@techreport{,

title = {Dugongs (Dugong dugon) of the Bazaruto Archipelago, Mozambique},

author = {Cockcroft, V.,Guissamulo, A.,Findlay, K.},

issn = {Final Report to SASOL Mozambique},

year  = {2008},

date = {2008-01-01},

number = {72},

pages = {1-84},

abstract = {Historically, dugongs occurred in almost all coastal areas from southern Mozambique in the

west to Australia in the east, including many of the island States in the Indian and Pacific

Oceans between. However, through much of this range, dugongs have all but disappeared

and where they occur, are represented by relict populations. This is especially true for

Eastern Africa and the Indian Ocean and Mascerine Islands. In most areas dugongs have

been extirpated and only in southern, central Mozambique, the Bazaruto Archipelago, does a

relatively large number still survive.

Between April 2006 and December 2007 a series of 27 aerial surveys was flown over the

Bazaruto Bay region, from Cabo Sao Sebastio in the south to north of the Save River Mouth

in the north and between the coast and the 30 m isobath (extending to eastwards of the

Bazaruto Archipelago islands in the south). A total of 9052 nautical miles of survey was flown

during the 27 surveys. During these, 355 sightings of an estimated 760 dugongs were made.

Two core areas of distribution were apparent; a northern core region, between the Save River

Mouth and 21°24’ S, and a southern core region aligned with the shallow sandbanks to the

north and south of Santa Carolina Island. The mean group size of dugongs encountered was

2.22 and comparable to mean group sizes observed elsewhere. However, the maximum

group size observed was considerably smaller than those observed in other areas of dugong

abundance. Likewise, calculated dugong densities were substantially lower in the Bazaruto

area than they are in the Arabian Gulf or Australia. There was no evidence of any seasonal

variation in occurrence, group size, density or proportion of calves. The proportion of calves

was similar to that in other areas surveyed for dugongs.

Statistical manipulation of the data from aerial surveys suggests that the dugong population

inhabiting the area surveyed is around 250 animals.

The extent of dugong habitat was gauged by mapping seagrass occurrence, diversity and

distribution. Mapping was accomplished through extensive dive sampling of 982 stations

within the surveyed area, within 5m to about 25 m depth. Five species of seagrass were

identified within the area. These covered some 4.68 % of the area and occurred generally in

the north and close to the shore in waters less than 8 m deep. In the north, the areas of

greatest seagrass abundance and diversity coincide with the areas of greatest dugong

abundance. However, in the area to the north and south of Santa Carolina, seagresses were

almost absent, suggesting that this area is used by dugongs for reasons other than feeding.

Population Viability Analysis (PVA) modelling was used to assess the viability of the dugong

population in the Bazaruto Archipelago under various scenarios. PVA is a statistical method

of understanding the influence of a species’ natural history, in combination with demographic,

environmental, and random genetic (stochastic) events, on the dynamics of its future. The

species’ population dynamics is modelled as a set of discrete, sequential events (e.g., births,

deaths, catastrophes, etc.), which occur according to defined probabilities. Population

dynamics models provide a predictive tool, where the impacts of variables (threats), in this

instance – environmental threats (cyclones, floods); hydrocarbon exploration and incidental

fishing catch, can be theoretically modelled.

Overall, the PVA models suggest strongly that the dugongs inhabiting Bazaruto Bay are in a

precarious position and face extirpation. The models suggest that dugongs are particularly

susceptible to long term stressors that might reduce overall reproductive success. Further, the

analyses clearly show that regular deaths, from anthropogenic sources, are the major cause

of dugong decline.

A number of scenarios, covering options for hydrocarbon exploration and exploitation, are

presented and discussed, in terms of their potential impacts on the dugongs inhabiting the

Bazaruto area.},

keywords = {abundance, Distribution, dugong, East Africa, Mozambique},

pubstate = {published},

tppubtype = {techreport}

}