Contributions to Zoology, 67 (1) 71-77 (1997)H.H. de Iongh; B. Bierhuizen; B. van Orden: Observations on the behaviour of the Dugong (Dugong dugon Müller, 1776) from waters of the Lease Islands, eastern Indonesia

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Results

The results of the dugong observations in Saparua Bay are listed in Table I. The results of all field observations are summarized below. Table IIpresents a summary of submergence times from the present study, compared with other authors.

 

Table I. Summary of field observations on individual dugongs in Saparua Bay, during June-October 1992. No. = observation number, Date:= date of observation, Time = time of observation, site = Observation took place from a cliff, from the boat or while diving, Swimming direction = swimming direction of the dugong when leaving the feeding sward.

No.

Date

Time

Site

Juvenile/
adult

Swimming
direction

Remarks

Grazing inside sward

Tide

1

24/6/92

10:30 am

cliff

adult

N

Rough sea

+

H

2

4/7/92

1:05 pm

cliff

adult

N

Rough sea

+

L

3

5/7/92

9:20 am

cliff

juvenile

NE

Swimming small circles

+

H

4

18/7/92

10:50 am

cliff

juvenile

N

None

+

L

5

20/7/92

9:40 am

diving

juvenile

E

Swimming circle around diver

+

H

6

13/9/92

4:25 pm

boat

adult

E

Loud noise while breathing (exhalation)

_

H

7

13/9/92

4:35 pm

boat

adult

E

Rough sea

_

H

8

14/9/92

1:05 pm

boat

juvenile

E

Rough sea

+

L

9

15/9/92

1:00 pm

boat

adult

N

2 Dugongs/vertical surfacing

_

L

10

16/9/92

10:00 am

diving

juvenile

NE

Approaching diver (2 times)

+

L

11

19/9/92

10:15 am

diving

juvenile

E

Swimming in high speed while being followed

+

H

12

1/10/92

9:45 am

diving

adult

NW

Approaching diver

+

H

13

1/10/92

1:20 am

boat

adult

W

Rough sea

+

L

14

23/10/92

11:00 am

diving

juvenile

S

Approaching diver (3 times)/ grazing behaviour

+

L

 

Table II: Submergence times (minutes) of free-ranging dugongs in Saparua Bay, compared with other authors

Number of observations

Submerged time
(mean ± SD)

Location

Author

17

4.6 (± 1.3)

Free-ranging dugongs at the Bay of Saparua

Present study

162

2.7 (± 0.07)

Free-ranging cow with calf in Shark Bay

Anderson, 1982

346

0.9 (± 0.04)

Free-ranging adults without calves in Shark Bay

Anderson, 1982

370

1.2 (± 0.04)

Free-ranging adults without calves in Shoalwater and Cleveland Bays

Anderson & Birtles, 1978

70

2.5 (± 0.05)

Adults

Allen et al., 1976

Feeding behaviour

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During the research period fourteen free-ranging dugongs were observed, all occurring in the outer part of Saparua Bay. Observations of dugongs were done both in the morning and afternoon (Table I), not indicating regular feeding times. We observed that dugongs forage at depths down to 10 m in the bay. During several observations dugongs foraged within 50 m of a ten meter high cliff (which was used as an observation site). Dugongs were spotted during both at high and low tide (Table I). During a diving session we observed that three distinct feeding tracks were made by a dugong within a single dive. The dugong moved from one place to another in the seagrass meadow without surfacing. It grazed upon the vegetation, hovering above the substratum with only the rostrum bent down. Dugongs foraged also during periods of rough seas (Table I). During snorkling transects over the Paperu meadow, concentrated grazing in a monospecific Halophila ovalis meadow was observed in a distinct sward of approximately 260 m2 with a high density of feeding tracks, surrounded by an undisturbed seagrass bed at the southern part of the meadow (Table I and Fig. 2). During most consecutive observation periods, individual dugongs visited this feeding sward to forage. This regular pattern of visiting the sward was consistent during the study period (Table I).

Although local fishermen claimed that dugongs would forage deliberately on a lingulid brachiopod and Sipunculus sp. (identification Dr. K. den Hartog, National Museum of Natural History, Leiden) this was not confirmed by our own field observations.

Surfacing and diving behaviour

All three modes of surfacing have been observed. Rolling was the most frequently observed mode of submerging. Each time the back was clearly seen and sometimes even the tail cleared the water. The sinking forward mode was observed a few times when the animals were seen swimming. Sinking back was seen on only one occasion. Submergence times of animals feeding in deep rough seas were recorded of 1.8 to 6.4 minutes. The mean submergence time was 4.6 (±1.3) minutes (n = 17) (Table II). Generally, an animal appeared clearly visible under the surface for a few seconds before taking its first breath. After that it cleared the water with its nostrils to breath and either submerged totally or stayed just beneath the surface to breath again. The dugongs breathed up to four times while at the surface, spending 3 to 62 seconds just beneath the surface.

Response to approach

Animals continued to feed when approached in a small outrigger boat (without an outboard engine). On one occasion, the approach of a large rowing boat caused a fright response. The animal surfaced vertically a few meters from the boat, facing the boat. Most of the time, dugongs appeared to display curiosity towards divers. They either swam in circles around the divers or approached while swimming, at a distance of approximately three meters. Once a diver was able to approach an adult dugong at c. 1 m distance. On another occasion, an adult dugong of approximately 3 m in length approached the diver, who was lying at the bottom, swimming in a zigzag fashion. It swung around its long axis, holding its flippers wide-open and passed the diver, turned and passed again in the same way. Suddenly the dugong swam upwards to the sea surface. The animal rested just beneath the surface, turned again and swam in a straight line towards the diver, changing its course only at close range (c. 2 m distance).