Griffiths Life f:istory of Thyrsttes atun 



701 



10 





10 



Females 



Inshore 

 Offshore 



39 



25 



1 215 

 ^130 186 



113 



I 



13 



63 



86 



B 



Males 



65 



72 



49 39 ><, Y 



J J A S O N D 



Month 



Figure 8 



Mean monthly gonadosomatic indices (GSI; ±2 standard errors) for mature 

 (>75 cm) (A) female and (B) male Thyrsites atun sampled both inshore (cir- 

 cles) and offshore (squares), 1994-97. Inshore and offshore sample sizes are 

 given above and below the error bars, respectively. 



nificant (P<0.01 ), but differences between the two offshore 

 areas — WC and WAB — in winter-spring were not. Mean 

 stomach content mass was, nevertheless, substantially 

 higher (60.1 g vs. 44.6 g.) offshore on the WAB than off- 

 shore of the WC during winter-spring (Table 5). 



Condition 



Relative condition (Kn) of adult snoek depicted a clearly 

 seasonal cycle: highest between March and May, declin- 

 ing steeply through June to October (spawning season), 

 increasing again from November through to March (Fig. 

 14). The proportion of stomachs without any fat was 

 inversely related to Kn (Fig. 14): it was lowest from March 

 to May but increased dramatically during the spawning 



season. The rate of fat decline was, however, higher in 

 females than males; by September 90^7^ of females and 

 58% of males had no mesenteric fat reserves. 



Discussion 



Crawford and De Villiers (1985) postulated that the 

 snoek of the Benguela ecosystem comprise a single stock 

 that undergoes a seasonal longshore migration — moving 

 southwards into South African waters to spawn in winter 

 and returning north, as far as southern Angola, in spring- 

 summer. Although this theory has become widely accepted 

 (Crawford et al., 1987; Crawford, 1995), this study showed 

 that adult snoek are available to South African line fisher- 



