Bangor University DOCTOR of PHILOSOPHY Using Local Informant Data and Boat-Based Surveys to Improve Knowledge on the Status of T
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Bangor University DOCTOR OF PHILOSOPHY Using local informant data and boat-based surveys to improve knowledge on the status of the Ganges River dolphin (Platanista gangetica gangetica) Richman, Nadia Award date: 2015 Awarding institution: Bangor University Link to publication General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Download date: 04. Oct. 2021 Using local informant data and boat-based surveys to improve knowledge on the status of the Ganges River dolphin (Platanista gangetica gangetica) Nadia Irene Richman School of Environment, Natural Resources and Geography, Bangor University A dissertation submitted for the degree of Doctor of Philosophy Bangor University October 2014 Abstract Freshwater cetaceans are one of the most threatened groups of mammals on earth. Limited resources for monitoring and low power to detect trends hinder the development of effective conservation. Using the southern Bangladesh subpopulation of Ganges River dolphins (Platanista gangetica gangetica), previously thought to be a closed population, I investigate cost-effective boat-based methods for monitoring and estimating population size, and the value of local informant data for contributing to knowledge on the status of this poorly-known subpopulation. Detectability must be accounted for during surveys to make inferences on species’ trends. However, many surveys use methods that do not account for detectability, assuming such approaches to be cheaper. I demonstrate that a combined visual-acoustic survey is a robust and cost-effective approach for monitoring. Using data from multiple seasons and marine surveys, I show the population may not be closed. I develop correction factors to account for imperfect detectability during past visual-only surveys and use these to show there is no detectable long-term (1999-2012) change in the abundance of this subpopulation. Local informant data are sometimes considered to have the potential to provide information of value to monitoring population trends. A comparison of the long-term and seasonal trends from boat-based surveys and those reported by fishers showed poor agreement. Memory- related biases are likely to have impacted informant recall. However, local informant data proved useful in identifying causal mechanisms underlying dolphin susceptibility to bycatch in gillnets, in particular river depth and net mesh size. Furthermore, local informant data provided a minimum estimate of annual mortality that is deemed unsustainable, but is based on a number of assumptions and potential biases that are discussed. Combined visual-acoustic surveys and local informant data represent cost-effective tools for addressing some of the significant knowledge gaps on freshwater cetacean status, aiding the development of evidence-based conservation strategies. i Contents Abstract ...................................................................................................................................... i List of Figures .......................................................................................................................... 1 List of Tables ............................................................................................................................ 5 Acknowledgements ................................................................................................................. 7 Chapter 1: Introduction ......................................................................................................... 10 1.1. Knowledge on species status ........................................................................................... 10 1.1.1 Why monitor the status of species? ........................................................................ 10 1.1.2 How to monitor species status? .............................................................................. 11 1.1.3 Data gaps in monitoring effort? ............................................................................... 12 1.1.4 Addressing data gaps in monitoring effort .............................................................. 12 1.2. Knowledge on the status of freshwater cetaceans ........................................................... 13 1.2.1 Introduction to freshwater cetaceans ...................................................................... 13 1.2.2 Status of freshwater cetaceans ............................................................................... 14 1.2.3 Status of the Ganges River dolphin ........................................................................ 14 1.2.4 Conservation of Ganges River dolphins ................................................................. 17 1.2.5 Research needs for river dolphin conservation ...................................................... 17 1.3. Objectives ......................................................................................................................... 18 Chapter 2: To see or not to see: investigating detectability of Ganges River dolphins using a combined visual-acoustic survey ........................................................................................ 20 2.1. Abstract ............................................................................................................................. 20 2.2. Introduction ....................................................................................................................... 21 2.3. Methods ............................................................................................................................ 25 2.3.1 Study site ................................................................................................................ 25 2.3.2 Pilot surveys ............................................................................................................ 27 2.3.3 Visual and acoustic survey ..................................................................................... 27 2.3.4 Calculating detectability .......................................................................................... 30 2.3.5 Matching acoustic and visual detections ................................................................ 31 2.3.6 Power to detect population trends .......................................................................... 34 2.3.7 Investigating factors affecting visual detection of river dolphins ............................. 35 2.3.8 Cost analysis ........................................................................................................... 35 2.4. Results .............................................................................................................................. 36 2.4.1 Visual and acoustic detections................................................................................ 36 2.4.2 Matching detections ................................................................................................ 36 2.4.3 Detection probabilities ............................................................................................. 38 1 2.4.4 Surveys required to detect trends ........................................................................... 39 2.4.5 Factors affecting detection by observers ................................................................ 39 2.4.6 Cost analysis of methods ........................................................................................ 40 2.5. Discussion ......................................................................................................................... 41 2.5.1 Visual availability bias ............................................................................................. 41 2.5.2 Perception bias ....................................................................................................... 43 2.5.3 Trend detection ....................................................................................................... 44 2.5.4 Cost of survey methods .......................................................................................... 44 2.5.5 Limitations of acoustic surveys ............................................................................... 45 2.5.6 Recommendations for future surveys ..................................................................... 46 2.5.7 Conclusions ............................................................................................................. 46 Chapter 3: Temporal patterns in the abundance of Ganges River dolphins in the southern rivers of Bangladesh .............................................................................................................. 48 3.1. Abstract ............................................................................................................................. 48 3.2. Introduction