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Journal of Advanced Marine Science and Technology Society. Vol.15. No.l. pp.23-27. 2009

Short report

The relationship between dugong grazing pressure on seagrass meadows and seagrass adaptations

Yoshiei Nakanishi *', Kanjana Adulyanukosol *% Nobuaki Arai *3, Seiichi Hosoya *', Yoshiko Nakanishi *', Kazuhiko Kotani *' and Kiyonori Katsukoshi *'

* 1 Okinawa Branch, IDEA Consultants Inc., 2-6-19 Aja, Nana, Okinawa 900-0003, Japan

* 2 Phuket Marine Biological Center, P.O. Box 60 Phuket 83000, Thailand

*3 Graduate School ofInformatics, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501, Japan

♦4 Okinawa Environmental Research Inc., 2-6-19 Aja, Naha, Okinawa 900-0003, Japan

Received : December. 22. 2008, Accepted : July, 14. 2009

Abstract

We compared the depth of horizontal rhizomes of small seagrass species to the depth of dugong feeding trails in seagrass meadows on the southeast coast of Talibong Island. Trang Province. Thailand, from 27 to 30 October 2005. The horizontal rhizomes of Halophila ovalis were within the depth range that was almost completely grazed by dugongs. Halophila ovalis populations in seagrass meadows appear to be maintained by their fast rate of growth. On the other hand, the horizontal rhizomes of Thalassia hemprichii. which has a slower growth rate, were deeper than the dugong grazing depth, thus reducing grazing pressure on T. hemprichii.

Keywords : Dugong dugon. small seagrass. Halophila ovalis. Thalassia hemprichii. horizontal rhizome, dugong feeding trail, grazing pressure

1. Introduction

The dugong (Dugong dugon) is a marine mammal

that is distributed in the tropical-subtropical areas of the Indo-Pacific Ocean (Nishiwaki and Marsh. 1985).

Analytical studies on both dung and contents of the stomachs and mouths (Heinsohn and Birch, 1972;

Lipkin, 1975; Johnstone and Hudson, 1981; Marsh et al., 1982; Erftemeijer et al., 1993; Preen, 1995;

Adulyanukosol, et al., 2001; Andre et al., 2005). as well as field observations of dugong grazing scars (Aragones. 1994; De Iongh et al., 1995, 1997; Mukai et al., 2000; Nakanishi et al., 2005; Nakanishi et al., 2008). have revealed that dugongs feed on seagrasses.

Seagrass meadows are very important feeding areas (Anderson. 1994; Preen and Marsh. 1995). but little is known about how dugongs use these meadows.

Here, we describe the relationship between dugong grazing pressure on seagrass meadows and seagrass adaptations.

2. Materials and Methods

In Thailand, dugongs are found along coastlines of both the Gulf of Thailand and the Andaman Sea

(Adulyanukosol, 2000). Talibong Island, Trang Province, is home to the largest dugong population in Thailand (Adulyanukosol et al., 1997, 1999; Hines et al., 2005).

The seagrass meadows on the southeast coast

of Talibong Island cover an area of approximately 2,141 ha (Fig. 1), with a fairly uniform sandy mud substratum. These seagrass meadows are critical dugong feeding areas (Nakanishi et al., 2005).

Our survey was performed from 27 to 30 October 2005 by scuba divers. In this area, ten small seagrass species were observed: Halophila beccarii, Halophila decipiens, Halophila ovalis, Halophila minor, Thalassia hemprichii, Cymodocea serrulata, Cymodocea rotundata, Halodule pinifolia, Halodule uninervis, and Syringodium isoetifolium, as well as one large species, Enhalus acoroides (Nakanishi et al.. 2005. 2006).

The presence of horizontal rhizomes usually allows the leaves of small seagrasses to recover from dugong grazing. If these rhizomes are lost due to dugong grazing, the vegetation can recover only if horizontal rhizomes from neighboring areas colonize the gap (Supanwanid. 1996). Thus, an important factor in the maintenance of the small seagrass community is whether dugongs graze the seagrasses to the depth of their horizontal rhizomes below the substrate. We measured the depth of 30 dugong feeding trails below the substrate in small seagrass meadows and the depth of 30 horizontal rhizomes of each of seven small

seagrass species (H. ovalis, T. hemprichii, C. serrulata,

C. rotundata, S. isoetifolium, H. pinifolia, and H.

uninervis) below the substrate (Fig. 2).

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—N7° 10.500'

E99° 20.000'

"ig. I Study site in southeast of Talibong Island. Trang Province. Thailand. The hatched area indicates the seagrass meadow study area.

bottom

I

Section of dugong feeding trail

Dugong feeding trail

J* , L J

Depth of dugong feeding trail Depth of horizontal rhizome

Fig. 2 Measurement points of the depth of dugong feeding trails and horizontal rhizomes of seagrasses.

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The relationship between dugong grazing pressure on seagrass meadows and seagrass adaptations

3. Results and Discussion

Fig. 3 shows the horizontal rhizome depth of each small seagrass species. Horizontal rhizomes of T.

hemprichii had the greatest mean depth at 4.8 ± 0.5 cm, which was significantly greater than the 3.1 ± 0.5 cm mean depth of dugong feeding trails (Mann-

Whitney U-test, P<0.05). The shallowest T. hemprichii

horizontal rhizome was 3.8 cm. which was deeper than the deepest dugong feeding trail (3.7 cm), suggesting that T. hemprichii horizontal rhizomes were minimally grazed by dugongs.

Horizontal rhizomes of C. rotundata were the second deepest, with a mean depth of 4.0 ± 1.0 cm. which was also significantly deeper than the mean depth of dugong feeding trails (Mann-Whitney U-test. P<0.05), indicating that C. rotundata horizontal rhizomes were comparatively difficult for dugongs to graze.

The mean depths of H. ovalis, C. serrulata, H.

pinifolia, and S. isoetifolium horizontal rhizomes were significantly shallower than that of dugong feeding trails, at 1.3 ± 0.4 cm, 2.3 ± 0.9 cm. 2.4 ± 0.7 cm. and 2.6 ± 0.9 cm, respectively (Mann-Whitney U-test, PO.05). The mean depth of H. uninervis horizontal

Dugong feeding trail

rhizomes (3.0 ± 0.9 cm) was not significantly different from that of dugong feeding trails. Thus, in comparison to T. hemprichii and C. rotundata, the horizontal rhizomes of these five species were more easily grazed by dugongs.

We found that dugongs almost completely grazed H.

ovalis horizontal rhizomes, which were the shallowest.

Nakanishi et al. (2005) observed concentric dugong feeding trails in H. ovalis meadows and reported that horizontal rhizomes of H. ovalis were grazed completely by dugongs during the dry season at our study site. Dugong grazing pressure was estimated to be greater on H. ovalis than on other small seagrass species. However. H. ovalis also had the highest ability to regenerate after dugong grazing (Aragones and Marsh, 2000). Yamamuro and Chirapart (2005) concluded that the fast growth rate of H. ovalis was important in sustaining populations as a food resource for dugongs.

Thus, H. ovalis communities are maintained by the fast growth rate of the plant despite dugong grazing pressure. Thalassia hemprichii, which has a slower growth rate than H. ovalis (Yamamuro and Chirapart,

-| n==30

MEAN _l MEAN±SD H MIN-MAX

Halophila ovalis

^n=30|

Cymodocea serrulata • *-n=30

Halodule pinifolia • *-n=30

Syringodium isoetifolium

-| ^n#30

Halodule uninervis

-| n=30

Cymodocea rotundata

h-

Thalassia hemprichii

0.0 1.0 2.0 3.0 4.0 5.0

Depth below substrate (cm)

X n=30

M n=30

6.0 7.0

Fig. 3 Depth of the horizontal rhizomes of small seagrass species and depth of dugong feeding trails in small seagrass meadows.

T ^ J : Species with mean horizontal rhizome depths that were significantly different from the mean depth of dugong

feeding trails.

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2005). maintains its populations by the greater depth

of its horizontal rhizomes, thus reducing the impact of dugong grazing pressure.

Although the growth rate of E. acoroides is less

than that of T. hemprichii (Cebrian et al., 1998), the

former has thick leaves, which dugongs cannot easily graze (Nakanishi et al., 2008), thus reducing dugong grazing pressure on E. acoroides populations.

4. Acknowledgements

We thank Dr. Takeshi Hara, and all who helped in this research.

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References

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