47 
AbStraCt.-Fishery dependent and 
fishery independent distribution analy- 
ses together reveal that there are three 
discrete areas of Argyrosomus inodorus 
abundance between Cape Point and the 
Kei River: one in the southeastern 
Cape, one in the southern Cape, and one 
in the southwestern Cape. On the ba- 
sis of migratory patterns determined 
from tagging and catch data, differences 
in growth rates, otolith-dimension and 
fish-length relationships, growth zone 
structure, sizes at maturity and sex ra- 
tios, and on the fact that each region 
has nursery and spawning areas, the 
conclusion has been drawn that these 
areas of abundance represent three 
separate stocks. Each stock apparently 
disperses offshore in winter (to ca. 100 
m depth) and concentrates nearshore in 
summer (<60 m depth) in response to 
oceanographic patterns. Although there 
is evidence of spawning activity through- 
out the year, the main spawning season 
for silver kob is from August to Decem- 
ber, with a peak in spring (Sep-Nov). 
Size at sexual maturity for silver kob 
was smaller in the southeastern Cape 
than in the southern Cape, and in both 
regions males matured before females. 
Median sizes at maturity (L g0 ) for fe- 
males and males were 310 mm TL (1.3 
yr) and 290 mm TL (1 yr) respectively 
in the southeastern Cape and 375 mm 
TL (2.4 yr) and 325 mm TL (1.5 yr) re- 
spectively in the southern Cape. East 
of Cape Agulhas, A. inodorus are found 
just beyond the surf zone to depths of 
120 m. Adults occur predominantly on 
reefs (>20 m), whereas juveniles are 
found mainly over soft substrata of sand 
or mud (5-120 m depth). Young juve- 
niles recruit to nurseries immediately 
seaward of the surf zone (5-10 m depth) 
but move deeper with growth. Because 
of lower water temperatures west of 
Cape Agulhas, the adults in this area 
are found from the surf zone to depths 
of only 20 m in summer. 
Manuscript accepted 22 August 1996. 
Fishery Bulletin 95:47-67 ( 1997). 
The life history and stock separation 
of silver kob, Argyrosomus inodorus, 
in South African waters 
Marc H. Griffiths 
Sea Fisheries Research Institute 
Private Bag X2, Roggebaai 80 1 2, Cape Town, South Africa 
and 
Department of Ichthyology and Fisheries Science, Rhodes University 
PO. Box 94, Grahamstown 6140, South Africa 
E-mail address, mgriffith@sfri.sfri.ac.za 
Silver kob, Argyrosomus inodorus , 
is an important commercial and rec- 
reational sciaenid fish (max. size 34 
kg) that is known from northern 
Namibia on the west coast of south- 
ern Africa to the Kei River on the 
east coast of South Africa (Griffiths 
and Heemstra, 1995). It is not com- 
mon between Cape Point and cen- 
tral Namibia; therefore it is likely 
that the Namibian populations are 
not continuous with those off the 
eastern seaboard of South Africa 
(Griffiths and Heemstra, 1995). 
Until recently A. inodorus was 
misidentified as A. hololepidotus 
throughout its distribution; off 
South Africa it was also confused 
with a sympatric species, A. japoni- 
cus (Griffiths and Heemstra, 1995). 
The South African line fishery 
consists of about 2,900 commercial 
(Kroon 1 ) and some 4,000 club-affili- 
ated recreational (Ferreira, 1993) 
vessels. These vary from 5 to 15 m 
in length and operate on both east 
and west coasts. Silver kob is prob- 
ably the most valuable species 
caught by the line fishery between 
Cape Point and East London if mar- 
ket value and annual catch are com- 
bined; A. inodorus is also landed as 
a bycatch of the sole- and hake-di- 
rected inshore trawl fishery be- 
tween Cape Agulhas and Port Alfred 
(Japp et al., 1994) and is caught by 
rock and surf anglers and commer- 
cial beach-seine fishermen in the 
southwestern Cape. Although an 
important species, trawl and line 
catch per unit of effort for this spe- 
cies has declined substantially dur- 
ing the last three decades, and con- 
cern has been expressed over the 
large contribution of recruits to line 
catches in the southeastern Cape 
(Smale, 1985; Hecht and Tilney, 
1989). 
Knowledge of the life history of 
fishes “is an almost essential pre- 
requisite to successful identification 
of stocks” (Pawson and Jennings, 
1996) and is fundamental to stock 
assessment and to the formulation 
of effective management strategies 
for their sustainable use. Despite 
the importance of A. inodorus and 
evidence for declining catches, little 
has been published on its life his- 
tory; wise management has there- 
fore not been possible. Smale ( 1985) 
investigated the sex ratio and 
spawning seasonality of “A. holo- 
lepidotus" based on the catches of 
lineboat fishermen in Algoa Bay but 
inadvertently included both A. 
inodorus and A. japotiicus in his 
study (established via voucher 
specimens and otoliths). Griffiths 
(in press, a) recently described the 
growth of A. inodorus from three 
geographical regions between Cape 
1 Kroon, W. 1995. Sea Fisheries, Permit 
Division, P. BagX2, Roggebaai, 8012, Cape 
Town, South Africa. Personal commun. 
