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The Natal Sharks Board
PINGERS
Richards Bay hump-backed dolphin project
Incidental deaths of dolphins and porpoises in the world's fishing nets are a major global problem. Unfortunately, the shark nets off KwaZulu-Natal also catch occasional dolphins.
As part of its attempts to reduce any environmental impact of the shark nets, the Natal Sharks Board has initiated an investigation of factors affecting dolphin catches and methods of reducing these catches.
The 1998 Richards Bay Hump-backed Dolphin Project was one of a suite of dolphin projects addressing this aim, with particular emphasis on determining the efficacy of active acoustic devices, popularly known as "pingers", in reducing dolphin entanglements. Surprisingly, this appears to be the first project that has ever investigated reactions of free-ranging dolphins (as opposed to porpoises) to pingers!
Indo-Pacific hump-backed dolphin (Sousa chinensis) population dynamics and behaviour around permanently set shark nets off Richards Bay, KwaZulu-Natal, South Africa were investigated. Photo-identification of the dolphins indicated that the population was open in nature, but that several individuals displayed medium-term residency.
Dolphins and Shark Nets
Analysis of residency indices for photo-identified dolphins showed considerable variation in how individuals used the research area. Individuals with high residency indices, however, were all regularly observed in the region around the harbour entrance and those nets which take a high catch.
The dolphins captured in the shark nets during the study were all individuals with low residency indices, suggesting that local knowledge of the shark net placing is important in reducing susceptibility to capture.
Interestingly, behavioural analyses indicated that behaviour was significantly different when within 200m of the shark nets than elsewhere in the study area. Social behaviour was never observed within this 200m zone, while feeding behaviour was significantly greater than elsewhere, particularly when within 100m of the nets.
These data suggest that the dolphins may be aware of the danger of nets, resulting in their reduced social behaviour to concentrate on their environment. However, the increased feeding around the nets may be due to their acting as an artificial reef or fish aggregating device, thereby negating the effect of heightened environmental awareness and increasing susceptibility to capture during fish chases.
Subsequent random incorporation of PICE© pingers and concomitant data collection indicated that these pingers had no affect on dolphin utilisation of the netted area, with no significant difference in the activity indices between periods of active and non-active pingers.
Similarly, there was no significant difference in the time spent by the observers searching for dolphins, the mean duration of dolphin observations, nor the proportion of time spent observing dolphins relative to time spent searching for them, for periods when pingers were active or not active.
Although the average surfacing distance from activated pingers was slightly smaller, it was not significant. Similarly, there was a trend towards closer approaches to nets containing activated pingers with increased exposure to them.
The data set was too small to test for significance, however. These results indicate that activated PICE© pingers had little effect on Indo-Pacific hump-backed dolphins. The capture of three dolphins during periods when pingers were inactive, however, suggests that they may function to heighten dolphin awareness of the presence of nets.
This project was supported by Richards Bay Minerals, AGFA (South Africa), and Kitt's Photos through the Endangered Wildlife Trust. Results have proved sufficiently promising to purchase sufficient Dukane NetMark™1000 pingers for installation on the entire Richards Bay shark net installation.
Continued monitoring of all catches in this shark net installation will continue in the hope that dolphin captures will be reduced significantly.
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