Scombridae: L II A7

Scomber japonicus Houttuyn, 1782

Mackerel

Egg diameter in µm	Number of oil globules	Diameter of oil globule in µm	Yolk texture	Perivitelline space	Position of oil globule at hatch	Gut length   at eye- pigment stage	Myomeres
960-1225	1	240-310	clear	narrow	stern	38% of NL	31-35

Egg: Having an incubation period of about 2 days, this egg was usually seen at roughly 24 hours incubation (A), or close to hatching (B). In the former state they are suffused with a pale yellow to amber to pink hue that makes them easy to separate from other like-sized species, by sliding a sheet of white paper under the glass dish containing the size-sorted eggs. The oil globule is clear to pale amber, with an oil globule measuring 25% of egg diameter. The only egg I consistently confused with this, was LIIB5, the triglid Lepidotrigla faurei, but careful use of white paper was usually successful. When close to hatching (B), the pair of yellow spots behind the eyes, the blotch of yellow at the oil globule, and a yellow mid-tail blotch, are characteristic for an egg this size, and the lack of pigment spots on the yolk immediately separated them from LIIB5. Incubation is about 48 hours (23°C). Black pigment dots are more evenly distributed on the larva and oil globule. Eggs often have a big range in size within a sample.

Larva: The distribution of yellow pigment on the early larva (C), and myomere count, quickly identify this species. Pigment has all disappeared by day 5, except for a line of black pigment ventrally on the notochord, from above the gut, to the notochord tip (E & F). Flexion is completed in 18 days (G). C: 1 day, D: 3 days, E & F: 5 days, G: 18 days, H: 45 days (22-23°C).

This species proved easy to rear, particularly to about 20 days, and a voucher series is preserved. After 20 days my simple rearing facility could not keep up with, and supply a suitable prey, to satisfy the voracious appetite of these larvae. Six larval barcodes match sequences from 6 adult S. japonicus collected off Park Rynie and on the Agulhas Bank (BOLD).

Linked samples	Offshore	Inshore
Eggs	81298	27557
Hits	241	216

This was the second most common egg collected off Park Rynie (Table 3: Introductory Notes), yielding a classic spawning intensity graph with a spring maximum (blue graph). The egg was seen only once in the DHM samples, as a single egg. Spawning intensity in the Park Rynie area increased markedly in the two years following the 1987 floods, but appeared to be suppressed through the dry periods of 1993-1995 and 2003-2007, followed by an increasing trend in the past 4 years (white graph). See also Section 6: Introductory Notes, for more on this subject. The Park Rynie linked samples yielded more eggs offshore (75%), suggesting the bulk of spawning occurs around the 40-50m depth contours. During times of heavy spawning intensity, big shoals of mackerel have been seen in 20-40m water depth (personal observation). The species is known to have a wide distribution across the shelf (Collette, 1986), and commercial fishermen report catching large specimens (up to 2kg) along the shelf edge in 150m (FC Clark pers. comm.). Thus this species probably spawns across a wide section of the shelf off Park Rynie. See Section 7.3 and Table 1 of the Introductory Notes, for more information on the linked samples.