Griffiths: Life history and stock separation of silver kob, Argyrosomus modorus 
49 
Classification 
Table 1 
and description of the macroscopic gonad maturity stages of Argyrosomus inodorus. 
Stage 
Description 
1 
Juvenile 
This stage is generally only found in fish < 200 mm TL. Testes are threadlike, and the ovaries 
appear as transparent pinkish flaccid sacs, about half the length of those at stage 2. 
2 
Immature or resting Testes are extremely thin, flat, and pinkish white. Ovaries appear as translucent orange tubes. 
Eggs are not visible to the naked eye. 
3 
Active 
Testes are wider, triangular in cross section and beige. Sperm are visible if the gonad is cut and 
gently squeezed. Eggs become visible to the naked eye as tiny yellow granules in a gelatinous 
orange matrix. There is very little increase in the diameter of the ovary. 
4 
Developing 
Testes become wider and deeper and are mottled and creamy beige. They are also softer in 
texture, rupturing when lightly pinched. Besides the obvious presence of sperm in the main 
sperm duct, some sperm are also present in the tissue. Ovaries become larger in diameter and 
opaque yellow in color. Clearly discernible eggs occupy the entire ovary. 
5 
Ripe 
Testes still larger in cross section and softer in texture. They become creamier in color owing to 
considerable quantities of sperm. The ovaries are larger in diameter as a result of an increase 
in egg size. 
6 
Ripe and running 
Testes even larger in cross section and uniformly cream in color. They are extremely delicate at 
this stage and rupture easily when handled. Sperm are freely extruded when pressure is applied 
to the abdomen of the whole fish. Ovaries amber in color and have a substantial proportion of 
hydrated eggs. 
7 
Spent 
Testes are shrivelled and a mottled beige and cream. A little viscous semen may still ooze from 
the genital pore when pressure is applied to the abdomen. Ovaries are reduced in size, similar 
in appearance to those at stage 2 and have a few remaining yolked oocytes. These yolked oocytes 
are generally aspherical and appear to be undergoing resorption. 
mean numbers of silver kob per 30-min trawl per 
grid block. The SCBS methods are fully described by 
Badenhorst and Smale ( 1991); therefore only a sum- 
mary is given here. The survey area extended from 
Cape Agulhas to Port Alfred and seawards to a depth 
of 500 m. This area was divided into four depth zones 
(0-50 m, 51-100 m, 101-200 m, and 200-500 m), 
which were in turn subdivided into blocks of 5 x 5 
nautical miles. The blocks trawled during each sur- 
vey were determined semirandomly according to the 
ratio of blocks per stratum. Bobbins were not used; 
therefore trawling was limited to nonreef substrata. 
The shallowest depth over which the research vessel 
( F.R.S . Africana ) could operate was 20 m. A 180-ft 
German trawler with a 25-mm-mesh (bar) liner at- 
tached to the trawl bag was used. Trawl duration was 
limited to 30 min, and the results of shorter trawls 
(owing to technical reasons or to hitting the reef) were 
standardized to that time. Bottom temperature was 
recorded immediately after most trawls with a Neil 
Brown MK III-B conductivity, temperature, and depth 
probe (CTD). Mean numbers of A. modorus per trawl 
for each 1°C of bottom temperature were plotted to ob- 
tain the preferred temperature range of this species. 
Migration of A. inodorus was studied by using tag- 
ging and catch data. A tagging program was initi- 
ated in February 1994. Silver kob captured with hook 
Table 2 
Number of trawls in which juvenile A. inodorus were 
caught during South Coast Biomass Surveys between Cape 
Agulhas (20°E) and Port Alfred (27°E) during the period 
1987-95. 
Total 
Trawls with 
Cruise 
trawls 
silver kob 
9 Sep-4 Oct 1987 
88 
24 
11 May-2 Jun 1988 
93 
8 
11 May-28 May 1989 
62 
12 
24 May-12 Jun 1990 
58 
12 
8 Sep-26 Sep 1990 
73 
21 
8 Jun-1 Jul 1991 
91 
24 
14 Sep-2 Oct 1991 
75 
30 
1 Apr-20 Apr 1992 
82 
6 
3 Sep-20 Sep 1992 
87 
32 
19 Apr-10 May 1993 
109 
9 
2 Sep-28 Sep 1993 
106 
30 
8 Jun-3 Jul 1994 
89 
11 
22 Sep- 16 Oct 1994 
92 
18 
23 Apr-15 May 1995 
95 
9 
All cruises 
1,200 
246 
and line were tagged with plastic T-bar tags in False 
Bay (n = l,034), off Struis Bay (/?=750), and off Stil 
Bay (t?= 291). The data for recaptured silver kob 
