DOLPHIN PINGERS AND WHALE ALARMS
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.