694 



Fishery Bulletin 100(4) 



29< 



30° 



31 > 



32° 



33° 



34° 



35° 



36° 





'^-V -^Orange River 



::'^ 



X ■•:■■'  



Port Nolloth 



•I'THondeklip Bay 



SOUTH AFRICA 



•t 



■-•'■ 



St Helena Bay 

 ^Cape Columbine 



SUMMER 

 (January - February) 



KEY 

 = 



• < 10 



• 10-50 



• >50 



t 



15° 



16° 



18° 



19° 



E20° 



15° 



16° 



17° 



18° 



19° 



E20° 



Figure 2A 



Mean number ofThyrsites atun per trawl per grid block (5x5 nmi) by season for (Al the west coast (July 1985-Jan 1991; 

 n=l& surveys and 1624 trawls) and (B) the south coast (September 1987-April 1996; n=18 surveys and 1554 trawls) 

 demersal biomass surveys off South Africa. 



proportions of stomachs with no fat were calculated sepa- 

 rately for adult (>75 cm) males and females. 



Results 



Migration 



Spatial analysis of CPUE from fishery-independent 

 DBSs indicated that T. atun are distributed farther off- 

 shore and farther southeast in winter and spring than 

 in summer and autumn (Fig. 2). Fishery-dependent data 

 support this pattern; commercial catches from the trawl 

 grounds (i.e. >150 m) were highest from June to October, 

 and those in the southern regions (3-6) lagged behind 

 catches in the north (regions 0-2) by approximately one 

 month (Fig. 3). Analysis of trawl CPUE by depth, area, 

 and season (Fig. 4) further confirmed offshore movement 

 during the spawning period (June-October) and further- 

 more revealed that T. atun were most abundant between 

 bottom depths of 150 m and 350 m while on the trawl 

 grounds. Snoek found on the trawl grounds were gener- 

 ally >65 cm FL (Fig. 5A). 



Line-based catch and CPUE in regions 0-2 was dis- 

 tinctly seasonal; most of the catch was made from April 

 to June (Fig. 3) and a slight southwards progression in 

 peak catch and CPUE occurred within this period (Fig. 

 3). Line-caught snoek from regions 1 and 2 were substan- 

 tially smaller (50-75 cm) and younger ( 1-3 years; author, 

 unpubl. data) than those from region 3 (80-95 cm; 3-7 

 years) (Fig. 5B). 



Line-caught snoek in region 3 were mostly >L5q (Fig. 5B). 

 Monthly catch and CPUE statistics from this region de- 

 picted no trend, indicating that adult snoek are available 

 to line fishermen throughout the year (Fig. 3). Catch rates 

 in region 4 were highest between July and October, which 

 is consistent with the winter-spring south-eastward 

 dispersal of adult snoek evident from trawl data (above). 

 Handline catch and CPUE in regions 5 and 6 were highest 

 during the first half of the year but declined dramatically 

 in winter. 



Size at maturity 



The ratio of active (stage-2) to ripe (stage-3) ovaries 

 during the spawning season decreased with fish length 



