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Facultative river dolphins : conservation and social ecology of freshwater and coastal Irrawaddy dolphins in Indonesia Kreb, D.

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Conservation of riverine Irrawaddy dolphins in Borneo

CHAPTER 6 Conservation management of small core areas: key to survival of a critically endangered population of riverine Irrawaddy dolphins in Borneo Daniëlle Kreb and Budiono In press: Oryx, 2004

Dolphins preference for fish-rich but human-crowded areas makes them vulnerable to many human-induced threats. Awareness campaigns therefore form a critical factor in their survival. Photo: Daniëlle Kreb

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Chapter 6

ABSTRACT In order to clarify the previous status of the facultative Irrawaddy River Dolphin, Orcaella brevirostris, in the Mahakam River in East Kalimantan, which was ‘insufficiently known’ following IUCN criteria, we collected data from early 1999 until mid 2002 on abundance, habitat use, population dynamics and threats relevant to the conservation of Indonesia’s only freshwater dolphin population. Our best estimates of total population size varied between 33 and 55 dolphins (95% confidence limits: 3176) based on direct counts, strip-transect analysis, and Petersen and Jolly-Seber markrecapture analyses of photo-identified dolphins. Mean minimum annual birth and mortality rates were nearly similar, i.e. 13.6% and 11.4% and no changes in abundance > 8% were detected over 2.5 years. Dolphins primarily died after gillnet entanglement (73% of deaths). Dolphins’ main habitat includes confluence areas between the main river and tributaries or lakes. Dolphins daily intensively use small areas mostly including confluences, moving up and downstream over an average length of 10 km of river and within a 1.1 km2 - area size. These areas are also primary fishing grounds for fishermen and subject to intensive motorized vessel traffic. Sixty-four percent of deaths (from 1995-2001) with known location (n=36) occurred in these areas. Formal interviews with local residents revealed a generally positive attitude towards the establishment of protected dolphin areas. Because of the dolphins’ dependence on areas that are also used intensively by people, primary conservation strategies should be to increase local awareness and introduce alternative fishing techniques.

RINGKASAN Dalam usaha memperjelas kondisi lumba-lumba Irrawaddy (Orcaella brevirostris) di Sungai Mahakam Kalimantan Timur, yang mana “belum banyak diketahui” berdasarkan kriteria IUCN, kami mengumpulkan data-data sejak awal tahun 1999 hingga pertengahan 2002 tentang jumlah, penggunaan habitat, perubahan populasi, dan ancaman yang berkaitan dengan upaya konservasi satu-satunya lumba-lumba air tawar di Indonesia. Diperkirakan jumlah populasi Pesut Mahakam berkisar antara 33 hingga 55 ekor (tingkat kepercayaan 95%: 31-76) berdasarkan perhitungan langsung, analisis strip-transek, dan analisis penandaan-ulang Peterson dan Jolly-Seber dari identifikasi foto lumba-lumba. Rata-rata terendah tingkat kelahiran dan kematian pertahun hampir sama yakni 13,6% dan 11,4% dan tidak ada perubahan jumlah lebih dari 8% selama 2,5 tahun. Kematian utama lumba-lumba adalah terperangkap rengge (73%). Habitat utama lumba-lumba termasuk pertemuan antara sungai utama dan anak sungai atau danau. Sehari-hari lumba-lumba secara intensif menggunakan daerah yang kecil kebanyakan merupakan daerah pertemuan sungai, bergerak ke hulu dan ke hilir dengan jarak tempuh rata-rata 10 km dan dalam ukuran areal 1.1 km2. Tempattempat ini juga daerah utama penangkapan ikan dan lalu lintas kapal bermotor. Enam puluh empat persen (64%) kematian (dari 1995-2001) dengan lokasi yang diketahui 82

Conservation of riverine Irrawaddy dolphins in Borneo

(n = 36) terjadi di daerah ini. Wawancara formal dengan penduduk lokal umumnya menyatakan sikap positif terhadap pembentukan daerah perlindungan lumba-lumba. Karena ketergantungan lumba-lumba pada tempat yang juga digunakan intensif oleh masyarakat, strategi utama konservasi adalah meningkatkan keperdulian dan memperkenalkan cara alternatif menangkap ikan kepada masyarakat lokal.

INTRODUCTION River dolphins and porpoises are among the world’s most threatened mammal species. The habitat of these animals has been highly modified and degraded by human activities, often resulting in dramatic declines in their abundance and range (Reeves et al., 2000). Protection of freshwater dolphins and their habitat is a major challenge since river systems are the veins of human activities in terms of transport, fishing, and industrial processes, and are also heavily affected by forest fires, which were more likely to occur near rivers (Fuller & Fulk, 1998) and likely caused a large increase in sedimentation rates together with large-scale illegal logging practices (Anon, 2000) with disrupting consequences for the aquatic ecosystem (e.g. Mackinnon et al., 1997). In Indonesia one representative freshwater dolphin population occurs in the Mahakam River in East Kalimantan, i.e., the facultative river dolphin Orcaella brevirostris, commonly and locally referred to as Irrawaddy dolphin and pesut, respectively. The species is found in shallow, coastal waters of the tropical and subtropical Indo-Pacific but also in three major river systems: the Mahakam in Indonesia, the Ayeyarwady in Myanmar, and the Mekong crossing through Vietnam, Cambodia and Laos (Stacey & Arnold, 1999). These river populations were all identified to consist of less than 100 individuals based on preliminary studies and faced ongoing and pervasive threats to their long-term persistence (Kreb, 2002; Smith et al., 2003). In order to identify and monitor the population status and threats more thoroughly and set a rationale for conservation action, a 3.5-years study from February 1999 until August 2002 was initiated. This article presents the results of this study and an in-depth analysis of habitat preferences, population dynamics, threats and recommendations for future conservation activities of the Irrawaddy dolphin population in the Mahakam River. Since 1990 the species has been fully protected by law in Indonesia and is adopted as a symbol of East Kalimantan Province. Prior to the present study, no systematic data had been collected before on the Mahakam population. A two-month preliminary study in 1997 revealed that sighting rates (0.06 dolphins/ km) in the middle Mahakam river segment (with highest dolphin densities) were very low (Kreb, 1999). Based on data we collected during 1999 and 2000, the IUCN status of this freshwater population was raised from ‘Insufficiently Known’ to ‘Critically Endangered’ (IUCN, 2003)

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Study area The Mahakam River is one of the major river systems of Borneo and runs from 118º east to 113º west and between 1º north and 1º south (Fig 1). Regional climate is characterised by two seasons, i.e. dry (from July-October, southeast monsoon) and wet (November-June, northwest monsoon) (MacKinnon et al., 1997). The river measures about 800 km from its origin in the Müller Mountains to the river mouth. Rapids start upstream of Long Bagun at c. 600 km from the mouth, which limit the dolphins from ranging further upstream. Three major lakes and nearly all major tributaries and many smaller swamp lakes are connected to the main river system in the Middle Mahakam Area (MMA) between 180 km until 375 km from the mouth. These lakes are very important fish-spawning grounds and replenish the main river seasonally. Therefore, the MMA is an area of intensive fishing activity with an annual catch of 25.000 to 35.000 metric tons since 1970 (MacKinnon et al., 1997). Coal mining and logging companies occur along the entire length of the Mahakam River, especially in the tributaries. A large gold digging company is located in the upper Mahakam River segment together with several small-scale, illegal gold mines. Infrastructure is poorly developed in East Kalimantan and the Mahakam River is the main transport system. METHODS

Field methods We searched the Mahakam River from the delta to upper rapid streams (600 km from the mouth) by boat from February 1999 until August 2002 for a total of 8925 km (837 hours), and observed river dolphins for a total of 549 h. We conducted 12 involved extensive monitoring surveys in 6 survey periods, which covered the entire distribution range (average duration 10 days; SD ± 2 days) during all types of water levels (high, low, medium, increasing, decreasing) to invest distribution patterns, annual recruitment and estimate population abundance using strip-transects, direct counts and mark-recapture techniques through photo-identification, which are more described in detail in Kreb (2002 & in press a). The distribution range was divided in 15 strip- transects (main river and tributaries) and 2 line-transects (Melintang and Semayang Lakes). Each transect could be finished in one day. Another six intensive surveys (average duration 12 days; SD ± 3 days) were conducted in areas of high dolphin density to investigate preferred habitat and to locate dolphin groups for further focal group follows (see below) to assess daily home ranges (Figure 1). To monitor abundance and locate groups, surveys were conducted with 12-16 m long motorised vessels (12-21 hp), travelling at an average speed of 10 km/ h.

84

85 Kedang Pahu

Damai

Rambayan

Muara Semayang Pahu Melintang

Tepian Muara Ulak Jempang Batuq Jelau

Bohoq

Muyub Ulu

Muara Benangak

Datah Bilang

Muara Kaman

Kedang Rantau

Kota Bangun Loa Kulu

Kedang Kepala

85

Figure 1. Study area with a) total dolphin distribution area, b) areas of high dolphin density and c) coastal Irrawaddy dolphin area.

Long Bagun

Conservation of riverine Irrawaddy dolphins in Borneo

Conservation of riverine Irrawaddy dolphins in Borneo

Chapter 6

The photographic effort during the extensive monitoring surveys was one hour per sighting with a total observation effort of 545 h. Durin observation team existed of three active observers: two front observers and one rear observer. The average observation time and g all surveys 2074 photographs were made of dorsal fins. For each sighting, the duration, location, group behaviour, group size, group composition and environmental data, i.e. depth, clarity, surface flow rate, temperature, pH and type of river section (river bend, straight stretch or confluence area) were collected. On average five times a day, similar random samples were collected as those obtained during sightings, whereas type of river section was recorded every fifteen min. In order to assess daily home ranges, 58 groups were followed for 321 h in total and on average 5.5 h daily (range 1.5-13 h) using a motorized canoe of 5 hp outboard engine maintaining an average distance of 50 m. In addition, land-based observations were made in the confluence area of Muara Pahu, c. 300 km upstream, which was frequented daily by different dolphin groups. On average, five sequential days (32 days in total) of land-based observation have been completed by two observers, which overlooked the area some 7 until 10 meter above the water surface (depending on water levels) during six different survey periods for a total of 286 h. When a group of dolphins was sighted, we recorded group size and composition (Kreb, in press a), changes in group-composition, and time spent in the area. Formal interviews were conducted using questionnaires with mainly open questions with residents and fishermen (n = 258) in six important dolphin areas to determine their knowledge and attitude with regards to the dolphins and their conservation. Respondents were questioned separately to ensure independence of data. In order to assess the minimum annual birth rates between November 2000 and November 2001, the total number of different newborns were counted during 5 different surveys (both extensive and intensive surveys), which were more or less equally distributed over the year with an average 2.5-month gap in between the surveys. Newborns encountered during each of these surveys were assumed different than those encountered during an earlier survey. We defined newborns to be of less than one month of age if they complied with all three categories: 1) exhibited an awkward manner of swimming and surfacing, 2) spent all their time in close proximity to an adult and 3) were of less than ½ the average length of an adult, following Bearzi et al. (1997). Mortality was estimated from own observations and semi-structured interviews conducted during a preliminary survey in 1997 and during the surveys from February 1999 until August 2002. Mortality was traced back as far as 1995. Incomplete or untrustworthy accounts with missing locality, date, and traceable eyewitnesses were disregarded (14% of n = 44). Dolphin reactions towards different types of boat traffic were tested by comparing dolphin group surfacing frequencies in presence and absence of different types of boats (see Kreb & Rahadi, in press b, for a more detailed method description)

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Conservation of riverine Irrawaddy dolphins in Borneo

Analysis To assess the importance of different river areas in terms of dolphin densities, the river was divided in seven areas where at least one dolphin sighting was made. Total sightings made in each of these different areas during 10 extensive surveys were compared using a chi-square test. Sightings in tributaries within 1 km of the confluence area were considered main river sightings. Sighting rates, densities and abundance estimates based on strip-transects and direct counts were calculated according to the formula described in Kreb (2002). Since no sightings were made in any of the lakes during these extensive surveys, no analysis of the conducted linetransects was required. The mean abundance estimates and coefficient of variation (CVs) of two replicated surveys within each survey period, were added for all survey periods and averaged to obtain the total mean population size (and mean CV). In addition, abundance estimates were calculated per water level condition (combining different years) averaging the estimates of each replicated survey. This was done since there was no trend in abundance (see Results, Trends in abundance), and there was no difference between the variation in abundance estimates per replicated survey within the same time period and the variation in abundance estimates of surveys conducted in different periods but at similar water level conditions. Because all rear sightings (n=9) were associated with the dolphins’ positions in river bends (which is an unpredictable variable), no detection correction factor g(0) was used to calculate abundance and associated CVs. Instead, rear sightings were directly included in the abundance estimates. Also, no seasonal variation was found in sizes of groups (see Results, Population composition) so this component was also not included in the calculation of CVs. Abundance was also estimated using both the Jolly-Seber and Petersen markrecapture methods based on 728 selected identifiable dorsal fin pictures (Kreb, in press a). Mean population size was calculated as the average of the mean abundance estimates from strip-transects, direct counts and mark-recapture analysis. Mean group size in the Mahakam was based on all on-effort sightings made during nine extensive abundance surveys covering the entire distribution range. Groups were considered different if a group joined after 15 min of observation or groups split during observation time. To detect any trend in abundance, regression analysis was applied to the natural logarithm of 5 mean strip- and direct count abundance estimates over a 2.5 years period (early 1999 until mid 2001). Statistical power was calculated by means of a linear regression program TRENDS (Gerrodette, 1993). The same analysis was performed to detect if there was any trend in mortality using data from 1995 until 2001. Random environmental samples i.e., depth, flow rates, pH, temperature and clarity were compared with samples collected at dolphin locations per water level using a two sample T-test, prior to which a two-tailed F-test was applied to test for similar variances, which were equal for all sample comparisons. Dolphin-preferred areas within the main river were investigated by comparing the percentage of dolphin 87

Chapter 6

sightings made per water level in straight stretches, river bends and confluences during ten strip-transects surveys that covered the same area using a chi-square test. The relative random availability of straight stretches, river bends and confluences were tested using a chi-square test. When df was 1, Yates correction factor was applied. To identify the year-round significance of a confluence area, for which highest dolphin densities were found, the numbers of identified dolphins per water level were compared using a chi-square test. To prevent biases, the correlation between the number of pictures obtained and number of identified dolphins was tested using the Product Moment Correlation Coefficient. Daily home ranges were estimated by measuring the distance between the two most widely separated sighting locations of the focal group. Minimum annual birth rate was estimated by dividing the total number of newborns encountered in one year (see Methods) with the mean population size. Minimum annual mortality rate was estimated by dividing the number of dead dolphins during the study period (interviews + observations) by years and mean population size.

RESULTS Abundance and distribution During ten extensive surveys, we made 76 on effort sightings of Irrawaddy dolphins in the Mahakam (Table 1). The actual dolphin sightings were confined to the area in the main river between Muara Kaman (c. 180 km from the mouth) and Datah Bilang (c. 480 km from the mouth) and the tributaries Belayan, Kedang Rantau, Kedang Kepala, Kedang Pahu, Ratah and Semayang Lake. The cross-shaded area (Figure 1) of 195 km length in the main river from Muara Kaman until Muara Benangak (c. 375 km from the mouth) represents an area of high dolphin densities. The total dolphin distribution area in the Mahakam, based on sightings and interviews with fishermen, starts about 90 km upstream of the mouth at Loa Kulu and ends some 600 km upstream at the rapids past Long Bagun, including several tributaries and two lakes (Figure 1, singleshaded areas). Significant differences were found in sighting density among eight survey areas where we made sightings (X2 = 35.91, df = 7, P < 0.01) (Table 1). The three areas where most sightings were made include several confluence areas with tributaries and lakes. Seasonal variation in distribution pattern is summarized in Table 2 and illustrated in Figure 2. At medium water levels sighting rates in the main river and tributaries are similar. At prolonged high-water levels dolphins were more often found in the main river than in the tributaries, whereas at rising high water levels (data not tabulated since incomplete total area coverage) a lowest mean sighting rate (0.03 dolphins/ km) was recorded in the main river indicating that dolphins had moved upstream into the tributaries. At low water levels no dolphins were sighted in the tributaries.

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Conservation of riverine Irrawaddy dolphins in Borneo

All but one sighting of Irrawaddy dolphins in and near the Mahakam delta were offshore the delta at low tide (n = 4), whereas one sighting was made 10 km upstream of the delta at high tide. A mean salinity of 21 ppt was measured at dolphin Table 1 Priority areas for conservation based on the combination of dolphin densities, presence of newborns, observed matings and mortality, with low numbers indicating high priority. River survey segments of 40 km length

Priority area Dolphins/ Newborns Mating Deaths * km < 2 months events ** 12 Muara Kaman – Kota Bangun 2 0.13 1 1 Kota Bangun – Batuq 3 0.16 1 1 Batuq – Tepian Ulak 5 0.1 8 1 13 Tepian Ulak – Rambayan – Muara Jelau 1 0.31 2 Rambayan – Bohoq 6 0.04 Bohoq – Muara Muyub Ulu Ratah Muara Jelau – Damai

6 4 6

0.04 0.12 0.04

1 1

* Actual proposed conservation areas (1–3) are confined to smaller areas based on frequent sighting locations (see results); ** Dolphins that died between 1995 – 2001 in the survey area; 5 dolphins died outside the survey areas and 2 dolphins died with unknown location.

Number of dolphins

Main River

Tributaries

Lake

Rapid area

35 30 25 20 15 10 5 0 Medium

Medium-high*

High

Low

Water Level

Figure 2. Seasonal habitat use of dolphins based on strip-transect analysis. The main river includes confluence areas of up to 1 km upstream tributaries. * At medium-high water levels, tributary data is lacking and thus not presented in the graph.

89

90

228 152 304 33** 33** 33**

3 2 4

9 6 12

3 2 4

621 414 828

Transects

75 75 75

43 43 43

200 200 200

Mean strip width (m)

3 2 4

7 2 0

16 12 28

No. sightings

4.5 4.6 4.3

4.5 4.6 4.3

4.5 4.6 4.3

Mean group size

-

0.14 0.06 0

0.12 0.13 0.15

No. dolphins/ km

-

3.2 1.4 0

0.58 0.67 0.73

No. of dolphins/ km2

5 5 4

12 5 0

23 28 30

Abundance (strip – transects)

90

** in cross-shaded river area of Figure 1. ; ** distance from mouth of tributary until rapids, however the dolphins sighted there occupy a 'closed' area in between two rapid streams of only 2 km, so no sighting rates have been calculated.

Medium-water levels High-water levels Low-water levels Upper tributary Medium-water levels High-water levels Low-water levels

Survey area Middle main river* Medium-water levels High-water levels Low-water levels Middle tributary**

Total length (km)

Table 2. Number of sightings, dolphin densities and abundance per river section and water level condition based on strip-transects.

Chapter 6

0% 0% 0%

62% 0% 0%

27% 24% 9%

Coefficient of Variation

Chapter 6

Conservation of riverine Irrawaddy dolphins in Borneo

positions in the delta and is associated with brackish waters. Their most inshore occurrence is about 20km upstream of the mouth at high tide according to interviews with fishermen. Since the coastal dolphins have not been sighted or reported to move further upstream than 20 km from the mouth and only enter the delta at high tide, we consider these to belong to a different, coastal stock than the Mahakam population, which range starts 180 km upstream the mouth according to our sightings. The coastal and freshwater populations thus seem isolated from each other. Total mean abundance estimates for the entire dolphin population in the Mahakam derived from strip-transect analysis (method 1) and direct counts (method 2) made during nine extensive monitoring surveys, arrive at 37 individuals (mean CV = 13%; 95% CL = 33-41) and 33 individuals (CV = 8%; 95% CL = 31-35), respectively (Table 3). Independent abundance analyses based on photo-identification and mark-recapture techniques gave total estimates of 55 dolphins (CV = 6%; 95% CL = 44-76) following Petersen’s method (3) and 48 dolphins (CV = 15%; 95% CL = 35-63) according to the Jolly-Seber method (4). The total number of identified dolphins during the study period is 59 individuals. Table 3 Total abundance of dolphins in the Mahakam based on strip-transect analysis an direct counts. Water levels No. conditions T

Total T length (km)*

No. Si

No. Si/ Mean surveys ** G

N striptransect CV

95% CL

Medium

882

26

4.5

40

21-58 33

15

8.7

19%

N count CV

High

10

599

16

4.6

8

37

17%

1-94

Low

20

1165

32

4.3

8

34

9%

30-39 32

36

Combined 45

2646

74

4.4

8.2

37

15%

33-41 33

8%

95% CV 26-40

12%

1-74

2% 8%

31-33 31-35

* Only including transect length of those sections where dolphins were sighted, excluding search effort in areas where no dolphins were sighted during these extensive monitoring surveys (e.g. lakes and upper and lower river section); ** For medium, high and low water levels 3, 2, and 4 surveys were conducted respectively; T = transects; Si = Sightings; G = group size; N = abundance; CV = Coefficient of Variation; 95% CL = 95% Confidence Limit

Regression analysis of the natural logarithm of five mean strip-transect abundance estimates showed a non-significant 1% increase in abundance (b=0.01, t=0.52, df=3, P>0.05) (Figure 3). Direct count abundance estimates for the same study period revealed no changes in abundance (b=0.001, t=0.18, df=3, P>0.05). Power analysis revealed that in order to detect a 5% change (either positive or negative) with high statistical power (90%) and mean CV of 13% or 6% (mean of the CVs of each replicated strip-transect or direct count estimates per survey period) 28 “strip”-samples or seven “count”-samples are needed. In our study period (with five

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Chapter 6

samples) only changes as large as 20% (strip-transect estimates) or 8% (direct counts) would have been detected with 90% power.

Abundance estimates

Trend in dolphin abundance in the Mahakam River 100 Y = 34.4 + 0.6x r2 = 0.070

80 60 40 20 0

Med 99

Low 99

Med 00

High 01

Low

Time period

Figue 3. Trend in abundance estimates based on strip-transects. No significant population changes occurred during the study period. The vertical bars represent 95% confidence intervals of the estimates.

Population dynamics The dolphin population consisted on average of 61% adults, 30% juveniles and 9% calves and neonates (Table 4). The mean group size observed during the extensive monitoring surveys was 4.4 (SD = 2.2; range = 1-10). Population composition and group size did not fluctuate during different water levels (H = 0.17, df = 2). Minimum annual number of newborns during the study period was six dolphins. Newborns (< one month of age) were observed at all water levels and in all months of the year. Birth rates between 11 - 18% may apply (of N = 55 and 33 dolphins, respectively). With six newborns per year, the minimum numbers of breeding females within this population are 12 or 18 individuals if a 2- or 3-year reproduction cycle applies, respectively. During the 3.5 year study period minimally 17 dolphins died an unnatural death (interviews and own observations). Minimum average annual mortality during the study period is five dolphins (SD = 2, range = 3-8), which is between 9 and 15 % of maximum and minimum abundance estimates, respectively. True mortality rates, including deaths from natural causes, are unknown.

92

Conservation of riverine Irrawaddy dolphins in Borneo Table 4.

Group composition during three different surveys with different water level conditions.

Group composition Adults Juveniles Calves & neonates

Number of dolphins per water level during one survey Medium 2000 High 2001 Low 2001 Mean numbers 19 58 % 22 61 % 20 63 % 20 61 % 10 30 % 11 31 % 9 28 % 10 30 % 4 12 % 3 8% 3 9% 3 9%

Habitat preferences and home ranges Environmental characteristics for different river sections at medium water levels are presented in Table 5. All freshwater fish trade production comes from the middle river section (including tributaries and lakes), which has the highest dolphin densities. No significant differences were found between random samples and samples collected at dolphin locations for most parameters and water levels. Only for depth measurements at low water levels in the tributaries of the middle river section did we find a significant difference in mean depth of random samples 7.5 m and of samples at dolphin locations 16.7 m (t = 2.85, df = 16, P < 0.05). This suggests that dolphins prefer to remain in deep water pools, such as confluence areas, during the dry season. Their dependence upon confluence areas in particular during the dry season is indicated in Figure 4. At low water levels, significantly more sightings occurred in these areas than in river bends (X2 = 8.5, df = 1, P < 0.01), in spite of the fact that river bends are significantly more numerous (X2 = 24.3, df = 1, P 0.05). Also, during the other water level conditions the number of dolphins identified in this area is still high and no significant differences were found in seasonal presence (X2 = 5.1, df = 3, P >0.05). Dolphins occurred in the confluence area of Muara Pahu on average 42% of daytime during every observation day and at all water levels (Table 7). The highest daily occupancy was found at high water levels (65%). Dolphins still remained nearby (< 10 km) at medium and low water levels, but they spent less time milling in the confluence area itself. On average three different groups (range 2–6 groups) consisting of a combined mean number of 12 individuals (range 5–19 individuals), frequented the confluence area daily. Moreover, 44 (75%) of the 59 photo- identified dolphins were sighted at least once in the confluence area (mean = 6 days, max = 17 days of 49 photo/ observation days in that area).

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Straight stretch

River bend

Confluence

% of dolphin sightings

70 60 50 40 30 20 10 0 High

Medium

Low

Water levels

Figure 4. Dolphins preferred areas within the main river. NB random relative Availability of straight stretches was significantly higher than that of bends and confluences (X2 = 112.3, df =2, P