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ORIGINAL RESEARCH
published: 03 September 2015
doi: 10.3389/fmars.2015.00063
Frontiers in Marine Science | www.frontiersin.org 1September 2015 | Volume 2 | Article 63
Edited by:
Romuald Lipcius,
College of William & Mary, USA
Reviewed by:
Laura J. May-Collado,
University of Vermont, USA
Christine Nicole Meynard,
College of William & Mary, USA
*Correspondence:
Ellen M. Hines,
Department of Geography and
Environment, Romberg Tiburon
Center for Environmental Studies, San
Francisco State University, 3150
Paradise Dr., Tiburon, CA 94920, USA
ehines@sfsu.edu
Specialty section:
This article was submitted to
Marine Conservation and
Sustainability,
a section of the journal
Frontiers in Marine Science
Received: 24 May 2015
Accepted: 20 August 2015
Published: 03 September 2015
Citation:
Hines EM, Strindberg S, Junchumpoo
C, Ponnampalam LS, Ilangakoon AD,
Jackson-Ricketts J and Monanunsap
S (2015) Line transect estimates of
Irrawaddy dolphin abundance along
the eastern Gulf Coast of Thailand.
Front. Mar. Sci. 2:63.
doi: 10.3389/fmars.2015.00063
Line transect estimates of Irrawaddy
dolphin abundance along the eastern
Gulf Coast of Thailand
Ellen M. Hines 1*, Samantha Strindberg 2, Chalatip Junchumpoo 3,
Louisa S. Ponnampalam 4, 5, Anoukchika D. Ilangakoon 6, Justine Jackson-Ricketts 7and
Somchai Monanunsap 8
1Department of Geography and Environment, Romberg Tiburon Center for Environmental Studies, San Francisco State
University, Tiburon, CA, USA, 2Wildlife Conservation Society, Bronx, NY, USA, 3Department of Marine and Coastal
Resources, Eastern Marine and Coastal Resources Research Center, Rayong, Thailand, 4Institute of Ocean and Earth
Sciences, University of Malaya, Kuala Lumpur, Malaysia, 5The MareCet Research Organization, Kuala Lumpur, Malaysia,
6Independent Researcher, Maharagama, Sri Lanka, 7Department of Ecology and Evolutionary Biology, University of
California, Santa Cruz, Santa Cruz, CA, USA, 8Department of Marine and Coastal Resources, Southern Marine and Coastal
Resources Research Center, Songkhla, Thailand
Effective conservation of coastal marine mammals is largely dependent on reliable
knowledge of their abundance, as well as the ecological and human factors driving their
distribution. In developing countries, lack of resources and capacity frequently impedes
research needed to estimate abundance and to determine the ecological requirements
of coastal marine mammals and the impact of threats related to coastal development
and fisheries. Over the course of 5 years, we developed practical research methods
and trained local scientists in Thailand to use accepted line transect distance sampling
methods for abundance assessment. The study focused on a little-known coastal and
freshwater species found throughout Southeast Asia, namely the Irrawaddy dolphin,
which has been sighted regularly along the coast of the eastern Gulf of Thailand. During
5 years of line transect boat surveys in Trat Province, the eastern-most province in
Thailand, we found an average of 423 dolphins distributed within 12 km of the coast.
Compared to other abundance estimates of coastal Irrawaddy dolphins in Southeast
Asia, this is a relatively large number. This population could extend into the northern
coast of Cambodia, where surveys are currently being planned. The Thai government
has begun talks with Cambodia about a transboundary marine protected area that would
include areas in both countries where coastal Irrawaddy dolphins are found. Collaboration
between scientists in Thailand, Cambodia and Vietnam is further needed to determine
dolphin movement and habitat use across borders.
Keywords: Irrawaddy dolphins, Orcaella brevirostris, eastern Gulf of Thailand, distance sampling, coastal dolphin
abundance, line transects, small boat surveys
Introduction
As coastal areas are increasingly impacted by human population growth and development, gaps in
knowledge about the abundance and habitat extents of coastal marine mammals obstruct effective
management and conservation (Dawson et al., 2008; Dick and Hines, 2011). In many developing
coastal regions, the extent and severity of threats (i.e., bycatch, pollution, overfishing) is largely
Hines et al. Irrawaddy dolphins in the eastern Gulf of Thailand
unknown, and cannot be quantitatively evaluated without high
quality abundance data (Dawson et al., 2008). Repeatable and
practical methods have been outlined (Dawson et al., 2008) and
tested (see for example Kreb, 2004; Hines et al., 2005; Dick
and Hines, 2011; Minton et al., 2013) in remote areas and
developing countries for line transect distance sampling surveys
(Buckland et al., 2001). As part of a long-term research project to
assess marine mammal abundance, distribution and threats along
the eastern Gulf coast of Thailand, primarily in Trat Province,
we found a relatively large population of Irrawaddy dolphins
(Orcaella brevirostris) and have had repeated sightings of two
other species:Indo-Pacific humpback dolphins (Sousa chinensis)
and Indo-Pacific finless porpoises (Neophocaena phocaenoides).
None of these species had been studied in this area previous
to our project’s commencement in 2008. However, the numbers
of sightings of humpback dolphins and finless porpoises were
not large enough to permit a distance sampling analysis.
In this paper we report on the results of our line transect
surveys for Irrawaddy dolphins for 5 years: 2008, 2009, 2012,
2013, and 2014.
Irrawaddy dolphins can be found in riverine, estuarine and
coastal waters throughout the tropical and subtropical Indo-
Pacific region (Stacey and Arnold, 1999; Minton et al., 2013).
Throughout Southeast Asia, the Irrawaddy dolphin is generally
found in shallow estuaries and coastal waters (Reeves et al.,
2008). For example, in the Philippines, most sightings were
made at a depth of <6 m (Dolar et al., 2002). Largely because
they are distributed close to coastal areas, Irrawaddy dolphins
are exposed to coastal development and fishing activities likely
to result in them becoming incidental bycatch (Dolar et al.,
2002; Reeves et al., 2003). Little is known about nearshore
Irrawaddy dolphins in the Gulf of Thailand. Chantrapornsyl
et al. (1996),Stacey and Leatherwood (1997); Stacey and Arnold
(1999), and Anderson and Kinze (1999) note that while records
of occurrence are few, Irrawaddy dolphins have been reported
along the coast of the Gulf of Thailand, with an early report of a
skull found in Khlong Yai, in Trat province, in 1914 (Anderson
and Kinze, 1999). The International Union for the Conservation
of Nature (IUCN) classes the Irrawaddy dolphin as a vulnerable
species with a decreasing trend on its Red List of Threatened
Species (Reeves et al., 2008). In Thailand, the Irrawaddy dolphin
is included under the 1992 Wild Animals Preservation and
Protection Act. In the IUCN/SSC Cetacean Specialist Groups’
2002–2010 Conservation Action Plan for the World’s Cetaceans,
one of the recommended research initiatives concerning the
status of coastal cetaceans in Thailand addresses a need to
identify areas of “cetacean abundance for special conservation
attention,” as well as to document fishing intensity in these areas
(Reeves et al., 2003, p. 60). These dolphins are a species of
concern in Thailand, as evidenced by Thailand’s sponsorship
of Irrawaddy dolphins for CITES Appendix I protection in
Bangkok at the Conference of the Parties in 2004 (CITES,
2004).
Within Thailand, Irrawaddy dolphins are distributed in
Phang Nga and Krabi Bays as well as Satun Province along
the Andaman Sea coast, Songkhla Lake, and several locations
around the Gulf of Thailand (Bonhote, 1904; Pilleri and Gihr,
1974; Chantrapornsyl et al., 1996; Stacey, 1996; Adulyanukosol,
1999; Anderson and Kinze, 1999; Chantrapornsyl et al., 1999;
Beasley et al., 2002; Smith et al., 2003; Perrin et al., 2005;
Kittiwattanawong et al., 2007; Reeves et al., 2008) (Figure 1).
The species has additionally been reported to occur within the
Chao Phraya and Bang Prakong rivers (Adulyanukosol, 1999).
Little is known about Irrawaddy dolphins along the Andaman
Sea coast of Thailand. Occurrence records for Phang Nga and
Krabi Bays are limited to fishermen’s reports (Chantrapornsyl
et al., 1996), while this species has been sighted off the Satun coast
around Tarutao Island (Adulyanukosol, 1999; Perrin et al., 2005).
The subpopulation in Songkhla Lake is considered Critically
Endangered by the IUCN (Reeves et al., 2008). It is currently
thought that Irrawaddy dolphins in the Lake number fewer
than 30 individuals (Jutapruet, 2013), and are threatened by
direct capture for Oceanaria, entanglement in fishing gear,
habitat degradation, pollution, prey depletion due to overfishing,
and inbreeding depression due to small population size and
demographic isolation (Stacey, 1996; Beasley et al., 2002; Perrin
et al., 2005; Kittiwattanawong et al., 2007; Reeves et al., 2008;
Jutapruet, 2013).
Irrawaddy dolphins occur in many locations along the coast
surrounding the Gulf of Thailand, from Surat Thani and Pattani
Provinces on the western Gulf coast, through Samut Songkhram,
Samut Sakhon, and Samut Prakan Provinces in the northern
Gulf, to Chonburi, Rayong, Chanthaburi, and Trat Provinces
on the eastern Gulf coast (Chantrapornsyl et al., 1996, 1999;
Adulyanukosol, 1999; Anderson and Kinze, 1999). Generally,
threats to Irrawaddy dolphins around the Gulf of Thailand
include entanglement in fishing gear; habitat degradation due
to coastal zone development, tourism, agriculture, aquaculture,
and manufacturing; pollution from waste runoff; and live capture
for Oceanaria (Perrin et al., 2005). In August of 1994, two
Irrawaddy dolphins were found stranded in Surat Thani Province
and one more stranded between 1996 and 1999 (Chantrapornsyl
et al., 1996; Adulyanukosol, 1999). One stranding was recorded
in Samut Songkhram Province (Adulyanukosol, 1999). In Trat
Province, where this research took place, bycatch seems to be
the most critical threat to Irrawaddy dolphins. Between 2010
and 2012, fishermen reported accidentally catching and killing
21 Irrawaddy dolphins, 10 of which were entangled in gillnets
(Whitty, 2014). Over a period of 14 days in January and
February 2013, 14 Irrawaddy dolphins and 3 finless porpoises
were discovered stranded or dead in illegal fishing gear along
the Trat coast. Necropsies confirmed that 10 of the animals were
incidentally entangled in illegal fishing gear, although one of the
finless porpoises had died due to parasites. For six, the cause of
death was not determined due to decomposition (Junchompoo
et al., 2014).
The goal of this research was to assess the conservation
status of coastal cetaceans in Trat province. Information about
the abundance of these dolphins is necessary to gauge the
effects of bycatch, habitat degradation and other threats on the
sustainability of this population. This assessment is urgently
needed to contribute effectively to local/regional conservation
and possible protected area management schemes, especially in
a rapidly developing area.
Frontiers in Marine Science | www.frontiersin.org 2September 2015 | Volume 2 | Article 63
Hines et al. Irrawaddy dolphins in the eastern Gulf of Thailand
FIGURE 1 | Map of Thailand with Irrawaddy dolphin sighting areas mentioned in the text.
Materials and Methods
Trat Province is at the eastern boundary of Thailand,
approximately 315 km east of Bangkok, extending to the
Cambodian border (Figure 1). The northern coast of the
Province is adjacent to an archipelago of islands. The
northernmost island, Ko Chang, is a popular tourist area.
Further east, into our field site, is Trat Bay, a wide, shallow bay
with two large rivers emptying into it. The eastern extent of the
Province is a narrow strip, at one point 1.5 km wide and 45 km
long, with the steep Cardamom Mountains in Cambodia forming
a natural border. Besides the city of Trat, above Trat Bay, Khlong
Yai, a major fishing port, and Hat Yai, at the Cambodian border,
the coast is relatively sparsely populated with fishing villages.
The climate is tropical, with a dry season between November
and March, and a wet southwest monsoon from April through
November. According to tidal charts from the Royal Thai Navy,
the tidal range is at maximum 3.0 m.
For analysis of patterns of distribution and habitat use in
further research, we divided our survey area into three strata,
based on variations of geography and human use. The north
stratum was in eastern Trat Bay, the middle, further down the
Frontiers in Marine Science | www.frontiersin.org 3September 2015 | Volume 2 | Article 63
Hines et al. Irrawaddy dolphins in the eastern Gulf of Thailand
strip, near the entrances of several rivers into coastal waters, and
the south stratum near the fishing port of Khlong Yai toward the
Cambodian border. During the surveys, our boat moved along
zig-zag transect lines in each of the three survey strata: north,
middle, and south (see Figure 2).
Distance sampling surveys along line transects are widely
used to estimate density and abundance of cetacean populations
(Buckland et al., 2001). We used Distance software (Thomas
et al., 2010) and a geographic information system to design a
line transect survey design for Irrawaddy dolphins in a stratified
study area in the eastern Gulf coast of Thailand during five
field seasons in 2008, 2009, 2012, 2013, and 2014. Our zig-zag
survey design for boat-based transects is based on Strindberg
and Buckland (2004). The design ensured reasonably even
sampling probabilities across the study area (even sampling
probabilities are an assumption underlying a standard distance
sampling analysis), while maximizing on-effort time, or time
spent following the transect looking for animals. Field ground-
truthing of each line was conducted to account or adjust for
shallow or impassable areas and to ensure the transect lines
could be run in perpetuity with minimal difficulty. Our goal was
to design a practical and repeatable systematic survey for this
area that would allow for quantitative analysis of abundance and
long-term monitoring of the dolphins in this area.
Surveys were conducted in January or February of each year,
during the north-east monsoon, which has a lower change of
rainfall in the eastern Gulf. Each year, the number of transect
days varied, according to weather. Transects were completed in
all strata in the survey area either two (2012, 2013, 2014) or three
times (2008, 2009) in a field season. All surveys were run from
a 12-m fishing boat equipped with inboard engines, boat speed
was kept at 10 kmh−1. A team of three observers continuously
scanned from the bow to 90◦Port and Starboard with the naked
eye and 7 ×50 binoculars while on effort. For each dolphin
group sighting detected during on-effort searching, waypoints
were recorded into a handheld GPS unit at the sighting location,
then three variables necessary to estimate density and abundance
were immediately recorded: the estimated radial distance from
the survey boat (r) using binoculars with reticles, the angle (θ)
between the transect line and the line of the detection using
angle boards, and the number of animals seen. Once the initial
data were recorded, the boat deviated from the transect to
approach the dolphin group off effort, to within 10 m to record
their location using the GPS unit. We also collected data on
the species and number of animals in the group, general age
composition (adult or calf), photographs for photo-identification
when possible, behavioral data and environmental parameters.
Afterwards, we returned to the location where the sighting was
made and resumed transect effort. Environmental parameters
were collected for each sighting, as well as every 30 min while on
transect.
Density of Irrawaddy dolphin groups within the area surveyed
is estimated as ˆ
Ds=nˆ
f(0)
2L, where Ldenotes the aggregate
length of the transects, nis the number of groups observed,
f(0) is the probability density function of observed perpendicular
distances evaluated at zero distance from the line. The density
of groups ˆ
Dsis multiplied by the estimated expected group
size ˆ
E(s) to obtain density of individuals ˆ
D, and this estimate
is multiplied by the surface area of each survey stratum to
obtain the corresponding abundance estimate of dolphins by
stratum ( ˆ
N). Overall density for the study area per year was
obtained by calculating a mean of stratum estimates weighted by
stratum area.
The Distance 6 software (Thomas et al., 2010) was used
to analyze the data. The on-effort observations of Irrawaddy
dolphin groups made from the line transects (Figure 2) were
used for the distance sampling analysis. A stratified (by stratum
or year) or pooled data approach was considered in fitting the
detection function. Various combinations of key functions and
adjustment term were considered to model the detection function
(e.g., uniform +cosine or simple polynomial, half-normal +
cosine or simple polynomial, hazard rate +cosine, or hermite
polynomial). Goodness of fit tests were used to identify violations
of assumptions. Exploratory analyses were conducted to examine
options for truncation and grouping intervals to improve model
fit for the detection function. Akaike’s Information Criterion
adjusted for small sample size (AICc) was used in final model
selection. Encounter rate was estimated within each of the survey
strata per year and its variance was estimated empirically using
the replicate transect lines as samples. There may be a tendency
for smaller dolphin groups to be missed more often than larger
groups at large distances from the transect line, which can
lead to “size bias” if the average group size is simply used
during the estimation process. To test for bias in the estimate
of group size, we applied a statistical hypothesis test at the 15%
α−level to the regression of natural logarithm of group size
against the probability of detection at distance xfrom the line
within the Distance software. If the regression was statistically
significant the expected group size [E(s)] was used, otherwise
average group size was used to estimate density and abundance.
Group size estimation was also stratified by survey stratum and
year. Individual Irrawaddy dolphin density per stratum for each
year and between sequential years was tested for statistically
significant difference at the 5% significance level using a two-
sided t-test (Buckland et al., 2004). This t-test explicitly accounts
for the lack of independence in the data that went into the
densities being compared due to using a common detection
function across strata and years.
Results
Table 1 summarizes the numbers of sightings and species for
each year. We had between 45 and 83 on effort sightings of
dolphins each year, which included between 151 and 341 animals.
The geographic distribution of sightings for each year distributed
across the strata are in Figure 2A (2008 and 2009), Figure 2B
(2012 and 2013) and Figure 2C (2014). Even with all years
combined, we did not have enough sightings of Indo-Pacific
humpback dolphins or Indo-Pacific finless porpoises to fit a
detection function to obtain an accurate abundance estimate. The
encounter rate for Indo-Pacific humpback dolphins, for example,
was 0.06 sightings/km−1of effort.
Frontiers in Marine Science | www.frontiersin.org 4September 2015 | Volume 2 | Article 63
Hines et al. Irrawaddy dolphins in the eastern Gulf of Thailand
FIGURE 2 | (A) All on-effort sightings of Orcaella brevirostris in 2008 and 2009 surveys along the coast of Trat Province, on the eastern Gulf coast of Thailand.
The purple area is the north strata closer to the shallow Bay. The green area is the shallow middle strata near several river mouths. The orange area is the
south strata, a deeper area closer to the major fishing port of Khlong Yai. (B) All on-effort sightings of Orcaella brevirostris in 2012 and 2013 surveys along the
coast of Trat Province, on the eastern Gulf coast of Thailand. (C) All on-effort sightings of Orcaella brevirostris in 2014 surveys along the coast of Trat
Province, on the eastern Gulf coast of Thailand.
Frontiers in Marine Science | www.frontiersin.org 5September 2015 | Volume 2 | Article 63
Hines et al. Irrawaddy dolphins in the eastern Gulf of Thailand
The total effort per stratum and year for Irrawaddy dolphins
are detailed in Table 2. In the final models, data were pooled
across survey strata and years and grouped into five equal-
spaced intervals with right truncation at 450 m (10% of the
data). Pooling provided a more robust detection function (see
Figure 3) due to some sample size issues when stratifying per
year or survey stratum, and also due to issues with the data
measurements. In the early years there was a heaping of angle
estimates at zero which indicates that particularly small angle
sizes were not being measured accurately. In the later years it
seemed that observers may have been overcompensating as there
were more observations than expected some distance from the
line (with no movement of dolphins away from the observers
before measurements were taken).
The final model was a half-normal with cosine adjustment
terms, which gave a detection probability of about 38% and an
effective half strip width of 171.26 m (see Figure 3). Estimates
of encounter rate and expected group size for the Irrawaddy
dolphins and survey stratum are also detailed in Table 2. Based
TABLE 1 | Sightings and numbers of Irrawaddy dolphins seen in 5 years of
surveys along the eastern Gulf coast of Thailand.
Year Species No. of on-effort Numbers of
sightings animals seen
2008 Orcaella brevirostris 62 248
2009 Orcaella brevirostris 83 341
2012 Orcaella brevirostris 52 266
2013 Orcaella brevirostris 45 161
2014 Orcaella brevirostris 67 151
on the t-test p-value being smaller than the specified significance
level of 15%, there is some indication of “size bias” in the
North (p=0.113) and Middle (p=0.124) strata in 2009
and in the South (p=0.125) in 2012, so the expected
group size was used in those cases and the average group size
for the remainder of the stratum-year combinations. The final
estimates of Irrawaddy dolphin group and individual density, and
abundance for each survey stratum per year and overall per year
is shown in Table 3. Although there were differences between
the strata per year and overall by sequential survey years, given
the fairly high variance associated with the estimates, individual
Irrawaddy dolphin densities overall were only significantly
lower in 2012 compared to 2009. Only when considering
one-tailed t-test results was the South stratum significantly
lower than the Middle stratum in 2009, and the South
stratum was also significantly lower than the North stratum
in 2013.
Discussion
The average relative abundance estimate of 423 Irrawaddy
dolphins within 12 km of the coast reflects the estimate of animals
in our survey area based on the number of sightings averaged
over the 5 years (Table 3). Averaged over these years, based on
our sighting patterns, the top and middle strata had slightly more
sightings and groups of animals, and the bottom strata the least,
with the exception of 2012 (Table 3). During this year there was
increased fishing activity near the port of Khlong Yai. Now that
we have an abundance estimate, the next step is to continue to
monitor the population for trends; to see whether the numbers
increase or decrease or remain the same over the next 5 years.
TABLE 2 | Details of the total effort (L) per year in each survey stratum.
Year Stratum L(km) nn/L(km−1) 95% CI E(ˆ
S)95% CI
2008 North 125.89 10 0.079 (0.042–0.151) 3.7 (2.19–6.25)
Middle 363.77 36 0.099 (0.057–0.171) 3.53 (2.91–4.27)
South 240.36 11 0.046 (0.022–0.096) 4.64 (3.01–7.14)
2009 North 125.89 17 0.135 (0.079–0.230) 3.10 (2.09–4.61)
Middle 363.77 50 0.137 (0.094–0.201) 3.74 (2.91–4.81)
South 240.36 16 0.066 (0.038–0.116) 3.81 (2.71–5.36)
2012 North 125.88 20 0.159 (0.061–0.415) 2.90 (2.00–4.20)
Middle 241.14 13 0.054 (0.021–0.138) 2.85 (2.07–3.92)
South 160.24 10 0.062 (0.037–0.106) 5.67 (2.16–14.89)
2013 North 83.92 12 0.143 (0.061–0.333) 3.67 (2.40–5.60)
Middle 241.14 22 0.091 (0.049–0.171) 3.64 (2.57–5.14)
South 160.24 8 0.050 (0.022–0.114) 2.38 (1.51–3.74)
2014 North 83.92 10 0.119 (0.043–0.331) 1.60 (1.00–2.574)
Middle 241.14 31 0.129 (0.093–0.179) 2.74 (1.90–3.96)
South 160.24 22 0.137 (0.089–0.212) 2.32 (1.83–2.94)
The number of observed Irrawaddy dolphin groups (n) post right truncation, the estimate of group encounter rate (n/L), and the expected group size [E(ˆ
S)] for each surveystratum per
year with the corresponding 95% confidence intervals (95% CI).
Frontiers in Marine Science | www.frontiersin.org 6September 2015 | Volume 2 | Article 63
Hines et al. Irrawaddy dolphins in the eastern Gulf of Thailand
FIGURE 3 | Detection functions fitted to the perpendicular distances of observations of groups of Irrawaddy dolphins. On-effort sightings were pooled
across survey strata and survey years. Data were grouped for final analysis using six equal-spaced intervals with right truncation at 450m.
Setting up the field methods and obtaining these estimates is a
first step—knowing the population trends will provide us with a
clearer idea of what the long-term prospects are for this species
and whether conservation activities are effective.
Compared to other transect-based abundance estimates of
coastal Irrawaddy dolphins in Southeast Asia, our present
estimate is relatively large. A comparable project in the region is
a 2 year line transect survey with similar methods for Irrawaddy
dolphins, conducted in Sarawak, Malaysia in 2010 and 2011. The
population estimate for this area was 129 dolphins, but included
a river which contributed most of the sightings. The density of
animals in coastal areas averaged 0.279 per km2, in contrast to our
average density of 0.982 per km2(Minton et al., 2013). Smith et al.
(2004) surveyed a geographically isolated group of Irrawaddy’s
in the Philippines in 2001 and estimated an overall abundance
of 77 animals. Research on coastal Irrawaddy’s in the region is
irregular and fragmented. Dolar and her collaborators have been
investigating Irrawaddy dolphins in the Philippines and have
recently discovered their presence in previously undocumented
locations (Hines et al., in press). Kuit et al. (2014) are researching
coastal dolphins in the Matang estuary of western Malaysia,
but have not published abundance statistics. Kreb and Lim
(2009) have estimated between 68 and 79 Irrawaddy dolphins
are in Balikpapan Bay, in eastern Kalimantan, Indonesia. For
a complete report on Irrawaddy dolphin presence and research
throughout Southeast Asia, see Hines et al. (in press).
Despite having once been considered a common area of
Irrawaddy dolphin occurrence (Stacey, 1996), few dolphins
were seen in the islands west of the Trat coast and no
dolphins were observed during a January 2014 survey in
Chanthaburi and Rayong Provinces (Hines et al., 2013, 2014)
(Figure 1). Modeling is underway to ascertain variations in
environmental covariates and human use to help explain this
marked difference in sightings. For now, the presence of so
many dolphins is an indication that the Trat coast contains
critical habitat for coastal Irrawaddy dolphins. We are working
with the Thai Department of Marine and Coastal Resources
(DMCR) on a long-term project to plan protective management
areas.
We still need more data from nearby coastlines to determine
the range of this population group. In Thailand, sightings of
between 20 and 25 individuals were found by Tongnunui et al.
(2011) in the Bang Pakong Estuary, approximately 250 km north
of our fieldsite. We would recommend stable isotope analysis
and photo-identification research to see if these animals mix with
the dolphins we have seen in Trat. The Trat dolphins could also
travel into the northern coast of Cambodia. In 2004, while on
aerial surveys along the Cambodian coast, Hines et al. (2004)
saw Irrawaddy dolphins on the northwestern coast near the Thai
border. They had 24 sightings of approximately 79 dolphins,
with a maximum group size of 15. There have been reports
of Irrawaddy dolphins in southwestern Vietnam, close to the
Cambodian border (Vu pers.comm). Coordination of efforts,
particularly photo-identification, between scientists in Thailand,
Cambodia and Vietnam is further needed to determine dolphin
movement and habitat use across borders.
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Hines et al. Irrawaddy dolphins in the eastern Gulf of Thailand
TABLE 3 | Estimates of density ( ˙
D) in number/km2(group density ˆ
Dsin parentheses) and abundance ( ˆ
N) of Irrawaddy dolphins for each survey stratum per
year and overall per year with their corresponding 95% confidence intervals (95% CI), and the percent coefficient of variation (%CV).
Year Stratum ˙
D(ˆ
Ds)95% CI ˆ
N95% CI (%CV)
2008 North 0.858 (0.232) (0.410–1.795) 120 (58–252) 35.27
Middle 1.019 (0.289) (0.571–1.821) 182 (102–326) 27.92
South 0.619 (0.134) (0.277–1.388) 73 (33–164) 39.39
Total 0.860 (0.229) (0.577–1.281) 376 (252–561) 20.02
2009 North 1.222 (0.394) (0.667–2.237) 172 (94–314) 28.97
Middle 1.501 (0.401) (0.953–2.362) 269 (171–423) 22.48
South 0.741 (0.194) (0.396–1.388) 87 (47–164) 30.57
Total 1.206 (0.343) (0.868–1.676) 528 (380–733) 16.60
2012 North 1.422 (0.491) (0.544–3.719) 200 (76–522) 43.07
Middle 0.475 (0.166) (0.180–1.250) 85 (32–224) 47.55
South 1.089 (0.193) (0.390–3.040) 129 (46–359) 50.26
Total 0.994 (0.275) (0.531–1.679) 413 (232–734) 28.21
2013 North 1.531 (0.417) (0.645–3.636) 215 (90–510) 39.65
Middle 0.969 (0.266) (0.483–1.943) 173 (86–348) 34.34
South 0.346 (0.146) (0.143–0.839) 41 (17–99) 43.08
Total 0.981 (0.282) (0.586–1.642) 429 (256–718) 25.14
2014 North 0.589 (0.368) (0.210–1.649) 83 (29–231) 46.92
Middle 1.088 (0.397) (0.671–1.763) 195 (120–316) 24.27
South 0.982 (0.424) (0.605–1.594) 116 (71–188) 23.33
Total 0.899 (0.395) (0.633–1.277) 393 (277–559) 17.63
Over 5 years of research we were successful in creating
a practical and repeatable systematic survey that allows for
quantitative statistical analysis and long-term monitoring of the
Irrawaddy dolphin population. Enough Irrawaddy dolphins were
seen in the combined years for robust statistical analysis, with
fewer issues in the sample data in later years, which shows the
increasing skills of the international and Thai scientists trained
in these methods. Also critical, each year the visibility of the
scientists and our research increased conservation awareness
in local villages. We stayed in local hotels run by villagers
for all 5 years, hired a driver who is an active community
member and conducted our research on a local fishing boat. We
have given presentations and lectures at elementary schools in
Trat Province, attended village council meetings and explained
our research and concerns about cetacean bycatch and plastic
pollution. Local teachers and students came on our boats
and assisted with our fieldwork, and we worked with a local
conservation group to take samples and teach about incidental
bycatch in a local village after bycaught animals were found both
on several beaches and floating in the water during our surveys.
This is a critical first step, however we still have important
questions: What are population trends in this group of
animals? What are changes in foraging and habitat patterns in
different seasons, and years? What is the role of environmental
covariates in distribution? One of our biggest questions is the
possible change of foraging and habitat patterns in various
seasons. Habitat modeling is needed to analyze relationships
between sightings of dolphins, possible influences of varying
riverine input and land use, and other environmental variables,
such as temperature, salinity, distance to river mouths, and
number/type of fishing vessels. Based on increased tourism and
a growing coastal population, recommended research includes
an assessment of threats, such as bycatch, plastic trash in the
water, coastal development, pollution from groundwater runoff,
and overfishing. Our goal is to continue to monitor this area with
transect surveys every 5 years.
Although the abundance of dolphins in Trat is comparatively
large, is this a sustainable population? The number of animals
recently caught in nets is high compared to the abundance
estimate, as evidenced by the bycatch we encountered during our
years in the field. Between 2002 and 2013, 84 Irrawaddy dolphins
have been found stranded in the eastern Gulf of Thailand (Teh
et al., in press). This number, a probable underestimate, as not
all bycaught animals are found, averages 10.5 dolphins per year.
One method commonly used by marine mammal scientists to
estimate if anthropogenic mortality is sustainable is the potential
biological removal (PBR) (Wade, 1998). The equation,
PBR =Nmin
1
2RMAXFR
Frontiers in Marine Science | www.frontiersin.org 8September 2015 | Volume 2 | Article 63
Hines et al. Irrawaddy dolphins in the eastern Gulf of Thailand
is based on Nmin, the minimum population estimate, ½RMAX ,
half of the maximum theoretical net productivity rate of the
population group (the default for cetaceans is 0.04; Wade, 1998),
and a recovery factor FR. After Whitty (2014), we used 0.1, 0.5,
and 1. Using the average population estimate of 423, PBR ranges
from 0.85 (FR=0.1), to 4.23 (FR=0.5), to 8.46 (FR=1).
Wade (1998) concluded that an FRof 0.5 was least biased. An
underestimated average of 10.5 bycaught dolphins per year is
over twice the calculated PBR of 4.23. Such a large estimate
of bycatch is unsustainable and could lead to the depletion of
this population (Wade, 1998). These calculations could be used
as the basis for goals in bycatch management. This issue is
common, and reflects the important threat of marine mammal
bycatch, identified as a serious regional problem during the
recent Southeast Asian Marine Mammal Workshop (Hines et al.,
in press).
Another question is how education and environmental change
are shifting the way that local fishers see and value their
environment and the role of these top predators, and how their
ideas of conservation and stewardship are changing. Values can
change as the result of various factors, such as the perception
of threats and scarcity, political and economic circumstances,
community cohesion and pressures, generational memories, and
exposure to media. Marine mammals have a living presence in the
lives and imaginations of the communities where they are seen
year after year. These associations between wildlife and humans
are especially valuable, as it is only through local solutions, and
with local cooperation that these animals can survive.
Our close collaboration with the Thai DMCR has enabled us
to create a good working relationship with local scientists. This
has enabled our work to be meaningfully applied in management
planning. The long-term nature of this project has brought local
and national attention to these marine mammals and the threats
they face. Incidents of bycatch are featured in national media, and
are communicated widely by local conservation organizations.
DMCR scientists and Directors concerned are convinced that
protected area zoning and increased protection from bycatch
are needed, and have begun pressing for conservation planning.
The Thai government has begun talks with Cambodia about a
transboundary marine protected area that would include areas in
both countries where coastal Irrawaddy dolphins are found.
Author Contributions
All authors have made substantial contributions to the
conception and design of the work, and the acquisition
and interpretation of these data. All authors were involved in
drafting and revising this work, approving it for publication,
and agree to be accountable for the accuracy and integrity of the
work.
Acknowledgments
This research was conducted with the approval of the
Institutional Animal Care and Use Committee of San Francisco
State University. We would like to thank the National Research
Council of Thailand for their cooperation and support through
the national permitting process. Many scientists from the DMCR
joined this project over the years and made it possible. We
apologize that we do not have room to name you all! We
acknowledge Ocean Park Conservation Foundation and the
Indo-Pacific Cetacean Research and Conservation Fund for
grants that enabled this project. Our work in Thailand was always
supported by Kanjana Adulyanukosol, whose visits to our boat
were so wonderful. We miss you. Many thanks to Tara Whitty,
Satoko Kimura, Long Vu, Hoang Minh Duc, Cindy Peter and
Isabelle Groc. Our volunteers were Michael Yuen, Jennifer Siu,
Laura Morse, Jen McGowan and Andrea Dransfield. Our research
vessel was driven beautifully each year by Captain Leigh and his
son Aa. Pi Hom, our driver, is a community member dedicated to
marine conservation who was instrumental to our work.
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Conflict of Interest Statement: The authors declare that the research was
conducted in the absence of any commercial or financial relationships that could
be construed as a potential conflict of interest.
Copyright © 2015 Hines, Strindberg, Junchumpoo, Ponnampalam, Ilangakoon,
Jackson-Ricketts and Monanunsap. This is an open-access article distributed under
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