Sciaenidae: L II A6

Atractoscion aequidens (Cuvier, 1830)

Geelbek

 

Egg diameter in m

Number of oil globules

Diameter of oil globule in m

Yolk texture

Perivitelline space

Position of oil globule at hatch

Gut length at eye- pigment stage

Myomeres

1125-1250

1

280-320

clear

narrow

stern

40% of NL

25

 

Egg: The oil globule is light amber. In 1-day incubated eggs (A), there are round yellow spots on the oil globule and embryo, with a few on the yolk, mostly near the oil globule. Black pigment has a similar distribution, and stellate black spots are ventral on the oil globule. Incubation is about 48 hours (23C).

Larva: The 1 day larva has the finfolds and parts of the notochord lined with yellow pigment (C), which consolidates into two bands of yellow by day 4-5 (D). Note the yellow pigment extending into the dorsal finfold, above the gut. At day 11 the larva is still preflexion (E), and the gut area pigment has turned brown. The 25-day larva is postflexion (F). C: 1 day, D: 5 days, E: 11 days, F: 26 days, G: 38 days, H: 55 days, I: 135 days (23-25C).

The egg and early larva are quite easily confused with Tripterodon orbis (LIIA6A) the spadefish, but the later larvae are distinctive, and T. orbis eggs usually only appear in December. This species proved quite easy to rear, and a voucher series is preserved. Three larval sequences are available, which match sequences from 6 adult A. aequidens collected in the area (BOLD).

Atractoscion is an important commercial and recreational fish, which moves into KZN waters from the south in winter to spawn. Movement up the coast is inshore, and during the 1950s, shoals were occasionally netted with large seine nets set from the shore, by hand-rowed surfboats. The spawning shoals congregate on reefs in the area, at water depths of 40m-60m. Commercial ski-boat fishermen target them while in KZN waters, by fishing at night in the spawning aggregations, a practice which, in many fisheries around the world, is regarded as foolish. The dilemma is worsened by the presence of large numbers of big copper sharks (Carcharhinus brachyurus) and blacktip sharks (C. limbatus) in the area at that time of the year, which congregate under the boats and can take more than 50% of the fish hooked.

 

Linked samples Offshore Inshore
Eggs 2570 470
Hits 47 32

Geelbek spawn from June to November off Park Rynie (blue graph). The egg was not seen in the DHM samples. Eggs were collected at a fairly constant level during the early part of the study, then showed a significant increase in the 3 years (2003-2005, after which they seem to have collapsed (white graph). The Park Rynie linked samples had more eggs offshore (85%). This has been used as a benchmark, in assessing the approximate area where other species are spawning, using data from this study. See Section 7.3 and Table 1 of the Introductory Notes, for more information on the linked samples