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Estimates of bycatch and field experiment using pingers to reduce bycatch of dolphins in drift- and bottom set gillnets in Menai Bay, Zanzibar, Tanzania
Stockholm University, Faculty of Science, Department of Zoology. (Marine Mammal)
Stockholm University, Faculty of Science, Department of Zoology. (Marine Mammal)
(English)Manuscript (preprint) (Other academic)
Abstract [en]

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.

Keyword [en]
Indo-Pacific bottlenose dolphin, humpback dolphin, fisheries bycatch, acoustic alarms, pingers, gillnets, conservation, management, Zanzibar, East Africa.
National Category
Ecology
Research subject
Animal Ecology
Identifiers
URN: urn:nbn:se:su:diva-34465OAI: oai:DiVA.org:su-34465DiVA: diva2:284850
Available from: 2010-01-08 Created: 2010-01-08 Last updated: 2010-09-24Bibliographically approved
In thesis
1. Biology, ecology and anthropogenic threats of Indo-Pacific bottlenose dolphins in east Africa
Open this publication in new window or tab >>Biology, ecology and anthropogenic threats of Indo-Pacific bottlenose dolphins in east Africa
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis examines the biology, ecology and anthropogenic threats of Indo-Pacific bottlenose dolphins (Tursiops aduncus) off Zanzibar, Tanzania, based on research conducted and samples collected between 2000 and 2008. Distribution and occurrence are described based on incidental catches (bycatch) in gillnet fisheries. Biology and ecology are examined by ageing and studying the reproductive biology and stomach contents of collected specimens. The composition of organohalogen compounds is determined in blubber samples, and assessment and mitigation of bycatch are conducted using observers onboard fishing vessels. Fisheries bycatch data showed that Indo-Pacific bottlenose dolphins occur year round in all areas around Zanzibar. Sexual maturity was attained between 7 and 8 years and body length 190-200 cm in females and at 16 years and body length 213 cm in males. The gestation period was estimated to be 12.3 months, with calving occurring throughout the year, peaking November-March and with an interval of 2.7 years. The estimated pregnancy rate was between 0.10 and 0.58 depending on methods used. Stomach contents revealed a relatively large number of prey species, but that only a few small- and medium-sized neritic fish and cephalopods contribute substantially to the diet. Estimates of total annual bycatch were >9% which is not considered sustainable. An experiment showed that pingers can be a short term mitigation measure to reduce bycatch of dolphins in both drift- and bottom set gillnets. Methoxylated polybrominated diphenyl ethers (Meo-BDEs) were found at higher concentrations than anthropogenic organic pesticides (OCPs), with only traces of polychlorinated biphenyls (PCBs) detected. This study reveals the magnitude and apparent susceptibility of Indo-Pacific bottlenose dolphins off Zanzibar to anthropogenic threats, especially fisheries bycatch, and it is clear that immediate conservation and management measures are needed to reduce bycatch.

Place, publisher, year, edition, pages
Stockholm: Department of Zoology, Stockholm University, 2010. 31 p.
Keyword
Growth, reproduction, feeding ecology, bycatch, organohalogen compounds, dolphins, Tursiops aduncus, Zanzibar, East Africa
National Category
Ecology
Research subject
Animal Ecology
Identifiers
urn:nbn:se:su:diva-34472 (URN)978-91-7447-002-4 (ISBN)
Public defence
2010-02-12, Ahlmansalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Submitted. Paper 4: Manuscript. Paper 5: Submitted. Available from: 2010-01-21 Created: 2010-01-08 Last updated: 2010-10-14Bibliographically approved

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