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Reproductive and Trophic Ecology of an Assemblage of Aquatic and Semi-Aquatic Snakes in Tonle Sap, Cambodia

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Reproductive and Trophic Ecology of an Assemblage of Aquatic and Semi-Aquatic Snakes in Tonle Sap, Cambodia Copeia 2009, No. 1, 7–20 Reproductive and Trophic Ecology of an Assemblage of Aquatic and Semi-Aquatic Snakes in Tonle Sap, Cambodia Sharon E. Brooks1, Edward H. Allison2, Jennifer A. Gill3, and John D. Reynolds4 We studied the reproductive and trophic ecology of a group of aquatic and semi-aquatic snakes that face severe hunting pressure in Cambodia. Over a two-year period we sampled hunters’ catches, measuring and dissecting a total of 8982 specimens of seven snake species, five of which belong to the family Homalopsidae. The seven species—Enhydris enhydris, Enhydris longicauda, Homalopsis buccata, Enhydris bocourti, Erpeton tentaculatus, Xenochrophis piscator, and Cylindrophis ruffus—all inhabit Tonle Sap Lake, the largest lake in South-East Asia. All species are sexually dimorphic in either body size or tail length. The larger species, E. bocourti and H. buccata, have a larger size at maturity, and the non- homalopsids, X. piscator and C. ruffus, have the highest and lowest fecundities, respectively. Clutch size increases significantly with female body size in all species, and with body conditioninE. enhydris. Our data also suggest that relative investment in reproduction increases with size in E. enhydris, which has the largest sample size. All species except one are synchronized in their timing of reproduction with the seasonally receding flood waters of the lake. There was variation in both the frequency of feeding and the prey size and type among species, with the homalopsids more similar to one another than to the other non-homalopsid species. The prey to predator mass ratio ranged from 0.04 to 0.1 in the homalopsids, compared to 0.15 to 0.17 in the non-homalopsids. There was also variation in the feeding frequency between the sexes that differed between species and six species continued to feed while gravid. These detailed life history analyses can help provide a basis for assessing conservation options for these heavily exploited species. ONLESapLakeinCambodiaishometoan in the 1970’s (Saint Girons and Pfeffer, 1971; Saint Girons, assemblage of aquatic and semi-aquatic snakes that 1972). T are heavily exploited and traded as a food supply for Tonle Sap Lake is the largest wetland in South-East Asia the numerous crocodile farms surrounding the lake (Stuart and exhibits an extraordinary seasonal fluctuation in water et al., 2000). As a result of the growing demand from this level and size. As a result of the rising waters of the Mekong industry over the last 20 years, this exploitation now River during the monsoon season each year, the Tonle Sap represents the world’s largest snake hunting operation, with River reverses and Mekong water flows into the Tonle Sap an estimated 6.9 million snakes removed annually (Brooks Lake, inundating a large expanse of forest (Bonheur and et al., 2007a, 2007b), yet it proceeds without knowledge of Lane, 2002). This lake receives intense fishing pressure, and the basic population biology of the species involved. This it has been suggested that homalopsids thrive in such areas study aims to increase our understanding of the reproduc- due to reductions in size composition of fish communities, tive and trophic ecology of these snake populations as part increasing the snakes’ food supply while decreasing the of a program to assess the severity of this threat. abundance of their natural predators (Murphy, 2007). Five of the species in this community belong to the family However, in recent years, decreasing fish catches from the Homalopsidae. There is also a member of the Natricidae lake have led to human exploitation of several snake species, (Xenochrophis) and a member of the Cylindrophiidae primarily to provide food for the growing number of (Cylindrophis). Homalopsidae consists of approximately 37 crocodile farms, and some snake species now face the viviparous species distributed throughout South and South- likelihood of population declines (Brooks et al., 2007a). East Asia from India to North Australia (Gyi, 1970; Voris et While most of the species in this study are widely distributed al., 2002; Murphy, 2007). The greatest diversity and throughout the region, the Tonle Sap Water Snake, Enhydris abundance is in South-East Asia, where the snakes are longicauda, is endemic to the Tonle Sap Lake and River. The largely found in lowland freshwater habitats (Voris and status of this species has not been assessed, but its limited Karns, 1996). Homalopsids can comprise a large part of the distribution and high level of exploitation throughout its vertebrate biomass of the aquatic systems they inhabit range raises strong conservation concerns. (Voris and Karns, 1996), and there is speculation that some The extreme level of exploitation of snakes at this study species may thrive in areas dominated by human activities site allowed assessment of a large number of specimens that such as rice cultivation, which may provide suitable habitat had been killed during routine hunting operations. Here we and food supplies (Gyi, 1970; Murphy and Voris, 1994). explore life history variation within and among species While there is a growing literature on the ecology of throughout the year, to identify breeding seasons and to homalopsid species (Jayne et al., 1995; Voris and Karns, describe the ecological traits of this aquatic and semi-aquatic 1996; Murphy et al., 1999; Karns et al., 1999–2000), snake community. Inter-specific comparisons are included reviewed by Murphy (2007), the Cambodian populations as they can be useful for predicting how species will differ in have received little attention since the work of Saint Girons their response to exploitation. The large sample sizes 1 Department of Geography, University of Cambridge, CB2 3EN, Cambridge, U.K.; E-mail: [email protected]. Send reprint requests to this address. 2 The WorldFish Center, P.O. Box 500, GPO, 10670 Penang, Malaysia, and School of Development Studies, University of East Anglia, Norwich NR4 7TJ, U.K. 3 School of Biological Sciences, University of East Anglia, Norwich NR4 7TJ, U.K. 4 Department of Biological Sciences, Simon Fraser University, Burnaby, BC, V5A 1S6, Canada. Submitted: 20 April 2007. Accepted: 11 June 2008. Associate Editor: G. Haenel. F 2009 by the American Society of Ichthyologists and Herpetologists DOI: 10.1643/CE-07-102 8 Copeia 2009, No. 1 available for some species have allowed us to adopt some used as an indication, which we include here for compar- analytical approaches used by fish biologists for studies of ative purposes (Fitzgerald et al., 1993; Blouin-Demers et al., population dynamics. 2002). However, variation between individuals in size at maturity makes this method heavily sample-size dependent. MATERIALS AND METHODS We therefore also adopted a more representative and statistically robust method, which is widely used in fisheries research (Jennings et al., 2001), to estimate the length at Study site.—Biological monitoring programs were based at which 50% of the female population is mature (SVL50), Chong Khneas landing site in Siem Reap province on the based on a logistic regression of the length of mature versus northern side of Tonle Sap Lake in Cambodia. This is the immature females: most important location for snakes being transported from . { z the lake as a result of the high number of crocodile farms in PMðÞ~ 1 1 z e ðÞa bðÞSVL , Siem Reap province. where P(M) is the probability of being mature, a is the y- Study species.—We purchased a total of 8982 dead snakes intercept, and b is the coefficient for the predictor variable from traders on a weekly basis during two hunting seasons (SVL). The SVL when the probability of being mature is 50% that extended from June 2004 to March 2005 and June 2005 was then calculated as: to March 2006. Seven species that occur regularly in the ~ trade were included in the study, with sample sizes that vary SVL a=b: according to their abundance in catches. We measured (snout to vent length [SVL] and total length), weighed, and An alternative indicator of size is mass, which might be sexed 4356 Enhydris enhydris, 1634 Enhydris longicauda, 1609 more appropriate for comparisons among species, due to Homalopsis buccata, 141 Enhydris bocourti,869Erpeton differences in shape. As our mass measurements included tentaculatus, 234 Xenochrophis piscator, and 139 Cylindrophis gravid females, we could not use the above method to make ruffus. We analyzed each species for sexual dimorphism of direct calculations of the mass at which 50% of the female body mass, length, and tail length and for the relationship population is mature. Instead, we used length–mass rela- between body length and mass, which we square-root tionships within each species to convert the length at 50% transformed. Only non-reproductive females were used in female maturity to mass at 50% female maturity. analyses using mass, as the mass of the clutch could bias the We measured the right testis of dissected males and results. estimated volume from length and width based on the formula for a prolate spheroid (vol 5 4/3p (K teste length) N 2 Sampling issues.—We sampled snakes by haphazardly select- (K teste width) (Harlow and Taylor, 2000). As the mass of ing crates of snakes as they arrived at the landing site to the gonads was unknown, the gonadosomatic index (GSI), ensure the data were as representative of catch composition which is a measure of the mass of the testis relative to the as possible. The size composition of the catch is influenced mass of the animal, could not be calculated. Instead we by the size-selective capture technique. The gill nets used in calculated the testis volume per 100 g snake in order to take Tonle Sap capture a large range of sizes of reproductive-aged account of the varying size of males. individuals of most species, but large adults of the two larger species, H.
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