Estimates of bycatch and field experiment using pingers to reduce bycatch of dolphins in drift- and bottom set gillnets in Menai Bay, Zanzibar, Tanzania
(English)Manuscript (preprint) (Other academic)
In order to estimate the total bycatch in gillnet fisheries and to assess the impact on dolphin populations in the Menai Bay Conservation Area, a survey using independent observers aboard the fishing vessels was conducted in 2003/2004. The observer programme covered 23.6% and 24.5% of the drift- and bottom set gillnets effort, respectively. The estimated total bycatch was 13 Indo-Pacific bottlenose dolphins in drift gillnets and 4 humpback dolphins in bottom set gillnets representing 9.6% and 6.3%, respectively of the estimated 136 Indo-Pacific bottlenose and 63 humpback dolphins resident in the area in 2002. These bycatch levels were not considered sustainable. In 2007/2008 a second observer programme was conducted in the same area to investigate the effectiveness of acoustic alarms (pingers) in reducing the bycatch of dolphins in the drift- and bottom set gillnets. The observed effort in the drift gillnets was 257 sets without pingers and 251 sets with pingers representing 21% and 20% of the total recorded effort, respectively. Six dolphins were bycaught during the pinger experiment in the drift gillnets (1 Indo-Pacific bottlenose dolphin in sets with pingers and 4 Indo-Pacific bottlenose dolphins and 1 spinner dolphin in the sets without pingers). In the bottom set gillnet fishery, the observed fishing effort was 236 sets without pingers and 224 sets with pingers, representing 28% and 27% of the total recorded effort, respectively. In the bottom set gillnets, one humpback dolphin was bycaught in the sets without pingers and no dolphin was bycaught in the sets with pingers. Pingers reduced the bycatch of dolphins in both drift- and bottom set gillnets, however the reduction was only significant in the drift gillnets. Estimates of the total bycatch in the sets without pingers in 2007/2008 fishing season were 16 Indo-Pacific bottlenose dolphins in drift gillnets and 3 humpback dolphins in the bottom set gillnet fishery, representing 11.8% and 4.8% of estimated population size for respective species in the area in 2002. Given the documented unsustainable removal levels in the drift- and bottom set gillnets, immediate management actions are needed to reduce dolphin bycatch in these fisheries.
Indo-Pacific bottlenose dolphin, humpback dolphin, fisheries bycatch, acoustic alarms, pingers, gillnets, conservation, management, Zanzibar, East Africa.
Research subject Animal Ecology
IdentifiersURN: urn:nbn:se:su:diva-34465OAI: oai:DiVA.org:su-34465DiVA: diva2:284850