The Order Sirenia (manatees and dugongs) consists of two Families—the Trichechidae (manatees), which comprise four species, and the Dugongidae, the sole extant member of which is the dugong (Dugong dugon). The dugong is the most abundant and widespread of the Sirenia. They are classified as vulnerable to extinction in the World Conservation Union (IUCN) Red Data Book of Threatened Species (Bryden et al. 1998). The closest non-Sirenian relative of the dugong is the elephant (Marsh et al. 2000).
Dugongs are large herbivorous marine mammals. They inhabit tropical and subtropical coastal and inland waters in the Indo-West Pacific, between about 26–27° north and south of the equator. With relatively thin blubber layers compared to cetaceans and pinnipeds, they are intolerant of cold and are generally found in waters at and above 18°C. At the southern end of their distribution in Australia, dugongs may undertake seasonal movements that assist with thermoregulation (Bryden et al. 1998; Packard et al. 1984).
The most important dugong habitat in Australia is Torres Strait (Blanshard 2001), but dugongs are found from Moreton Bay in the east to Shark Bay in the west (Marsh et al. 2000). Aerial surveys have indicated that dugongs are the most numerous marine mammal in the in-shore waters of northern Australia. However, evidence suggests that dugong numbers have declined along most of the urbanised coast of eastern Queensland since the mid 1980s (Bryden et al. 1998). Habitat loss, drowning in gill or shark nets and traditional hunting have been implicated in the decline (Marsh 1997).
Dugongs are long-lived. The age-determination method developed for pinnipeds and toothed cetaceans has been adapted for dugongs, enabling their age to be determined from the number of growth layer groups in the tusks (Marsh 1980). The oldest dugong whose tusks have been examined for age determination was estimated as 73 yr old when she died (Marsh et al. 2000).
2 ANATOMY AND PHYSIOLOGY
Adult dugongs typically range from 2.4–3.0 m in length and 250–420 kg in weight. Females are slightly larger than males. There are reports of animals up to 3.31 m in length and weighing up to 600 kg (Blanshard 2001; Bryden et al. 1998). The relationship between body weight and length has been described by the equation:
y = (-34.251) – 14.976x + 55.218x2
where y is the body weight (kg) and x is the body length from the tip of the snout to the notch in the tail fluke (in metres) (Spain & Heinsohn 1975). A selection of predicted body weights, together with an indication of the likely stage of maturity at certain body lengths, is presented in Table 19.1. Body length is an unsatisfactory indication of age except in young animals (Marsh et al. 1984a).
Estimated body weight (kg)
Likely stage of maturity
<1.8 m—probably suckling
<2.4 m—probably immature
Anatomy of the dugong has been reviewed by Marsh (1989) and a detailed guide to the necropsy of dugongs is available through the Australian Government Great Barrier Reef Marine Park Authority (Eros et al. 2007). The necropsy guide presents good diagrams and images of dugong anatomy and is available online at www.gbrmpa.gov.au.
Dugongs show a number of anatomical and physiological adaptations associated with life in a marine environment. They have fusiform bodies, fin-like fore limbs and horizontally expanded tail flukes, and they lack hind limbs. The flippers are short, the testes abdominal. The skull has a remarkably enlarged and sharply downturned premaxilla and a correspondingly elongated symphysial region of the mandible. The skeleton is extremely dense bone which presumably helps the animal overcome buoyancy problems when bottom-feeding in salt water. The feeding apparatus of the dugong reflects its role as a herbivore specialising on benthic seagrasses. The upper lip takes the form of a horseshoe-shaped disc or muzzle, which is a very complex and mobile structure (Marsh 1989).
Two pairs of upper incisors are present in juveniles. The deciduous incisors are small, do not erupt and are resorbed. Posterior growth of the permanent incisors (tusks) is similar in both sexes until about puberty, when the tusks are about 100 mm long. Female tusks continue to grow throughout life and may reach 180 mm. The tusks of males erupt after puberty and may reach about 150 mm, but never protrude more than a couple of centimetres. The erupted end is worn into a chisel shape. Apart from the tusks, the only functional teeth are the cheek teeth. Dugongs have a total of six cheek teeth in each jaw quadrant during their lifespan, but they are never all erupted and in occlusion at once. The dugong has a simple stomach and is considered to be a non-ruminant herbivore (Marsh 1989). It is a hindgut fermenter (Marsh et al. 2000).
Variable, average 2–4 breaths/5 min in water, 3–15 breaths/5 min out of water
40–60 bpm (can be as low as 30 bpm with diving)
Source: Chittick & Walsh (2007).
There is a double ventricular apex to the heart and the lungs are long and extend posteriorly almost as far as the kidneys. The diaphragm is large and obliquely sloped. The main bronchus runs almost the entire length of the lung with only a few side branches. Even the most peripheral bronchioli possess cartilage within their walls (Marsh 1989).
Unrinsed seagrasses consumed by dugongs contain about 15 times more chloride and 30 times more sodium than most terrestrial pasture plants. Dugongs do not drink fresh water and presumably acquire their entire water intake from seagrass. Kidneys are elongated. The brain is relatively small (Marsh 1989).
Although they can be long-lived (>70 yr), dugongs do not reach sexual maturity until 10–17 yr (females). Females are polyovular and polyoestrus and may undergo a number of sterile cycles before becoming pregnant (Marsh 1989). Gestation lasts approximately 13 mo. After the birth of a single calf there is an extended period of maternal investment and a prolonged intercalving interval (3–7 yr). Size at birth is variable with neonates ranging from about 1.10–1.25 m long and about 27–35 kg in weight. Calves begin to consume seagrass soon after birth but continue to suckle from their mothers for at least 18 mo (Bryden et al. 1998).
The dugong breeding season around Townsville or Cairns extends from August/September to December (Marsh et al. 1984a, b). Southern populations may lag a little behind their northern counterparts, with both mating activity and births concentrated in a few months at the end of each calendar year (Bryden et al. 1998).
The anal and genital apertures are contiguous in female dugongs. In males the genital aperture is distinctly separated from and anterior to the anus (Marsh et al. 2000).
Some information is available on husbandry of manatees (Walsh & Bossart 1999) but very little information is available on dugongs.
Dugongs should be kept in natural sea water enclosures. Artificially produced salt water can be used to house dugongs but it is expensive and there may be problems with the availability of the constituents. Enclosures can be natural or artificial. If artificial, the enclosure walls and floor should be durable, watertight, non-porous, non-abrasive, non-toxic and easily cleaned and disinfected. Sharp projections and loose fittings should be avoided. Artificial pools have the advantage of being able to maintain water parameters better than natural pools. Natural pools, however, can be constructed much larger for less cost than artificial pools. The pools should be large enough to accommodate the natural behaviours of the animals and deep enough for the animals to dive and avoid the effects of ultraviolet radiation. Shade should be supplied and the temperature of the water should be maintained between 23°C and 30°C. If chemical treatment is used to maintain water quality and cleanliness, it should not cause harm to the animals (see Chapters 17 and 18 for further information on water quality). Minimum depth for long-term housing of one adult animal should be 4 m. Minimum length should be 11 m and minimum width should be 9 m. If more than one animal is to be housed, the length and the width should be increased by 50%.
Dugongs can be housed for short periods in small pools. The size of the animal determines the size of pool that it will require. Infant dugongs can be housed in above-ground pools as long as the water is turned over quickly enough to avoid build-up of bacteria, phosphates and ammonia. It is advantageous if the pool can be drained completely as this will allow capture and restraint of the animal far more easily than if it has to be caught in deep water.
Little published information is available on transport of dugongs. Manatees are generally secured in a stretcher and transported by truck on a 15 cm thick foam pad (Geraci & Lounsbury 1993). Dugongs should be kept wet and shaded from the sun. If possible they should be kept at 23–28°C during the transport. Temperatures outside this range must be avoided.
4.4 Individual marking and identification
Generally, individual dugongs are identified by their markings or other obvious physical differences. These differences may include the shape of the tail fin or any visible scars or notches. Microchips or passive integrated transponder (PIT) tags can be inserted IM into the epaxial muscles.
5.1 Wild diets
Dugongs feed almost exclusively on several species of tropical and temperate seagrasses. Marine algae may be eaten if seagrass is scarce. Of the 10 genera of seagrasses occurring within the dugong’s Australasian range, nine are known to be eaten (Lanyon 1991).
5.2 Captive diets
There is a reasonable amount of information on captive diets of manatees (Walsh & Bossart 1999), but very little information available on dugongs. A typical manatee diet may incorporate 75–85% leafy green vegetables such as romaine and leaf lettuce, cabbage, kale, hydroponic wheat, oats and sprouts; 15–20% dried forage such as timothy/alfalfa hay; and a small percentage of vegetables (e.g. carrots) and concentrates such as those designed for elephants. Lettuce by itself (romaine and green leaf) has been used for extended periods but has numerous drawbacks, including poor nutrient content and expense. Issues associated with captivity and nutrition include blockage of pool filtration associated with large amounts of particulate matter and fungal contamination of grains (Walsh & Bossart 1999).
Few dugongs have been maintained successfully in captivity. Ingredients of captive dugong diets include seagrass, cos lettuce, snowpea sprouts, cabbage, endive, wheatgrass and spinach. Dugongs feed from the ocean bed in the wild; efforts should be made to feed captive animals from the bottom of their enclosures. Food items can be anchored into a weighted frame that sinks to the bottom of the pool. Some captive dugongs may adapt to feeding from the surface. Advice should be sought from institutions that have held these animals.
5.3 Feeding sick animals
Dugongs have been reported to be notoriously difficult to maintain in captivity. If natural diets are used, animals will need to be hand-fed 40+ kg of seagrass a day. Long-term rehabilitation has been described as an expensive, labour-intensive and probably impractical option with little chance of success (Smith 1997).
It has been stated that, for manatees, diets should be designed with future release in mind and that food items vastly different from wild varieties (e.g. some fruits and vegetable) should not be used (Walsh & Bossart 1999). Nutritional supplementation has been administered by gastric tube to manatees (Walsh & Bossart 1999). About 2 L of gruel composed of monkey chow, lettuce and spinach was administered twice a day. A thin mixture is administered initially. Over time it is thickened, and may be delivered via stomach pump. Read Walsh and Bossart (1999) if supplementary feeding of dugongs is being considered.
Little information is available on restraint of dugongs.
6.1 Physical restraint
One method of capture involves surrounding the manatee with a net (30–35 m long and 4 m deep, of 10 cm stretch mesh) that is gradually drawn towards the shore until the animal is in water shallow enough for handlers to physically restrain it. A healthy animal may require 10–15 people (Geraci & Lounsbury 1993). Handlers must use extreme caution to avoid becoming entangled in the net and pulled into the water if the animal attempts to escape. Once in shallow water, the animal is manoeuvred onto a padded platform and secured with straps. A 3.5 m stretcher is usually appropriate. Once out of the water, manatees generally become calm and may need little restraint. Covering the eyes and wrapping a small piece of netting over the snout (do not obstruct breathing) helps to calm the animal. Attempts to assist unrestrained manatees are ill-advised and dangerous. One person cannot safely lift a manatee calf. Calves should be placed securely in a stretcher before removal from the water. Handlers must be prepared for sudden thrashing at all stages of handling (Geraci & Lounsbury 1993).
6.2 Chemical restraint
The unique anatomy, physiology and aquatic environment of sirenians make sedation and anaesthesia challenging (Chittick & Walsh 2007). Little information is available on chemical restraint of dugongs. However, a wide variety of anaesthetic agents and techniques have been used to restrain and anaesthetise Florida manatees (Chittick & Walsh 2007) and readers are encouraged to review that source prior to attempting chemical restraint of dugongs. There are anatomical and physiological differences between dugongs and manatees and it is unknown how applicable the techniques used in manatees may be to dugongs. An expert veterinary anaesthetist should be involved in any chemical restraint procedures.