Griffiths: Life history and stock separation of silver kob, Argyrosomus inodorus 
51 
Figure 2 
Annual catch per unit of effort and mean annual catch for commercial linefishermen operating in 12 subregions between 
Cape Point and the Kei River, 1986-94. See Figure 1 for localities. 
are also found on predominantly untrawlable rocky sub- 
strata during their offshore winter distribution. 
Size at maturity 
Silver kob were found to mature at a smaller size in 
the southeastern Cape than in the southern Cape, 
and in both regions males matured at a smaller size 
than did females. Females began to mature at about 
250 mm in both regions, but the percentages of ma- 
ture fish in consecutive size classes increased more 
rapidly in the southeastern Cape than in the south- 
ern Cape (Fig. 8, Aand B). Estimated median lengths 
at maturity (L 50 ) were 310 mm and 375 mm for the 
two regions respectively. All females in the south- 
eastern Cape larger than 450 mm and all females in 
the southern Cape larger than 550 mm were mature 
(Fig. 8, A and B). 
A comparison of the testes method with the drum- 
ming muscle method for estimating male maturity 
indicated that, within each region, the two methods 
produced similar estimates for length at total matu- 
rity but that the testes method produced higher es- 
timates for the proportions of mature fish in size 
classes below this length. In the southeastern Cape, 
males began to mature at 150 mm (testes method) 
and at 200 mm (drumming muscle method), L 50 was 
calculated at 205 mm (testes method) and 290 mm 
(drumming muscle method), and total maturity was 
attained at 400 mm (both methodsXFig. 8, C and E). 
In the southern Cape, males began to mature at 200 
mm (testes method) and at 250 mm (drumming 
muscle method), L r)0 was calculated at 270 mm (tes- 
tes method) and 325 mm (drumming muscle method), 
and total maturity was attained at 450 mm (both 
methodsXFig. 8, B and F). 
Many of the smaller males (<300 mm) classified 
as mature (i.e. testes contained sperm), had dispro- 
portionately smaller gonads and also lacked drum- 
ming muscles. Because male drumming plays an 
important role in sciaenid spawning behavior 
(Takemura et al., 1978; Saucier and Baltz, 1993; 
Connaughton and Taylor, 1995; Connaughton, 1996), 
it is not known whether these fish would actually 
spawn. Even if the small males (without drumming 
muscles) managed to spawn with a communal spawn- 
