Research

Reducing catches

The KZN Sharks Board Act requires the organisation to provide measures to reduce the risk of shark attack but also to attempt to reduce the environmental cost associated with those measures. As a result, the Board is continuously looking at ways to reduce the catch of sharks and other non-shark species. Since the late 1990s the strategies outlined below have reduced shark mortalities in the nets by 64%!

The following are some of the measures it has introduced:

1. Extensive net reduction

Between 1952 and 1961, Durban was the only net installation. There was a steep rise in the number of protected beaches and the length of netting in the 1960s. After 1970, few new installations were added, although the length of netting continued to increase, peaking at 45 km in 1992, when there were 44 protected beaches. Between 1999 and 2004, a phase of intensive net reduction at most beaches resulted in a decline in the length of netting down to 27 km, a reduction of 40%.

2. Release of all large sharks

Initially, all large sharks found alive were killed. This practice changed very gradually with the release of smaller individuals of those species that posed little or no threat. By 1989, all sharks were released   including the species identified as posing the greatest threat, namely Zambezi shark, Carcharhinus leucas, white shark, Carcharodon carcharias, and tiger shark, Galeocerdo cuvier.  Between 1984 and 2014, KZNSB staff tagged and released over 4250 sharks.

3. Experimentation with nets of a larger mesh size

In 1991, a series of experiments was initiated with nets of a larger mesh size (70 cm) to try and reduce the catch of smaller sharks, rays, turtles and fish. There was no significant difference in the number of harmless animals caught, however, the larger mesh (70 cm) caught fewer large sharks raising concerns that the deployment of these nets would result in an unacceptable reduction in bather-safety levels. As a result, the experiments were abandoned and the mesh size remained at 51 cm.

4. Reducing catches associated with the sardine run

In a phenomenon known as ‘the sardine run’, shoals of sardines Sardinops sagax enter southern KZN coastal waters in June and July. Following the sardines are a host of predators including sharks and dolphins, which are susceptible to capture in the shark nets. In an attempt to reduce shark catches associated with the sardine run, nets are removed from beaches between June and July every year.

5. Reducing cetacean catches

In 1996, small air-filled floats were introduced into the meshes of half of the nets deployed at Margate to establish whether the sonar of bottlenose dolphins could detect the air-filled floats and thereby avoid accidental entanglement. In 11 years, more bottlenose dolphins (16) were caught in the Margate nets with small, air-filled floats than without (11) and so the experiment was terminated.

Acoustic deterrents, termed ‘pingers’, are widely used in the fishing industry to alert dolphins to the presence of fishing nets. In 1999, commercially manufactured 10-kHz deterrents were introduced at Richards Bay to prevent catches of Indo-Pacific humpback dolphins, Sousa chinensis. The program was expanded to five other beaches in an attempt to reduce catches of bottlenose dolphins. On several occasions, a bottlenose or humpback dolphin was caught within 10m of a dolphin pinger, suggesting that the animal may have been attracted to the sound source. This prompted a decision to move the two pingers out of each net and to attach each pinger to one of the ropes demarcating the net’s anchors. Catches of humpback dolphins persisted at Richards Bay, so the pingers were replaced with louder whale alarms, operating at lower frequencies. An analysis is currently being undertaken to assess the efficacy of these alarms in reducing catches of bottlenose dolphins, and initial impressions suggest that their deployment has not had the desired effect.

In 2005, whale alarms were manufactured in-house and deployed each whale season (June–December) at beaches with the highest sightings of whales. The alarms operate over a frequency range of 2.9–3.4 kHz. In 2009, 48 alarms were used at 17 beaches. Preliminary results, however, suggest that the alarms are of little deterrent to whales.

6. Replacement of nets with drumlines

A major initiative was the introduction of three drumlines in 2005 at Richards Bay, after several years of local experimentation. In 2007, 76 drumlines replaced almost half (4 km) of the nets at the 17 southernmost protected beaches on the Hibiscus Coast , with a replacement ratio of four drumlines to one net. A comparison in catches along the Hibiscus Coast Pre-2007 (before drumlines were introduced) and Post-2007 (nets and drumlines) indicate the following:

a. There was no significant difference in the mean annual catch of white, tiger or Zambezi sharks (species historically responsible for shark attacks in KZN) Pre-2007 (nets only) and Post-2007 (nets and drumlines). This indicates that the use of mixed gear has not reduced the level of protection offered to bathers.

b. The introduction of drumlines and the use of mixed gear Post-2007 brought about a statistically significant reduction (by 47.5%) in the annual bycatch of non-shark animals. It also resulted in a marked, although not significant reduction (33.7%) in suspected whale entanglements.

c. Probably the major disadvantage associated with the introduction of drumlines is the increased catch of juvenile dusky sharks. Also, the drumlines do not appear to catch Zambezi sharks as efficiently as nets. As a result, the optimal solution to ensure bather safety is a combination of both nets and drumlines.

d. There was no significant difference in the Catch-Per-Unit-Effort (CPUE) for white, tiger or Zambezi sharks caught in the nets between Pre and Post-2007 periods. This strongly suggests that the baited drumlines deployed Post-2007 are not acting as a major attractant, drawing sharks into the near shore zone.