School of Natural and Environmental Sciences


Mitigation of dolphin bycatch in gillnet fisheries in Zanzibar, Tanzania

Bycatch in fishing gear, especially drift and bottom set gillnets, likely poses the greatest direct threat to coastal cetaceans in the South-West Indian Ocean region. Off Zanzibar, bycatch primarily affect Indo-Pacific bottlenose dolphins, Indo-Pacific humpback dolphins and spinner dolphins, but pantropical spotted dolphins, common bottlenose dolphins, Risso’s dolphins and humpback whales are also affected. The recorded large number of dolphin bycatch in gillnets off Zanzibar is a serious conservation concern, e.g. unsustainable bycatch levels (>5% per year of population size estimates) of Indo-Pacific bottlenose and humpback dolphins have been documented in artisanal drift-and bottom set gillnets. The aim of this project is to investigate the effectiveness of acoustic alarms (pingers) in reducing the bycatch of dolphins and their potential effect on target fish catch in drift gillnet fisheries in Zanzibar, Tanzania.

The pinger used is the Fumunda FMDP-2000 which is a small (150 mm) pinger which emits a 10 kHz, 132 dB sound levels “ping” during 300 ms every 4s.

The field experiment is conducted off the north coast Zanzibar where 24 drift gillnet boats are participating. 12 boats are using pingers on their nets and 12 fish without pingers as control. Pingers are attached every 100m along the float line of the nets with a pinger at each end of the drift gillnet. In this set-up, a typical drift gillnet with a length of 700m require eight pingers. The aim is to achieve a minimum of 500 hauls with both pinger nets and control nets.

The results will inform fishery conservation and management whether pingers are an effective mitigation measure to reduce dolphin bycatch in Zanzibar drift gillnet fisheries. The results will also be relevant to other areas where similar gears are used.