REVIEW
Electrofishing as a potential threatto freshwater cetaceans
Peter O. Thomas1,*, Frances M. D. Gulland1, Randall R. Reeves2, Danielle Kreb3, Wang Ding4, Brian Smith5, Muhammad Imran Malik6, Gerard E. Ryan7, Somany Phay8
1Marine Mammal Commission, 4340 East-West Highway, Bethesda, Maryland 20814, USA2Okapi Wildlife Associates, 27 Chandler Lane, Hudson, Quebec J0P1H0, Canada
3Yayasan Konservasi Rasi, Komplek Pandan Harum Indah, Samarinda, 75124 Kalimantan Timur, Indonesia4Institute of Hydrobiology, The Chinese Academy of Sciences, Wuhan, Hubei 430072, PR China
5Wildlife Conservation Society, Asian Coastal Cetacean Program, IUCN SSC Cetacean Specialist Group, Arcata, California 95518, USA
6Indus River Dolphin Conservation Project (IRDCP), WWF-Pakistan, Sukkur 65310, Pakistan7School of BioSciences, University of Melbourne, Melbourne, Victoria 3052, Australia
8WWFund Cambodia, #21, St. 322, Boeung Keng Kang I, Chamkar Morn, Phnom Penh 12300, Cambodia
ENDANGERED SPECIES RESEARCHEndang Species Res
Vol. 39: 207–220, 2019https://doi.org/10.3354/esr00962
Published July 11
1. INTRODUCTION
Several species of cetaceans occur solely or partly infreshwater systems of South America (Amazon,Orinoco, Tocantins/Araguaia, Madeira), southern Asia(Indus, Ganges, Brahmaputra, Karnaphuli), South east
Asia (Ayeyarwady, Mekong, Mahakam), and EastAsia (Yangtze), and many of these populations are en -dangered. The Amazon River dolphin Inia geoffrensis isendemic to the 4 aforementioned South American riversystems and was recently Red-Listed by the IUCN asEndangered (da Silva et al. 2018a). The tucuxi Sotalia
*Corresponding author: [email protected]
ABSTRACT: Electrofishing is an accepted practice for legal fish sampling and surveying, but its usefor subsistence food and market fishing has long been illegal in most countries. Illegal use affectsfreshwater fish populations in many parts of the world, and has been cited as a cause of mortality forendangered freshwater cetaceans in China (Yangtze dolphins and finless porpoises) and SoutheastAsia (Ayeyarwady, Mekong, and Mahakam dolphins in Myanmar, Cambodia, and Indo nesia, respec-tively), although the extent of this threat to cetaceans is unclear. Given their threatened status, thesepopulations can ill afford such mortality in addition to the other threats they face (e.g. entanglementin gillnets, habitat deterioration and loss, declines in prey). Here, we review the evidence that elec-trofishing is a serious threat to freshwater cetaceans. It may alter the behavior of dolphins and por-poises, and contact with electrical currents may even directly kill or injure these animals, althoughquestions remain unanswered concerning the exact nature and scale of the impacts. While otherthreats may appear more certain and urgent, electrofishing could be playing a significant role in driv-ing the declines of some critically endangered freshwater cetaceans in Asia. Due to ethical and logis-tical 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 damagecaused by electrofishing, to be more certain about cause and effect beyond spatiotemporal associa-tions, 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, highpriority should be given to enforcing electrofishing bans in the freshwater habitat of dolphins and fin-less porpoises.
KEY WORDS: Freshwater cetaceans · River dolphins · Finless porpoises · Electrofishing · Cetaceanconservation
OPENPEN ACCESSCCESS
Contribution to the Special ‘Marine vertebrate bycatch: problems and solutions’
© The authors 2019. Open Access under Creative Commons byAttribution Licence. Use, distribution and reproduction are un -restricted. Authors and original publication must be credited.
Publisher: Inter-Research · www.int-res.com
Endang Species Res 39: 207–220, 2019
fluviatilis, which is endemic to the Amazon system, iscurrently listed as Data Deficient (Secchi 2012) but ur-gently needs reassessment as its conservation statushas deteriorated rapidly in recent years, at least in Brazil(da Silva et al. 2018b). The South Asian river dolphinPlatanista gangetica is Endangered (Braulik & Smith2017), with 2 subspecies (both Endangered): the bhulanP. g. minor in the Indus system (Braulik et al. 2012)and the susu P. g. gangetica in the Ganges, Brahmapu-tra, and Karnaphuli systems (Smith et al. 2012). TheIrra waddy dolphin Orcaella brevirostris, an Endangeredand primarily coastal marine species (Minton et al.2017), has 3 entirely freshwater subpopulations, all ofthem Red-Listed as Critically Endangered — one in theAyeyarwady of Myanmar (Smith 2004), one in theMekong of Cambodia and Laos (Smith & Beasley2004), and one in the Mahakam of Indonesia (Jeffer-son et al. 2008). Finally, 2 fresh water cetaceans histori-cally inhabited the Yangtze River and adjoiningDongting and Poyang lakes. The Yangtze River dol-phin, or baiji Lipotes vexillifer, continues to be Red-Listed as Critically Endangered (possibly Extinct), butit has been at least functionally extinct since the early21st century (Smith et al. 2017). The Yangtze finlessporpoise, a subspecies of the En dan gered narrow-ridged finless porpoise Neophocaena asiaeorientalis(Wang & Reeves 2017), is Red-Listed as Critically En-dangered (Wang et al. 2013).
All freshwater cetaceans face numerous types anddegrees of anthropogenic threats, and their threatenedstate is, in every case, due to multiple factors — thevast majority of which are being driven by human ac-tion, whether deliberate or incidental. The primaryfactors threatening their survival are direct injury andmortality in legal and illegal fisheries (e.g. entangle-ment in gear, deliberate retaliation to protect gear andcatch from depredation); deliberate hunting to obtainfish bait (particularly for the Amazon River dolphin),and loss and fragmentation of cetacean and prey habi-tat due to water development projects (e.g. dams andbarrages, embankments, conversion of wetlands). Ad-ditional threats in some areas include vessel traffic(e.g. dredging, noise, boat strikes), harbor construction(e.g. noise, land reclamation for infrastructure), andexposure to contaminants (e.g. from industrial efflu-ent, urban and agricultural runoff, gold mining).
The focus of this paper is on a specific form of fish-ing — electrofishing — that has been characterized,but not well-documented or well-described, as a seri-ous threat to some freshwater cetacean populationsin Asia. Electrofishing has not (yet) been identified asa threat to freshwater cetaceans in South America(Secchi 2012, da Silva et al. 2018a).
Our objectives were four-fold: (1) to explain whatelectrofishing is and how it might be expected to affectcetaceans; (2) to summarize available evidence con-cerning mortality as well as responses of the animalsto exposure; (3) to critically evaluate such evidence;and (4) to identify gaps in our understanding of howelectrofishing threatens freshwater cetaceans.
2. INFORMATION SOURCES AND METHODS
Information on the mechanics and physics of electro - fishing was obtained by requesting advice from ex -perts on the subject who work (or worked) with theUS Fish and Wildlife Service (Alan Temple and JanDean) and by reviewing manuals and publications onits use and the potential hazard it poses to mammals,including humans.
All the authors of this paper have field experience withfreshwater cetaceans and have participated in con feren -ces, workshops, and informal discussions with fisher -men, fishery managers, and government officials inthe range countries. Some of us have ob served electro -fishing in the vicinity of dolphins or porpoises, somehave examined dead cetaceans that were thought tohave died of electrocution, and some have seen electro -fishing equipment deployed in areas inhabited bycetaceans. As a result, much of the information in thispaper, as well as many of the ideas discussed and theconclusions reached, come from direct experience.
In addition, we canvassed colleagues in the variousrange countries to determine where electrofishingdoes and does not occur and to obtain as much detailas possible regarding spatial and temporal overlapwith dolphins and porpoises. This included an initialinformal questionnaire to researchers in Cambodia,Myanmar, Indonesia, and China to identify concerns,equipment used, and perspectives on the nature ofeffects of electrofishing on freshwater cetaceans.Wherever we found reference to mortality of cetaceansattributed to electrofishing in the literature or mediaoutlets, we made an effort to follow this up with theoriginal source or sources to learn more about the cir-cumstances and how causation was diagnosed.
3. RESULTS
3.1. Electrofishing
3.1.1. What it means
Electrofishing is an important and commonly usedtool that allows fish biologists to collect a variety of
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data in the field, e.g. to assess species composition,estimate populations and growth rates, and collectindividual fish for analysis (Bohlin et al. 1989, Snyder2003). Generally, legal electrofishing for scientificpurposes is not intended to kill fish but to stun themso that they can be counted or so that individuals orsamples can be collected. When used for fish sam-pling, the type, power, and form of the electrical cur-rent are adjusted, and the size and shape of anodesare designed to reduce or eliminate the likelihood offish being injured or killed by the electrical current(Kolz 1989).
3.1.2. How it works
A basic manual (www.efish-solutions.com/ support/library- intro/) describes electrofishing as:
the process of using electricity to catch fish by creatingan electrical-field through water, around an anode (usu-ally on a hand-held pole or hanging from a boat) andcathode (trailing in the water behind the operator or boat).The electric-field between the 2 electrodes develops avoltage along the length of fish exposed to it, such thatgalvanotaxis stimulates their nervous system, and theyswim ‘up current’ towards the anode (the source of thefield). At a point approaching the source of the field, thefish enter a zone where the field is then of sufficientstrength to temporarily immobilize them and thus aidin their capture.
Elec trofishing manuals define 3 primary zones ofinfluence around the electrodes based on fish be -havior and physiological/muscular response to electri-cal fields, the dimensions of which depend upon thevoltage used and the conductivity of the environ-ment. The ‘zone of detection’ (or ‘perception’) is theportion of the electrical field where the fish is awareof it but can move away; the ‘effective zone’ (for fishcapture) is where fish may be forced to swim towardthe electrode and become immobilized but are notharmed; and the ‘injury zone’ is where fish experi-ence such high voltage gradients and power (andcurrent) densities that they are injured or killed (J.Dean pers. comm.). Understanding these zones andthe equipment and electrical currents used to createthem provides the operational basis for capturingfish. For boat-mounted equipment, the effective zonetypically has a radius of 2−3 m.
The voltages required to immobilize fish of differentsizes and types have been carefully studied, althoughmany details of the effects on fish are still uncertain.In general, larger fish (in terms of total body surfacearea) are more likely to be paralyzed or injured(spinal injuries from sudden muscular contractions),
and thus are more easily captured than smaller ones,because larger size allows a larger voltage differenceto develop across the body (Emery 1984, Mahoney etal. 1993, Dalbey et al. 1996, Dolan & Miranda 2003,Snyder 2003). Thin-scaled and unscaled fishes, in-cluding large catfishes, are less resistant to electricalcurrents than are thick-scaled fishes (Green 2011).
Electrofishing gear is generally powered by DCbatteries (carried in backpack units) or small genera-tors on boats. The need for mobility usually limits thesize of the power supply and the amount of powerthat can be applied in the water (Mahoney et al.1993, Reynolds & Harlan 2011).
3.1.3. How it could affect cetaceans
Cetaceans could be injured or killed directly byelectrofishing when they come into (1) direct contactwith the electrodes, thus becoming part of the electri-cal circuit or (2) contact with the electrical fields pro-duced in the water by electrofishing devices. Theimpact of a given electrical field depends on the dis-tance of an animal from the source and the nature ofthe field itself.
There is no question that direct contact with elec-trodes in water is potentially fatal for mammals ofall sizes. For example, when an animal makesphysical contact with a metal electrode, whetheraccidentally or deliberately (e.g. out of curiosity), itbecomes ‘directly “wired” to the power source and issubject to possible electrocution’ (Kolz & Johnson1995, p. 211). Large-scale experiments with electricharpoons in the 1930s proved this in a dramatic man-ner with blue whales Balaenoptera musculus (Mitchell1986). The prototypes used in field trials under theauspices of the Norwegian Association of WhalingCompanies proved extremely effective, paralyzingthe whales and causing them to roll over onto theirsides with flippers and flukes above water. It report-edly did not matter where the whale was hit — theresult was usually a rapid death. Although the ‘killingcurrent’ was 220 V 50 Hz AC, resistance was so lowthat on average a potential of 20 V passing throughthe body was claimed to be sufficient for killing theselarge creatures.
Electrocution, cardio-respiratory failure, and drown-ing can also be the fate of fishermen who make mis-takes when using electrofishing equipment (DiNunno et al. 2003). In descriptions of potential humaninjury from electrofishing it is often difficult to discernwhether authors are referring to direct physical con-tact with electrical circuits or with electrical fields,
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but the vulnerability is clear, regardless. Reynolds &Kolz (2013, p. 337), for example, confirmed that thebatteries and generators used in electrofishing aspracticed for scientific fish monitoring deliver ‘morethan enough energy to electrocute a person.’ Theynote that grasping a wire carrying as little as 0.01 Acan cause muscular tetany and inability to releaseone’s grip, leading to death from respiratory arrest,asphyxia, or ventricular fibrillation. Primavesi (2009)emphasized that in human electrical injuries, skinthickness and moisture are crucial factors because skinresistance impedes current flow and protects againstelectrocution.
There is greater uncertainty about the harm thatcould come to freshwater cetaceans when they comeinto contact with the electrical fields in the water pro-duced by electrofishing devices. Fish behavior andsize, water conductivity and temperature, substrate,and current strength are among the well-studied vari-ables that influence the effectiveness of electro fishing(Emery 1984, Mahoney et al. 1993), but we have littleknowledge of the be havioral, physiological, and neu-rological responses of cetaceans as they enter an elec-trically charged zone, or of the distances or voltagesat which they would begin to detect it. Experiencedelectrofishing personnel reported that geese, ducks,beavers, muskrats, and nutrias did not appear to be‘inadvertently shocked at the power levels normallyused for fish’ (Kolz & Johnson 1995, p. 208).
Pictures and descriptions of electrofishing equip-ment used by artisanal fishermen in the rivers andlakes of East and Southeast Asia do not suggest thatthe electrical outputs or effective distances producedwould be of signi ficantly greater magnitude thanthose produced by standard devices used for legal fishsampling elsewhere. There is no evidence of tech no - logical difference, other than the power of the electri-cal source, that would significantly increase the powerand range of influence of such basic equipment orchange the basic behavior of electrical fields. Thisleads electrofishing experts to discount general state-ments such as ‘the lethal electric charge was able to killanything within a range of 20 meters…’ (Turvey 2008,p. 39), but the larger apparatus described as being usedin the Yangtze River may have greater reach (A. Tem-ple pers. comm.).
3.1.4. Sensitivity and response of freshwatercetaceans and other mammals to electrical fields
There are few descriptions of the behavior of smallcetaceans or other aquatic mammals reacting to elec-
trical fields. Research on dolphin electro-receptivityand pinniped deterrence indicates that some marinemammals can detect low-level electrical fields. Guianadolphins Sotalia guianensis can easily detect low-level electrical fields produced by electric fish andhave specialized sensory anatomy (‘vibrissal crypts’)for doing so (Czech-Damal et al. 2012). Deterrenceexperiments on California sea lions Zalophus califor-nianus (Burger 2008) and harbor seals Phoca vitulina(Forrest et al. 2009) showed that pinnipeds detect andare actively deterred from weak electrical fields infreshwater environments. An expert in electrofishing(A. Temple pers. comm.) suggested that if free-swim-ming dolphins have the ability to detect extremelylow electrical field strengths, they should be able toavoid entering the ‘effective’ and ‘injury’ zones. Thisassumes that they recognize the low electrical fieldstrengths as a danger, and that they are as resistant aspinnipeds to them. Cetacean skin, however, is less ker-atinized and hairless, potentially increasing the electri-cal conductance of this external barrier in comparisonwith other mammals (Harrison & Thurley 1974).
3.1.5. Legal status of electrofishing
Electrofishing is banned or restricted in most of therivers and lakes where freshwater cetaceans occur inAsia (Table 1), but the economic incentives for its useare strong. In Cambodia, for example, legal fishingwith a cast net (cost: USD $10−12) yielded a fisher-man 6−7 kg of small fish, or $3−4 d−1 (Kurien 2007),whereas with a $30−40 investment in illegal electro -fishing equipment, an unskilled and untrained per-son could catch 20−30 kg of large fish worth $10−13d−1 and a skilled electro-fisherman could catch 30−40 kg in a single haul (Kurien 2007). The equipmentfor electro fishing is relatively inexpensive (and thebattery can be used in the home for other purposes),little maintenance is required (unlike for nets, long-lines, bamboo traps, and fishing fences which requireconstant repair), and relatively large catches can bemade with little effort (Smith & Tun 2007).
3.1.6. Illegal electrofishing gear
We have not found detailed technical descriptionsof apparatus used for illegal electrofishing in therivers of Asia, nor have we had the opportunity toexamine such equipment, except through photos.Table 1 summarizes descriptions of electrofishingoperations and apparatus that we found in fisheries
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reports and news accounts or obtained from theinformal responses of researchers to our simplequestion naire.
3.2. Impacts of electrofishing
3.2.1. Impacts on prey species and the ecosystemsof freshwater cetaceans
Like dynamite and poison fishing, electrofishing, ifused without controls, is a potentially destructive andindiscriminate fishing method with lethal or sub-lethal impacts on both target and non-target spe-cies. The entire ‘ecological footprint’ of freshwaterelectro fishing falls outside established managementregimes, catch limits, and other restrictions thatmight apply to legal fishing activity (Garcia et al.2003, Kreb et al. 2007, Phen & Nam 2011, FAO 2016).
In China, Zhou et al. (1998) reported that electro -fishing, though ‘strictly banned by the fishery agency,’was pursued at night, in small boats that hid duringthe day in small channels adjacent to the YangtzeRiver. In Myanmar, electrofishing is used illegally forcommercial fishing on the Ayeyarwady River, reportedlywith wide-ranging impacts on fish stocks and bio di -versity, including riverine fish and turtles (https:// www.mmtimes. com/ news/ working- save- symbol- human-animal- cooperation. html).
The presence of multiple illegal operators, some-times working in organized gangs to avoid detec-tion and intimidate enforcement officers, as occurson the Ayeyarwady River, greatly increases the over-all impact on fishery resources and aquatic biodi-versity (Smith 2004, Smith & Tun 2007, Holland 2015,http:// news. nationalgeographic. com/ news/ 2015/ 02/150217- irrawaddy- dolphins- myanmar- electro- fishing-mandalay/).
3.2.2. Behavioral impacts on freshwater cetaceans
Interactions between fishermen and dolphins arelikely because freshwater cetaceans generally feedin relatively confined and specific areas (e.g. deeppools, eddies, confluences) that are also prime fishinggrounds exploited by fishermen using nets and linesas well as electricity.
In addition to prey depletion, reports from theAyeyarwady River suggest that electrofishing and as -sociated activities by gangs of fishermen in motorizedvessels have disrupted the traditional mutualistic fish-ing relationship between humans and Irrawaddy
dolphins (Smith et al. 2009). In this relationship, thedolphins, upon receiving an acoustic signal from net-fishermen in non-motorized boats, approachedthe boats and assisted the fishing operation by herd-ing fish toward the nets (Smith & Tun 2007). Alreadyby the mid-2000s, the dolphins reportedly had begunto stay away from certain areas to avoid beingshocked (Smith & Tun 2007), and more recent obser-vations (in 2012 and 2014) indicate that they rarelyapproach oar-powered boats to engage in ‘coopera-tive’ fishing (Holland 2015, https://www.marine-mammalscience.org/letters/letter-to-myanmar-offi-cials-regarding-irrawaddy-dolphins/).
Anecdotal evidence suggests that electro-fishermenhave taken advantage of the normal fish-herdingbehavior of dolphins in the Ayeyarwady by signalingto the dolphins using acoustic means, similar to thecooperative cast-net fishermen (see Smith et al.2009), so that they can deploy their electrical gearnear the dolphins to catch more fish (A. M. Chit pers.comm.). Exploiting what used to function as mutuallybeneficial interspecies signaling, the electro-fishermenmight switch off the electrical current as dolphinsmove into an area (generally an indication of fishpresence) and then turn it back on to stun the fishthat try to escape ahead of the approaching dolphins.This situation could put dolphins in close proximity toelectrodes as the electrical current is activated, a sit-uation that could lead to injury or even death of thedolphins (Myanmar Times 6−12 August 2012 https://www. mmtimes. com/ national- news/ mandalay- upper-myanmar/ 239- battery- fishing- rise- threatens- unique-dolphin- cooperation. html).
In some circumstances, Mahakam river dolphinsappear to co-exist with electrofishing operationsand approach the boats and gear without apparentharm. On several occasions, one of the authors (D.Kreb) observed dolphins as close as 15 m to a boatthat was actively electrofishing. The dolphins showedno obvious reaction but appeared to be eating fishthat had been stunned or escaped from the elec-trical field.
3.2.3. Pathology and diagnosis of electrofishing ascause of death of freshwater cetaceans
It is widely assumed, implied, or asserted in the lit-erature that exposure to electrofishing can electro-cute or stun freshwater cetaceans. In the river-by-riverreview below, we consider scientific and popular ac -counts that cite electrofishing as a possible or knowncause of death of cetaceans.
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ower
sou
rces
. T
he
equ
ipm
ent
is u
sed
to
stu
n f
ish
in
th
e op
enw
ater
of
the
rive
r, s
wam
p o
r la
ke
and
is
rep
orte
d t
o b
e ef
fect
ive
from
1.5−
3 m
wit
hb
atte
ry p
ower
an
d, d
epen
din
g o
n t
he
dep
th, u
p t
o 10−
15 m
wit
h a
gen
erat
or (
wit
h a
smal
ler
ran
ge
in d
eep
er w
ater
s).
Kre
b e
t al
. (20
07, 2
010)
,D
. Kre
b p
ers.
ob
s.
Mek
ong
Orc
aell
a b
revi
rost
ris
Ille
gal
: Cam
bod
ia,
Lao
s‘T
wo
typ
es o
f el
ectr
o fi
shin
g [
gea
r] u
sed
:(1
) co
nsi
der
ed a
s sm
all
scal
e, c
ond
uct
ed b
y 1−
2 p
eop
le p
er o
utf
it,
wit
h o
r w
ith
out
ab
oat.
Fis
her
men
pu
t th
e el
ectr
ic s
hoc
ker
in t
he
wat
er m
anu
ally
(lo
cal
peo
ple
cal
l th
ism
eth
od ‘h
ot e
lect
rofi
shin
g’)
,(2
) co
nd
uct
ed b
y 2−
3 p
eop
le w
ith
1−
2 b
oats
, th
e el
ectr
ic s
hoc
ker
cab
les
are
low
ered
in th
e w
ater
col
um
n fr
om o
ne
boa
t wh
ich
mov
es o
n g
rad
ual
ly a
nd
th
e el
ectr
ic s
hoc
ker
is a
ctiv
e co
nti
nu
ousl
y. T
he
oth
er b
oat
foll
ows
beh
ind
col
lect
ing
th
e fi
sh (
loca
l peo
ple
call
th
is m
eth
od ‘c
ool
elec
tro f
ish
ing
’). C
ool
elec
trof
ish
ing
cou
ld k
ill
dol
ph
ins.
’
Ph
en &
Nam
(20
11, p
. 34)
Fis
her
men
use
d D
C c
urr
ent f
rom
a b
oat t
hat
‘was
eff
ecti
ve fo
r fi
sh c
aptu
re f
or 2
5−30
m’
and
wh
en c
arri
ed o
n t
he
bac
k w
as e
ffec
tive
for
2−
3 m
.M
ekon
g R
iver
Gu
ard
s (i
nfo
rmal
inte
rvie
w)
Bat
teri
es f
or e
lect
rofi
shin
g d
evic
es a
re o
ften
‘d
ual
-pu
rpos
ed’
to l
igh
t h
omes
an
dp
ower
tel
evis
ion
s in
a r
egio
n w
her
e th
ere
is n
o ru
ral
elec
trif
icat
ion
.
Com
mu
nit
y an
d lo
cal o
ffic
ial a
ttri
bu
tion
s of
ele
ctro
fish
ing
as
the
sou
rce
of d
olp
hin
mor
-ta
lity
du
e to
lack
of
evid
ence
for
oth
er c
ause
s an
d s
igh
tin
gs
of e
lect
rofi
shin
g in
are
asu
sed
by
dol
ph
ins.
Ku
rien
(20
07)
G. R
yan
an
d S
. Ph
ay p
ers.
com
m.
Tab
le 1
. Riv
er b
y ri
ver
acco
un
ts o
f el
ectr
ofis
hin
g in
th
e vi
cin
ity
of f
resh
wat
er c
etac
ean
sTable continued on next page
Thomas et al.: Electrofishing and freshwater cetaceans 213Y
ang
tze
Neo
ph
ocae
na
asia
e -or
ien
tali
sas
iaeo
rien
tali
s,L
ipot
esve
xill
ifer
Ille
gal
: Ch
ina
‘In
rec
ent
year
s th
e u
se o
f el
ectr
ic f
ish
ing
gea
r h
as b
een
in
crea
sin
g.
Su
ch g
ear
isco
mp
osed
of
a st
orag
e b
atte
ry a
nd
tra
nsf
orm
er w
hic
h a
re c
arri
ed b
y a
smal
l b
oat.
Ah
igh
vol
tag
e is
pro
du
ced
bet
wee
n a
n e
lect
rod
e lo
wer
ed f
rom
th
e b
oat
and
an
oth
eron
e se
t in
th
e ri
ver
du
rin
g o
per
atio
n.
Ele
ctri
c fi
shin
g g
ear
ind
iscr
imin
atel
y d
estr
oys
fish
erie
s re
sou
rces
an
d i
s th
eref
ore
stri
ctly
ban
ned
by
the
fish
ery
agen
cy. H
owev
er,
the
smal
l b
oats
th
at c
arry
th
is t
ype
of g
ear
hid
e d
uri
ng
th
e d
ay i
n s
mal
l ch
ann
els
alon
g t
he
Yan
gtz
e R
iver
, an
d o
per
ate
at n
igh
t.’
Zh
ou e
t al
. (19
98, p
. 128
)
‘Th
e p
rim
ary
fact
or r
esp
onsi
ble
for
th
e b
aiji’
s ex
tin
ctio
n w
as p
rob
ably
un
sust
ain
able
inci
den
tal
by-
catc
h i
n l
ocal
fish
erie
s, p
rim
arily
fro
m i
lleg
al m
eth
ods
such
as
‘rol
ling
-h
ook
’ lo
ng
-lin
es a
nd
pos
sib
ly e
lect
ro-fi
shin
g (
Zh
ou &
Wan
g 1
994;
Zh
ang
et
al.
2003
),w
hic
h in
volv
es to
win
g a
n e
lect
rifi
ed (>
300
volt
) met
al-f
ram
ed n
et b
ehin
d a
fish
ing
boa
tat
nig
ht
in t
he
mai
n r
iver
ch
ann
el a
nd
oth
er m
ajor
wat
er b
odie
s.’
Tu
rvey
et
al. (
2013
, p. 3
53)
‘Th
ere
are
not
on
ly b
ig g
ang
s of
ele
ctro
fish
ing
, in
div
idu
al f
ish
erm
an a
lso
use
th
ism
eth
od t
o ca
tch
fis
h.
Th
ey w
ill
firs
t u
se a
pu
mp
to
low
er t
he
wat
er l
evel
an
d c
has
efi
shes
in
to s
mal
ler
area
; th
en p
ut
elec
tric
ch
arg
es i
nto
th
e ri
ver;
fin
ally
col
lect
th
efl
oate
d b
ig f
ish
es t
o sh
ip t
o th
e m
ark
et a
nd
lef
t th
e ot
her
s on
th
e g
rou
nd
.’
htt
p:/
/yan
gtz
efin
less
por
poi
se.
wee
bly
. com
/ in
dex
.htm
A w
eb a
rtic
lere
por
ted
that
12
dea
d fi
nle
ss p
orp
oise
s fo
un
d d
uri
ng
an
ext
rem
ely
low
-w
ater
per
iod
fro
m 3
Mar
ch−
15 A
pri
l 20
12 a
t or
nea
r D
ong
tin
g L
ake,
th
e se
con
dla
rges
t la
ke
in C
hin
a, a
t le
ast
one
of w
hic
h h
ad f
ish
in it
s m
outh
at
the
tim
e of
dea
th,
had
‘d
ied
fro
m e
lect
rofi
shin
g.’
Th
e ar
ticl
e st
ated
th
at s
ome
of t
he
por
poi
ses
die
dd
irec
tly
from
ele
ctri
cal
shoc
k w
hil
e ot
her
s ‘f
ain
ted
’ fr
om t
he
shoc
k a
nd
th
en d
ied
‘fro
m d
row
nin
g’.
htt
p:/
/yan
gtz
efin
less
por
poi
se.
wee
bly
. com
/
Ind
us
Pla
tan
ista
gan
get
ica
min
orIl
leg
al: P
akis
tan
Ele
ctro
fish
ing
is
an e
mer
gin
g t
hre
at t
o th
e In
du
s d
olp
hin
s as
we
coll
ecte
d 6
car
-ca
sses
of
the
dol
ph
ins
last
yea
r (2
017)
fro
m t
he
up
stre
am G
ud
du
Bar
rag
e. W
hen
we
inve
stig
ated
th
e m
orta
lity
an
d i
nte
rvie
wed
nea
rby
inh
abit
ants
th
ey s
aid
th
e ca
r-ca
sses
wer
e b
elie
ved
to
hav
e d
ied
fro
m e
lect
ric
shoc
k.
Th
e lo
cal
fish
erm
en a
lso
said
th
at t
he
elec
trof
ish
ers
wer
e n
ot t
he
fish
erm
en,
they
wer
e th
e cr
imin
als,
wh
o h
ad b
een
hid
den
in t
he
thic
k r
iver
ine
fore
st s
ince
mor
e th
ana
year
an
d u
sed
12
V 1
60 A
bat
teri
es o
r el
ectr
ic g
ener
ator
s to
cat
ch th
e m
axim
um
fish
in n
o ti
me.
Mor
e re
cen
t fi
eld
in
vest
igat
ion
s in
201
8 re
veal
ed t
hat
loc
al f
ish
erm
en a
re a
lso
invo
lved
in e
lect
rofi
shin
g, w
ork
ing
in g
rou
ps
in s
hal
low
poo
ls a
nd
lak
es u
pst
ream
of
the
Gu
dd
u &
Su
kk
ur
Bar
rag
es. T
he
dol
ph
ins
ente
r th
ese
lak
es d
ue
to t
he
abu
nd
ance
of f
ish
, b
ut
in l
ow f
low
mos
t of
th
e la
kes
an
d w
ater
bod
ies
bec
ome
cut
off
from
th
em
ain
stre
am a
nd
the
dol
ph
ins
bec
ome
trap
ped
. Th
e sh
allo
w w
ater
an
d a
bu
nd
ant f
ish
also
att
ract
th
e fi
sher
men
wh
o u
se t
he
pow
erfu
l AC
gen
erat
ors
to f
ish
in t
hes
e la
kes
.
M. I
. Mal
ik p
ers.
ob
s.
Gan
ges
-B
rah
map
utr
aP
lata
nis
ta g
ang
etic
ag
ang
etic
aIl
leg
al: N
epal
, In
dia
Nor
thea
ster
n I
nd
ia:
'Usi
ng
som
e cr
ud
e d
evic
es,
an e
lect
ric
fiel
d i
s cr
eate
d i
n w
ater
to i
mm
obil
ize
and
col
lect
ed f
ish
. C
urr
ent
is p
asse
d t
hro
ug
h a
nak
ed w
ire/
pla
te a
nd
the
fish
es a
re c
olle
cted
wit
h h
and
s or
net
s. T
his
met
hod
is
pop
ula
r in
mos
t p
arts
of
Man
ipu
r.'
Gu
rum
ay
am
& C
ho
ud
hu
ry (
20
09
,p
. 239
), D
. Su
tari
a p
ers.
com
m.,
N.
Kel
kar
per
s. c
omm
., R
. K. S
inh
ap
ers.
com
m.
Ku
lsi R
iver
: 'C
hem
ical
poi
son
ing
, dyn
amit
ing
, ele
ctro
-fis
hin
g, e
tc.,
bec
ome
very
pop
u-
lar
bu
t d
estr
uct
ive
for
aqu
atic
eco
syst
em i
ncl
ud
ing
fis
hes
ag
ain
st t
he
dis
cuss
ed t
rad
i-ti
onal
met
hod
s.'
Isla
m e
t al
. (20
13, p
. 291
)
Up
per
Gan
ga
Riv
er s
yste
m o
f C
entr
al H
imal
aya,
In
dia
: 'e
lect
ric
wir
e is
con
nec
ted
to
the
mai
n p
ower
on
an
ele
ctri
c p
ole
or s
omet
imes
con
nec
ted
to
the
por
tab
le g
ener
a-to
r. W
ire
is t
hen
pas
sed
alo
ng
th
e ri
ver
ban
k a
nd
is
con
nec
ted
wit
h n
aked
wir
ed
ipp
ed i
n t
he
stre
am w
ater
. A
s th
e cu
rren
t is
pas
sed
to
the
stre
am,
elec
tric
fie
ld i
sd
evel
oped
up
to
cert
ain
dis
tan
ce i
n t
he
stre
am.
In t
he
reg
ion
of
elec
tric
fie
ld a
ll t
he
fish
es (
fry,
fin
ger
lin
gs,
ju
ven
ile
and
bro
oder
) ei
ther
get
kil
led
or
par
alyz
ed;
floa
tin
gto
the
wat
er s
urf
ace
and
eas
y to
cat
ch w
ith
han
ds.
Fis
hes
wh
ich
nor
mal
ly e
scap
e n
et-
tin
g b
y h
idin
g u
nd
er t
he
rock
s ar
e ca
ug
ht
easi
ly u
sin
g e
lect
ric
curr
ent.
Man
y p
re-
cau
tion
s ar
e n
eed
ed f
or a
pp
lyin
g t
he
elec
tric
cu
rren
t.'
Sin
gh
& A
gar
wal
(20
14, p
. 201
)
Nep
al:
‘We
can
usu
ally
see
use
of
‘tra
dit
ion
al’
typ
e of
ele
ctri
c fi
shin
g i
n N
epal
. W
eh
ave
seen
it
ofte
n i
n t
he
fiel
d d
uri
ng
ou
r su
rvey
tim
es.
It i
s m
ore
com
mon
in
th
eS
apta
Kos
hi
and
Kar
nal
i ri
vers
th
an i
n N
aray
ani
rive
r sy
stem
s. A
ll th
ese
rive
r sy
stem
scu
rren
tly
hav
e ri
ver
dol
ph
in p
opu
lati
ons.
I d
on’t
th
ink
th
ere
are
reco
rd o
f in
cid
ents
du
e to
ele
ctri
c fi
shin
g. W
e h
ave
only
fis
her
ies
byc
atch
.’
S. P
aud
el p
ers.
com
m.
Table 1 continued
Endang Species Res 39: 207–220, 2019
As with most cetacean deaths other than thosecaused by deliberate hunting or fishery bycatch (atleast when the animal is found in the gear), the causeof death of freshwater cetaceans usually has to bedetermined from examining tissues obtained fromcarcasses. Death by electricity is often invoked as thecause when carcasses are unmarked or bear littleexternal or internal evidence of fishery interaction,disease, or trauma (Zhou et al. 1998). Cause of deathhas been attributed to electrofishing for Yangtze fin-less porpoises, baijis, Indus River dolphins (bhulans),and Irrawaddy dolphins in the Mahakam, Mekong,and Ayeyarwady rivers (Table 1).
Conclusively identifying electric shock as a causeof death is difficult, and the absence of signs causedby other factors such as net entanglements or propellerwounds does not necessarily imply electric shock asthe cause of death. In humans, electrocution is iden-tified by histological detection of damage to cardiacmuscle and, in cases of severe electric shock (e.g.lightning strike), burning of surface tissue (Wright &Davis 1980, Fineschi et al. 2006). Ideally, a sample offresh cardiac muscle tissue is available that can befixed in formalin and examined histologically to detectcharacteristic lesions of cardiomyopathy. However, itis rare to recover carcasses of freshwater cetaceans,and even more rare for someone to recover sampleswithin 24 h of death and fix them in formalin for his-tological examination.
3.3. River-by-river reports
Table 1 provides the details of reports of electro -fishing and operational interactions with freshwatercetaceans in the rivers of Asia.
3.3.1. Yangtze River
Yangtze River finless porpoises. Reports of electro -fishing as the possible cause of death of cetaceanscome most frequently from China and involve Yangtzefinless porpoises and baijis in the Yangtze River. Themost commonly reported sources of mortality of fin-less porpoises in the Yangtze are boat strikes andentanglement in fishing gear.
The most recent reports of finless porpoise mortalityfrom electrofishing and other illegal fishing activitiescome from the 2 large lakes (Poyang and Dongtinglakes) adjoining the Yangtze River (Fig. 1). Mei et.al. (2019) reported that of the 29 out of 60 dead por-poises collected from 2008 to 2013 from Poyang
Lake (the largest lake in China) for which cause ofdeath could be determined, 11 (37.9%) were killeddirectly by illegal fishing (1 by rolling hooks, 3 byset nets, and 7 by electrofishing gear) and only 1was killed by a propeller strike. In 2009 during alow-water period at Poyang Lake, several porpoiseswere found that had been injured or killed byentanglement in fixed stake nets (http:// english. ihb.cas. cn/ rh/ rp/ 201003/ t20100319_51725. html).
The cause of death was determined for 7 out of 22dead porpoises collected from 2008 to 2012 in Dong -ting Lake: 4 were killed by illegal fishing (2 by elec-trofishing, 1 by rolling hooks, and 1 by set nets) and 3were killed by propeller strikes. In addition to risksposed by illegal fishing gear and methods, the finlessporpoises in the lakes adjoining the Yangtze Riverare exposed to other threats, including toxic pollutionand disturbance from sand mining and dredgingactivities (Wang & Reeves 2017). There was concernthat the increasing use of both stake nets and elec-trofishing during times of low water would increasethe risk of injury to the porpoises.
In a personal communication to F. M. D. Gulland(10 July 2017), Dr. Zheng Jinsong of the Institute ofHydrobiology, Chinese Academy of Sciences, notedthat electrofishing was inferred to be the cause ofdeath for Yangtze finless porpoises by ruling outother possible causes, such as boat impact or pro-peller wounds, entanglement in fishing gear, disease,or starvation, and on the basis of the presence of elec-trofishing in the area. Usually, the carcasses forwhich electrocution was assigned as the cause ofdeath were intact without any obvious trauma orwounds, although sometimes there were lesions suchas blood spots in the blubber, signs of ‘sclerosis’ ofthe heart (distended blood vessels), symptoms ofasphyxiation in the lungs, and intact prey fish in theforestomach, all supportive of a diagnosis of suddendeath due to electrocution. For example, investiga-tors concluded that an adult female finless porpoisethat stranded with no obvious evidence of external orinternal trauma, appeared to be in good health, andhad fed just prior to death, was most likely killedby electrofishing (Turvey 2009, https:// www. edge ofexistence. org/ blog/ what- is- killing- finless- porpoises-in- the- yangtze/).
Baijis. Electrofishing has been widely cited as acontributing factor in the decline and extinction ofthe baiji (Chen & Liu 1992, Turvey et al. 2007, Smithet al. 2017), although mortality caused by rollinghook longlines, gillnets, fish traps, boat propellers,and explosives was more conclusively documented(Perrin & Brownell 1989, Zhou & Zhang 1991). Death
214
Thomas et al.: Electrofishing and freshwater cetaceans
from electrofishing was usually inferred from a lackof external marks or internal injuries on carcasses —when animals were found dead with no observedlesions, electrofishing was often judged to be themost probable cause of death. The most direct state-ment related to the baiji comes from the 1980s, whenChen & Hua (1989, p. 84) stated, ‘In some cases, theelectro fishing teams are formed by several electrofish-ing vessels cooperating with small boats. One dolphinwas killed in February 1981 near Paizhou in this way.’
The limited information on causes of baiji deaths,summarized below, suggests that the overall con -tribution of electrofishing to the species’ demise isless certain than is sometimes implied or stated in theliterature.
In the middle reaches of the Yangtze, 15 of 28 recor -ded baiji deaths from 1973 to 1983 were caused byrolling hook longlines (Zhou & Wang 1994). Of 31baiji carcasses from the lower Yangtze examined todetermine cause of death between 1978 and 1985, 13were entangled in rolling hook longlines or caught innets, 6 were killed by illegal fishing with explosives,10 were struck by propellers, 1 was trapped in asluice gate, and 1 was simply reported as stranded(Zhou & Li 1989). Of 13 carcasses from the lowerYangtze examined between 1989 and 1996, 3 werefound entangled with hooks, 1 bore propeller wounds,and 4 had no evidence of entanglement in nets orrolling hook longlines (Zhou et al. 1998, p. 128). Inthe absence of an obvious cause, Zhou et al. (1998)
concluded that these last 4 were ‘pos-sibly killed by shocks of electric fish-ing,’ which reportedly was increasingon the river at the time. In a sample of12 baiji carcasses examined between1990 and 1999 (apparently distinctfrom those reported by Zhou et al.1998), a cause of death was given for10, of which 4 were said to have diedfrom electrocution (Zhang et al. 2003).This appears to be the source of thecon clusion that electrofishing ac -counted for 40% of baiji deaths in the1990s and was ‘apparently the biggestthreat to baiji survival’ (Turvey 2008,p. 39). While the possibility of deathsfrom electrofishing cannot be dis-counted, little or no direct evidencewas provided to substantiate thesecause-of-death determinations (Wanget al. 2006, Smith et al. 2017).
The reports on the 2 species (baijiand finless porpoise) from the Yangtze
River suggest 2 possible scenarios in which electro-cution of cetaceans could occur: (1) the animals areentrapped by declining water levels in segments of alake or side-channel of the river, and (2) the dolphinsor porpoises are corralled, intentionally or un in ten -tion ally, by multiple electro fishing boats workingtogether. The entrapped or encircled animals mightthen be ex po sed to electric shock at close proximityas the fishermen seek to stun the fish near them (Linet al. 1985, Chen & Hua 1989, Zhou & Li 1989, Zhou& Wang 1994, Zhou et al. 1998, Zhang et al. 2003).
3.3.2. Indus River dolphins
Electrofishing is regarded as an emerging threatin Pakistan. Six dolphin carcasses (4 males, 2 fe -males) were collected between 10 October and 12December 2017 from an area 5−10 km upstream ofthe Guddu Barrage (M. I. Malik unpubl. data).Nearby villagers who were interviewed said theybelieved that the animals had died from electricshock in the vicinity of electrofishing conducted bycriminals operating out of the thick riparian forest.More recent field investigations in 2018 revealedthat local fishermen are also involved in electro -fishing, working in groups in shallow pools andlakes upstream of the Guddu and Sukkur barrageswhere they might encounter dolphins (M. I. Malikunpubl. data).
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Fig. 1. Yangtze River finless porpoise (foreground) near large electrofishing boatwith an electrified trawl in Poyang Lake, China, December 2017. Electric currentfrom the wires on the net stuns or kills fish, which are then collected by thenet. Despite the presence of electrofishing, entanglement in fishing gear-remains the greatest threat to freshwater cetaceans. Photo credit: Dr. X. Yijie
Endang Species Res 39: 207–220, 2019
3.3.3. Ganges River dolphins
Electrofishing is practiced in various forms in theGanges River (see Table 1) but we have found noreports of Ganges River dolphins being affected.
3.3.4. Irrawaddy dolphins in the Mahakam River
Fishermen in the Mahakam River reported 2deaths of Irrawaddy dolphins suspected to haveresulted from electric shock, one in 2003 (Kreb etal. 2007) and one in 2008 (Kreb et al. 2010). Onthe first occasion, a dead juvenile dolphin wasseen floating with a blue bruise on its head afterit came in contact with an electrofishing boat. Onthe second occasion, some people were observedfishing with high-voltage AC generators in smallpools and channels of the upstream section of theMahakam. They were deliberately chasing andharassing dolphins, and not long afterward, a fish-erman observed a dead calf floating downstream.According to his statement, the dead calf’s bellywas blue.
In the first instance, the dolphin apparently camein direct contact with the electrodes of the electro -fishing apparatus and was electrocuted. The secondof these reports suggests that in the small pools andchannels of the upper Mahakam it is more difficultfor dolphins to avoid the fishing activity than it is inbroader downstream reaches. D. Kreb has also ob -served dolphins approaching and then moving awayfrom electrofishing operations without harm, indica-ting that proximity alone does not necessarily leadto death or injury (Fig. 2).
3.3.5. Irrawaddy dolphins in the Ayeyarwady River
Electrofishing has been mentioned repeatedly as acause of dolphin mortality in the Ayeyarwady River,and some recent observations were mentioned above.Smith & Tun (2007) cited electrocution as a directthreat, but provided few details. On 4 December 2014,2 dead young dolphins (male and female) were foundon the same day in a protected stretch of the rivernorth of Mandalay with no sign of entanglement and‘no sign of toxics.’ According to a journalist (Holland2015), sources he interviewed stated that necropsiesshowed the dolphins to have been victims of elec-trofishing. This report, like so many that cite electro-cution as the cause of death of freshwater cetaceans,has proven impossible to validate.
3.3.6. Irrawaddy dolphins in the Mekong River
Electrofishing has been reported and considered asa possible cause of death for Irrawaddy dolphins inthe Mekong River. It is reported as a widespread andcommon practice within and around dolphin habitat(Phen & Nam 2011) and has been the subject of sig-nificant enforcement efforts. From October 2013 toMay 2017, river guards aggressively enforced the banon electrofishing, with 53 cases of interdiction andconfiscation of electroshock units (including in -verters, batteries, and wires) (S. Keo pers. comm.).This enforcement effort was centered on what arecalled core dolphin areas and buffer zones. The riverguards encountered electrofishing during day andnight, but mostly at night. Electro-fishermen weresaid to fish from motorboats in mid-stream, not alongthe shore.
Carcass recovery and investigations of the causesof dolphin mortality have been carried out in theMekong in southern Laos and Cambodia since theearly to mid 1990s (Baird & Mounsouphom 1994, 1997,Gilbert & Beasley 2005, WWF & FiA 2014, 2017). Thepresence of external net marks with no internal lesionsand full stomachs is almost always interpreted toindicate gillnet entanglement as the cause of death.Although electrofishing has never been demon-strated as having a role in the death of dolphins in theMekong River, the appearance of un marked car-casses (often neonates or very young calves) that lackevidence of net entanglement or other wounding hasled to speculation that electrofishing is a source ofmortality (G. Ryan and S. Phay pers. obs.). Since 2012there has been an effort (led by WWF-Cambodia) tosecure and perform necropsies on the carcasses ofthese young dolphins. Roughly 20% ex hibit internalevidence of blunt trauma that, while still poorly doc-umented, may be the result of intra specific interac-tions in which adult dolphins rush at and hit calves,sometimes throwing them out of the water; at leastsome of these interactions occur at or near the time ofbirth (WWF & FiA 2017). Until other possible causessuch as infectious diseases and toxicosis have notbeen ruled out, death from electrofishing interactionis not indicated for these young animals.
4. CONCLUSIONS
While we do not have any information on the elec-trical power required to injure or kill a dolphin orporpoise, or how closely an animal would have toapproach an electrical field to be injured or killed,
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Thomas et al.: Electrofishing and freshwater cetaceans
the general factors of increasing impact of electricityon organisms of larger body size, mammalian sus-ceptibility to respiratory and cardiac impacts fromelectrical currents, and decreased electrical resist-ance of the hairless, poorly keratinized cetacean skinin water, all suggest that freshwater cetaceans are atrisk of injury or death if they come in close enoughcontact with electrical fields. The reporting of illegalelectrofishing, confiscation of gear, and anecdotalreports of carcasses with no external evidence oftrauma from nets or lines or vessel strike all supportthe concern that electrofishing poses a threat toendangered cetacean populations.
Unlike electric whaling, electrofishing techniquesare usually designed to stun fish as they encounterelectrical fields, not to electrocute the fish by directcontact with electrodes. We have found no evidenceof intentional electrocution of dolphins or porpoisesby fishermen in the rivers of Asia and are unaware ofany incentive for such a practice to evolve (but seeRobards & Reeves [2011] for discussion of how theconsumption of marine mammals, and in turn theirdeliberate capture, has arisen out of other types ofbycatch mortality).
Whether, and under what conditions, free-swim-ming dolphins or porpoises would move into thevicinity of electrofishing operations that could harmor kill them and how close they would have to cometo be harmed remain open questions. The evidencecited above, that cetaceans can detect low-level elec-
trical fields and that pinnipeds andother aquatic mammals are deterredby them, suggests that free-swimmingfreshwater cetaceans could detect andavoid such fields as long as they recog-nize the danger. This capability ofdetection and avoidance would dependon how the electrofishing equipment isdeployed in the vicinity of cetaceans:whether the electric current is on con-tinuously, turned on and off to con-serve energy, or deployed in a mannerto optimize the number of fish killed orstunned. It also may depend on theearlier experiences of the cetaceansswimming close to electrical fields — anovel experience could arouse curios-ity, particularly in young animals,inducing them to swim closer to inves-tigate. The few observations that wehave to go on suggest the followingscenarios where cetaceans might beput at risk: (1) entrapment in shallow
water or restricted river or lake areas (resulting fromdrought, or the intentional draining or enclosure ofwater bodies to entrap fish) and subsequent inabilityto move away from electrofishing activities (e.g.Yangtze finless porpoises); (2) encounters betweencetaceans and electrofishing activities in naturallyrestricted river reaches such as shallow upstreamareas and narrow tributaries where it is difficult tomove away from the electrical charges in the water(e.g. Mahakam and Indus dolphins); (3) incidentalor deliberate corralling of cetaceans into the prox-imity of electrofishing apparatus by multiple fishingboats working together (e.g. baijis and Ayeyarwadyand Mahakham dolphins); (4) appropriation by electro-fishermen of the mutualistic behavioral cues usedby dolphins and throw-net fishermen (‘cooperative fishing’) to get near fish concentrations (e.g.Ayeyarwady dolphins); (5) intentionally leaving elec-trofishing gear switched off and allowing cetaceansto approach an area while chasing fish, then turningthe current back on when the cetaceans are insidethe ‘effective’ danger zone to stun the fish herded inby the dolphins (e.g. Ayeyarwady dolphins); and (6)attraction of cetaceans to fish that have been immo-bilized by electrofishing activities (e.g. Mahakamdolphins).
Except possibly in the case of Ayeyarwady throw-net fishermen exploiting the mutualistic relationshipbetween them and dolphins, and the single reportedcase of harassment in the Mahakam, we found no
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Fig. 2. Small boat fishing with electrofishing gear powered by a portable electricgenerator in the vicinity of an Irrawaddy dolphin mother and calf pair, MahakamRiver, Indonesia. Areas of abundant fish such as deep pools, confluences, andeddies may draw freshwater cetaceans and fishermen into close proximity.
Photo credit: D. Kreb,Yayasan Rasi
Endang Species Res 39: 207–220, 2019
evidence of people intentionally drawing cetaceansinto the proximity of electrofishing operations inorder to capture, kill, or harass them.
Mortality of cetaceans from exposure to electrofish-ing appears to be uncommon, but this must be con-sidered in the context of populations with fewer than100 individuals in several river systems (Irrawaddydolphins in the Mahakam, Mekong, and Ayeyarwadyrivers) or that are surrounded by human activityand subject to numerous threats (finless porpoises inthe Yangtze River). Diagnosis of cause of death forstranded cetaceans is generally difficult, but particu-larly so for determining death by electrocution (orelectrically induced paralysis and subsequent drown-ing). Fresh carcasses suitable for histological exami-nation of tissues, and collection and formalin fixationof heart and lung tissue are needed, but in most cir-cumstances, there is little or no local expertise in fieldnecropsy techniques.
This review indicates a need for further assessmentof electrofishing practices and of when and how suchfishing might threaten freshwater cetaceans. Knowingmore about the nature of interactions between theanimals and the operations, the ability of cetaceans todetect and respond to electrical currents, the impacton immersed mammals of the electrical fields pro-duced by electrofishing, and diagnostic histology ontissues from dead animals would enable improvedassessment of the implications for conservation. Weacknowledge, however, that pursuing research onthe effects of electrical currents on cetaceans and theirability to detect currents is ethically and logisticallychallenging, and thus not a recommended avenue topursue. Rather, efforts should focus on obtaining freshcarcasses to determine cause of death through histo-logical examination of fresh tissues fixed in formalin,as this will also enhance detection of other threats.
Acknowledgements. The authors thank Jan Dean and AlanTemple, who teach legal electrofishing sampling techniquesat the US Fish and Wildlife Service, for sharing their ex -pertise. We thank Ravindra K. Sinha, Dipani Sutaria, andNatchiket Kelkar for information from India and ShambhuPaudel for information from Nepal. F. M.D.G. thanks the Mar-ine Mammal Center where she was employed during thedevelopment of this paper. R.R.R. thanks the Marine Mam-mal Commission for covering part of his time while workingon this paper.
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Editorial responsibility: Jeff Mangel (Guest Editor),Lima, Peru
Submitted: January 15, 2019; Accepted: April 17, 2019Proofs received from author(s): July 4, 2019
https://doi.org/10.1007/s11160-004-1095-9https://doi.org/10.1098/rsbl.2007.0292https://doi.org/10.1016/j.biocon.2012.07.016https://doi.org/10.2305/IUCN.UK.2017-3.RLTS.T41754A50381766.enhttps://doi.org/10.1002/aqc.547www.iucn-csg.org/wp-content/uploads/2010/03/Report-of-the-2017-International-Workshop-on-the-Conservation-of-Irrawaddy-Dolphins-in-the-Mekong-River.pdfhttps://doi.org/10.1520/JFS11252Jhttps://doi.org/10.2305/IUCN.UK.2013-1.RLTS.T43205774A45893487.enhttps://doi.org/10.1065/espr2006.10.350
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