Shark Nets Research Articles

Protection against shark attack in South Africa, 1952-90

Historically, most shark attacks in South Africa have occurred off Natal, where the water is relatively warm and where there are many holiday resorts. The first shark nets were installed in 1952. This, followed by the establishment of the Natal Sharks Board and the widespread installation of shark nets in the 1960s, has reduced the incidence of shark attack considerably. Forty-four kilometres of permanently maintained nets catch an annual average of 1470 sharks and 536 other animals, which are largely dolphins, turtles and rays. The impact of netting on both the species caught and the inshore environment as a whole is not well understood and is a cause for concern. The release from the nets of live animals, including sharks, and the removal of nets during periods of highest catches may reduce such impact. Experiments with an increased mesh size are in progress. Tests of an electrical barrier as an alternative to nets have not been encouraging but are continuing. In the cooler waters to the south of Natal, where there are no shark nets, a marked increase in the number of shark attacks has taken place in recent years. There appear to be no plans to expand the netting operations into those waters.

Incidental capture of bottlenose dolphins (Tursiops truncatus) in shark nets: an assessment of some possible causes

A minimum of 250 bottlenose dolphins, Tursiops truncatus, was caught in anti‐shark nets off Natal, South Africa between January 1980 and December 1988. Males and females constituted 45.4% and 54.6% of the catch, respectively. Sexually mature males and females constituted 18.2% and 24% of the total catch, respectively. Of the latter, 65% were lactating and a further 10.3% were pregnant. Adolescents, three or more years old but not mature, contributed 12.4% of the catch. Calves, two growth layer groups (GLGs) or less in the dentine, constituted the greatest portion of the catch, almost 43%, and of these almost 70% were either weaned or weaning at capture. Analyses of biological, environmental and physiographic data for each capture suggest a number of reasons for the catch of bottlenose dolphins. The distribution of catches was random but catch rates were proportional to the number of nets. The stomachs of most dolphins were almost full at the time of capture, suggesting that enmeshing occurred either during or subsequent to feeding. Most captures were of single animals, but lactating females with calves constituted more than 25% of catches. The direction of the prevailing current was significantly related to captures. These data are examined in relation to existing knowledge of bottlenose dolphins in this area and suggest that capture may be a consequence of prey abundance at and around the nets, dolphin capture occurring either during or directly after feeding. Possible methods of reducing captures are proposed.

Sharks caught in the protective gill nets off Natal, South Africa. 5. The Java sharkCarcharhinus amboinensis(müller & Henle)

Between 1978 and 1990, 209 Java sharks Carcharhinus amboinensis were caught in the shark nets that protect the swimming beaches of Natal, thereby constituting 0,5 per cent of the total shark catch. The catch rate was relatively constant at about 0,4 sharks·km net−1·year−1. Catches were highest at the northernmost beaches in summer and autumn and they consisted mostly of juvenile, but not newborn, and adolescent sharks. Specimens ranged from 99 to 176 cm PCL, with a mode of 131–135 cm for males and 131–145 cm for females. Males matured at 150 cm and females at 160 cm. Of the eight mature females caught, five were pregnant. Mating takes place in late summer and gestation is thought to last approximately 12 months. The largest embryo found was 53 cm. Teleosts were identified in 62,1 per cent of the 103 stomachs that contained food. Elasmobranchs (44,7%), crustaceans (12,6%) and cephalopods (11,6%) were the other major prey groups. Most of the fish prey were demersal, associated with soft bottoms.

Sharks caught in the protective gill nets off Natal, South Africa. 4. The bull sharkCarcharhinus leucasValenciennes

A total of 772 bull sharks Carcharhinus leucas was caught in Natal's protective "shark nets" between 1978 and 1990. Confusion in distinguishing C. leucas from C. amboinensis resulted in their catch data being combined from 1966, when data collection began, to 1977. The catch rate of the species pair declined until 1977, recovered until the mid 1980s, but subsequently declined again. The trend in catch rate of C. leucas alone for the period 1978–1990 was similar, with minima of 0,70 sharks·km-net−1·year−1 in 1978 and 0,95 in 1990, and a maximum of 2,08 in 1986. Recaptures of six tagged sharks suggest that the species is not highly migratory. Catches, particularly of immature sharks, were highest at the northernmost beaches. Most bull sharks were caught in summer and in turbid water (mean water clarity 2,0 m). The sex ratio of the catch was 1 male to 1,3 females. Sizes ranged from 74 to 213 cm precaudal length, with modes of 141–145 cm (males) and 171–175 cm (females). Size at maturity for both sexes was between 180 and 190 cm. The mating season was prolonged but with a summer peak. Seven gravid females were examined; the mean litter size was 8,7 embryos and size at birth was approximately 55 cm. Fluke infections were observed on 9 per cent of animals examined. As size increased there was a shift in diet, in terms of frequency of occurrence, from teleost to elasmobranch predominance. There was a high incidence of benthic and demersal species in the stomachs. Minor prey groups included mammals, birds, turtles, molluscs and crustaceans. Scavenging appeared to be important.

Bottlenose dolphinTursiops truncatusdistribution in Natal's coastal waters

The distribution of bottlenose dolphins Tursiops truncatus between the Tugela River mouth, Natal, and the Transkei border was investigated. Comparison of sighting rates between the periods 1974–1978 and 1984–1988, accumulated by personnel servicing shark nets at 42 localities, indicated six areas where dolphins were sighted more frequently. These preferred areas were similar in length (32,7–41,9 km). Behavioural data suggested that the preferred area centred on Margate represents the home range of a local dolphin population. Analysis of dolphin catches in the nets indicated that they are caught at random along the coast, although there was an association between catch and sighting rates. Mean water clarity at each beach was not correlated with either sighting or capture rates at the same beach for the period 1984–1988. Sighting rates of groups of bottlenose dolphins decreased some five- to tenfold in the decade between the periods 1974–1978 and 1984–1988, suggesting a decline in the number of bottlenose d...

Organochlorines in common dolphins caught in shark nets during the Natal ‘sardine run’

The concentrations of organochlorines were determined in blubber and liver samples from common dolphins inhabiting the coastal waters of the south-east coast of southern Africa. Liver levels of PCBs and DDTs are far lower and do not appear directly associated with those in blubber. In males, blubber residue concentrations increased with age but in females a marked rapid decline in concentrations of PCBs and t-DDT was evident between the eighth and eleventh growth layer groups, approximately the age of sexual maturity and subsequent to their first or second ovulation. Lower levels in females are explained through transfer to the new-born calf during lactation and the consequences of this for the newborn calf are discussed.

Analysis of Stomach Contents of Dugongs From Queensland.

Quantitative analysis of samples of the stomach contents of 95 north Queensland dugongs, Dugong dugon, (63 from the Townsville area, 30 from the Mornington Island area and 2 from the Torres Strait area) and one dugong from south Queensland indicates that the diet of most consisted almost entirely of seagrasses of all available genera. Halodule was found in 95% of stomachs, followed by Halophila (89%). Cymodocea (61%) and Thalassia (39%). Seagrass rhizomes were present in all stomachs examined including that of a newborn calf. The generic composition of the stomach contents probably reflects that of the seagrass beds in the areas where the dugongs were captured and is not necessarily indicative of discrimination in selecting food. Non-epiphytic algae occurred in 51% of the stomachs, usually in small amounts. When seagrasses were abundant, dugongs appeared to eat algae often but only in very small amounts (<2% of food volume). Dugongs do not appear to be well adapted to utilize algae as food and probably usually consume them incidentally with seagrasses. Many of the dugongs which drowned in the shark nets near Townsville in the year after a cyclone had severely damaged the seagrass beds in the area, had eaten more of the seagrasses Cymodocea and Thalassia, less of the seagrasses Halodule and Halophila, and more algae than animals which died in other years.