Sharks caught in the protective gill nets off KwaZulu-Natal,South Africa. 11. The scalloped hammerhead shark Sphyrna lewini (Griffith and Smith)

Between 1978 and 1998, a total of 3 385 scalloped hammerhead sharks Sphyrna lewini was caught in the protective nets off KwaZulu-Natal. The mean annual catch was 166 sharks (range 60–279). There was a significant decrease in catch rate with time, but the relationship with the population size in KwaZulu-Natal waters is unknown. Size and sex segregation is indicated. Sizes-at-50% maturity were 161.5cm (males) and 183.1cm (females). Most of the catch (91%) was immature, but neonates are poorly sampled by the 51-cm meshed nets. In small animals (<160cm precaudal length, PCL), males significantly outnumbered females by 2.2:1, and in large animals by 3.6:1. The length-mass relationship differed between sexes. Catches of both small and large animals were highest in summer. Most of the large males were caught in November and December, consistent with an inshore movement of mature animals to breed in summer, but no evidence of recent mating was observed in either sex. Females pregnant with term embryos (median embryo length per litter 30.4–36.2cm) were caught between October and March. These females tended to have maximum ovarian follicle diameters of ≥30mm, indicating that mating would occur shortly after parturition. The median size of 11 litters was 10 embryos. The Tugela Bank to the north of the netted region appears to be a nursery ground. Teleosts, comprising 42 families and 60 identified species, dominated the diet in terms of frequency of occurrence (77%), followed by cephalopods (25%).     

Comparison of horse mackerel length frequencies obtained from research vessels and commercial midwater trawlers: implications for biomass estimation

The validity of abundance estimates from hydroacoustic surveys relies, inter alia, on the ability of the fishing gear on the research vessel to sample non-selectively. This study compares the length frequencies of Cape horse mackerel Trachurus trachurus capensis taken in Namibian waters by the R.V. Welwitchia and commercial midwater trawlers. Results indicate that the length distributions of catches taken by the Welwitchia were significantly different from those from commercial trawlers, with a greater proportion of fish >25 cm being sampled by the latter. Also, the biomass estimated per length-class from hydroacoustic surveys over the period 1994–2000 was compared with those from annual landings of the commercial fishery. The length distribution of horse mackerel in purse-seine catches compares favourably with those from the research vessel. However, comparisons with the midwater trawl catches indicated that the length frequencies obtained from research vessels during the years 1994–1997 underestimated the number of large fish in the population and biased the biomass in those years. From 1998 to 2000 the bias was negligible. The magnitude of the error varied between years, but it remained fairly low. To compensate for this bias, length distribution data from midwater trawlers should be integrated into the acoustic biomass calculation procedure.     

The seasonal intrusion of relatively saline water on the shelf off northern and central Namibia

The surface and subsurface waters of the Angola and Agulhas Current systems significantly influence the Benguela region and its living resources, and it is probable that the movement of Central Water, which plays a key role in the coastal upwelling process, is controlled by circulation of underlying Antarctic Intermediate Water (AIW) as well as by the dynamics of the overlying subtropical water. The movement of AIW can be inferred from a study of the t-s characteristics, and the data holdings and data base of the South African Data Centre for Oceanography facilitated this investigation. Key findings of the investigation, some confirming earlier theories and hypotheses, are as follows. The mean depth of the AIW core in the South-East Atlantic is 750 m, and in the South-West Indian Ocean, 1 100 m. Agulhas Current AIW, which is modified by Red Sea Water, becomes fresher en route because of entrainment and mixing of water from the south and west. Most of the Agulhas Current AIW per se retroflects east of 18°E. A poleward movement of AIW along the West Coast to around 32°S may be inferred from the salinity and oxygen data, with a freshening en route analogous to the Agulhas Current. Relatively fresh AIW (s < 34,35 × 10^?3) is present off the South-Western Cape, the only part of the Benguela where the overlying virgin Central Water upwells. No statistically significant seasonal differences could be resolved.     

Sharks caught in the protective gill nets off KwaZulu-Natal,South Africa. 12. The African angel shark Squatina africana (Regan)

Between 1980 and 2001, a total of 661 African angel sharks Squatina africana was caught in the protective nets off KwaZulu-Natal, South Africa. The mean annual catch was 30 sharks (range = 11–69, SD = 12.4), with no trend in catch rate over the study period. Individuals were caught throughout the year and through much of the netted region, with a higher catch to the south. The sex ratio of the catch was 2.9 females:1 male. Males matured between 640mm and 700mm and females at about 700mm precaudal length (PCL). Mature males had clusters of thorn-shaped denticles, each about 2mm high, near the anterio-dorsal margins of both pectoral and pelvic fins. Of the mature females, 44% were pregnant, many of which contained only ova in utero. Embryos were present from April through to January. The average litter size was six, with length at parturition at least 240mm PCL. Most early-term pregnant females and all mature males were caught in the south. The reproductive cycle showed some seasonality and appeared to be biennial, with a gestation of about one year. Teleosts were the most common prey (76% of stomachs with food), followed by cephalopods (52%).     

Biochemical genetic variation in the Cape anchovy Engraulis capensis Gilchrist

The extent of biochemical genetic variation in the Cape anchovy Engraulis capensis was estimated from starch-gel protein electrophoresis. The gene products of 31 protein-coding loci were identified and Mendelian polymorphisms were observed at 21 (68 per cent). Ten of these loci (32 per cent) had common-allele frequencies less than 0,95 and hence were useful for testing hypotheses of geographic stock structure. The average population heterozygosity, a measure of genetic variation, ranged from 0,110 to 0,128 and averaged 0,115 over samples from 31 locations. Cape anchovies and other clupeiform fishes appear, on average, to have higher levels of genetic variation than many other marine fishes. This may result from the large population sizes and weak population subdivision that characterize them.