Griffiths: Life history and stock separation of silver kob, Argyrosomus inodorus 
63 
for subadult Atractoscion aequidens (Sciaenidae), 
occurring in the southwestern Cape (Griffiths and 
Hecht, 1995a). 
Data from all 14 SCBS’s revealed that the bottom 
mixed layer (<12°C) extends farther up the shelf (and 
is closer to the coast) in the area between Knysna 
and Cape St Francis (Fig. 15; see also Le Clus and 
Roberts, 1995), thus inhibiting exchange between the 
silver kob stock in the southern Cape and that in the 
southeastern Cape. Along the eastern and western 
sides of False Bay, the 20-m isobath is found <500 m 
from the shore (van Ballegooyen, 1991). Because suit- 
able temperatures for silver kob are found at depths 
shallower than 20 m in the southwestern Cape dur- 
ing spring to autumn, the movement of silver kob 
into or out of False Bay (the focus of the southwest- 
ern Cape stock) during this period is therefore re- 
stricted. In addition, the upwelled bottom mixed layer 
frequently extends to the shore between Cape 
Hangklip and Cape Agulhas, particularly from De- 
cember to April (Boyd et al., 1985; Largier et al., 
1992), thus further limiting exchange between in 
False Bay and in the southern Cape. 
Spawning occurred throughout the distributional 
ranges of all three stocks and peaked in spring ( Sept- 
Nov) in all regions. Sizes and ages at maturity were, 
however, substantially smaller in the southeastern 
Cape than in the southern Cape. Female L 50 was 310 
mm TL (1.3 yr) in the former and 375 mm TL (2.4 yr) 
in the latter region. Changes in ages and sizes at 
maturity have been correlated with exploitation rate 
for several fish species (Healey, 1975; Borisov, 1978; 
Ricker, 1981; Beacham, 1983; Wysokinski 1984; 
Armstrong et al., 1989). Because fishing mortality is 
significantly higher in the southeastern Cape than 
in the southern Cape (F= 0.67 vs. 0. 42)( Griffiths, in 
press, c), the smaller sizes at maturity recorded for 
the southeastern Cape are possibly due to fishing 
pressure. The mechanisms accounting for the de- 
creases in the size and age at maturity in the south- 
eastern Cape, however, remain to be identified. One 
explanation is that the younger ages and smaller 
sizes at maturity for silver kob in the southeastern 
Cape could be the result of density-dependent effects; 
higher mortality results in more food for surviving 
fish, in additional energy for gonad growth, and con- 
sequently in earlier maturity. In several other spe- 
cies, ages or sizes (or both) at maturity have been 
correlated with the amount of accumulated surplus 
energy within a fish (Armstrong et al., 1989; Rowe 
et al., 1991; Berglund, 1992; Kerstan 3 ). On the other 
hand, Ricker (1981) stated that “If a fish matures 
before it is large enough to be vulnerable to fishing, 
its expectation of contributing to future generations 
will be greater than that of a sibling of the same size 
that does not mature until a year later. The result 
can be a gradual decrease in the mean size at matu- 
rity.” Female silver kob attain the minimum size limit 
for the line fishery (400 mm TL) at ca. 2.8 yr in both 
the southeastern Cape and the southern Cape 
(Griffiths, in press, a). Approximately 95% of these 
3 Kerstan, M. 1995. Sex ratios and maturation patterns of 
horse mackerel (Trachurus trachurus) from the NE-and SE- 
Atlantic and the Indian Ocean — a comparison. ICES council 
meeting H:6, 20 p. (Mimeo.) 
