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9th December 2011

Abu N. M. Hossain, Petra Lahann, Adam C. D. Barlow, Md. Anwarul Islam, Christina J. Greenwood, Ishtiaq U. Ahmed

BANGLADESH SUNDARBANS RELATIVE TIGER ABUNDANCE SURVEY

TECHNICAL REPORT

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ACKNOWLEDGEMENTS

The 2010 and 2011 surveys were conducted by the Bangladesh Forest Department (FD), Wildlife Trust of Bangladesh (WTB), and the Zoological Society of London (ZSL). The funds for this survey were provided by the United States Fish and Wildlife (USFWS) service and ZSL.

We are grateful for the permission to carry out these surveys granted by the Ministry of Environment and Forest. We are also particularly grateful to the support of the following FD staff: Ishtiaq U. Ahmad (CCF), Md. Akbar Hossain (CF, Khulna), Dr Tapan K. Dey (CF, Wildlife Circle), Aboni Bhushon Thakur (DCF), Zahir Uddin Ahmad (DCF), Mihir Kumar Tho (DFO), Rajesh Chakma (ACF), Bipul Krishna Das (ACF), Zahirul Haque (ACF), Towfiqul Islam (ACF), and Md Shahabuddin (Forest Ranger).

The field work and data collection would not have been possible without the hard work of Mijanur Rahman, Alam Hawladar, Abul Kalam, Md Zihad, and Md Nurul Alam. We are also grateful to our boat cook Montu.

We would like to acknowledge Anwarul Islam, Tommaso Savini, George A. Gale, and J.L. David Smith for their support and supervision.

Finally, we would like to thank Yadvendradev Jhala (Wildlife Institute of India), John Goodrich (Wildlife Conservation Society), Linda Kerley (ZSL), Ben Collen (ZSL), and Nathalie Pettorelli (ZSL) for their scientific review and helpful comments on former drafts of this report.

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EXECUTIVE SUMMARY

Starting in 2007, a tiger monitoring programme has been developed and successfully implemented for the Sundarbans Reserved Forest (SRF) of Bangladesh. The aim of this monitoring programme is to periodically collect data on the relative tiger abundances in the four ranges of the SRF by recording the number of tiger track sets along the banks of creeks (khals). Based on these data, general trends and short-term changes in the SRF tiger abundance can be identified, which helps to evaluate the effectiveness of tiger conservation strategies, and to identify forest areas in need of additional protection measures.

The first survey was conducted in 2007, followed by a second survey in 2009. In this report we present new findings from the 2010 and 2011 surveys and compare these results with the data sets from 2007 and 2009.

The 2010 and 2011 surveys were both conducted over two months by three teams of two observers, with relative tiger abundances recorded for 65 sample units covering all four ranges of the SRF.

In 2010 a total of 675 tiger track sets were recorded from 1053 km of surveyed khals, with an average of 0.57 tiger track sets/km of khal surveyed/sample unit. In 2011 a total of 556 tiger track sets were recorded from 923 km of khals surveyed, with an average of 0.47 tiger track sets/km of khal surveyed/sample unit.

Between 2007 and 2011, the index of relative tiger abundance (tiger track sets/km of khal surveyed/sample unit) has declined by 47%. The highest decline in relative tiger abundance during this period was found in Sarankhola range (62% decline), followed by Chandpai range (61% decline), and Satkhira range (44% decline). Only Khulna ranges remained relatively stable in relative tiger abundance. If the current trend continues, there is a risk that tigers become locally extinct within the next 5-10 years.

The reasons for this overall rapid and steep decline in relative tiger abundance have not been identified, but we speculate that this may have been caused by tiger/prey poaching or tiger/prey diseases.

This study highlights the importance of assessing the threats to tigers, prey, and habitat. Of particularly high priority for research is the need to further assess tiger poaching, prey poaching, tiger disease and prey disease.

While research and monitoring is continuing, it is essential to immediately start management activities that are likely to have an impact on reversing the decline in relative tiger abundance. Considering the possible causes of decline, we strongly recommend that the Bangladesh government carry out targeted enforcement measures to tackle tiger and prey poaching, consumption and trade and also to reduce the number of domestic animals inside the SRF thereby reducing disease transmission potential.

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CONTENTS

1. INTRODUCTION .................................................................................................. 4 2. METHODS ............................................................................................................ 5 2.1. Study area ............................................................................................................ 5 2.2. Data collection .................................................................................................... 5 2.2. Data analysis ....................................................................................................... 6 3. RESULTS .............................................................................................................. 7 3.1. Relative tiger abundance for 2010 and 2011 ..................................................... 7 3.2. Comparison of relative tiger abundance between years and ranges ............. 7 4. DISCUSSION ........................................................................................................ 9 4.1. Change in relative tiger abundance ................................................................... 9 4.2. Possible causes of decline in relative tiger abundance ................................ 10 4.3. Management recommendations ...................................................................... 12 References ................................................................................................................ 13 Tables........................................................................................................................ 15 Figures ...................................................................................................................... 16 APPENDIX 1: 2007, 2009, 2010 and 2011 survey results ...................................... 25

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1. INTRODUCTION

The Sundarbans of Bangladesh and India have probably one of the largest tiger (Panthera tigris) populations in the world (Barlow 2009). As outlined by the Bangladesh Tiger Action Plan, a monitoring programme to track changes in the tiger population is needed to help evaluate the impact of conservation activities (Ahmed et al. 2009). A stable or increasing tiger population would be an indication of success, whereas a decreasing population would be a cause for alarm, requiring additional and threat specific management interventions (Barlow et al. 2008, Barlow et al. 2009a).

Counting tigers in any habitat is notoriously difficult due to their elusive nature and low natural densities. The Indian pug mark method has been used in both the Bangladesh and Indian side of the Sundarbans but the results of this method are not accepted by the scientific community; among a wide range of problems, there is no account taken of tigers that are present but not detected and not all individual tigers can be correctly identified from their tracks (Karanth et al. 2003). Camera-trapping is a widely used technique for estimating tiger density, but it has been found to be an impractical approach in the Sundarbans (Karanth and Nichols 2000). Camera-trapping requires placing the camera-trap units along paths used by tigers but, because of frequent inundation, such paths are difficult to identify in the Sundarbans. Furthermore, even if this issue of camera trap placement could be overcome, monitoring change in the tiger population across the whole Sundarbans using camera-traps would be extremely expensive (Barlow et al. 2008).

Researchers in Russia developed a way to monitor changes in tiger abundance by counting the frequency of tiger tracks along transects (Hayward 2002). This approach was modified and applied to Sundarbans Reserve Forest (SRF) to give the first ever map of relative tiger abundance for this area (Barlow et al. 2008, Barlow et al. 2009a). The Sundarbans approach relies on counting tiger track sets along the banks of khals (creeks) to give an index of relative tiger abundance for each of 65 sample units covering the whole SRF with its four ranges (Fig. 1 and 2). The survey does not record the number of individual tigers because there are no direct evidence from the SRF to quantify the relationship between the number of tiger track sets/km of khal surveyed and the actual tiger population number. However, there is strong evidence from a large-scale survey in India, that the frequency of tiger tracks is a very good indication of the actual tiger population (Jhala et al. 2011).

The objectives of this study were to (1) map the current relative tiger abundance across the SRF and to (2) investigate if there have been any changes/trends in relative tiger abundance between 2007 and 2011.

To achieve the objectives we conducted khal surveys in 2010 and 2011, using a comparable methodology to the 2007 and 2009 surveys. The data set offers us the first opportunity to access tiger abundance over a 5 year time period to investigate trends in the SRF tiger population and to evaluate the effectiveness of conservation measures.

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2. METHODS

2.1. Study area

The 6017 km2 SRF has three areas declared as Wildlife Sanctuaries (West, South, and East), covering a total area of 1397 km2 (Fig. 1). The SRF is administered by the Bangladesh Forest Department (FD) and split into two divisions, four ranges, and 55 compartments. Each division (East and West) is overseen by a Divisional Forest Officer (DFO). The four ranges, each supervised by an Assistant Conservator of Forest (ACF), are Satkhira and Khulna (under the West Division), and Chandpai and Sarankhola (under the East Division) (Fig. 1). Activities within the forest are supervised by FD staff assigned to patrol posts.

2.2. Data collection

Data collection was carried out in February, 2010 and in February, 2011. Surveys were carried out at this time of year due to its low average precipitation rate, as rainfall may degrade tiger tracks at a faster and less uniform rate than by tidal action alone (Barlow et al. 2008).

Surveys were conducted by three teams of two observers. Each team was led by a person had experience in detecting tiger signs in the previous 2007 and 2009 surveys. Training on safety and standardized data collection was carried out in the first three days of field work for each survey. During the survey, observers were swapped between teams on a daily basis to reduce observer bias.

The number of tiger track sets/km of surveyed khal was used as an index of relative tiger abundance in the SRF. Tigers make deep, distinctive tracks in the soft mud bank of khals. A tiger track set was classified as a group of tiger tracks going up or down a khal bank. Tiger tracks can be confidently identified as tiger sign for up to a mean 10 days (range = 6–14) after they have first been made, after which they become too degraded to be recognizable (Barlow et al. 2008). Occupying a home range of 10-14 km², a female tiger moves on average 1.65-1.72 km/day with a maximum of 10-11.3 km/day (Barlow 2009). Distances moved include traversing both terrestrial habitat and waterways; a SRF female tiger crosses a khal 10-15 times/month (equivalent to one crossing every two to three days (Barlow 2009). This frequent rate of water way crossing suggests that a monitoring survey, based on track counts along creek banks, would have a high chance of detecting tiger presence.

To improve spatial definition during the survey, the Sundarbans was delineated into sample units. To allow for detection of changes in abundance (rather than presence/absence), sample units need to be large enough so that they can be occupied by several tigers. We used the current framework of 55 management compartments to delineate sample units. Compartments <40 km2 were joined to adjacent units and those >100 km2 were split along watercourses. A total of 65 sample units was created with a mean area of 63.3 ± 14.1 km2 (range = 40–100 km2), which means they are large enough to encompass at least more than one female tiger territory (Barlow et al. 2008).

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For the 2007 and 2008 surveys, a 1.7 km width buffer (calculated from the mean straight line distance moved/day by two satellite collared tigers) around all surveyed khals, was used to assess the survey coverage of each sample unit (Barlow et al. 2008, 2011). To ensure that all tigers present would have some probability of being detected, the aim of khal selection was to have no areas of >20 km2 not surveyed; which would represent an approximate home range of an adult female tiger (Barlow et al. 2008).

For the 2010 and 2011 surveys we selected a smaller number of khals to survey for each sample unit, but surveyed each sample unit twice, using the accumulated total km of khal surveyed and the total number of tiger tracks to calculate the number of tiger track sets/km of khal surveyed/sample unit.

Because the width of a waterway may at some stage become a barrier to normal tiger movements within a home range, 50 m was selected as a maximum width of khal to be surveyed. Khals were checked approximately three hours either side of low tide when tiger tracks below the high water mark could be detected in the soft mud of the khal banks. To ensure high track detection rate, the boat was kept between 1 and 3 m from the khal bank with a speed of 4–6 km/h, and all obscure signs were checked by inspecting the suspected tracks on foot. Only one side of each khal was surveyed. All tiger track sets and khals were marked using Geographical Positioning Systems (Barlow et al. 2008).

2.2. Data analysis

The number of tiger track sets/km of surveyed khals was compared between ranges and years after first log transformation to normalize the data. Data were tested for normality using the Shapiro-Wilk test for normality. Since data were not normally distributed, median values, quartiles (and ranges) are presented. Data were tested for relations between relative tiger abundance and length of surveyed khals with the Spearman's rank correlation test. The non parametric Kruskal–Wallis test was used to compare median tiger track sets per kilometre of survey (a relative measure of tiger abundance) between ranges and years. Trends in SRF relative tiger abundance were calculated with a non linear (polynomial) regression model using the method of least squares. The Cox regression model was used to determine the time point of reaching zero in relative tiger abundance. Statistical significance was accepted for p < 0.05 for all tests. All statistical tests were carried out with SYSTAT 11.

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3. RESULTS

3.1. Relative tiger abundance for 2010 and 2011

In 2010 a total of 1053 km of khals were surveyed in the SRF and 675 tiger track sets were recorded, giving a median 0.57 tiger track sets/km of khal surveyed/sample unit (n = 65, quartiles = 0.15/1.08, range = 0.00-2.64).

In 2010 the highest relative tiger abundance was found in Satkhira range with a median 1.19 tiger track sets/km of khal surveyed/sample unit (n = 19, quartiles = 0.77/1.66), followed by Khulna range with a median 0.60 tiger track sets/km of khal surveyed/sample unit (n = 18, quartiles = 0.28/1.05). The lowest relative tiger abundance was found in Sarankhola range, with a median 0.29 tiger track sets/km of khal surveyed/sample unit (n = 12, quartiles = 0.10/0.40), and Chandpai range with a median 0.12 tiger track sets/km of khal surveyed/sample unit (n = 17, quartiles = 0.09/0.49).

In 2011 a total of 923 km of khals were surveyed in the SRF, and 556 tiger track sets were recorded (Fig. 3), giving a median 0.47 tiger track sets/km of khal surveyed/sample unit (n = 65, quartiles = 0.16/0.96, ranges = 0.00-3.03).

In 2011 the highest relative tiger abundance was found in Satkhira range with a median 0.94 tiger track sets/km of khal surveyed/sample unit (n = 19, quartiles = 0.44/1.21), followed by Khulna range with a median 0.50 tiger track sets/km of khal surveyed/sample unit (n = 18, quartiles = 0.16/0.71). The lowest relative tiger abundance was found in Sarankhola range, with a median 0.29 tiger track sets/km of khal surveyed/sample unit (n = 12, quartiles = 0.18/0.84) and in Chandpai range, with a median 0.26 tiger track sets/km of khal surveyed/sample unit (n = 17, quartiles = 0.07/ 0.55).

3.2 Comparison of relative tiger abundance between years and ranges

Whereas the number of sample units (65) remained the same for all years, the total length of khals surveyed and the number of recorded tiger track sets differed slightly between years (Tables 1 and 2). In 2010 and 2011 the length of khals surveyed was 17-23% lower than in 2007 and 2009 (Table 1). Further analyses showed no positive correlations between the lengths of khals surveyed and relative tiger abundances for the sample units. In contrast, for 2011 we detected a low negative correlation (r = -0.31, p < 0.05) indicating that in sample units with reduced khal survey lengths relative higher tiger abundances were found.

The comparison of tiger track sets/km of khal surveyed/sample unit, revealed a highly significant 47% decline in the relative tiger abundance from 2007 to 2011 (df = 3, χ2 =

28.26, p < 0.001) (Table 2, Fig. 4). The 2007 median 0.88 tiger track sets/km of khal surveyed/sample unit (n = 65, quartiles = 0.48/1.62) initially increased in 2009 to a median 1.1 tiger track sets/km of khal surveyed/sample unit (n = 65, quartiles = 0.54/2.07), but then decreased in 2010 to a median 0.57 tiger track sets/km of khal surveyed/sample unit (n = 65, quartiles = 0.15/1.08), and further decreased in 2011 to

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a median 0.47 tiger track sets/km of khal surveyed/sample unit (n = 65, quartiles = 0.16/0.96) (Fig. 4).

Results of the trend and survival analysis suggested that if the current trend continues, there is a high risk that tiger become locally extinct within the next 5-10 years.

Comparing the relative tiger abundances for the different ranges between years, we found significant declines in relative tiger abundances in three of four ranges (Table 3, Fig. 4).

The highest decline was found in Sarankhola range, where the median 0.75 tiger track sets/km of khal surveyed/sample unit (n = 19, quartiles = 0.50/1.20) in 2007 decreased to a mean 0.29 tiger track sets/km of khal surveyed/sample unit (n = 19, quartiles = 0.18/0.84) in 2011 (df = 3, χ2 = 12.38, p < 0.01). This is equivalent to an overall 62% decline in relative tiger abundance between 2007 and 2011 in the Sarankhola range.

In Satkhira range, the relative tiger abundance decreased from a median 1.67 tiger track sets/km of khal surveyed/sample unit (n = 19, quartiles = 1.10/1.96) in 2007, to a median of 0.94 tiger track sets/km of khal surveyed/sample unit (n = 19, quartiles = 0.44/1.21) in 2011 (df = 3, χ2 = 12.38, p < 0.01). This is equivalent to an overall 44% decline in relative tiger abundance between 2007 and 2011 in Satkhira range.

Chandpai range, the relative tiger abundance decreased from a median 0.67 tiger track sets/km of khal surveyed/sample unit (n = 3, quartiles = 0.30/1.14) in 2007 to a median 0.26 tiger track sets/km of khal surveyed/sample unit (n = 17, quartiles = 0.07/0.55) in 2011 (df = 3, χ2 = 11.23, p < 0.05). This is equivalent to an overall 61% decline in relative tiger abundance between 2007 and 2011 in Chandpai range.

Only in Khulna range the relative tiger abundance remained stable over the last 5 years (n = 3, χ2 = 2.59, p = 0.46), but with consistently low relative tiger abundance compared to the other three ranges (Table 3).

As indicated in the last report from 2009 and now again confirmed by the two surveys in 2010 and 2011, the relative tiger abundances differ significantly between areas, with highest relative tiger abundance in Satkhira range and lower relative tiger abundance in Chandpai and Sarankhola ranges (Table 3, Fig. 5).

In terms of absolute declines from 2007 to 2011, we found the highest decline in recorded track sets/km of khal surveyed/sample units in the Satkhira range (Fig, 6, Fig. 7). In terms of percentage declines from 2007 to 2011, we found 14 sample units with decreases of relative tiger abundance of more than 75%, 19 sample units with a decline of 50-75% of relative tiger abundance, in 13 sample units a decline of 25-50%, and in 4 sample units a decline of up to 25%. 15 sample units showed no change, or an increase (Fig. 8).

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4. DISCUSSION

4.1. Change in relative tiger abundance

The survey results suggest that in the SRF from 2007 to 2011 there has been an overall 47% decline in relative tiger abundance, as indicated by the drop in the number of tiger track sets/km of khal surveyed. This is in contrast to the last report from 2009, which showed an increase in relative tiger abundance between 2007 and 2009 (Barlow et al. 2009a). If the current trend continues, there is a high risk that tiger become locally extinct within the next 5-10 years.

Currently, we do not have direct evidence from the SRF to quantify the relationship between the number of tiger track sets/km of khal surveyed and the actual tiger population number. However, there is strong evidence from a large-scale survey in India that the frequency of tiger tracks is a very good indication of the actual tiger population; this study found that the tiger track encounter rates from 21 sites explained 84% of the variability in the tiger densities for those sites (Jhala et al. 2011). This means that the changes in the index of relative tiger abundance recorded in the SRF survey is very likely to be a direct reflection in the change in the actual tiger population. However, it is important to consider sources of error in case these may account for or contribute to the detected trend in relative tiger abundance.

Error due to differences in sampling effort: One potential source of error is the differences in the length of khal surveyed between years; approximately 20% less km of khals were surveyed in the 2010 (1053 km) and 2011 (923 km), compared to 2007 (1201 km) and 2009 (1207 km). However, less sampling of khals would not lead to a measured decrease in relative tiger abundance, because the index of relative tiger abundance is calculated by dividing the tiger track sets by the km of khal surveyed. We found no positive correlation between km of khal surveyed/sample unit and number of tiger track sets recorded for 2007, 2009, and 2010 surveys. There was even a low negative correlation (2011) between length of khal surveyed and number of recorded tiger track sets/km of khal surveyed. Thus it seems unlikely that there is any substantial linkage between the reduced survey length in 2010 and 2011 and the lower relative tiger abundances recorded for those years.

Less km of khal surveyed may also lead to additional variance in the tiger track sets/km of khal recorded, which could decrease the power of the survey to detect a trend. However, in this case a significant trend was detected despite this potential source of higher variance, i.e. the size of the drop in tigers is too large to be attributed to scientific error. Furthermore, even in sample units where the length of khal surveyed stayed stable, the number of recorded tiger track sets decreased from 2007/2009 to 2010/2011. Therefore, the 47% overall decrease in recorded tiger track sets cannot be simply explained by a 20% decrease in the km of khals surveyed.

Error due to differences in rainfall: Another potential explanation for a reduced number of recorded tiger track sets might be a difference in the amount rainfall between years; higher rainfall may degrade tiger track sets faster, resulting in less tiger track sets available to record. A comparison of precipitation data showed little variation in rainfall between years, in fact there were more rainy days during the 2007

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survey period compared to the 2010 and 2011 survey periods (Fig. 9). Therefore the overall decline in relative tiger abundance between years cannot be attributed to differences in rainfall.

Error due to observer bias: A change between years in the number of recorded tiger track sets/km of khal surveyed also could potentially be explained by observer bias, but this is unlikely considering that the same observers took part in all surveys. The observers’ abilities to detect tiger tracks could also potentially change over time, but it would be more likely that the observers’ tiger track detection would improve over time, which would contribute to a positive (rather than negative) trend in relative tiger abundance.

Therefore, although there are some potential sources of error, we feel they are unlikely to account by any substantial degree for the overall 47% decline in tiger track sets/km of khal surveyed/sample unit between 2007 and 2011. It can therefore be concluded that this decline is a reflection of a true decline in relative tiger abundance, and also of a true decline in the actual SRF tiger population.

4.2. Possible causes of decline in relative tiger abundance

We do not know definitively the reasons for the decline in the SRF relative tiger abundance, but the steep and rapid nature of this decline suggests that it is attributable to threats which can have high impact over a short time i.e. tiger poaching, prey poaching, and or disease in either tiger or prey. These along with other possible causes are discussed below.

Natural fluctuations in the population: Studies found that natural fluctuations in a small tiger population (7-9 breeding adults) can be recorded as a result of changes in the number of non-breeding adults, even when the number of breeding adults has remained stable (Barlow et al. 2009a; Kerley et al. 2003). However, the large fluctuations in abundance due to the synchrony of litter birth in a small population are likely to be smoothed out in a larger tiger population like that in the SRF (Barlow et al. 2009b), so this effect is very unlikely to explain the overall decline in relative tiger abundance recorded in this study.

Tiger poaching: Tiger poaching is driven in part by the traditional Asian medicine trade is documented as the most important threat to tiger populations in nearly all landscapes where they occur (Nowell 2000; Newman 2004; Shepherd and Nolan 2004; Sanderson et al. 2006; Nowell and Ling 2007). Recently Verheij et al. (2010) showed that tiger poaching and an increasing illegal trade in tiger parts in recent years is greatly contributing to the current rapid worldwide decline of tigers in the wild. There is evidence of tiger poaching in the Bangladesh Sundarbans area: in early 2011 a poacher was caught with three tiger skins and four tiger skulls in a village adjacent to the SRF, in September 2011 fishers reported that they had observed three tiger skulls and a lot of tiger and deer bones on a pirate boat (VTRT, personal communication), and in December 2011 another poacher was caught with one tiger skin in a village adjacent to the SRF. Given that most incidents are unlikely to be detected due to the secretive nature of poaching, this is likely indicative of a larger poaching problem.

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Prey poaching: A recent study on the scale of deer consumption in the village areas immediately adjacent to the SRF suggests that >10000 deer are consumed/year (Mohasin et al. 2010). Tiger prey animals, such as deer, may be poached from any part of the SRF due to the widespread distribution of forest users and the current limited protection capacity of the FD. It is therefore reasonable to assume that the current poaching level of deer may have a serious impact on the prey population, which in turn would have a negative impact on the tiger population. Followed by tiger poaching, the excessive poaching of key prey species is known to contribute to a high amount to the current worldwide decline in tiger numbers (Verheij et al. 2010).

Disease: It also may be possible that the decline in the SRF tiger population was caused by the outbreak of disease. Tigers are potentially at risk from a variety of pathogens transmitted by domestic animals; captive tigers have died from avian influenza and captive and wild tigers have died from canine distemper (Appel and Summers 1995; Myers et al. 1997; Keawcharoen et al. 2004; Goodrich et al. in press; Quigley et al. 2010). Potential sources of disease are domestic cats, which are kept in FD guard posts throughout the forest to control vermin, and some posts also keep chickens (Hossain et al. 2009). Domestic animals are also present in the villages along the northern and eastern boundaries of the forest and here especially dogs might be a critical source of canine distemper for tigers. Livestock enters inside the forest in some areas and some tigers predate on livestock in both forest and villages areas (Rahman et al. 2010). Prey animals could also contract diseases introduced by domestic animals; in some northern parts of the forest deer share habitat with cows, buffalo, goats, and dogs (Rahman et al. 2010). However, the current information base on tiger and prey disease in the SRF is not sufficient to assess the risk and impact of this threat.

Natural disasters: Another possibility is that the tigers were negatively impacted by cyclones during the five year period. Cylone Sidr hit the SRF in November 2007 and resulted in substantial tree damage and inundation, particularly in the Sarankhola range of the SRF. Cyclone Aila hit the SRF in June 2009, mainly in the Satkhira range, and resulted in inundation but much less tress damage relative to cyclone Sidr. However, it is difficult to rationalise how temporary inundation would have a large impact on the tiger or prey populations, as both tiger and prey are adapted to inundations as a regular feature of the tidal mangrove ecosystem. The low impact of cyclones on tigers is supported by evidence from the surveys, which showed an increase in relative tiger abundance from January-February 2007 to January-February 2009 (when the surveys took place), despite cyclone Sidr hitting in November 2007. It therefore seems very unlikely that these cyclones are contributing in any meaningful way to the overall decline in the relative tiger abundance.

Loss and degradation of tiger habitat: Loss and or degradation of tiger habitat could also lead to a drop in the tiger population. However, due to the long-term nature of these habitat threats (e.g. sea level rise, overexploitation of forest resources, invasive species, encroachment) it is unlikely to create such a large decline in relative tiger abundance in such a short (5 year) time span.

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4.3. Management recommendations

This study highlights the importance of assessing the threats to tigers, prey, and habitat as outlined in the BTAP Research Agenda (Barlow et al. 2011). Of particularly high priority are the threats of tiger poaching, prey poaching, tiger disease and prey disease, which all need to be further assessed to better understand and mitigate the decline in relative tiger abundance.

These threats cannot be fully assessed, however, without being able to monitor the tiger and prey populations over time. A tiger monitoring system is in place, but is scheduled to be carried out every two years. In times like these of substantial decline, we recommend yearly surveys until the relative tiger abundance has reached at least its 2007 level. This would mean carrying out the next survey in 2012. Management would also benefit from an additional tiger monitoring approach, to verify the khal survey and provide an additional means to detect trends and identify emerging threats. Based on these studies, suitable tiger management protection strategies should be develop to, at least, stabilize the tiger population and to prevent further declines.

While research and monitoring is going on to better assess the threats to the tiger population, it is essential to start management activities that are likely to have an impact on reversing the decline in relative tiger abundance. Considering the possible causes of decline, we strongly recommend the immediate implementation of the following:

• The FD to carry out additional patrolling measures in the SRF to combat tiger and deer poaching.

• The FD to set up a wildlife crime hotline to help collect anonymous information on deer and tiger poaching in the SRF.

• The Bangladesh Government to set up a special taskforce to combat tiger and deer poaching, consumption, and trade.

• The Bangladesh Government to set up a cross-boundary taskforce with the Indian government to combat tiger and deer poaching, consumption, and trade.

• The FD to update the Wildlife Act to contain stiffer penalties for deer and tiger poaching, consumption, and trade. The penalties must be in line with market prices and be expanded to include compound and repeat offences.

• The FD to remove and prevent entrance of all domestic animals inside the SRF to combat the risk of disease transmission to tigers and prey.

• The Government and NGO partners to carry out awareness campaigns to tackle the social drivers of tiger and prey poaching and consumption.

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Appel, M. J. G., and B. A. Summers. 1995. Pathogenicity of morbilliviruses for terrestrial carnivores. Veterinary Microbiology 44: 187-191.

Barlow, A. C. D., M. I. U. Ahmed, M. M. Rahman, A. Howlader, A. C. Smith, and J. L. D. Smith. 2008. Linking monitoring and intervention for improved management of tigers in the Sundarbans of Bangladesh. Biological Conservation 141: 2031-2040.

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Barlow, A. C. D., Chakma, S., Hossain, A. N. M.. Rahman, M., Howlader, C. J Greenwood, A. Islam, I. U. Ahmed, and J. L. D. Smith. 2009a. Bangladesh Sundarbans relative tiger abundance survey report. Wildlife Trust of Bangladesh.

Barlow, A. C. D., C. McDougal, J. L. D. Smith, B. Gurung, S. R. Bhatta, S. Kumal, B. Mahato, and D. B. Tamang. 2009b. Temporal variation in tiger (Panthera tigris) populations and its implications to monitoring. Journal of Mammalogy 90: 472-478.

Barlow, A. D, C. J. Greenwood, M. A. Aziz, I. U. Ahmed, K. D. Tapan, H. Akbar, M. Islam, and A. Islam. 2011. Research Agenda 1.9. Bangaldesh Tiger Action Plan 2009-2017. Bangladesh Forest Department, Ministry of Environment and Forest, Government of the People’s Republic of Bangladesh, Dhaka, Bangladesh.

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TABLES

Table 1: Data collection 2007-2011

2007 2009 2010 2011

Number of sample units 65 65 65 65

Total length of khals surveyed in km 1201 1207 1053 923

Total number of recorded tiger track sets 1338 1652 675 556

Table 2: Comparison of relative tiger abundance survey data 2007-2011.

Year Median tiger track sets/km of khal surveyed/sample unit

Quartiles

2007 0.88 0.48/1.62

2009 1.11 0.54/2.07

2010 0.57 0.15/1.08

2011 0.47 0.16/0.96

Kruskal-Wallis Test χ2 = 28.26, df = 3, p < 0.001

Table 3: Comparison of relative tiger abundance (tiger track sets/km of khal surveyed/sample unit) survey data 2007-2011, separated for the four ranges

Tiger track

sets/km of khal

surveyed

Satkhira Khulna Chandpai Sarankhola Test between ranges, df = 3 Median Quartiles Median Quartiles Median Quartiles Median Quartiles

2007 1.67 1.10/ 1.96

0.54 0.25/ 1.39

0.67 0.30/ 1. 14

0.75 0.50/ 1.20

χ2=16.97, p<0.01

2009 2.22 1.31/ 3.10 0.85 0.45/

1.26 0.59 0.25/ 1.75 1.10 0.71/

1.79 χ

2=16.84, p<0.01

2010 1.19 0.77/ 1.66 0.60 0.28/

1.05 0.12 0.09/ 0.49 0.29 0.10/

0.40 χ

2=28.35, p<0.001

2011 0.94 0.44/ 1.21

0.50 0.16/ 0.71

0.26 0.07/ 0.55

0.29 0.180/ 0.84

χ2= 8.68, p<0.05

Test between years, df = 3

χ2 = 23.81,

p < 0.001 χ

2 = 2.59, p = 0.459

χ2 = 11.23, p < 0.05

χ2 = 12.38, p < 0.01

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FIGURES

Figure 1. The administrative ranges, guard posts and wildlife sanctuaries of the SRF of Bangladesh

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Figure 2. Distribution of sample units

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Figure 3. Distribution of khals surveyed and Coverage (left) and recorded tiger track sets (right) in the SRF for the 2011 survey. Coverage was calculated using 1.7 km buffer around surveyed khals (Barlow et al. 2008).

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Figure 4. Overall trend of relative tiger abundance In the SRF from 2007 to 2011, shown as recorded tiger track sets/km of khal surveyed/sample unit. Illustrated are box plots with 50% medians, 25% and 75% percentiles (boxes). The red line indicates the 5-year trend.

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Figure 5. Range-specific trends in relative tiger abundance from 2007 to 2011, shown as recorded tiger track sets/km of khal surveyed/sample unit for each range. Illustrated are box plots with 50% medians, 25% and 75% percentiles (boxes). The red line indicates the 5-year trend.

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Figure 6. Recorded tiger track sets/km of khal surveyed/sample units for the 65 sample units in different years.

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Figure 7. Level of absolute decline in reported tiger track sets/km of khal surveyed/sample unit from 2007 to 2011. The decline in tiger track sets/km of surveyed ranges from no decline (white) to declines of up to 0.5 (rose), 0.5-1 (red), 1-1.5 (dark red) and > 1.5 (black).

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Figure 8. Level of relative decline in tiger abundance from 2007 to 2011, measured in percentage of loss.

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Figure 9. Monthly precipitation in mm (bars) and number of rainy days for January and February between 2007 and 2011.

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APPENDIX 1: 2007, 2009, 2010 and 2011 survey results

2007 2009 2010 2011

Sample Unit

Area (km²)

Khal surveyed

(km)

Tiger track sets

Tiger track

sets/km of khal

Khal surveyed

(km)

Tiger track sets

Tiger Tracks sets/km of khal

Khal surveyed

(km)

Tiger track sets

Tiger Track

sets/km of khal

Khal surveyed

(km)

Tiger track sets

Tiger Track

sets/km of khal

1 82.07 16.38 15 0.92 13.85 3 0.22 33.47 13 0.39 26.95 6 0.22

2 81.81 27.13 13 0.48 17.96 34 1.89 15.31 1 0.07 10.33 1 0.10

3 57.95 14.93 11 0.74 15.71 18 1.15 15.01 0 0.00 11.55 3 0.26

4 101.09 46.37 102 2.20 37.70 37 0.98 21.14 6 0.28 14.25 10 0.70

5 68.33 21.45 12 0.56 16.10 33 2.05 16.03 6 0.37 14.82 14 0.94

6 58.76 48.96 96 1.96 28.16 64 2.27 19.78 6 0.30 18.25 39 2.14

7 79.51 39.51 20 0.51 36.45 38 1.04 20.74 19 0.92 9.25 3 0.32

8 99.03 24.94 37 1.48 28.61 48 1.68 12.17 10 0.82 13.35 23 1.72

9 71.25 30.05 24 0.80 26.06 31 1.19 24.71 10 0.40 19.40 4 0.21

10 77.84 19.77 21 1.06 16.12 54 3.35 11.89 21 1.77 9.91 30 3.03

11 57.96 18.56 26 1.40 17.58 5 0.28 23.86 15 0.63 11.51 9 0.78

12 60.61 14.15 55 3.89 14.64 30 2.05 15.44 13 0.84 12.01 23 1.91

13 56.73 22.40 17 0.76 16.74 15 0.90 31.90 4 0.13 28.92 4 0.14

14 57.76 17.34 5 0.29 17.06 21 1.23 6.71 1 0.15 13.46 1 0.07

15 55.71 14.16 11 0.78 11.79 17 1.44 17.06 2 0.12 11.48 5 0.44

16 43.15 14.55 8 0.55 12.00 7 0.58 11.48 9 0.78 27.13 2 0.07

17 55.87 18.89 23 1.22 20.32 60 2.95 9.11 1 0.11 12.71 6 0.47

18 62.93 18.47 4 0.22 29.16 18 0.62 19.27 19 0.99 15.04 8 0.53

19 77.07 9.70 14 1.44 12.29 12 0.98 11.14 25 2.24 8.73 6 0.69

20 68.73 11.99 17 1.42 15.09 19 1.26 9.47 16 1.69 11.02 27 2.45

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2007 2009 2010 2011

Sample Unit

Area (km²)

Khal surveyed

(km)

Tiger track sets

Tiger Track

sets/km of khal

Khal surveyed

(km)

Tiger track sets

Tiger Track

sets/km of khal

Khal surveyed

(km)

Tiger track sets

Tiger track

sets/km of khal

Khal surveyed

(km)

Tiger track sets

Tiger Track

sets/km of khal

21 58.27 22.26 31 1.39 20.75 60 2.89 12.35 7 0.57 15.00 15 1.00

22 79.49 26.14 14 0.54 30.66 34 1.11 9.74 6 0.62 15.01 16 1.07

23 79.20 26.52 16 0.60 29.46 16 0.54 17.07 18 1.05 12.80 2 0.16

24 46.57 9.67 16 1.65 10.41 22 2.11 8.92 3 0.34 9.47 6 0.63

25 48.42 14.91 15 1.01 10.60 7 0.66 11.21 1 0.09 13.54 2 0.15

26 76.65 18.88 10 0.53 25.13 15 0.60 21.95 2 0.09 19.25 8 0.42

27 53.44 13.67 5 0.37 10.42 1 0.10 16.91 0 0.00 19.98 0 0.00

28 45.94 4.49 2 0.45 8.91 2 0.22 8.23 1 0.12 16.54 1 0.06

29 40.87 12.10 1 0.08 11.23 2 0.18 18.91 1 0.05 25.59 1 0.04

30 42.49 12.49 1 0.08 6.22 1 0.16 15.81 0 0.00 16.04 0 0.00

31 44.35 5.88 5 0.85 7.82 3 0.38 11.10 2 0.18 13.14 4 0.30

32 52.41 20.37 5 0.25 13.16 7 0.53 21.95 0 0.00 9.28 2 0.22

33 64.55 19.74 6 0.30 18.90 1 0.05 21.47 2 0.09 21.81 1 0.05

34 55.55 19.66 2 0.10 18.76 0 0.00 19.70 2 0.10 14.71 2 0.14

35 53.43 24.83 4 0.16 23.48 7 0.30 21.28 6 0.28 11.36 1 0.09

36 47.93 18.86 16 0.85 17.77 13 0.73 20.40 11 0.54 12.72 9 0.71

37 65.90 23.98 6 0.25 21.45 3 0.14 16.88 2 0.12 9.11 6 0.66

38 72.34 27.10 5 0.18 31.06 14 0.45 20.73 3 0.14 10.79 1 0.09

39 58.12 18.83 10 0.53 20.58 7 0.34 14.36 10 0.70 12.63 2 0.16

40 63.61 16.71 8 0.48 25.83 13 0.50 17.14 22 1.28 10.87 5 0.46

41 61.64 19.87 9 0.45 24.41 38 1.56 11.08 3 0.27 12.69 7 0.55

42 40.91 14.49 7 0.48 14.57 14 0.96 8.37 4 0.48 10.44 11 1.05

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2007 2009 2010 2011

Sample Unit

Area (km²)

Khal surveyed

(km)

Tiger track sets

Tiger track

sets/km of khal

Khal surveyed

(km)

Tiger track sets

Tiger track

sets/km of khal

Khal surveyed

(km)

Tiger track sets

Tiger Track

sets/km of khal

Khal surveyed

(km)

Tiger track sets

Tiger Track

sets/km of khal

43 71.48 13.03 21 1.61 23.36 41 1.76 8.59 6 0.70 11.33 11 0.97

44 71.80 18.33 20 1.09 17.96 33 1.84 11.12 19 1.71 15.83 6 0.38

45 46.98 5.02 11 2.19 3.17 8 2.52 7.67 4 0.52 12.89 5 0.39

46 64.68 17.00 15 0.88 13.20 13 0.98 10.63 10 0.94 14.72 5 0.34

47 86.09 28.97 102 3.52 28.15 36 1.28 19.65 21 1.07 19.74 7 0.35

48 71.67 25.21 2 0.08 23.72 12 0.51 23.48 4 0.17 13.68 10 0.73

49 41.42 13.29 13 0.98 14.56 11 0.76 27.85 21 0.75 15.16 11 0.73

50 70.10 19.74 20 1.01 18.96 19 1.00 32.71 14 0.43 12.50 5 0.40

51 56.03 13.30 15 1.13 9.76 22 2.25 20.25 19 0.94 10.27 13 1.27

52 48.23 12.24 8 0.65 10.73 12 1.12 19.80 22 1.11 11.54 13 1.13

53 80.72 12.61 38 3.01 23.77 79 3.32 10.44 13 1.25 8.45 19 2.25

54 69.72 11.56 17 1.47 12.05 47 3.90 14.39 26 1.81 15.00 11 0.73

55 45.94 16.44 37 2.25 14.68 53 3.61 9.77 20 2.05 12.67 17 1.34

56 53.69 11.49 20 1.74 14.22 24 1.69 13.11 11 0.84 8.91 11 1.23

57 60.47 19.39 38 1.96 16.09 27 1.68 22.22 32 1.44 16.02 21 1.31

58 60.83 18.22 57 3.13 18.53 53 2.86 14.03 16 1.14 15.82 11 0.70

59 69.18 17.29 31 1.79 22.07 26 1.18 11.24 17 1.51 10.47 6 0.57

60 48.75 15.32 30 1.96 15.89 39 2.45 8.32 22 2.64 15.76 16 1.02

61 82.70 14.83 39 2.63 29.00 84 2.90 9.92 17 1.71 10.44 10 0.96

62 55.26 19.02 27 1.42 18.11 15 0.83 20.03 13 0.65 12.19 1 0.08

63 69.63 11.69 8 0.68 19.84 44 2.22 5.05 6 1.19 10.79 1 0.09

64 78.96 7.30 13 1.78 12.49 41 3.28 17.08 25 1.46 15.63 12 0.77

65 81.62 18.59 31 1.67 21.76 69 3.17 24.54 14 0.57 20.43 8 0.39