Aquatic Habitat Features within Rivers in Western Sarawak (Borneo) and their Relationship to Habitat Use and Spatial Distribution of Crocodylians

• Main Author: Anthony, Pine
• Format: E-Thesis
• Published: Universiti Malaysia Sarawak (UNIMAS) 2019
• Subjects: Q Science (General); QL Zoology
• Online Access: http://ir.unimas.my/id/eprint/26593/
• Institution: Universiti Malaysia Sarawak

Crocodylians are important ecological components of riverine ecosystems in Sarawak, acting as an important prey resource as hatchlings and helping to maintain balance within the food web as they grow. The Saltwater Crocodile (Crocodylus porosus) and Tomistoma (Tomistoma schlegelii) are two species of crocodylian found in Sarawak, unique in many ways to one another in relationship to their morphology, habitat use and distribution. The waterways and wetlands where crocodylians exist in Sarawak can be equally diverse, spanning from coastal estuaries, inland rivers and peat swamps. Across their range, crocodylians offer unique and challenging management requirements in order to conserve healthy population numbers and to reduce conflict that may exist with humans. In order to better understand and contribute towards research, management and conservation priorities, there is a need to better understand the distribution and habitat use of crocodylians in Sarawak. This study investigates ecological and morphological patterns in Saltwater Crocodiles (C. porosus) and Tomistoma (T. schlegelii) at four study sites in western Sarawak, and aims at better understanding habitat use, relative densities, spatial distributions and morphometrics of crocodylians. A major objective was to better understand relationships existing for crocodylians to aquatic habitats, an area that has not been fully explored. Standardised research techniques used during this study included vessel-based spotlights surveys, crocodylian captures and side-scan sonar mapping of river areas. A total of 573 C. porosus and 54 T. schlegelii were sighted during spotlight surveys, with measurement data collected from captures for 47 C. porosus and 15 T. schlegelii. In relationship to side-scan sonar mapping of habitat, a total of 78.5 km of river length, equaling a total of 3,827.45 km2 of river area, was mapped across study sites. It was found that crocodylian species did significantly differ in their morphologies, spatial distributions and habitat use within study areas. C. porosus had a robust, wide head shape and overall body plan, found in many rivers without significant influence from factors assessed in this study. Proportional aquatic habitat associations for C. porosus were 62.94% clay/silt, 19.18% bank vegetation, 1.15% decaying vegetation, 1.53% sedimentary rock and 0.58% anthropogenic. T. schlegelii on the other hand was found to have a much more elongated head shape and body plan suited towards the densely vegetated, complex and restricted habitat areas of uppermost reaches of the rivers and peat swamps where they were found. Proportional aquatic habitat associations for T. schlegelii were 6.81% clay/silt, 13.47% bank vegetation and 65.62% decaying vegetation. Findings from this study have significant importance and implications related to addressing research, management and conservation priorities for crocodylians in Sarawak. In relationship to future research objectives, emphasis should be placed on better understanding distributions and habitat use of crocodylians at different sites and regions within Sarawak to better understand variation in densities, regional biomass and habitat availability. For T. schlegelii specifically, effort should be placed on identifying additional sites where breeding populations exist and that would allow for long-term research, management and conservation effort. From a management perspective, crocodylians exist at varying distributions, densities and population structures across their range. Ensuring that management effort accurately represent these differences would be important for maintaining healthy crocodylian populations and regional biomass into the future. For T. schlegelii, specifically, habitat areas that are found to be important for survival should have highest management priorities and allocation of resources to monitor and protect. In terms of conservation, efforts need to be made to make more people, especially children, in Sarawak interested and inspired by crocodylians for their ecological importance and unique diversity. This can be achieved through greater allocation of resources towards public awareness, youth education and involving local communities in conservation efforts for crocodylians. Keywords: Sarawak, Crocodylus porosus, Tomistoma schlegelii, crocodylian ecology, morphometric ratios, crocodylian eyeshine surveys, side-scan sonar, river habitat mapping