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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
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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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Thomas et al.: Electrofishing and freshwater cetaceans
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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Endang Species Res 39: 207–220, 2019
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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Thomas et al.: Electrofishing and freshwater cetaceans
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.
211
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Endang Species Res 39: 207–220, 2019212
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bat
tery
(or
bat
teri
es)
org
ener
ator
. Th
ese
are
atta
ched
to
a w
ood
en p
ole
that
is h
and
-hel
d o
ff t
he
fron
t of
th
eb
oat
and
use
d t
o sc
oop
ele
ctro
cute
d f
ish
ou
t of
th
e w
ater
. S
imp
le b
atte
ry o
per
ated
gea
r u
ses
a p
ole
wit
h a
n a
nod
e in
on
e h
and
wit
h t
he
fish
ing
net
dep
loye
d w
ith
th
eot
her
han
d t
o sc
oop
ou
t fi
sh. A
rec
ent
gea
r in
nov
atio
n i
s to
att
ach
a s
coop
ing
net
on
2 w
ood
en p
oles
sti
ckin
g o
ut
in f
ron
t of
th
e b
oat
that
has
an
ele
ctri
c cu
rren
t g
oin
gth
rou
gh
th
e n
et i
tsel
f. T
he
woo
den
pol
es a
re m
anu
ally
low
ered
in
to t
he
wat
er a
nd
pu
lled
up
ag
ain
th
ereb
y ef
fect
ivel
y sc
oop
ing
sm
all
rive
r fi
sh a
lon
g t
he
rive
r b
ank
s.T
he
targ
et fi
sh s
pec
ies
is a
lso
kn
own
pre
y fo
r th
e d
olp
hin
s. B
atte
ries
(D
C-b
atte
ry w
ith
bet
wee
n 1
2−60
A, 1
2−24
V)
and
gen
erat
ors
(AC
400−
900
W, 2
20 V
, an
d D
C 1
2 V
8.3
Am
p)
are
use
d a
s p
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
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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).
215
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
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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,
216
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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
217
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
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