Towards a sustainable indigenous fishery for dugongs in Torres Strait : a contribution of empirical data analysis and process

Kwan, Donna (2002) Towards a sustainable indigenous fishery for dugongs in Torres Strait : a contribution of empirical data analysis and process. PhD thesis, James Cook University.

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Abstract

Given the globally threatened status of dugongs and the fact that Torres Strait supports the world's largest dugong population, there is international, national, regional and local interest in ensuring the sustainability of the Torres Strait traditional dugong fishery. This fishery is endorsed by the Torres Strait Treafy between Australia and Papua New Guinea (PNG).

This study investigated, within a biological context, the major factors (i.e., those additional to biological) that determine sustainability of the traditional dugong fishery in Torres Strait.

My thesis had the following specific aims:

(1) To provide information relevant to the development of community-based management for the dugong fishery in Torres Strait by: describing and quantifying the major factors that affect hunting pattern, hunting effort, hunting success and harvest levels in the major dugong hunting community of Mabuiag Island, and estimating the life history parameters of Torres Strait dugongs at the time of my study, and (i) comparing them with parameters obtained from parallel studies of dugong life history and reproductive biology in order to gain insights into the factors influencing these aspects of dugong ecology, and (ii) incorporating them into population models to predict the natural rate of change in the size of the population in the absence of hunting mortality.

(2) To assist in the development of community-based management in Torres Strait through contributing to capacity building by actively involving Torres Strait Islanders in my research and training them in the collection of catch statistics and biological samples from dugongs.

The fieldwork for this project was undertaken primarily at Mabuiag Island between September 1997 and November 1999 inclusive. All data and specimens from 127 female and 51 male dugongs were collected from animals butchered for food by Torres Strait Islander hunters.

Being able to live and work within the Mabuiag Island community provided me with the rare opportunity to develop a relationship with Islanders built upon the mutual trust, co-operation and commitment. Involving Islanders as active participants in my research allowed me to collect the valuable specimens on which my information is based. The active involvement of hunters and community members enhanced their development of a sound understanding of and trust in the information I obtained. By training Torres Strait Islanders as research assistants, 1 also increased the capacity of the community to actively participate in future research, monitoring and management programs for dugongs.

This study provided the opportunity to identify the main factors which impact catch rates of dugongs by Islanders. Only four to five hunters were responsible for 79% and 75% of the 145 and 170 dugongs taken at Mabuiag lsland during March to October in 1998 and 1999, respectively. The magnitude of the dugong catch was influenced by ecological, economic environmental and social factors that affected the temporal and spatial nature of hunting effort and hunting success.

When my results were compared with those of other studies using similar techniques, I showed that the age and size at sexual maturity and first reproduction, pregnancy rates of female dugongs and the sex ratio of their young exhibit considerable variability in both space and time. The age and size at sexual maturity and first reproduction of male dugongs exhibit similar variability. At Mabuiag lsland in 1997-99, female dugongs reached sexual maturity between six or seven and fourteen years and at body lengths of between 2.05 and 2.58 m. Male dugongs reached sexual maturity between four and thirteen years and at body lengths between 1.90 and 2.57 m. Calving intervals of adult female dugongs from Mabuiag lsland in 1998-99 ranged between 2.4 f s.e. 0.3 to 3.3 f s.e. 0.5 years. The parameters from Mabuiag lsland in 1997-98 are thus the lowest recorded for dugongs. At Mabuiag lsland in 1997-98, almost all sexually mature nulliparous females were either pregnant andlor lactating and the sex ratio of foetuses was 1:3 in favour of males.

Variability in life history parameters such as age at sexual maturity and first reproduction, pregnancy rates and calving intervals in dugongs is likely to result from spatial and temporal variation in seagrass community composition and biomass. Thus, food quality and availability influence dugong population dynamics. When their seagrass food disappears as a result of episodic disturbance, some dugongs suspend breeding. It is likely that the effects of food supply on life history parameters and hence fecundity in dugongs includes both density dependent and density independent components. The gaps in the age composition of the catch from Mabuiag in 1997-99 together with the low pregnancy rates of animals caught in the Daru market at that time suggest that a seagrass dieback in Torres Strait in the late 1970s had significant effects on dugong recruitment. These links between seagrass food availability and dugong life history parameters have very important implications for management given the dugong's susceptibility to large-scale episodic losses of seagrass.

Several findings in this study highlight that a regional approach to management will be required because the potential for episodic or longer-tern over-harvesting can be exacerbated in the event of a combination of: low dugong fecundity as a result of extensive seagrass dieback events such as that reported in the 1970s; movements of dugongs close to hunting communities to feed on intertidal seagrass in response to dieback of deepwater communities exacerbating high hunting pressure; increase in hunting pressure in response to reduction in important sources of income such as the cray fishery.

Management coordinated at a regional scale must include the Northern Peninsula Area of Cape York and PNG. Management will need to build on established processes which incorporate Ailan Kast'om (the customary way of life of Torres Strait Islanders) and institutional structures in the Torres Strait region which include the: Torres Sfraif Treafy, Torres Sfraif Fisheries Acf (1984), Native Title and regional Islander initiatives such as MaSTERS (Marine Strategy for Torres Strait).

A community-based management approach, which builds on the research processes, and the empirical information obtained in this study, has considerable potential to ensure that the Torres Strait dugong fishery is sustainable. The challenge now is to build on information and trust generated by this project by enabling Islanders to use these results to develop and implement appropriate management plans for their dugong resources.

Item ID: 1376
Item Type: Thesis (PhD)
Keywords: Torres Strait traditional dugong fishery, sustainability, hunting pattern, hunting effort, hunting success, harvest levels, Mabuiag Island, life history parameters, population models, age, size, sexual maturity, first reproduction, pregnancy rates, sex ratio of young, calving intervals, seagrass dieback, community-based management
Date Deposited: 26 Jun 2007
FoR Codes: 07 AGRICULTURAL AND VETERINARY SCIENCES > 0704 Fisheries Sciences > 070402 Aquatic Ecosystem Studies and Stock Assessment @ 0%
05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050211 Wildlife and Habitat Management @ 0%
06 BIOLOGICAL SCIENCES @ 0%
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