A new method to detect illegal oil use and estimate mortality rates of endangered Ganges river dolphins based on Clupisoma fish catches

Abstract

Gillnet entanglement is a major threat to endangered small cetacean species around the world, including Ganges river dolphins (Platanista gangetica). While conservation efforts have helped to reduce the prevalence of targeted hunts, bycatch still occurs. A serious concern is the illegal opportunistic exploitation of bycaught river dolphins to evade detection and potential penalties, where oil is extracted from the blubber and illegally traded as bait for catfish (Clupisoma garua). While this threat is well known, enforcement is weak and evidence difficult to collect as the molecular analyses required to confirm the use of dolphin oil are expensive and time‐intensive. Simpler detection methods are therefore needed to help improve enforcement. We developed a new visual test and decision‐making process to help identify the use of dolphin oil while fishing for catfish, which draws on a common understanding among local fishers that oil‐baited fish appear paler/whiter than net‐caught fish. We conducted colour analyses and visual‐perception tests, combining these results with other variables, such as catch weight, sale price and season, to identify a set of rules which can be used to determine the use of dolphin oil in a sampled catch. Based on our predicted oil‐use prevalence, we estimated that five to seven dolphins (~4% of the population in our study area) may be exploited each year to support catches landed at one of two sites in our study area. This system therefore provides a simple and efficient tool to identify instances of illegal exploitation of Ganges river dolphins.