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Biology, ecology and anthropogenic threats of Indo-Pacific bottlenose dolphins in east Africa
Stockholm University, Faculty of Science, Department of Zoology. (Marine Mammal)
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 [en]
Growth, reproduction, feeding ecology, bycatch, organohalogen compounds, dolphins, Tursiops aduncus, Zanzibar, East Africa
National Category
Ecology
Research subject
Animal Ecology
Identifiers
URN: urn:nbn:se:su:diva-34472ISBN: 978-91-7447-002-4 (print)OAI: oai:DiVA.org:su-34472DiVA: diva2:284939
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
List of papers
1. The occurrence and distribution of dolphins in Zanzibar, Tanzania, with comments on the differences between two species of Tursiops
Open this publication in new window or tab >>The occurrence and distribution of dolphins in Zanzibar, Tanzania, with comments on the differences between two species of Tursiops
2005 (English)In: Western Indian Ocean journal of marine science, ISSN 0856-860X, Vol. 4, no 1, 85-93 p.Article in journal (Refereed) Published
Abstract [en]

Incidental catches (bycatch) in gillnet fisheries off Zanzibar (Unguja Island), as asource of mortality among several species of dolphins, were reported in a questionnaire surveyconducted in 1999. As a follow-up to that survey, from January 2000 to August 2003, wemonitored the incidental catches of dolphins collected from 12 fish landing sites. Six species ofdolphins were recorded from 143 specimens retrieved from bycatches in drift- and bottom setgillnets. Of these, 68 (48%) were Indo-Pacific bottlenose dolphins (Tursiops aduncus), 44 (31%) spinner dolphins (Stenella longirostris), 12 (8%) Risso’s dolphins (Grampus griseus), 11 (8%) Indo-Pacific humpback dolphins (Sousa chinensis), 6 (4%) Pan-tropical spotted dolphins(Stenella attenuata) and 2 (1%) common bottlenose dolphins (Tursiops truncatus). Most of thebycatches (71%) were in nets set off the north coast of Unguja Island. In this paper, bycatchrecords are examined to describe the occurrence and distribution of dolphin species in UngujaIsland coastal waters. The relatively large numbers of bycatch dolphins recorded indicate thatbycatch may be a potential threat to local populations that need to be addressed in futureconservation and management efforts in the region.

Keyword
incidental catch, occurrence, distribution, dolphins, Unguja Island, Zanzibar
National Category
Ecology
Research subject
Animal Ecology
Identifiers
urn:nbn:se:su:diva-34438 (URN)
Projects
Sustainable dolphin tourism in east Africa
Available from: 2010-01-08 Created: 2010-01-08 Last updated: 2017-12-12Bibliographically approved
2. Growth and reproduction of Indo-Pacific bottlenose dolphins (Tursiops aduncus) incidentally caught in gillnets off Zanzibar, Tanzania
Open this publication in new window or tab >>Growth and reproduction of Indo-Pacific bottlenose dolphins (Tursiops aduncus) incidentally caught in gillnets off Zanzibar, Tanzania
(English)Article in journal (Refereed) Submitted
Abstract [en]

Life history parameters of Indo-Pacific bottlenose dolphin (Tursiops aduncus) were examined in 69 specimens incidentally caught in gillnet fisheries off Zanzibar, Tanzania between 2000 and 2008. Calves were born at a body length of 103 cm and a weight of 12-15 kg. Sexual maturity in females was reached at 7-8 years and body length 190-200 cm. Sexual maturity in males was attained at 16 years and a body length of 213 cm. Calving occurred throughout the year with a peak November-March, after a gestation period of 12.3 months. The estimated pregnancy rate was 0.10 based on the proportion pregnant mature females in the sample and 0.58 based on the occurrence of Corpora Lutea in the ovaries. The average calving interval was calculated to 2.7 years. The results are important for assessment of fisheries bycatch mortality and conservation and management of Indo-Pacific bottlenose dolphins in western Indian Ocean

Keyword
Indo-Pacific bottlenose dolphin, Tursiops aduncus, life history parameters, growth, reproduction, sexual maturity, ovary, Corpora Lutea, pregnancy rate, gestation period, calving interval, gillnet fisheries, byctach, Zanzibar, Tanzania, Indian Ocean.
National Category
Ecology
Research subject
Animal Ecology
Identifiers
urn:nbn:se:su:diva-34454 (URN)
Available from: 2010-01-08 Created: 2010-01-08 Last updated: 2010-10-14Bibliographically approved
3. Feeding ecology of the Indo-Pacific bottlenose dolphin (Tursiops aduncus) incidentally caught in the gillnet fisheriesoff Zanzibar, Tanzania
Open this publication in new window or tab >>Feeding ecology of the Indo-Pacific bottlenose dolphin (Tursiops aduncus) incidentally caught in the gillnet fisheriesoff Zanzibar, Tanzania
2005 (English)In: Estuarine, Coastal and Shelf Science, ISSN 0272-7714, E-ISSN 1096-0015, Vol. 63, no 3, 429-437 p.Article in journal (Refereed) Published
Abstract [en]

The stomach contents of 26 Indo-Pacific bottlenose dolphins (Tursiops aduncus) incidentally caught in gillnet fisheries aroundUnguja Island (Zanzibar) between February 2000 and August 2002 were examined. The relative importance of each prey species wasassessed through indices of relative importance. In total, 1403 prey items comprising 50 species of bony fish and three species ofsquid were identified from food remains. Five species of fish, Uroconger lepturus, Synaphobranchus kaupii, Apogon apogonides,Lethrinus crocineus, Lutjanus fulvus, and three species of squid, Sepioteuthis lessoniana, Sepia latimanus and Loligo duvauceli, werethe most important prey species. Based on an index that included frequency of occurrence, percentage by number and by weight,Uroconger lepturus proved to be the most important prey species of mature dolphins whereas Apogon apogonides was the preferredprey of immature dolphins. These results indicate that Indo-Pacific bottlenose dolphins off the coast of Zanzibar forage ona relatively large number of prey species, but that only a few small- and medium-sized neritic fish and cephalopods contributesubstantially to the diet. Further, the ecology and behavior of the preferred fish prey species indicate that the dolphins forage overreef or soft bottom substrata and near the shore.

Keyword
Indo-Pacific bottlenose dolphin; Tursiops aduncus; stomach contents; feeding ecology; diet; Zanzibar
National Category
Ecology
Research subject
Animal Ecology
Identifiers
urn:nbn:se:su:diva-34448 (URN)10.1016/j.ecss.2004.12.006 (DOI)
Projects
Sustainable dolphin tourism in east Africa
Available from: 2010-01-08 Created: 2010-01-08 Last updated: 2017-12-12Bibliographically approved
4. Estimates of bycatch and field experiment using pingers to reduce bycatch of dolphins in drift- and bottom set gillnets in Menai Bay, Zanzibar, Tanzania
Open this publication in new window or tab >>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)
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
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:nbn:se:su:diva-34465 (URN)
Available from: 2010-01-08 Created: 2010-01-08 Last updated: 2010-09-24Bibliographically approved
5. Organohalogen compounds in blubber of Indo-Pacific bottlenose dolphin (Tursiops aduncus) and spinner dolphin (Stenella longirostris) from Zanzibar, Tanzania
Open this publication in new window or tab >>Organohalogen compounds in blubber of Indo-Pacific bottlenose dolphin (Tursiops aduncus) and spinner dolphin (Stenella longirostris) from Zanzibar, Tanzania
Show others...
2010 (English)In: Environmental Pollution, ISSN 0269-7491, E-ISSN 1873-6424, Vol. 158, no 6, 2200-2207 p.Article in journal (Refereed) Published
Abstract [en]

Blubber samples of Indo-Pacific bottlenose (Tursiops aduncus) and spinner (Stenella longirostris) dolphins from Zanzibar, East Africa, were analysed for a wide range of organohalogen compounds. Methoxylated polybrominated diphenyl ethers (MeO-BDEs), presumably biogenic, were found at higher concentrations than anthropogenic organochlorine pesticides (OCPs). Only traces of industrial pollutants, such as polychlorinated biphenyls, were detected. The OCP levels found off Zanzibar were lower than those reported from other regions while MeO-BDE levels were higher. The relative composition of the OCPs indicated recent use of lindane (γ-hexachlorocyclohexane) and aged residues of DDT and technical HCH. Placental transfer was estimated to 2.5% and 0.5% of the total burden of OCPs and MeO-BDEs, respectively. Overall transfer from mother to calf in Indo-Pacific bottlenose dolphins was estimated to 72% and 85% for the OCPs and MeO-BDEs burdens, respectively. Health effects of MeO-BDEs are not known, but structural similarities with well-known environmental toxins are cause for concern.

Keyword
Methoxylated polybrominated diphenyl ethers, MeO-BDEs, organochlorine pesticides, maternal transfer
National Category
Pharmacology and Toxicology
Research subject
Animal Ecology
Identifiers
urn:nbn:se:su:diva-34468 (URN)10.1016/j.envpol.2010.02.027 (DOI)000278343000028 ()
Available from: 2010-01-08 Created: 2010-01-08 Last updated: 2017-12-12Bibliographically approved

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  • Other locale
More languages
Output format
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