Dugong Research Articles

Seascapes of fear: evaluating sublethal predator effects experienced and generated by marine mammals

AbstractThe notion that predators can affect their prey without killing them is widely supported in the ecological literature yet rarely applied by marine mammal studies. We present three case studies in which patterns of time allocation by individual marine mammal foragers were used to index the sublethal effects of predators on bottlenose dolphins (Tursiops sp.), harbor seals (Phoca vitulina), and dugongs (Dugong dugon). In each case, foraging individuals optimized energy gain and safety from predators by spending less time in more profitable but dangerous patches or decreasing their use of risky feeding tactics that would increase net energy gain. By implication, marine mammals are subject to the non consumptive effects of their predators (i.e., to intimidation), and fear can mediate their impacts on their resources. We suggest, therefore, that future studies quantify patterns of time allocation to measure sublethal effects of predators on marine mammals, as well as the capacity of marine mammals to have sublethal effects on their own prey. We argue that such an approach is important because non consumptive effects may be of greater magnitude than lethal effects of predators, and information on sublethal effects of predators can inform conservation plans and studies of community structure.

A review of research on the interactions between dugongs (Dugong dugon Müller 1776) and intertidal seagrass beds in Indonesia

Research on dugong-seagrass interactions in Indonesia was done during the period 1990 until 2005 in respectively East Aru, Maluku Province and East Kalimantan, Indonesia. This research investigated intensive rotational grazing by dugongs in intertidal inshore Halodule univervis seagrass meadows, dugong movements in relation to these grazing swards, and analyses of parameters explaining the temporal and spatial patterns of grazing in these meadows. In this paper, we report the findings of this long-term study. The patterns of movement and the results of snorkelling surveys confirmed a practice of regular recropping of restricted grazing swards by small feeding assemblages of dugongs. Dugong grazing showed a significant correlation with carbohydrate content of the below-ground biomass and no significant relation with total N. The timing of dugong grazing in these intertidal meadows coincides with high below-ground biomass and high carbohydrate content in the rhizomes of H. uninervis in the upper 0-4 cm sediment layer. Our findings support the hypothesis that temporal dugong grazing is ruled by carbohydrate content in below-ground biomass. The mechanisms of rotational grazing in restricted grazing swards are not yet well understood, and the maximisation of carbohydrates does not fully explain this phenomenon. Our research confirms that intertidal H. univervis seagrass meadows form a crucial resource for dugong survival. These relatively unknown biotopes need therefore more attention in research and conservation programmes.

Concentrations of Organotin Compounds in Tissues and Organs of Dugongs from Thai Coastal Waters

Concentrations of butyltin (BT) and phenyltin (PT) compounds were measured in organs and tissues of dugongs (Dugong dugon) from the coastal waters of Thailand. Concentrations of BTs and PTs were in the range of 14-14,468 and <1-30 microg kg(-1)(detection frequency: 79%), respectively. Although concentrations of BTs in dugongs were higher then reported concentrations in cetaceans and pinnipeds, PTs were lower in dugongs. In half of the dugongs in which measurements were made, the concentration of BTs in the liver was the highest among the all the tissues and organs tested. Dibutyltin (DBT) or monobutyltin (MBT) was found to be the dominant compounds among the BTs. The distribution in the body of PTs was not clear because of the lower levels of this compound. TPT was the dominant compound among PTs. The coastal area of Thailand is located off the Gulf of Thailand and the Andaman Sea. Concentrations of organotin (OT) compounds in dugongs collected from the Gulf of Thailand were compared to those from the Andaman Sea. No significant differences in BT or PT concentrations were observed between the two areas (p < 0.05). The concentrations of BTs and PTs in the livers of dugongs were decreased between 1998 and 2002, suggesting a decrease in OT concentrations in the surrounding environment.

Fear factor: do dugongs (Dugong dugon) trade food for safety from tiger sharks (Galeocerdo cuvier)?

Predators can influence plants indirectly by altering spatial patterns of herbivory, so studies assessing the relationship between perceived predation risk and habitat use by herbivores may improve our understanding of community organization. In marine systems, the effects of predation danger on space use by large herbivores have received little attention, despite the possibility that predator-mediated alterations in patterns of grazing by these animals influence benthic community structure. We evaluated the relationship between habitat use by foraging dugongs (Dugong dugon) and the threat of tiger shark predation in an Australian embayment (Shark Bay) between 1997 and 2004. Dugong densities were quantified in shallow (putatively dangerous) and deep (putatively safe) habitats (seven survey zones allocated to each habitat), and predation hazard was indexed using catch rates of tiger sharks (Galeocerdo cuvier); seagrass volume provided a measure of food biomass within each zone. Overall, dugongs selected shallow habitats, where their food is concentrated. Foragers used shallow and deep habitats in proportion to food availability (input matching) when large tiger sharks were scarce and overused deep habitats when sharks were common. Furthermore, strong synchrony existed between daily measures of shark abundance and the extent to which deep habitats were overused. Thus, dugongs appear to adaptively manage their risk of death by allocating time to safe but impoverished foraging patches in proportion to the likelihood of encountering predators in profitable but more dangerous areas. This apparent food-safety trade-off has important implications for seagrass community structure in Shark Bay, as it may result in marked temporal variability in grazing pressure.

Past and present marine mammal hunting rates and abundances: dugong ( Dugong dugon) evidence from Dabangai Bone Mound, Torres Strait

Dugongs ( Dugong dugon) are a key food item and a totemic animal with major spiritual significance for Torres Strait Islanders of northeastern Australia. These marine mammals are officially classed vulnerable to extinction which has placed hunters under considerable internal (cultural) and external (bureaucratic) pressure to lower hunting rates dramatically to sustainable levels. But did Torres Strait Islanders hunt dugongs at much lower rates in the pre-colonial past? Excavation of a ritual dugong bone mound on Mabuyag island revealed the remains of 10,000–11,000 dugongs hunted between c. 1600 and c. 1900AD. The translated hunting rate of 33–37 dugongs per year is surprisingly high and challenging as this single site represents one-third of what conservation biologists argue is the current mean sustainable hunting rate for the entire Torres Strait archipelago. These data suggest that dugong abundance was much higher in the pre-colonial past and that current hunting rates are uncharacteristically unsustainable primarily due to an unprecedented dugong population crash and not increased post-contact hunting rates.

Characterization of 26 new microsatellite loci in the dugong (Dugong dugon)

AbstractTwenty‐six microsatellite loci have been isolated from a dugong (Dugong dugon). The average heterozygosity was 0.52 with two to 10 alleles per locus surveyed from 50 individuals. The markers are suitable for genetic mark–recapture (PID = 5 × 10−16) in dugongs and they could also be used to quantify physical tag loss, estimate relatedness, assign paternity, elucidate population structure and identify migrants. The loci also amplified in Florida manatees (22/26) and Asian elephants (6/26).

CURRENT STATUS AND CONSERVATION NEEDS OF DUGONGS IN SOUTHERN JAPAN

AbstractWe conducted aerial surveys of dugongs (Dugong dugon) using the line‐transect method and snorkeling surveys of dugong feeding trails in 1998 and 1999 around Okinawa Island (26°30′N, 128°00′E) and the Sakishima Islands, southern Japan. A total of ten dugongs were sighted and feeding trails were confirmed in the sea grass beds off the east coast of Okinawa Island. In the Sakishima Islands, however, no dugongs were observed, and there was no evidence of feeding trails despite the existence of apparently suitable sea grass beds for feeding. The results of these surveys and other available information suggest that Okinawan dugongs represent a small, geographically isolated population. Our sightings of dugongs during the daytime, offshore of sea grass beds where feeding trails were recorded, suggest that Okinawan dugongs principally feed at night when human activities are limited. Survival of this remnant dugong population is threatened by habitat degradation and occasional entanglement mortality in fishing nets.

Long-Distance Oceanic Movement of a Solitary Dugong (&lt;I&gt;Dugong dugon&lt;/I&gt;) to the Cocos (Keeling) Islands

In this paper, we report on the arrival of a small (2 m long) male dugong (Dugong dugon) to the Cocos (Keeling) Islands (12° 12' S, 96° 54' E), Indian Ocean. The dugong arrived in June 2002 after travelling more than 1,000 km across deep open ocean, during which time it would have been vulnerable to predation and presumably unable to feed. This is the longest recorded dugong movement and demonstrates that dugongs have the capacity to make long-distance oceanic movements to colonise distant, unoccupied locations. The solitary dugong has remained at the Cocos Islands for at least four years and exhibits unusual habitat use and behaviour; it frequently occupies deep water on the edge of the coral reef and interacts with SCUBA divers.

Use of tusks in feeding by dugongid sirenians: Observations and tests of hypotheses

Most living and fossil sea cows of the subfamily Dugonginae (Dugongidae, Sirenia, Mammalia) are characterized by large upper incisor tusks, which are thought to play an important role (at least primitively) in feeding on seagrass rhizomes. Testing this hypothesis is difficult, because the only extant tusked sirenian (Dugong dugon) is morphologically and perhaps behaviorally aberrant. The tests attempted here involve examination of stomach contents of wild Recent dugongs, experiments using plastic replicas of diverse tusks to harvest seagrasses, gross anatomical observations on tusks and skulls, measurements of tusk tip geometry, and observations of microwear on tusks. We conclude that (a) male D. dugon (with erupted tusks) do not consume more rhizomes than females (without erupted tusks); (b) the tusks do not play a significant role in feeding in the modern dugong; (c) larger, more bladelike tusks are more effective at harvesting rhizomes, but the effect of shape was not experimentally separated from the effect of exposed tusk length; (d) some fossil dugongines show apparent cranial adaptations for downward and backward cutting motions of their large, bladelike tusks; (e) geometry of wear surfaces is consistent with use of at least the more bladelike tusks as cutting instruments; (f) preliminary observations of microwear in D. dugon do not indicate more than occasional use of the tusks in purposeful harvesting of rhizomes, and then only opportunistically by large adult males. The hypothesis of such tusk use by extinct dugongines (in contrast to the living species) is so far corroborated, but available data and tests do not suffice to establish this conclusively.

Vertebral anatomy in the Florida manatee, Trichechus manatus latirostris: A developmental and evolutionary analysis

The vertebral column of the Florida manatee presents an unusual suite of morphological traits. Key among these are a small precaudal count, elongate thoracic vertebrae, extremely short neural spines, lack of a sacral series, high lumbar variability, and the presence of six instead of seven cervical vertebrae. This study documents vertebral morphology, size, and lumbar variation in 71 skeletons of Trichechus manatus latirostris (Florida manatee) and uses the skeletons of Trichechus senegalensis (west African manatee) and Dugong dugon (dugong) in comparative analysis. Vertebral traits are used to define morphological, and by inference developmental, column modules and to propose their hierarchical relationships. A sequence of evolutionary innovations in column morphology is proposed. Results suggest that the origin of the fluke and low rates of cervical growth originated before separation of trichechids (manatees) and dugongids (dugongs). Meristic reduction in count is a later, trichechid innovation and is expressed across the entire precaudal column. Elongation of thoracic vertebrae may be an innovative strategy to generate an elongate column in an animal with a small precaudal count. Elimination of the lumbus through both meristic and homeotic reduction is currently in progress.

Observations of dugong reproductive behavior in Trang Province, Thailand: further evidence of intraspecific variation in dugong behavior

The seagrass beds at Muk Island and Talibong Island in Thailand are important areas for dugong (Dugong dugon) feeding and reproduction. We used opportunistic observations during aerial surveys to investigate dugong mating behavior in shallow water areas near Talibong Island on three different days. The mating pattern was categorized by five stages: (1) following: the male followed the female, (2) approaching and stimulating: the male approached and muzzled the female, (3) pairing: both male and female swam in parallel, ventral to ventral or dorsal to dorsal side, (4) mounting: the male copulated with the female, and (5) separating: the male and female swam in different directions. Parental care of calves included one observation of a cow embracing her neonatal calf by the flippers.

Atypical associations between dugongs (Dugong dugon) and dolphins in a tropical lagoon

Several types of relationships link organisms to each other, including competition, predation and various types of associations. This paper presents the first case of association/interaction between dugongs, Dugong dugon, the only strictly marine herbivorous mammal, and three species of tropical dolphins in and around the lagoon of Mayotte (45°10'E 12°50'S), in the western tropical Indian ocean. Data were collected opportunistically from 1999 to 2005 in the surrounding waters of Mayotte from boat (N=2 observations) and ULM (ultra-light motorized vessel, N=2). The dolphin species involved in associations with dugongs were Indo-Pacific bottlenose dolphin, Tursips aduncus, Indo-Pacific humpback dolphin, Sousa chinensis, and spinner dolphin, Stenella longirostris. In one case, dugongs were associated with both bottlenose and humpback dolphins. Associations were observed in the protected waters of the lagoon and outside, along the external slope of the barrier reef. Group size, activity and group structure of each species were recorded. Behavioural observations suggest that dugongs and dolphins were engaged in similar activities, such as travelling, on several occasions and were clearly associated when group formation was tight. If dolphins and dugongs may not associate for feeding purposes, then these interactions may occur: (1) for predation avoidance toward sharks; or (2) without any ecological reasons due to dolphin and dugong habitat overlap around the island.

Behavioral Indicators in Marine Conservation: Lessons from a Pristine Seagrass Ecosystem

In light of ongoing changes to marine ecosystems, there is a need for behavioral indicators that can identify critical habitats and assess anthropogenic impacts. Although the application of behavior to conservation has yielded mixed results, habitat selection behavior has promise as such an indicator. Terrestrial studies and a decade of work in Shark Bay's pristine seagrass ecosystem show that habitat selection theory based on the ideal free distribution can be used to assess critical habitats based on food and safety and signal impacts of anthropogenic disturbance. One lesson from our studies of dugongs (Dugong dugon) and other species at risk from tiger sharks (Galeocerdo cuvier) is that physical attributes of habitats may influence the effects of predators, and, by extension, human disturbance. These species generally prefer seagrass bank edges, which facilitate escape from sharks, even though shark density is lower over interior portions of the banks. Thus, habitat selection indicates that a bank's qu...

Feeding behavior of wild dugongs monitored by a passive acoustical method

Little is known about feeding behavior of wild dugongs (Dugong dugon) because direct measurements of feeding events in the water were scarcely feasible. In this study, the authors achieved the first successful feeding sound monitoring in a seagrass area using a full-band underwater recording system (called automatic underwater sound monitoring system for dugong: AUSOMS-D). In total, 175 feeding sounds were identified in 205 h of recording. Feeding sounds were only detected at night, implying diurnal differences in the feeding behavior of the studied dugong population. Differences in periodicity of feeding sounds suggested that two or more individuals were in the acoustically observable area. Furthermore, a feeding position monitored by two AUSOMS-Ds was used to calculate source levels of dugong feeding sounds. Assuming spherical_propagation, source levels were measured between 70.6 and 79.0 dB rms re 1 microPa/square root of Hz.

Indigenous Use and Trade of Dugong (Dugong dugon) in Sabah, Malaysia

Dugongs (Dugongidae: Dugong dugon) are represented by relic populations separated by areas where they are close to extinction if they not already extinct. However, in 1999, the dugong was determined to still exist in Malaysian waters after a spate of dugong deaths was reported in waters near Johors, in peninsular Malaysia. This incident focused attention on the status of dugongs in Malaysia, which has led to renewed interest in conducting research on dugongs. In eastern Malaysia, dugongs exist in small fragmented populations along the coastal seagrass regions of Sandakan District, Teluk Brunei, and Kudat District in Sabah (2, 3). In the past and, to some extent in the present, the dugong has been exploited by people for various uses. The remaining dugong populations are decreasing in Sabah due to various threats; namely, incidental catching and fish bombing, habitat loss as a result of land reclamation, and pollution from palm oil plantations and sedimentation (2-4). This report provides information on the traditional uses and anthropogenic threats to the remaining dugong populations in Sabah.

Distribution and significance of seagrass ecosystems in Malaysia

Seagrasses are the only flowering plants (monocotyledonous Angiosperms) that have adapted themselves to living in marine and estuarine habitats, and are submerged most of the time. They are rooted in sediments on the sea bottom, with shoots appearing above the substrate. Seagrasses occurred at 78 sites scattered in the west and east coasts of Peninsular Malaysia and in Sabah and Sarawak in East Malaysia. They are usually found along the coasts growing in association with shallow inter-tidal, mangroves, coral reefs, semi-enclosed lagoons and shoals. In these habitats, seagrass beds or meadows have distinct species assemblage completely adapted to the submerged life. There are fourteen major species of seagrasses recorded in Malaysia: Enhalus acoroides, Halophila beccarii, H. decipiens, H. ovalis, H. minor, H. spinulosa, Halodule pinifolia, H. uninervis, Cymodocea rotundata, C. serrulata, Thalassia hemprichii, Syringodium isoetifolium, Ruppia maritima and Thalassodendron ciliatum. Although seagrasses make up only a small portion of the marine ecosystem, the physical settings and their interactive community within and from outside account for their high diversity and ensure survival of an assortment of vertebrates (fish), invertebrates (shrimps, starfish, sea cucumbers, bivalves, gastropods), and seaweeds. Seagrass form the food and habitats for the vulnerable dugongs or sea cows (Dugong dugon), seahorses (Hippocampus spp.) and endangered green turtles (Chelonia mydas), and for other fish, and also a feeding ground for seasonal migratory birds, Egretta garzetta. Seagrasses provide conditions for the growth and abundance of invertebrates and fish that many local coastal communities collect and catch for their livelihood. Seagrass ecosystems are sources of food and yet they are continually threatened by human activities, causing their degradation and possible habitat loss. The purpose of this review is to give information on the significant linkages of seagrasses with the coastal inhabitants and to suggest recommendations for the protection and conservation of this important ecosystem and the associated resources.

Dugong (Dugong dugon) vocalization patterns recorded by automatic underwater sound monitoring systems

To quantitatively examine the diurnal, or tidal, effects on dugong behavior, we employed passive acoustic observation techniques to monitor the animals. Automatic underwater sound monitoring systems for dugongs (AUSOMS-D) were deployed on the sea floor at depths of about 5 m south of Talibong Island, Thailand. The AUSOMS-D recorded underwater sound in stereo at a sampling frequency of 44.1 kHz for more than 116 consecutive hours. Dugong calls were automatically detected by newly developed software with a detection rate of 36.1% and a false alarm rate of 2.9%. In total, 3453 calls were detected during the 164 h of recording. The autocorrelation of the call rate indicated an attendance cycle of about 24 or 25 h, and the most frequent vocalizations were observed from 0300 to 0500 h. The calculated bearings of the sound sources, i.e., dugongs, were used as an indicator to track the relative numbers of dugongs during the monitoring periods.

Factors influencing the sustainability of customary dugong hunting by a remote indigenous community

The sustainability of indigenous customary hunting and fishing in remote areas can be influenced by human factors operating at global as well as regional and local scales because of the hybrid nature and sectoral interactions of the local economic environment. The internationally significant population of dugongs (Dugong dugon or seacow) in Torres Strait between Australia and Papua New Guinea supports an important indigenous fishery. The economic, socio-cultural and environmental factors that influenced hunting activity in 1998 and 1999 by the members of the community of Mabuiag Island were investigated to inform the sustainable management of the fishery. The landed catch during the eight months March to October of 145 dugongs in 1998 and 170 dugongs in 1999 potentially provided the community with an average of 290 g of dugong meat per person per day. Fifty-seven per cent of adult males on the island participated in dugong hunting, but more than half the catch in each year was caught by only two hunters. The probability of at least one person from the community going dugong hunting in 1998 and 1999 was 0.59 ± 0.02 per day. This probability was influenced by local environmental factors, including the abundance of dugongs in the traditional hunting grounds (affected by wind speed, year, season and lunar day) and the size of the commercial crayfish catch (which is influenced by the global market price, as well as local conditions). Although dugong hunting remains a very important part of the islanders’ contemporary culture and customary economy, the capacity to hunt dugongs is facilitated by the ease with which some hunters move between the state, commercial and customary sectors of their local economy. The complexities of the economic, social and cultural environments need to be considered in planning for the sustainable harvesting of threatened species by remote indigenous communities.

The Dugong, Dugong dugon, in Egyptian waters: distribution, relative abundance and threats

This paper presents the results of an interview survey used to identify the distribution and relative abundance of Dugongs (Dugong dugon) in Egypt’s Red Sea during 2001, 2002 and 2003. The survey area ranged from Hurghada, immediately south of the Gulf of Suez, to El Shalatin, about 200 km north of the Sudan border. Respondents, such as fishermen, were asked preprepared questions to determine the number of Dugongs observed per sighting, the length class of individuals, the month of observation and the location. Important feeding habitat, defined in part by the presence of Dugong feeding trails, was identified during a concurrent seagrass survey. Dugongs occurred throughout the survey area, but apparently in very low numbers. The maximum and minimum numbers reported were 17 in 2002 and 12 each in 2001 and 2003. Some individuals may have been counted more than once per annum. There was no evidence of annual change in the number of Dugong sightings, but a statistically greater number of sightings was made in summer compared with winter. Two hypotheses are proposed to explain this variation, but they are not mutually exclusive. The first is that Dugongs undertake winter movements to warmer waters. The second is that winter sea conditions made it harder to observe Dugongs. Important feeding habitat in the Wadi El Gemal Hamata Protected Area was confirmed by the presence of Dugong feeding trails in seagrass meadows. Known and predicted threats to this species in Egypt are loss of seagrass meadows, net entrapment and boat strikes.

Estimation of Water Turnover Rate in Captive Dugongs (&lt;I&gt;Dugong dugon)&lt;/I&gt;

Water turnover rate was measured for two captive dugongs (Dugong dugon) using deuterated water. Body water content of 69.5% in a dugong was high compared to other marine mammals. A water turnover of 257.2 ml kg -1 day -1 measured in one dugong was almost twice as high as the highest rates measured in studies of captive West Indian manatees (Trichechus manatus) and was high compared to those measured in carnivorous marine mammals. The other dugong’s estimated water turnover rate of 134 ml kg -1 day -1 was comparable to the highest rates in manatees. Two alternative explanations are offered for the high water turnover: (1) the incidence of mariposia or voluntary drinking of sea water by the dugong or (2) a metabolic rate that is significantly higher than predicted, based on its phylogeny.