AEW-80439 MSc Thesis Aquatic Ecology and Water Quality Management The behaviour, activity pattern and substrate use of green turtles (Chelonia mydas) in a heavily grazed seagrass meadow, East Kalimantan, Indonesia

Iris (I.I.) de Winter

Registration number WU: 890216 963 090 Thesis serial number WU: 014/2012 Registration number RU: 4178696 Thesis serial number RU: 410 Date: 05-07-2012

Supervisors: Dr. Rudi (R.M.M.) Roijackers (Aquatic Ecology and Water Quality, Wageningen University, Wageningen, the Netherlands) Marjolijn (M.J.A.) Christianen (Environmental Science, Radboud University Nijmegen, Nijmegen, the Netherlands) Dr. Marieke (M.M.) van Katwijk (Environmental Science, Radboud University Nijmegen, Nijmegen, the Netherlands)

The behaviour, activity pattern and substrate use of green turtles (Chelonia mydas) in a heavily grazed seagrass meadow, East Kalimantan, Indonesia

Iris (I.I.) de Winter

Master Biology, specialization Ecology

Department of Aquatic Ecology and Water Quality Wageningen University and Research Centre

Department of Environmental Sciences Radboud University Nijmegen

Registration number WU: 890216 963 090 Thesis serial number WU: 014/2012 Registration number RU: 4178696 Thesis serial number RU: 410

Thesis code: AEW-80439

Supervisors:

Wageningen University Dr. Rudi (R.M.M.) Rooijackers

Radboud University Nijmegen Marjolijn (M.J.A.) Christianen Dr. Marieke (M.M.) van Katwijk

Title page photograph: Green turtle, Chelonia mydas (I. de Winter, 26-01-2012)

Table of contents

Preface ...... II Summary ...... III 1. Introduction ...... 1 2. Methods ...... 3 2.1 Study site ...... 3 2.2 Observations ...... 3 2.3 Statistical Analyses ...... 6 3. Results ...... 8 3.1 Sex and size classes ...... 8 3.2 Environmental factors ...... 8 3.3 General behaviour ...... 11 3.4 Uncommon behaviour ...... 13 3.5 Aerial photographs...... 14 3.6 Seagrass preferences ...... 14 4. Discussion ...... 16 4.1 Sex and size classes ...... 16 4.2 Environmental factors ...... 16 4.3 General behaviour ...... 17 4.4 Uncommon behaviour ...... 18 4.5 Seagrass preferences ...... 19 5. Conclusion ...... 20 Acknowledgement ...... 20 References ...... 21 Appendix 1 Tidal table ...... 25 Appendix 2 Moon Calendar ...... 26 Appendix 3 Dive and Snorkel transects ...... 28 Appendix 4 Output transformation depth sensor ...... 29 Appendix 5 Video-stills of uncommon behaviour ...... 30

Behaviour, activity pattern and substrate use of green turtles I

Preface For this master thesis I worked together with Peter van Leent (MSc student) within the project of Marjolijn Christianen (PhD candidate) from the Radboud University (RU) entitled: “Interactive effects of hydrodynamic stress and green turtle grazing on seagrass gap dynamics, Derawan, Indonesia”. Because the seagrass meadow around Derawan island has an extraordinary high density of green turtles and consequently a low seagrass biomass, I became interested in the potential behavioural adaptations of green turtles to these unique circumstances. Therefore, I was really motivated to perform this behavioural study on the endangered green turtles (Chelonia mydas) within this area, entitled: “The behaviour, activity pattern and substrate use of green turtles in a heavily grazed seagrass meadow, East Kalimantan, Indonesia”. My aim is to publish this work in a scientific journal.

By performing this second thesis project within my MSc Biology (Wageningen University) under the supervision of experienced researchers, I have gained more experience in all practical aspects of designing a detailed research project. Hereby, I want to thank all my supervisors (Marjolijn J.A. Christianen, Dr. Marieke M. van Katwijk and Dr. Rudi M. Roijackers) for their guidance within this project, my field partners and dive buddies (Peter van Leent, Jelco van Brakel, Sara Lambrecht, Hans Wolkers, Ger van Leent, Gerard/Inge/Martijn de Winter) for their support in the field and Anne Baauw for her support during the writing process. Furthermore, I want to thank Ibu Heldi, Combodia and Salsia and Pak Dirman, John, Udin and Wawan for their local and logistic support and the following funds for their financial support: Dr. Hendrik Muller, Dr. Christine Buisman, Alberta Mennega and the FONA.

Thank you all for giving me the opportunity to gain additional research skills and expertise through performing this thesis project, which will be very valuable for my future career in the field of biological research.

Iris I. de Winter

Behaviour, activity pattern and substrate use of green turtles II

Summary Green turtles (Chelonia mydas) spend most of their time in seagrass meadows, their main coastal foraging areas. The turtles’ behaviour, daily activity cycle and spatial use within these areas are still poorly understood, although understanding these patterns is of primary importance to improve the turtles’ conservation management. Our study has been conducted in a pristine Indo-Pacific seagrass meadow, which is prone to a very high turtle density and a relatively low seagrass biomass. This provides us with the opportunity to test for the first time the hypothesis that the daily activity cycle, behaviour and substrate use of green turtles is determined by a high turtle density, and therefore low seagrass biomass, under different environmental circumstances.

To this end, we have used snorkel and dive transects to record individual behavioural characteristics in combination with external environmental influences. Aerial photography was used to get more insight into the turtles’ spatial distribution on the seagrass meadow.

This study suggests that the green turtles in this system have maximized their grazing efforts by foraging during all daylight hours, as long as the water depth is sufficiently high to enter the seagrass. In addition, our results indicate that high moonlight intensities during the night stimulate feeding after daylight hours. Furthermore, the turtles have adopted some unique strategies, probably to meet their energy demands, such as digging and feeding in gaps on the belowground parts of seagrasses. They were also observed to use invertebrates as an alternative food source. The aerial photographs of the seagrass meadow show as well that turtles often feed in gaps.

We suggest that the high grazing pressure, specific behavioural patterns and physical disturbances to the seagrass will result in a persistent increase in the gap cover and erosion rate within the seagrass meadow. This will likely lower the resilience, productivity and carrying capacity of seagrass meadows, which could have disastrous consequences for the fitness of the complete green turtle population. We stress that more research on early warning signals and resilience of such dynamic and complex systems is needed, to take action accordingly.

Behaviour, activity pattern and substrate use of green turtles III

1. Introduction Seagrass meadows provide important ecosystem services and are therefore essential for the economy of many coastal zones worldwide (Costanza et al., 1997; Orth et al., 2006). Seagrasses in tropical marine systems are considered as primary producers (e.g. Riley and Kent, 1999; Green & Short, 2003). In addition, seagrass forms the main component of the diet of green turtles (Chelonia mydas) and other megaherbivores, like dugongs and manatees (e.g. Castelblanco-Martínez, 2012) and therefore, seagrasses are important for the survival and fitness of these species (Aragones, 1996).

In seagrass systems, grazing may change the seagrass meadow structure (Lal et al., 2010), can reduce the number of macroalgae in a system and may diminish overgrowth by epiphytes (e.g. Christianen et al., 2012). Al these effects of grazing result in a positive feedback mechanism between marine herbivores and the productivity of seagrass meadows, which is suggested to compensate for the biomass loss caused by grazing (McNaughton, 1984; Valentine et al., 1997; Moran & Bjorndal, 2005). Furthermore, seagrasses are able to regenerate their leaves quickly by using carbohydrates that are stored within their rhizomes (Valentine et al., 1997; Moran & Bjorndal, 2005; Christianen et al., 2012).

Both the valuable seagrass meadows and the endangered green turtles that rely on them are both of high priority to conserve (Seminoff, 2004). Despite the widespread conservation efforts of last decades, the current population densities of green turtles are still far below historical levels (e.g. Seminoff, 2004; Koch et al., 2007; Christianen et al., 2012). This decline of green turtle populations is primarily caused by the worldwide degradation of seagrass meadows and nesting habitats due to increasing anthropogenic disturbances, but is also caused by direct hunting and fisheries bycatch (Lutcavage et al., 1997; Nordlund, 2007; Unsworth & Cullen, 2010).

Worldwide, studies have focused mainly on female population abundance near the principal nesting sites, studying reproduction parameters, nesting activities and the long-term breeding migrations of green turtles (Pelletier et al., 2003; Taquet et al., 2006; Senko et al., 2010). Contrastingly, the short- term daily activity patterns of green turtles and their behaviour in coastal foraging areas are still poorly understood (Seminoff et al., 2002a; Seminoff & Jones, 2006; Hazel, 2009; Senko et al., 2010). It is suggested that the foraging behaviour of green turtles is mainly determined by food availability (Bjorndal, 1997; Seminoff et al., 2002b) and consequently, the turtles’ behaviour seem to vary highly between different foraging areas (compare e.g.: Williams, 1988; Seminoff et al., 2002b; Taquet, 2006; Senko et al., 2010). Furthermore, it is known that turtles generally alternate their presence on alternating substrates, mostly seagrass and coral. Their presence on one of those substrates is determined by several environmental factors, such as time of the day and tidal fluctuations (Taquet et al., 2006; Ballorain et al., 2010). Green turtles spend most of their lives in coastal foraging areas, where they face multiple anthropogenic impacts. This underlies the importance of a better understanding of the green turtles’ spatial use in foraging areas in combination with all aspects of their behaviour, activity patterns and feeding ecology to define and improve their conservation management.

Our study was conducted in a pristine Indo-Pacific seagrass meadow, which is part of a Marine Protected Area (MPA) that was established in 2005 (Hooker & Gerber, 2004; WWF, 2005). This legislation seemed to have resulted in a considerable increase in the number of green turtles, as the area now contains the world’s highest (reported) density of this species and incidental turtle density measurements suggest an increasing trend of their population densities (Christianen et al., in prep.). This high turtle density is accompanied with a relatively intense grazing pressure and a relatively low standing seagrass leaf biomass (Christianen et al., 2012). In this area, it was observed that the green

Behaviour, activity pattern and substrate use of green turtles 1

turtles showed a specific behaviour, digging, which causes gaps into the seagrass. A considerable increase in gap cover over the last years was recorded (Christianen et al., in prep.). The unique behavioural adaptations of the green turtles in this area may have profound consequences for the health of the seagrass meadow, and therefore, their foraging ground.

We will test whether the green turtles in this system are able to adjust their behaviour to the relatively low seagrass biomass by exhibiting new foraging strategies, like digging. This provides us with the opportunity to test for the first time the hypothesis that the daily activity cycle, behaviour and substrate use of green turtles is determined by a high turtle density, and therefore low seagrass biomass, under different environmental circumstances. The research questions underlying these hypotheses are 1) What temporal activity patterns and spatial preferences (seagrass-coral) do the green turtles show under varying environmental conditions (e.g. time of the day, tidal heights, varying moon light intensities and cloudiness)? 2) With what frequency do the turtles show unique foraging strategies, like digging, which seems to be a reaction to the low seagrass biomass? 3) Do their behavioural responses, activity patterns and substrate use differ from the behaviour of green turtles in foraging grounds with a higher seagrass biomass? For these questions we will focus on potential differences between green turtles from different sex and size classes.

To this end, we have combined our results with literature data from seagrass meadows with a relatively higher seagrass biomass. Our study is innovative in our assessment of the links between seagrass biomass and the behavioural responses of green turtles in their natural environment, both by direct observations and from aerial photography data.

Behaviour, activity pattern and substrate use of green turtles 2

2. Methods

2.1 Study site Our study was conducted in an Indo-Pacific seagrass meadow surrounding the island Derawan. Derawan is situated 16 km offshore from the Berau River delta and the mainland of East Kalimantan, Indonesia (2°17’19'' N, 118°14’53'' E) (Fig. 1). About 1600 people live in the only village of the island and the local economy of Derawan is mostly based on tourism and fisheries. The Derawan archipelago (circa 1.2 million ha) is a marine protected area (MPA) since 2005 (WWF, 2005) and it is suggested that this area is still relatively pristine (van Katwijk et al., 2004; 2011; Supriyadi & Kuriandewa, 2008). The seagrass meadow surrounding the island functions as a foraging ground for the highest observed density of green turtles in the world, a density that has been increased from 15.4 ± (SE) 1.5 turtles ha-1 in 2008 to 20.0 ± (SE) 1.4 turtles ha-1 in 2012 (Christianen et al., in prep.).

Figure 1 a) The location of the Derawan archipelago in front of the east coast of Kalimantan, and b) the location of Derawan island (Christianen et al., 2012)

The seagrass meadow is dominated by narrowleaf seagrass (Halodule uninervis), but spoon seagrass (Halophila ovalis) and sickle seagrass (Thalassia hemprichii) are also present, although in much lower densities. The seagrasses grow on a carbonate substrate (Christianen et al., 2012) and surround the island till 500-800 m off the coast, with coral reefs on the outer edges.

Fluctuations in the water height of the meadow can be strong (Madang & Yanagi, 2008) (Appendix 1), especially around full and new moon (Appendix 2), when tidal amplitudes are most extreme. Minimum tidal amplitudes in the meadow range from 0.9 to 1.7 m and maximal tidal amplitudes from 0.0 to 2.9 m (Christianen et al., in prep.).

2.2 Observations Three different types of observations were performed in this study, which was conducted in January and February 2012. Our main observations consist of snorkel and dive transects and aerial photography was used to support these observations and to get an overview over the turtles’ distribution over the seagrass meadow. During the snorkel transects, the behaviour of green turtles was observed following a fixed path (“transect”), which crossed two different substrates: seagrass and coral (Table 1, Appendix 3). The transects had a fixed distance, which was measured and mapped with a hand-held GPS (Garmin 60 Cx). In addition to these snorkelling transects we collected data using SCUBA to record the green turtles behaviour during the night and their estimated depth

Behaviour, activity pattern and substrate use of green turtles 3

by following transects that differed Table 1 Length of snorkel and dive transects divided up into from the snorkel transects as they

two different substrates (seagrass and coral) covered relatively less seagrass and Substrate Distance Snorkel Distance Dive more coral (Table 1, Appendix 3). Transect (m) Transect (m) Each snorkel transect (N= 63) lasted Seagrass (SG) 509 247 31 min 37 sec ± (SD) 5 min 12 sec Coral (SC) 269 402 and each dive transect (N= 28) 36 Total 778 659 min 40 sec ± (SD) 15 min 52 sec. All transects were started between 6:00-22:00 hr by one observer (IdW). During the dive transects, four alternating buddies accompanied IdW, and therefore we will test for potential observer bias. During our transects we stayed at a minimum distance of 4 m from turtles to avoid disturbance (Taquet et al., 2006) At this distance, the turtles did not seem to be sensitive to our presence as they did not interrupt their specific activities when we passed them.

For each green turtle that we encountered along the transects, we notated on which substrate it was present (seagrass or coral), the behaviour it exhibited, its sex and size class and all occurrences of uncommon behaviour (Table 2). Beforehand, the observer trained herself during dive and snorkel surveys in estimating the size and carapace length of the turtles by estimating the size of different objects in the water at varying distances and checking the size with a tape line afterwards. In addition to these observations, the exact depth (measured with a dive computer, Mares Puck) and sleeping locations of the green turtles were recorded as well during the dive transects. When the turtles were resting, their substrate material and potential shelter was notated (Table 2). In pilot studies it was noticed that the turtles in this area mostly rest and sleep at depths between 4 and 10 meters, which was also observed in another study on green turtles (e.g. Hazel et al., 2009; Senko et al., 2010). Therefore, we dove at a depth of about seven meters in order to cover this preferred depth range.

Our third observation, aerial photography of the seagrass meadow, was used to get a broad and complete overview of the green turtles’ spatial distribution over the seagrass meadow and their possible tendency to forage individually or in (small) groups. For this method we use a small (model) aircraft (N= 5 flights) (Swinglet CAM (Sensefly; Ecublens, Switserland)) for aerial photography of the seagrass meadow (N= 14 photos, height= 55 m, area= 5 000 m2, the pictures are made during sunny or partly cloudy days between 08:00 and 18:00 hr and during water heights of 1.4-1.9 m). The number of green turtles on the seagrass was counted and for each turtle we scored whether it was grazing in gaps or intact parts of the seagrass meadow. This was based on the location of their heads, which could be distinguished on the photographs. In addition, we counted the number of turtles that was present in close proximity to another turtle (at a distance of < two turtle lengths) and the number of turtles that was individually present (at a distance of > two turtle lengths of the nearest turtle).

All observations were done during varying times of the day, water heights, extreme and minimal fluctuating tides, cloudiness and moon light intensities, to get a broad overview of the distribution and behaviour of the turtles on the seagrass meadow and coral reef during varying environmental circumstances. These different environmental parameters were scored before each transect (Table 3). Moonlight intensities vary greatly throughout the lunar cycle and are determined by a combination of moon phase and cloudiness. Therefore, we corrected the data on moonlight intensity during clouded nights (Taquet et al., 2006). Due to tidal differences, the water height on each substrate differed throughout the day and month. To determine the water height we used tidal tables produced by the nearest known station (Teluk Sangkulirang) (Appendix 1). A depth profile of the transect was obtained by moving a pressure sensor (Sensus Ultra; Reef Net Inc.; Ontario, Canada) over the sediment at a fixed speed. Pressure was logged every second and averaged over a

Behaviour, activity pattern and substrate use of green turtles 4

5 second interval to exclude wave movement. This enabled us to extract the exact water height on each substrate from the tidal table (Appendix 4).

Table 2 Ethogram: classification and description of the categorized behaviours and individual characteristics of green turtles Parameter Class Description Behaviour Feeding Obtaining, chewing or swallowing food (Guillotin and Dubost, 1994; Kirika et al., 2008; Flörchinger et al., 2010). Swimming Movement through the water Resting All inactive behaviour (laying down or sleeping) Breathing air Swimming towards the surface with the intention to breath Other Other behaviour Sex and size Female Adult of >85 cm, short tail class Male Adult of >85 cm, long tail Middle sized Subadult of 50-85 cm (sex cannot be determined) Small Juvenile of <50 cm (sex cannot be determined) Uncommon Digging Feeding on the rhizomes and roots of seagrasses by digging behaviour into the ground with their forelimbs Feeding in gaps Eating uncommon food sources E.g. jellyfish and banana leaves Other Substrate Coral material (when Sand resting) Stone Other Shelter during Exposed No shelter resting Covered Finding shelter by (1) covering their heads, (2) covering half of their bodies or (3) covering their complete bodies with some structures

Behaviour, activity pattern and substrate use of green turtles 5

Table 3 Classification and description of the observed environmental parameters Parameter Class Description Water 0.7-1.0 m Classes of water height caused by the tidal regime (e.g. Ballorain et al., height 2010) 1.1-1.4 m 1.5-1.8 m 1.9-2.4 m Tidal High The tides four days before and after maximum spring tides extremes Low The remaining days that are not classified as ‘high’ Time of day Morning 06:00-10:30 hr Midday 10:30-13:50 hr Afternoon 13:50-18:30 hr Night 18:30-22:00 hr Daylight Day 6:10-18:10 hr hours Night 18:10-6:10 hr (following Ballorain et al., 2010; Senko et al., 2010) Cloudiness Sunny Less than 50% cloud cover Cloudy More than 50% cloud cover Rainy Moon light 0-25% Percentage illumination intensity 25-50% (http://www.timeanddate.com/calendar/moonphases.html?n=434, 50-75% visited: 12/2011) 75-100%

2.3 Statistical Analyses During pilot transects, the visibility in the water regularly differed between transects, but the visibility was assumed to be constant within each transect. Furthermore, the distance of each substrate (seagrass and coral) differed within both the dive and snorkel transects. To correct for these variations, the data were corrected for both the visibility and the proportional distances of each substrate (Table 1) as follows:

(1) = 1

푉푖푠푖푏푖푙푖푡푦 퐶표푟푟푒푐푡푖표푛 푉푖푠푖푏푖푙푖푡푦 = (2) 푆푢푏푠푡푟푎푡푒 퐷푖푠푡푎푛푐푒 퐶표푟푟푒푐푡푖표푛 퐷푖푠푡푎푛푐푒 푇표푡푎푙 퐷푖푠푡푎푛푐푒 (3) =

푇표푡푎푙 퐶표푟푟푒푐푡푖표푛 = 퐶표푟푟푒푐푡푖표푛 푉푖푠푖푏푖푙푖푡푦 ∗ 퐶표푟푟푒푐푡푖표푛 퐷푖푠푡푎푛푐푒 ∗ 퐶표푢푛푡푒푑 푡푢푟푡푙푒푠

푆푢푏푠푡푟푎푡푒 퐷푖푠푡푎푛푐푒

푉푖푠푖푏푖푙푖푡푦∗푇표푡푎푙 퐷푖푠푡푎푛푐푒 ∗ 퐶표푢푛푡푒푑 푡푢푟푡푙푒푠 Note that due to these corrections, values for the counted turtles in the results are not the absolute numbers of turtles that we observed.

A multiple regression analysis was conducted to predict which environmental variables significantly influenced the distribution of turtles over the different substrates and to test for multicollinearity. We used both 24 hour data to test for the effect of the multiple environmental variables on the number of turtles during the complete day used data collected solely during the day as well to test the effect within daylight hours. The five main environmental variables (time of the day, water height, extreme tides, moonlight intensity and cloudiness) were entered simultaneously into the

Behaviour, activity pattern and substrate use of green turtles 6

analysis and were one by one removed from the model when they were not significant influencing the number of turtles on the coral or seagrass (the two individually tested dependent variables). All standardized residuals were tested for normality and before each of these analyses we performed a General Linear Model (GLM) to test for potential observer bias.

After correcting for the differences in visibility and distances of each substrate, we analysed the differences in substrate preferences and the behaviour of green turtles (dependent variables) under varying environmental circumstances (Table 2), which were significant in the multiple regression analysis (included as independent variables). To test for these potential differences, we performed GLMs followed by a Tukey post-hoc test when the data were normally distributed with equal variances. If the data were not normally distributed, we tried to force the data into a normal distribution by applying a √(X+0.5) transformation. When this transformation did not succeed, we used non-parametric Mann-Whitney U (MW U) or Kruskal-Wallis (KW) tests to analyse the data (SPSS version 17.0). We used the same approach for the analyses of the depth occupation, the use of substrate material and cover during resting.

Individuals of different sex and size classes were tested for differences in the uncommon behaviours they showed (Table 3) by using Chi square and Fisher’s exact tests. Due to the low number of observations, we also used Fisher’s exact tests to test for individual preferences for the rhizomes, roots, shoots or leaves of seagrasses and potential preferences for the different seagrass species (Halodule uninervis, Halophila ovalis or Thalassia hemprichii).

Behaviour, activity pattern and substrate use of green turtles 7

3. Results No significant differences were found in the number of turtles encountered on the seagrass and coral between the different combinations of observers (GLM: F3,22= 1.24, P= 0.321).

3.1 Sex and size classes During the transects, we most frequently encountered adult females and middle sized individuals and significantly less small individuals and adult males (KW, for the snorkel transects: χ= 44.25, df= 3, P< 0.005) (Fig. 2).

Figure 2 (Corrected) number of turtles of different sex and size classes encountered along the transects. Error bars are displayed for the 95% confidence interval.

3.2 Environmental factors No multicollinearity between the environmental variables was found on both the seagrass and the coral (multiple regression analysis, variance inflation factor (VIF) < 5) (Table 4). On the seagrass, both throughout the 24-hour cycle and when focusing on daylight hours, the number of turtles that were present on the seagrass significantly decreased with time of the day and increased with water height. Extreme tides, moonlight intensities and cloudiness did not significantly influence the number of turtles on the seagrass and were therefore excluded from the model. On the coral, the number of turtles significantly increased with time of the day and moonlight intensities and a decreased with water height and during extreme tides (Table 4). Furthermore, the number of turtle present on the either seagrass or the coral was also negatively correlated with each other (rs= - 0.794; P< 0.005).

Behaviour, activity pattern and substrate use of green turtles 8

Table 4 Results of the multiple regression analysis on the effect of the environmental variables time of the day, water height, extreme tides and moonlight intensity on the number of turtles that were present on the seagrass and coral throughout the 24-hours cycle and during daylight hours. Part of Substrate Explained F Environmental Coeffic- t P VIF the day variance variable ient β (%) 24- Seagrass 23.2 24.79 Time of the day -0.30 -6.90 <0.005 1.04 hours Water height 0.27 2.66 0.009 1.04 cycle Coral 43.6 31.33 Time of the day 0.27 9.93 <0.005 1.07 Water height -0.23 -3.18 0.002 1.37 Extreme tides -0.25 3.34 0.034 3.51 Moonlight 0.006 -2.14 0.001 3.98

Daylight Seagrass 40.6 18.78 Time of the day -1.91 5.19 <0.005 1.01 hours Water height 5.30 -3.70 <0.005 1.01

Coral 25.8 9.23 Time of the day 0.27 4.46 <0.005 1.43 Water height -0.18 -2.33 0.022 1.43 Extreme tides -0.41 -3.08 0.003 3.96 Moonlight 0.006 3.38 0.001 3.30

Daily pattern Focussing on differences in the number of turtles encountered along each transect between different time classes, we found significant differences on both the seagrass and the coral (seagrass: GLM F3,62= 5.05, P= 0.004 and coral: GLM F3,27= 3.20, P= 0.011) (Fig. 3). The largest differences on both substrates were found between the number of turtles in the early morning (6:00-10:30 hr) and the night (after 18:30) (Tukey post-hoc test, on the seagrass; mean difference ± SE: 8.40 ± 2.74, P= 0.026 and on the coral: -4.05 ± 1.13, P= 0.008). On the coral we also found significantly less turtles during midday (10:30-13:50 hr) compared to the night (18:30-22:00 hr) (mean difference ± SE: -3.78 ± 1.13 turtles) (Fig. 3).

Furthermore, we tested for differences in the number of turtles between the seagrass and the coral within time classes. We found significant differences between those substrates throughout the day for each time class (GLM F3,326= 39.8, P< 0.005 and Tukey Post-hoc: 6:00-10:30 hr, P< 0.005; 10:30- 14:50 hr, P< 0.005; 14:50-18:30 hr, P=0.396; 18:30-22:00 hr, P< 0.005) (Fig. 3).

When splitting the day into daylight hours (6:10-18:10 hr) and night (18:10-6:10 hr), we found significantly more turtles on the seagrass than on the coral during daylight hours (GLM F1,157= 32.16, P< 0.005) and more turtles on the coral than on the seagrass during the night (GLM F1,21= 9.94, P< 0.005).

Behaviour, activity pattern and substrate use of green turtles 9

Figure 3 The number of turtles counted on the seagrass and coral during different times classes (morning: 6:00- 10:30 hr, midday: 10:30-14:50 hr, afternoon: 14:50-18:30 hr and night: 18:30-22:00 hr). The significant differences between time classes are indicated with lowercase letters for the seagrass and uppercase letters for the coral and significant differences between the number of turtles on the substrates within each time class are indicated with an asterix. Error bars are displayed for the 95% confidence interval.

Tides During relatively lower water height classes of 0.7-1.0 m and 1.1-1.4 m, significantly less turtles were present on the seagrass compared to during the water height class of 1.5-1.8 m (GLM F3,125= 7.89, P< 0.005). On the reef, the pattern is the other way around, with significantly more turtles on the reef during the relatively low water height classes of 0.7-1.0 m and 1.1-1.4 m compared to the higher water height classes (1.5-1.8 m and 1.9-2.4 m) (GLM F3,125= 8.00, P< 0.005) (Fig. 4, Table 5).

The number of turtles on the seagrass is also significantly different from the number on the coral at the higher water height classes (GLM F3,326 = 16.1, P< 0.005 and Tukey Post-hoc: 0.7-1.0 m, P= 0.309; 1.1-1.4 m, P= 0.324; 1.5-1.8 m, P< 0.005; 1.9-2.4 m, P< 0.005) (Fig. 4).

Behaviour, activity pattern and substrate use of green turtles 10

Figure 4 The number of turtles counted on the seagrass and coral during different water heights in meters. The significant differences between tide classes are indicated with lowercase letters for the seagrass and uppercase letters for the coral and significant differences between the number of turtles on the substrates within each tidal class are indicated with an asterix. Error bars are displayed for the 95% confidence interval.

Table 5 Significant differences between the number of turtles during different water height classes (Tukey post-hoc test). The classes represent the following water height: 1: 0.7-1.0 m, 2: 1.1-1.4 m, 3: 1.5-1.8 m, 4: 1.9-2.4 m. Substrate Water height classes P Seagrass 1 – 3 < 0.005 2 – 3 0.002 Coral 1 – 3 0.004 1 – 4 0.003 2 – 3 0.005 2 – 4 0.005

Moon light intensity Turtle densities on the coral reef during daytime were significantly higher when moonlight intensities were above 50 % (mean 5.15 ± SD 2.99), compared to days with preceding nights with moonlight intensities below 50 % (mean 1.85 ± SD 1.65) (GLM F1,16, P = 0.001).

3.3 General behaviour

Activity pattern The data indicated that the most common behaviour of the green turtles on the reef was resting, whereas on the seagrass the turtles were observed to significantly feed, swim and breathe air more frequently (Fig. 5, Table 6). Water height and time of the day did not seem to influence turtle behaviour on the different substrates. Focussing on different individually; resting was significantly more frequently performed by adult males and females compared to middle sized and small

Behaviour, activity pattern and substrate use of green turtles 11

individuals, while the latter were more frequently observed swimming (KW: N= 72, χ2= 12.33, df= 3, P= 0.006).

Figure 5 The number of turtles encountered feeding, swimming, resting and breathing air on the seagrass and coral during the day. Significant differences in the different behaviours between both substrates are indicated with an asterix. Error bars: 95% confidence interval.

Table 6 Results of the Mann-Whitney U test of the differences in behaviour on the seagrass and coral Feeding Swimming Resting Breathing Air Z -17.04 -7.67 -2.32 -6.19 P < 0.005 < 0.005 0.020 < 0.005

Depth occupation The minimum and maximum water height on the seagrass meadow, at which green turtles were present, was between 0.7 and 2.4 m, while the mean depth of the turtles on the coral during our transects was 7.33 ± SD 2.73.

Water height did not determine the depth at which turtles were present on the reef, while focussing on the 24 hour cycle (GLM F3,55= 0.60, P= 0.620) and daylight hours (GLM F3.31= 1.90, P= 0.152). Neither did we find differences in depth occupation between different sex and size classes. Contrastingly, the depth at which the turtles were present did differ with the time of the day (GLM F3,55= 2.91, P= 0.043), as they stayed down deeper during the night (7.96 ± (SD) 2.99 m) compared to the day (6.38 ± (SD) 1.90 m). Turtles were present at significantly larger depths during the morning compared to the midday and afternoon and during the night compared to midday (GLM F2,39= 7.19, P= 0.003) (Fig. 6, Table 7).

Behaviour, activity pattern and substrate use of green turtles 12

Figure 6 The mean depth in meters of the counted turtles on the coral reef during different time classes (morning: 6:00- 10:30 hr, midday: 10:30-14:50 hr, afternoon: 14:50-18:30 hr and night: 18:30-22:00 hr). The significant differences were pointed out by a Tukey post-hoc test and are indicated with lowercase letters. Error bars are displayed for the 95% confidence interval.

Table 7 Differences in the number of turtles between different parts of the day (early morning: 06:00-10:30, midday (10:30-14:50), afternoon (14:50-18:30) and night (18:30-22:00) (Tukey post-hoc test). Part of the day Mean difference ± SE (m) P Early morning – Midday 2.89 ± 0.78 0.002 Early morning – Afternoon 2.18 ± 0.76 0.018 Midday - Night -2.72 ± 0.95 0.031

Shelter and ground material during resting While resting, turtles were observed to find shelter under overhangs and rocks by covering themselves more often during the night than during the day (MW U: N= 44, Z= -3.65, P< 0.005). During the day, the turtles covered their head or complete body significantly less compared to during the night (MW U: N= 39, head: Z= -2.19, P= 0.028 and complete body: Z= -2.18, P= 0.029). No significant difference between day and night was found for the cover of half their body (MW U: N= 39, Z= 0.22, P= 0.823). Juveniles seek complete shelter more often compared to other size classes (GLM F3.36= 4.61, P= 0.008).

The substrate material where the turtles rested on did not significantly differ between individuals of different sex and size classes (GLM F3.35= 0.51, P= 0.678), but a coral substrate is more often occupied during the night compared to the day (GLM F1.37= 5.760, P= 0.022).

3.4 Uncommon behaviour During our transects (N=63), the most frequently observed uncommon behaviour was digging (N= 10). This behaviour was only exhibited by adult individuals with no differences between males and females (Fisher Exact test N= 10, P= 0.789). Digging occurred significantly more during relatively high water heights of 1.5-1.8 m (χ2= 9.80, P= 0.007), but we found no significant differences between different times of the day or moonlight intensities. Digging seem to occur more frequently during

Behaviour, activity pattern and substrate use of green turtles 13

extreme tides (N= 7) compared to normal tides (N= 3), however, this difference was not significant (Fisher Exact test: N= 10, P= 0.333) (Appendix 5a)

We noticed that turtles were using rubbish (N= 2), banana leaves (N= 2) and jellyfishes of up to 0.5 X 0.5 m (N= 4; during one transect, three turtles were observed feeding simultaneously on one big jellyfish), as alternative food sources. The jellyfishes were eaten by large females and middle sized individuals, but the samples sizes were too low to test for significant differences between turtles from different sex or size classes.

3.5 Aerial photographs The aerial photographs supported our results that most green turtles were present on the seagrass in the (early) morning and during high tides during the day. On the seagrass meadow, 34.5% of the turtles was foraging in gaps, while the remaining 65.5% was grazing in intact parts of the seagrass meadow. The majority of the turtles (79.1%) was grazing individually, but the remaining part (20.9%) was observed to forage close to another turtle. We never saw more than two turtles foraging together. It was remarkable that 50.6% of the turtles that were grazing in gaps were grazing in close proximity to another turtle (Fig. 7).

Figure 7 Aerial photograph of green turtles on the seagrass meadow. Four turtles are marked with a circle, (I) is a turtle grazing in an intact part of the seagrass meadow, (II) is grazing in a (re-growing) gap and (III) indicates two turtle grazing in close proximity to each other.

3.6 Seagrass preferences Next to the leaves and shoots, the belowground parts of the seagrass were eaten by the green turtles as well. Adults seemed to focus slightly more on the rhizomes and roots of the seagrass compared to the middle sized and small individuals, but these differences were not significant

Behaviour, activity pattern and substrate use of green turtles 14

(Fisher Exact N= 24, P= 0.085). Just as was observed in the aerial photographs, we observed turtles in an existing gap (N= 5) or a gap that had been digged by themselves (N=10), whilst feeding on the rhizomes and roots of the seagrass (Appendix 5b).

During our transects, we typically saw turtles feeding on Halodule uninervis and never specifically on algae or the other two seagrass species (Halophila ovalis and Thalassia hemprichii). Although we did not observe turtles specifically feeding on Halophyla ovalis, we observed that this species was regularly eaten together with bites of Halodule uninervis. Halophyla ovalis also floated in between seagrass fragments that were spilled by green turtles during feeding.

Behaviour, activity pattern and substrate use of green turtles 15

4. Discussion We believe that this study complements to our understanding of the behaviour, daily activity pattern and substrate use of green turtles in a region with a very high turtle population density and relatively low seagrass biomass. First, the green turtles in this system seem to have maximized their grazing efforts by using all the time during the day that the tides enable them to enter the seagrass for grazing. Furthermore, they have adopted some unique foraging strategies, which are not shown anywhere else in the world, such as foraging on the belowground parts of seagrass in existing gaps and actively digging for the roots and rhizomes of seagrasses. This behaviour was only exhibited by the relatively older, larger individuals. Another important observation was that the turtles in this area supplemented their diet with alternative food sources (e.g. jellyfish) to meet their energy demands.

4.1 Sex and size classes The population structure of green turtles in this area seems to be biased towards adult females and middle sized individuals, as these individuals were significantly more encountered along our transects compared to small individuals and adult males. Small (young) individuals were usually observed to be shyer compared to larger individuals, which may explain the observed pattern. Turtles use this area as a foraging and nesting site. Although migratory behaviour of males is suggested to be similar to that of females, mating systems may vary across populations (Fitzsimmons et al., 1997), which can explain the local bias towards females (Reich et al., 2007).

4.2 Environmental factors

Daily pattern As the number of turtles on the seagrass and coral were negatively correlated with each other, we can state that the turtles alternate both substrates throughout the day. We found that more turtles were present on the seagrass during the day, and on the coral during the night. This is in line with the findings of other studies that green turtles prefer to travel and feed during the day and to rest at night (Bjorndal, 1980; Ogden et al., 1983; Williams, 1988; Makowski et al., 2006; Taquet et al., 2006; Hazel et al., 2009). In another studies though, it is found that green turtles were active throughout the 24 hour cycle, with no difference in their diurnal and nocturnal net displacement (Senko et al., 2010).

Tides Turtles alternated their presence on the seagrass and coral depending on water heights. Significantly less turtles were sighted on the seagrass and more on the coral during the relatively lower water heights compared to higher water heights. Low tides during the day likely account for a restricted access to the seagrass and an increased number of turtles on the coral. This pattern was also observed in other green turtle foraging grounds (Taquet et al., 2006; Ballorain et al., 2010), just as in a periodically accessible mangrove area (Queenland, Australia) (Limpus and Limpus, 2000).

Large differences between turtles’ reaction to tidal differences between areas with low and high food availabilities seem to be present. The observations in this study were in line with our expectation that turtles use all the foraging time available to forage on the seagrass, as it seems that they entered the seagrass as soon as the tides enabled them to do so. It has been observed in other studies with relatively low food availabilities as well that green turtles spend a long time foraging every day (Williams, 1988; Taquet et al. 2006; Senko et al., 2010). The grazing efforts made by the turtles in this area seem to be intense compared to other foraging areas with higher seagrass biomasses, where turtles feed intermittently during the day, mostly during the early morning and late afternoon, while resting for several hours around noon (e.g. Bjorndal, 1980; Williams, 1988; Ogden et al., 1983).

Behaviour, activity pattern and substrate use of green turtles 16

Moon light intensity Variation in moon light intensities had a significant effect on the number of turtles on the coral during the day. It seems that when moon intensities were low during the night, turtles were present on the seagrass in much lower numbers during the day, even when circumstances were not optimal to feed on the seagrass (namely during low water heights). When moon intensities during the night were relatively high, the turtles are more often found on the coral, also during relatively high water heights that are favourable for feeding.

Therefore, it seems that variation in moonlight intensities had an effect on the activity cycle of green turtles in such way, that high moonlight intensities during the night stimulated feeding after daylight hours. The turtles compensated this behaviour by resting relatively more on the coral during the day after such nights and by foraging more on the seagrass during the day when they had not been active during the night. Significant positive correlations between moonlight intensities and the presence of green turtle that were foraging on the seagrass during the night were shown in another study as well (Taquet et al., 2006).

4.3 General behaviour

Activity pattern Adult males and females were more often observed resting compared to middle sized and small individuals, while the latter were more frequently observed swimming. This may be due to an observer effect, as middle and small individuals in general are less habituated in this area and might therefore be shyer when humans are approaching.

Feeding, swimming and breathing air were mostly done on the seagrass. Other studies found as well that on the seagrass, turtles were feeding for the majority of the time, which is only alternated by swimming slowly in between feeding bouts and breathing air at the surface (Taquet et al. 2006; Hazel et al., 2009; Ballorain et al., 2010; Senko et al., 2010). Because of their higher activity level on the seagrass, the turtles needed to breathe air more often compared to when they rested on the coral, as they needed to compensate their higher oxygen consumption (Schmidt-Nielsen, 1997).

The turtles mostly rested on the coral reef slopes and barely on the seagrass, which was also found in other studies (e.g. Hays et al., 2002 Taquet et al., 2006; Heithaus et al., 2007). First, the reason for this behaviour may be that water heights on the seagrass can become so low due to tidal effects, that the turtles would have become exposed when they would have stayed on the seagrass. This would lower their mobility and would make them more vulnerable for unwary speed boats that could seriously injure them. Second, they may prefer structured resting sites and shelter, which can only be found on the reef (e.g. Senko et al., 2010). This behaviour would lower their predation risk, especially at night (Heithaus et al., 2007; Hazel et al., 2009; Senko et al., 2010). Although their main predators in this area, tiger sharks, had recently become extinct (Christianen, personal information), their instinct to avoid predators is likely still present.

Depth occupation The depth occupation of turtles on the reef varied throughout the day. In general, turtles showed stronger selection for moderate (5-10 m) and deep locations (>10 m) at night and shallow water (<5 m) during the day. During the night the turtles occupied deeper parts of the reef compared to the day. This was observed in several other studies as well (Bjorndal, 1980; Ogden et al., 1983; Hays et al., 2002; Makowski et al., 2006; Taquet et al., 2006; Hazel et al., 2009). Nevertheless, the opposite pattern with a stronger selection for shallow water (and even a strong avoidance of deep water) at night was also found (Brill et al., 1995; Seminoff et al., 2002a; Southwood et al., 2003). Individual variation in depth selection has been found as well (Seminoff et al., 2001), just as a selection for

Behaviour, activity pattern and substrate use of green turtles 17

moderate water (5-10 m) instead of shallow water during the day (Hays et al., 2002; Senko et al., 2010).

Turtles can vary the degree of inflation of their lungs in order to regulate their buoyancy. Their depth occupation during resting was observed to be within the range at which they can be neutrally buoyant (Minamikawa et al., 2000). The turtles can reach their maximum depth while being neutrally buoyant with completely inflated lungs and can be neutrally buoyant at lesser depths by only partly inflating their lungs. In this way, they can rest passively at varying depth (Minamikawa et al., 2000; Hays et al., 2002). As totally inflated lungs are associated with greater oxygen stores, turtles can remain submerged longer when they dive with more inflated lungs (Hays et al., 2002; Hazel et al., 2009). Green turtles’ sleeping bouts are usually longer during the night compared to the day (e.g. Makowski et al., 2006), which explains our results that turtles lay deeper on the reef during the night compared to the day.

Shelter and ground material While resting, turtles were observed to find shelter by covering themselves with some (coral) structures. This behaviour was shown significantly more during the night than during the day. Because small individuals are usually more vulnerable for predation (if predators would be present), it is reasonable that they completely cover themselves more often than larger individuals.

All turtles mostly rested upon sand or coral, but a coral substrate is more often occupied during the night compared to the day. This is probably linked to their depth occupation, as coral is the dominant substrate at larger depths.

4.4 Uncommon behaviour

Feeding in gaps The most frequently observed and most worrisome uncommon behaviour was digging (Appendix 5b), after which the turtles feed on the below-ground parts of the seagrasses. Green turtles are usually described to feed only on the above-ground part of seagrasses (which is often called “cropping”) (e.g. Bjorndal, 1980; Aragones, 2006), which makes their foraging behaviour in this area unique.

Digging was only performed by adult turtles, probably because adults have more strength in their forelimbs compared to younger individuals to perform this behaviour. Besides, young individuals were reported to select the most digestible seagrasses in other seagrass areas (Brand-Gardner et al., 1999), which may explain why we did not observe them feeding on the below ground parts of the seagrass in this study. Nevertheless, more research on the feeding preferences of individual turtles and the digestibility of the below-ground parts of Halodule uninervis is needed in this specific area. Digging occurred significantly more during relatively high water levels, but not during extremely high tides. These preferred water levels are probably associated with a low current strength, which facilitates digging. After digging, turtles were always feeding on the rhizomes and roots of seagrasses, so it seems that the purpose of digging was to reach the below-ground parts of the seagrasses.

The aerial photographs showed that turtles quite often occurred in gaps, which might indicate that they were feeding on the young re-growing leaves. This is in line with their described preferences for the most digestible parts of seagrasses (e.g. Bjorndal, 1980; 1997; Ballorain et al., 2010). However, we regularly saw turtles feeding on the roots and rhizomes of the seagrasses within such gaps during our transects. These below-ground parts of the seagrass had become exposed and are therefore relatively easily accessible in these gaps compared to the more intact parts of the seagrass meadow.

Behaviour, activity pattern and substrate use of green turtles 18

The above-ground seagrass biomass is relatively limited in this area and green turtles had already been observed to eat 100% of the daily seagrass biomass production (Christianen et al., 2012). Therefore, obtaining the nutritious below-ground parts of the seagrasses seems to outweigh the costs of the efforts that need to be made to reach the below-ground parts and therefore, the turtles seem to implement this behaviour as a new foraging technique. Digging by green turtles was previously noticed in this area, but has not been recognized in other areas yet (Christianen et al., in prep.).

The same accounts for feeding on the below-ground parts of the seagrasses in existing gaps, which might be created by boats, anchors or turtles that has been digging. Uprooting and damaging the roots and rhizome of the seagrass is likely to affect the resiliency, stability and productivity of a seagrass meadow (Di Carlo and Kenworthy, 2008; Eklöf et al., 2009), as was for example shown in a meadow with a high disturbance caused by boat anchors (Williams, 1988). Erosion of the seagrass in between gaps would lower the health of the seagrass system even further, which may turn the system into a barren state with an extremely low seagrass biomass and productivity (e.g. Eklöf et al. 2009; Christianen et al., in prep.). Therefore, these new foraging behaviours could have disastrous consequences for the fitness of the complete green turtle population.

Although a large number of turtles were foraging on the seagrass at the same time, the majority of the turtles were grazing individually. More than half of the turtles that were observed in gaps though, were foraging in close proximity to another turtle, which underlines the preference of turtles to feed in such gaps. Literature data on their tendency to either forage individually or in groups is limited.

Alternative food sources As alternative food sources, rubbish and banana leafs that were fallen into the water were used. The most remarkable uncommon food source that was eaten were jellyfishes of up to 0.5 x 0.5 m. Green turtles are mainly carnivorous from hatching until juvenile, after which they gradually shift to an herbivorous diet (e.g. Bjorndal, 1997). In this study, both middle sized and adult green turtles regularly supplemented their seagrass diet with these jellyfishes. Although turtles are generally described as completely herbivorous (e.g. Bjorndal, 1997), it has been shown in another seagrass area with a very high abundance of algae (Central Gulf of California), that green turtles deliberately augmented their diet with animal matter, including sponges, tube worms, sea pens and sea hares (Seminoff et al. 2002b). We suggest that the opportunistic consumption of invertebrates may be an adaptation to the limited availability of seagrass biomass and may occur in other areas with high herbivore densities or low food availabilities as well.

4.5 Seagrass preferences In this foraging ground, we saw the turtles mainly feeding on Halodule uninervis, much less on Halophyla ovalis and never on Thalassia testudinum during the transects, although green turtles are known to eat all three species (e.g. Ballorain et al., 2010). Green turtles exhibit a combination of selective and opportunistic feeding (e.g. Arthur & Balazs, 2008) and as the biomass of Halodule uninervis in this seagrass area is much higher compared to the other seagrass species (Christianen et al., 2012), it has a great nutritional value (e.g. Arthur & Balazs, 2008) and is relatively easy to digest compared to most other topical seagrass species, (Aragones 1996; Sheppard et al. 2007; de Iongh et al., 2007), it logically ensues that Halodule uninervis performs the main component of the green turtles’ diet in this area.

Behaviour, activity pattern and substrate use of green turtles 19

5. Conclusion Based on this study, time and location-specific risks can be identified and mitigation strategies can be applied to maximize the protection of green turtles and seagrass, while minimising constraints to human activities. Tourism in this area has increased over the last century, just as the number of speedboats, rubbish and pollution. This study showed that the turtles prefer to be on the seagrass during the day and therefore, they have a greater exposure to human activities that are typically concentrated close to the shore. Speed boat traffic over the meadow and waste disposal should be limited, just as anchor damage and other physical disturbances to the seagrass.

The main problem in this system is that the green turtle population size around Derawan island is currently close to or above the estimated carrying capacity of the seagrass meadow and is still increasing, while migration possibilities to nearby foraging areas are limited (Christianen et al., in prep.). This study showed that green turtles adapt their behaviour to limited food availabilities, but the consequences of these behavioural responses are still unknown. The combination of an intense grazing pressure and physical disturbances of the below-ground part of the seagrass by green turtles may lower the recovery and growth rate of the seagrasses and consequently, the carrying capacity of the meadow as a whole. Particularly when erosion of the remaining seagrass in between gaps occurs, the seagrass system may pass a tipping point, which may lead to an irreversible state transition to a system with a very low biomass and productivity (“barren state”) (e.g. Eklöf et al. 2009; Christianen et al., in prep.). This can lead to an eventual loss of (part of) the seagrass bed, with disastrous consequences for the green turtle population. Therefore, we stress that research on the current state, resilience and early warning signals of such dynamic and complex systems as this green turtle-seagrass system is urgently needed, to be able to take action accordingly.

Acknowledgement I want to thank all my supervisors (Marjolijn J.A. Christianen, Dr. Marieke M. van Katwijk, Dr. Rudi M. Roijackers) for their guidance within this project and my fieldpartners and dive buddies (Peter van Leent, Jelco van Brakel, Sara Lambrecht, Hans Wolkers, Ger van Leent, Gerard/Inge/Martijn de Winter) for their support in the field and Anne Baauw for her support during the writing process. Furthermore, I want to thank Ibu Heldi, Combodia and Salsia and Pak Dirman, John, Udin and Wawan for their local and logistic support and the following funds for their financial support: Dr. Hendrik Muller, Dr. Christine Buisman, Alberta Mennega and the FONA. Thank you all for enabling this project.

Behaviour, activity pattern and substrate use of green turtles 20

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Appendix 1 Tidal table Table Tidal table with water heights in meters. The hours of the day can be found in the first row, while the days of the month can be found in the first column (Teluk Sangkulirang)

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Appendix 2 Moon Calendar Table Moon calendar for East-Kalimantan. All times are in local time for Balikpapan, (retrieved from: http://www.timeanddate.com/calendar/moonphases.html?n=434, visited: 12/2011) Meridian Passing Date Moon- Moonset Time Altitude Distance Illuminated Phase rise (km) 1 Jan 2012 11:59 - 18:08 78.8° 403,265 51.7% First Quarter at 14:14 2 Jan 2012 - 00:17 18:51 74.7° 404,486 61.3% 12:42 - 3 Jan 2012 - 01:00 19:35 71.1° 404,339 70.5% 13:26 - 4 Jan 2012 - 01:44 20:21 68.3° 402,918 78.9% 14:12 - 5 Jan 2012 - 02:31 21:10 66.3° 400,415 86.4% 15:00 - 6 Jan 2012 - 03:20 22:00 65.4° 397,096 92.5% 15:50 - 7 Jan 2012 - 04:11 22:52 65.6° 393,270 97.0% 16:42 - 8 Jan 2012 - 05:03 23:45 67.0° 389,250 99.5% 17:34 - 9 Jan 2012 - 05:55 Full Moon at 15:30 18:26 - 10 Jan 2012 - 06:48 00:37 69.7° 385,317 99.8% 19:17 - 11 Jan 2012 - 07:39 01:28 73.3° 381,691 97.6% 20:07 - 12 Jan 2012 - 08:29 02:18 77.9° 378,517 93.1% 20:56 - 13 Jan 2012 - 09:19 03:07 83.0° 375,863 86.2% 21:45 - 14 Jan 2012 - 10:08 03:56 88.4° 373,731 77.4% 22:33 - 15 Jan 2012 - 10:58 04:46 84.1° 372,086 67.0% 23:23 - 16 Jan 2012 11:50 05:37 79.0° 370,891 55.6% Third Quarter at 17:08 17 Jan 2012 00:16 12:45 06:30 74.5° 370,144 43.8% 18 Jan 2012 01:10 13:41 07:26 70.9° 369,900 32.3% 19 Jan 2012 02:08 14:40 08:24 68.6° 370,267 21.7% 20 Jan 2012 03:07 15:40 09:23 67.8° 371,373 12.8% 21 Jan 2012 04:06 16:38 10:22 68.5° 373,321 6.0% 22 Jan 2012 05:04 17:34 11:19 70.7° 376,140 1.8% 23 Jan 2012 05:59 18:27 12:13 74.0° 379,754 0.1% New Moon at 15:39 24 Jan 2012 06:51 19:17 13:04 78.2° 383,978 1.1% 25 Jan 2012 07:40 20:03 13:51 82.8° 388,534 4.4% 26 Jan 2012 08:25 20:47 14:36 87.8° 393,080 9.6% 27 Jan 2012 09:10 21:30 15:20 85.5° 397,248 16.4% 28 Jan 2012 09:53 22:12 16:02 80.8° 400,688 24.5% 29 Jan 2012 10:36 22:55 16:45 76.5° 403,095 33.4% 30 Jan 2012 11:20 23:38 17:29 72.7° 404,246 42.8% 31 Jan 2012 12:05 - 18:14 69.6° 404,016 52.5% First Quarter at 12:10

1 Feb 2012 - 00:24 19:01 67.2° 402,395 62.1% 12:52 - 2 Feb 2012 - 01:11 19:50 65.8° 399,489 71.5% 13:40 - 3 Feb 2012 - 02:00 20:41 65.5° 395,519 80.1% 14:31 - 4 Feb 2012 - 02:51 21:33 66.3° 390,805 87.7%

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Meridian Passing Date Moon- Moonset Time Altitude Distance Illuminated Phase rise (km) 15:23 - 5 Feb 2012 - 03:44 22:25 68.4° 385,741 93.8% 16:15 - 6 Feb 2012 - 04:36 23:18 71.6° 380,755 97.9% 17:07 - 7 Feb 2012 - 05:29 17:58 - 8 Feb 2012 - 06:20 00:09 75.8° 376,252 99.8% Full Moon at 05:54 18:49 - 9 Feb 2012 - 07:11 01:00 80.8° 372,566 99.0% 19:39 - 10 Feb 2012 - 08:02 01:51 86.3° 369,915 95.5% 20:29 - 11 Feb 2012 - 08:54 02:41 86.1° 368,384 89.4% 21:20 - 12 Feb 2012 - 09:47 03:33 80.8° 367,929 81.0% 22:12 - 13 Feb 2012 - 10:41 04:27 76.0° 368,409 70.8% 23:07 - 14 Feb 2012 11:37 05:22 72.1° 369,633 59.4% 15 Feb 2012 00:03 12:35 06:19 69.4° 371,413 47.7% Third Quarter at 01:04 16 Feb 2012 01:01 13:33 07:17 68.1° 373,603 36.1% 17 Feb 2012 01:59 14:31 08:15 68.3° 376,118 25.5% 18 Feb 2012 02:56 15:27 09:12 69.9° 378,921 16.3% 19 Feb 2012 03:51 16:19 10:05 72.7° 381,998 8.9% 20 Feb 2012 04:43 17:09 10:56 76.4° 385,328 3.7% 21 Feb 2012 05:32 17:56 11:44 80.8° 388,854 0.8% 22 Feb 2012 06:19 18:41 12:30 85.6° 392,471 0.3% New Moon at 06:35 23 Feb 2012 07:03 19:24 13:14 87.7° 396,017 1.9% 24 Feb 2012 07:47 20:07 13:57 82.9° 399,277 5.5% 25 Feb 2012 08:30 20:49 14:40 78.5° 402,001 10.8% 26 Feb 2012 09:14 21:33 15:24 74.4° 403,924 17.6% 27 Feb 2012 09:59 22:17 16:08 71.0° 404,807 25.5% 28 Feb 2012 10:45 23:04 16:54 68.3° 404,464 34.4% 29 Feb 2012 11:32 23:52 17:42 66.5° 402,795 43.9%

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Appendix 3 Dive and snorkel transects

Figure Aerial photograph of the overview of the dive and snorkel transects in front of the coastline of Derawan island (made by M. Christianen, 02/2012). The solid line indicates the first part of both the dive and snorkel transect over the seagrass and coral. The dive transect continues (not included in the picture) along the coral reef crest, in total covering 247 m of seagrass and 402 m of coral. The second part of the snorkel transect is indicated with dashed lines. The total snorkel transects covers 509 m of seagrass and 269 m of coral.

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Appendix 4 Output transformation depth sensor

The output of the depth sensor (sensus pro, ReefNet inc.) was in Pascal and therefore, we needed to transform this output to obtain the water height in meters by applying the following equations:

( ) ater height (m) = = 1) W ( ) ( ) P fuid Ptotal − Patmosphere

ɾ salt water ∗ g ɾ salt water ∗ g -3 -2 With Patmosphere= 1013 hPa (measured with depth sensor), ɾ= 1024 kgm and g= 9.81 ms

Behaviour, activity pattern and substrate use of green turtles 29

Appendix 5 Video-stills of uncommon behaviour

Figure a Series of video-stills of a green turtle that is digging a gap (made bij M.J.A. Christianen, 30- 05-2008).

Figure b Video-still of a green turtle feeding in an already existing gap. Note the second gap on the left side of the picture (made by I.I. de Winter, 20-02-2012).

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