Electrofishing as a potential threat to freshwater cetaceans

Po Thomas,S Phay,D Kreb,Ge Ryan,Fmd Gulland,B Smith,Mi Malik,Rr Reeves,W Ding

doi:10.3354/esr00962

Abstract

Electrofishing is an accepted practice for legal fish sampling and surveying, but its use for subsistence food and market fishing has long been illegal in most countries. Illegal use affects freshwater fish populations in many parts of the world, and has been cited as a cause of mortality for endangered freshwater cetaceans in China (Yangtze dolphins and finless porpoises) and Southeast Asia (Ayeyarwady, Mekong, and Mahakam dolphins in Myanmar, Cambodia, and Indonesia, respectively), although the extent of this threat to cetaceans is unclear. Given their threatened status, these populations can ill afford such mortality in addition to the other threats they face (e.g. entanglement in gillnets, habitat deterioration and loss, declines in prey). Here, we review the evidence that electrofishing is a serious threat to freshwater cetaceans. It may alter the behavior of dolphins and porpoises, and contact with electrical currents may even directly kill or injure these animals, although questions remain unanswered concerning the exact nature and scale of the impacts. While other threats may appear more certain and urgent, electrofishing could be playing a significant role in driving the declines of some critically endangered freshwater cetaceans in Asia. Due to ethical and logistical challenges to improving our understanding of the impacts of electrical currents on cetaceans, clear descriptions of lesions in dead animals found stranded are needed to characterize the damage caused by electrofishing, to be more certain about cause and effect beyond spatiotemporal associations, and to determine the extent of this threat. Mortality from electrofishing seems to be uncommon, but in face of the uncertainties and the numerous other threats to these small populations, high priority should be given to enforcing electrofishing bans in the freshwater habitat of dolphins and finless porpoises.

Highlights

Several species of cetaceans occur solely or partly in freshwater systems of South America (Amazon, Orinoco, Tocantins/Araguaia, Madeira), southern Asia (Indus, Ganges, Brahmaputra, Karnaphuli), SoutheastAsia (Ayeyarwady, Mekong, Mahakam), and East Asia (Yangtze), and many of these populations are endangered
The Irrawaddy dolphin Orcaella brevirostris, an Endangered and primarily coastal marine species (Minton et al 2017), has 3 entirely freshwater subpopulations, all of them Red-Listed as Critically Endangered — one in the Ayeyarwady of Myanmar (Smith 2004), one in the Mekong of Cambodia and Laos (Smith & Beasley 2004), and one in the Mahakam of Indonesia (Jefferson et al 2008)
While we do not have any information on the electrical power required to injure or kill a dolphin or porpoise, or how closely an animal would have to approach an electrical field to be injured or killed, trical fields and that pinnipeds and other aquatic mammals are deterred by them, suggests that free-swimming freshwater cetaceans could detect and avoid such fields as long as they recognize the danger

Summary

Introduction

Several species of cetaceans occur solely or partly in freshwater systems of South America (Amazon, Orinoco, Tocantins/Araguaia, Madeira), southern Asia (Indus, Ganges, Brahmaputra, Karnaphuli), SoutheastAsia (Ayeyarwady, Mekong, Mahakam), and East Asia (Yangtze), and many of these populations are endangered. Several species of cetaceans occur solely or partly in freshwater systems of South America (Amazon, Orinoco, Tocantins/Araguaia, Madeira), southern Asia (Indus, Ganges, Brahmaputra, Karnaphuli), Southeast. The Amazon River dolphin Inia geoffrensis is endemic to the 4 aforementioned South American river systems and was recently Red-Listed by the IUCN as Endangered (da Silva et al 2018a). The South Asian river dolphin Platanista gangetica is Endangered (Braulik & Smith 2017), with 2 subspecies (both Endangered): the bhulan P. g. The Irrawaddy dolphin Orcaella brevirostris, an Endangered and primarily coastal marine species (Minton et al 2017), has 3 entirely freshwater subpopulations, all of them Red-Listed as Critically Endangered — one in the Ayeyarwady of Myanmar (Smith 2004), one in the Mekong of Cambodia and Laos (Smith & Beasley 2004), and one in the Mahakam of Indonesia (Jefferson et al 2008). The Yangtze finless porpoise, a subspecies of the Endangered narrowridged finless porpoise Neophocaena asiaeorientalis (Wang & Reeves 2017), is Red-Listed as Critically Endangered (Wang et al 2013)

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