50 
Fishery Bulletin 95( 1 ), 1997 
(n = 157), predominantly adults, were analyzed ac- 
cording to tagging locality, days free, and the mini- 
mum aquatic distance travelled. 
Owners of commercial line boats and inshore trawl- 
ers are required to submit daily catch returns to the 
Sea Fisheries Research Institute. The monthly 
catches of A. inodorus made by commercial line- 
fishermen in each of the three regions and the 
monthly catches made by the inshore trawl fishery 
in the southern Cape and the southeastern Cape for 
the period 1986-94 were expressed as percentages 
of the respective annual totals. 
The median size at first maturity (L 50 ) for males 
and females was estimated by fitting a logistical func- 
tion (LOGIT) to the fractions of mature fish (gonad 
stage 3+) per 50-mm length class (midpoint) that 
were sampled in the southern Cape and the south- 
eastern Cape during the breeding season. Many of 
the smaller males with active testes lacked drum- 
ming muscles. Logistical functions were therefore 
also fitted to the fractions of males (per 50-mm length 
class) with fully developed drumming muscles. Be- 
cause A. inodorus are not trawled in the southwest- 
ern Cape, few juveniles were sampled and L 50 val- 
ues could not be calculated for that region. 
Reproductive seasonality was established in the 
southeastern Cape by calculating both gonado- 
somatic indices (GSI’s) and the monthly percent fre- 
quency of each maturity stage for fish >L 50 . 
GSI = gonad weight/ 
(fish weight - gonad weight) x 100. 
The extent of the spawning area was determined by 
computing the percent frequency of each maturity 
stage for fish ( >L 50 ) that were sampled during peak 
spawning (Oct and Nov 1991) off East London, Port 
Alfred, Mossel Bay, St Sebastian Bay, and False Bay. 
Sex ratios were tested statistically for significant 
deviations from unity with a chi-square test (P<0.05). 
Nursery areas were delineated by comparing the 
length-frequency distributions of silver kob caught 
1) during South Coast Biomass Surveys (SCBS), 2) 
during experimental linefishing expeditions (no mini- 
mum size) and 3) by the line fishery (1990-94) as 
well as by analyzing the catch and effort distributions 
generated for silver kob during SCBS’s (1987-95). 
Results 
Catch distribution and migration 
Geographically related catch and CPUE trends for 
the line fishery consisted of three modal groups 
(Fig. 2), indicating that there are three areas of adult 
abundance between Cape Point and the Kei River 
(one in each region). Data from SCBS’s showed that 
adult abundance trends were reflected in juvenile 
distribution, at least for the east coast (Fig. 3). Sub- 
stantial differences in growth rates, otolith-dimen- 
sion and fish-length relationships, and growth zone 
structure (Griffiths, in press, a) suggest that these 
areas of abundance represent three allopatric stocks. 
Tag returns from the present study revealed that 
South African silver kob are capable of migrations of 
240 km in six months but that most fish (84%) did 
not move more than 50 km from their tagging local- 
ity (Fig. 4). Only one fish tagged in False Bay was 
recaptured outside of that bay. Of the silver kob 
tagged in the Struis Bay vicinity, five (5.3%) had 
migrated westwards to False Bay, and the rest were 
recaptured either within 50 km of the tagging local- 
ity (77.3%) or had moved eastwards (17%), but only 
as far as Mossel Bay. None of the tagged fish were 
recaptured in the southeastern Cape. Tagging data 
therefore support the three-stock concept but sug- 
gest that there is limited exchange between silver 
kob in the southern Cape and those in the south- 
western Cape. Based on catch data, the foci of each 
stock are apparently False Bay, Stil Bay, and Port 
Alfred, which are separated by distances of 396 and 
630 km, respectively. Struis Bay is situated towards 
the westerly extreme of the area occupied by the 
southern Cape stock: therefore it is not surprising 
that of the recaptured silver kob that had moved 
substantial distances (>50 km) from this tagging lo- 
cality, most had moved to the east. 
Interviews with commercial linefishermen ([n=36]; 
also confirmed by author’s personal experience) in- 
dicated that their silver kob catch was made on reefs 
at depths of 20-60 m to the east and 5-20 m to the 
west of Cape Agulhas. Inshore trawling between 
Cape Agulhas and Port Alfred occurs on soft ground 
at depths of 50-120 m (Japp et al., 1994). Decreases 
in the line catches of all three stocks during winter 
(Fig. 5) and corresponding increases in the catches 
made by inshore trawlers (Fig. 6) suggest that silver 
kob move farther offshore at this time of the year. 
Because inshore trawlers fish over substrata that are 
different from those over which linefishermen fish 
and since they land mostly juvenile and young adult 
A. inodorus (Fig. 7), it could be argued that trawl 
catch data do not reflect the winter locality of the 
adult population. The offshore movement of adults 
is supported, however, by the recapture of four speci- 
mens (435-720 mm) tagged in 30 m of water off Struis 
Bay in summer 1995 by inshore trawlers operating 
in 80 m off Stil Bay and off Cape Infanta in the winter 
and early spring of that year. Presumably, large adults 
