lower than in region 3 or region 5, 

 indicating relatively low inshore abundance at that time of 

 the year High female mean stomach content mass (60.1 g) 

 and the significantly higher proportion of females than males 

 with food (shared only with the inshore feeding ground of 

 the WC ) suggest that the offshore WAB functions as both 

 spawning and feeding ground during winter-spring. In 

 addition, PBSs have revealed that clupeids — adult sardine 

 and round herring — are more abundant near the shelf 

 edge of the WAB in winter than along the WC (Coetzee'^), 

 and dietary comparison (this study) showed that clupeids 

 indeed represent a larger component of the offshore diet 



Coetzee, J. 2000. Personnal commun. Pelagic Section, 

 Marine & Coastal Management, Private Bag X2, Roggebaai, 

 8012, Cape Town. 



