Population Estimate and Foraging Niche of Dugong (Dugong
dugon) in Davao Gulf
Author(s)LUCERO, RUTH. S.
Issue Date 2010-02
KURENAI : Kyoto University Research Information Repository
Population Estimate and Foraging Niche of Dugong
(Dugong dugon) in Davao Gulf
RUTH. S LUCERO
Southern Philippines Agri-business & Marine & Aquatic School of Technology (SPAMAST)
Malita, Davao del Sur 8012 Philippines
This study focused on the seagrass habitat quality assessment and population estimate of dugong in
Davao Gulf. It was carried out through interview/survey and a three-month dugong daily monitoring
from an elevated facility set along the shoreline. Line transect quadrant method (LTQM) and SCUBA
diving were employed to assess the seagrass beds in the study sites.
Not less than 14 dugongs were counted in New Argao (12) and Talicud (2), mostly between 8AM-
12NN and 1PM-4PM. Simultaneous occurrence of dugong in New Argao and Talicud was observed
between November- December 2008.
Of the 14 seagrass species found in Davao Gulf, the Halophila species was the most preferred food
of dugong with a herbivory rate of 80%. Approximately, 1,705 sq. m were occupied by feeding
trenches within the three-hectare seagrass bed having a potential biomass production (wet weight) of
164.32 grams/sq. m. A dugong consumes 18.68 kg (wet weight) of seagrass per day. Thus, the
seagrass beds in New Argao can sufficiently sustain the food requirement of dugong in the area.
Protecting the foraging niche of dugong is significant to address the declining population of dugong
and to increase the chances of seeing them in the wild.
KEYWORDS: Dugong dugon, foraging niche, dugong feeding ground, seagrass habitat quality,
Dugong dugon is a large marine mammal of the
order Sirenia and the only survivor today of the
family Dugongidae. Dugongs have been listed as
vulnerable to extinction since 1982 by the World
Conservation Union (IUCN, 2004). It selectively
feeds on seagrasses. It is an animal which is so
very difficult to rear in an aquarium that only few
dugongs are reared in captivity in the world.
Dugongs, whales and dolphins are gaining interest
for many people and remarkably potential objects
of ecotourism (Lucero, 2005). It is perceived that
dugong watching can bring some kind of healing
and therapeutic effects. An ecologically friendly
and inexpensive way of watching dugong is to
watch them in their natural habitat. Thus,
protection of important and critical areas in Davao
Gulf providing natural refuge and foraging niche
for dugong may enhance success of watching
dugong in the wild.
Generally, the study aimed to investigate
the population and occurrence of dugong and
assess the seagrass habitat quality. Specifically, it
aimed to (1) come up with an estimate of dugong
population and (2) assess the status of seagrass beds
(which are utilized as a feeding ground of dugong),
seagrass species composition, species eaten by
dugong and trenches.
MATERIALS AND METHODS
The study was conducted in three stations
within the coast of Davao Gulf (Fig.1), namely
New Argao located between N 06° 22' 59"; E 125°
37' 22” and N 06°21‟52”; E125°38‟16.8”, Talicud
Island located between N 06° 57' 26.8”; E 125°
40‟35.4” and N 06°55‟40.4”; E 125°43‟24.4”;
Governor Generoso located between N 06° 37' 09”;
E 126° 04' 44.5” and N 06°27‟45.7”; E 126° 07‟
11.1”. The stations were established based on
reported dugong sighting and occurrence.
Fig. 1 Map of Davao Gulf showing the study sites.
New Argao, Malita, Davao del Sur
Talicud, Island Garden City of Samal
Governor Generoso, Davao Oriental
Survey and interview were conducted with
fisherfolks to obtain a historical record of dugong
sighting and occurrence in the gulf.
To come up with an estimate of dugong
population, daily visual observation and monitoring
of dugong occurrence was made between 8 AM – 5
PM from an elevated facility located on the
shoreline. An observer was assigned per station.
Observation was made simultaneously from the
To assess the seagrass quality habitat, Line
Transect and Quadrant Method (LTQM) along with
SCUBA and skin diving were employed. Manta
tow was also done and GPS was used to locate
Production potential was determined
through seagrass biomass determination. Seagrass
grazing by dugong was also assessed involving
difference in the seagrass biomass (wet weight)
inside and outside the feeding trench.
Records of dugong occurrence in Davao
Gulf can be traced back nine decades. They often
appeared in groups compared to its rarity in recent
times. Local taboos prevent people from harming
dugong. However, destructive fishing methods and
use of obnoxious chemicals prevail in the area.
An estimate of not less than 12 dugongs
(Fig. 2) inhabit the shallow coastal waters of New
Argao. In Talicud Island, at least a pair of dugong
frequents the channel. The frequency of dugong
occurrence is high in the morning between 9AM to
11AM for New Argao. In the afternoon,
occurrence is high between 1PM – 3PM for both
New Argao and Talicud. However, the difference
is not significant.
and determining the
Fig. 2 A photo of a pair of dugong taken on
December 21, 2008 at New Argao, Malita, Davao
Fig. 3 Comparative data of the frequency of dugong
occurrence in New Argao and Talicud
Feeding trenches (Fig. 4) are concentrated in New
Argao where Halophila species dominate. The
size of the trenches range from .13 m - .2 m wide
and 13 m – 15.9 m long. The average size of a
trench is 2.53 sq. m. Shape of feeding trenches vary
from oval, short to long narrow stretch, branching
Fig. 4 One of the feeding trenches found in the
Halophila dominated seagrass bed
On average some 674 trenches occurred in
one hectare (equivalent to 1,705 sq. meters). The
approximate area required to support a dugong is
estimated at 142 sq. m. of seagrass bed per day.
Each dugong is estimated to consume 18.68 kg of
seagrass a day.
Fourteen species of seagrasses were
accounted namely: Halodule pinifolia, Halodule
uninervis, Cymodocea rotundata, Cymodocea
serrulata, Syringodium filiforme, Syringodium
minor, Halophila ovalis and Halophila spinulosa.
New Argao has the highest species
richness of 12 dominated by Halophila species
followed closely by Talicud with 11 species
dominated by Thalassia and Halophila. Eight
species are found in Governor Generoso dominated
by Cymodocea species.
The seagrass biomass production is
highest in Talicud Island which is estimated at 2.4
tons/ha. Biomass production in New Argao is
approximately 1.6 tons/ha. while that in Governor
Generoso is approximately 1 ton/ha.
Physico-chemical parameters like salinity,
temperature, pH, tide and climate are comparable
among the three areas in New Argao, Talicud
Island and Governor Generoso and do not vary
Historical records derived from fisherfolks on the
rarity and low occurrence of dugong in recent times
coincide with the observed occurrence of dugong in
New Argao and Talicud. The frequency of
occurrence decreases as dugong count increases, an
indication that dugong seen in herds is a thing of
the past. In recent times dugongs are often seen
singly or in pairs. The prevalence of destructive
fishing activities within the gulf implies that man
and man-induced activities contribute and are
highly considered a threat that endangers the
remaining number of dugongs in the Gulf.
A viable population of dugongs are still
found in Davao Gulf with an estimate of not less
than 12 in New Argao and 2 in Talicud. Dugongs
found in these areas at a time are not the same
dugong but may belong to same population of
dugong that inhabit the shallow coastal waters of
It is also worth noting that the
Halophila species dominate in New Argao.
Results of this study shows that feeding trenches
were concentrated in New Argao where Halophila
species dominate. This finding implies that the
seagrass beds may be used as foraging area and
may explain the higher occurrence of dugong in
this site. This finding also confirms the
observation of Nakanishi, et al. (2006) where
among the seagrass meadows, concentration of
feeding trails were found at H. ovalis communities
in tidelands implying that dugongs selectively feed
on H. ovalis. It also supports the clarification
that seagrass meadows are important feeding
grounds of dugong (Preen and Marsh, 1995).
The seagrass biomass production of
1.6 tons/ha/day in New Argao is slightly lower than
that in Talicud, but it can sufficiently sustains the
18.68 kg feeding requirement of each dugong per
Halophila engelmannii, Halophila
day. This finding closely resembles the
consumption data of a dugong in captivity which is
25 kg of seagrass per day (wet weight) or 8% of its
body weight (Toba Aquarium and PCP Report,
Provision of healthy and quality seagrass
habitat for dugong may significantly contribute to
sustain the remaining population of dugong in the
Based on the findings of the study, the following
conclusions are drawn:
1. A viable population of dugong still exist in
2. An estimated population of not less than 12
dugongs are distributed in New Argao and at least a
pair of dugong is found in Talicud Island.
3. Fourteen species of seagrasses are found
inDavao Gulf dominated by Halophila species, the
most preferred food of dugong.
4. The seagrass beds in New Argao are the
foraging niche of dugong.
5. Current seagrass biomass production in New
Argao can sustain the feeding requirements of the
dugong population in Davao Gulf.
This study is partly supported by NAGAO-Natural
Environment Foundation-Japan, SPAMAST, LGU-
Malita, Davao del Sur, LGU- Gov. Generoso,
Davao Oriental and CGU-Island Garden City of
IUCN. 2004. Appendix 1 of Convention on
International Trade on Endangered Species (CITES)
of the Wild Flora and Fauna.
Lucero, Ruth. 2005. Dugong Sighting and Occurrence
in New Argao, Malita, Davao del Sur, Philippines in
Support to Ecotourism. 2005. SPAMAST Research
Journal. Vol. 2 Number 1. January – June 2005. ISSN
Nakanishi, Yoshiei, Seeichi Hosoyal, Nobuaki
Arai, Yoshiko Nakanishi, Kiyonori Katsukoshi and
Kanjana Adulyanukoso. 2006. The Distribution of
Seagrass Meadows and Dugong Feeding Trails Around
Talibong Island, Trang Province, Thailand. Phuket
Marine Biological Center. Department of Marine and
Preen, A. and Helen Marsh. 1995. Response of dugongs
to large-scale loss of seagrass from Hervey Bay,
Queensland, Australia. Wildl. Res. 22, 507-519.
Toba Aquarium and PCP Report. 1995. Dugongs of the
Philippines. A Report of the Joint Dugong Research
and Conservation Program.