RESEARCH ARTICLE
A No el Tool o Mi iga e By-Ca ch Mo ali y
o Bal ic Seals in Coas al Fyke Ne Fishe y
Sa i M. Oksanen
1
*, Ma kus P. Ahola
2
, Jy ki Oika inen
3
, Me i Kunnas an a
1
1Depa men o Biology, Uni e si y o Eas e n Finland, Joensuu, Finland, 2Na u al Resou ces Ins i u e
Finland, Tu ku, Finland, 3Pe äme en Kala alousyh eisöjen Lii o y, Oulu, Finland
*sa i.oksanen@ue . i
Abs ac
De eloping me hods o educe he inciden al ca ch o non- a ge species is impo an , as
by-ca ch mo ali y poses h ea s especially o la ge aqua ic p eda o s. We examined he e -
ec i eness o a no el de ice, a “seal sock”, in mi iga ing he by-ca ch mo ali y o seals in
coas al yke ne ishe ies in he Bal ic Sea. The seal sock de eloped and es ed in his s udy
was a cylind ical ne a ached o he yke ne , allowing he seals access o he su ace o
b ea he while apped inside ishing gea . The numbe o dead and li e seals caugh in yke
ne s wi hou a seal sock (yea s 2008–2010) and wi h a sock (yea s 2011–2013) was e-
co ded. The seals caugh in yke ne s we e mainly ju eniles. O inged seals (Phoca hispida
bo nica) bo h sexes we e equally ep esen ed, while o g ey seals (Halichoe us g ypus) he
a io was biased (71%) owa ds males. All he by-caugh seals we e dead in he yke ne s
wi hou a seal sock, whe eas 70% o inged seals and 11% o g ey seals su i ed when he
seal sock was used. The seal sock p o ed o be e ec i e in educing he by-ca ch mo ali y
o inged seals, bu did no pe o m as well wi h g ey seals.
In oduc ion
Fishe ies wo ldwide cap u e se e al non- a ge species as inciden al by-ca ch. Popula ions o
many la ge aqua ic p eda o s, such as ma ine mammals and sha ks a e especially ulne able o
by-ca ch mo ali y in consequence o hei slow ep oduc i e a es [1]. Mi iga ion o by-ca ch
mo ali y has he e o e become inc easingly impo an in bo h ishe ies managemen [2] and
wildli e conse a ion [3]. Howe e , he in e ac ions be ween aqua ic p eda o s and ishe ies
can be complex, and in pa icula , many seal species cause losses o ish ca ches and damage o
ishing gea [4,5]. De eloping ishing me hods can p o ide means o mi iga e bo h aspec s o
he seal- ishe ies con lic . Fo example, physical ba ie s moun ed on he ish aps, such as
wi e g ids, p e en seals om en e ing he ap [6–8]. Al e na i ely, exclusion de ices ha acil-
i a e he seals’exi om he ishing gea [9] and echniques ha ap seals ali e inside he gea
ha e also been de eloped [10].
The Bal ic g ey seal (Halichoe us g ypus) and inged seal (Phoca hispida bo nica) ha e had
signi ican in e ac ions wi h human ac i i ies. Bo h popula ions declined d as ically o only
abou 5000 seals in he 1970s as a esul o excessi e hun ing and ep oduc i e diso de s caused
PLOS ONE | DOI:10.1371/jou nal.pone.0127510 May 18, 2015 1/9
OPEN ACCESS
Ci a ion: Oksanen SM, Ahola MP, Oika inen J,
Kunnas an a M (2015) A No el Tool o Mi iga e By-
Ca ch Mo ali y o Bal ic Seals in Coas al Fyke Ne
Fishe y. PLoS ONE 10(5): e0127510. doi:10.1371/
jou nal.pone.0127510
Academic Edi o : Jan Gee Hiddink, Bango
Uni e si y, UNITED KINGDOM
Recei ed: Decembe 30, 2014
Accep ed: Ap il 16, 2015
Published: May 18, 2015
Copy igh : © 2015 Oksanen e al. This is an open
access a icle dis ibu ed unde he e ms o he
C ea i e Commons A ibu ion License, which pe mi s
un es ic ed use, dis ibu ion, and ep oduc ion in any
medium, p o ided he o iginal au ho and sou ce a e
c edi ed.
Da a A ailabili y S a emen : All ele an da a a e
wi hin he pape and i s Suppo ing In o ma ion iles.
Funding: This s udy was pa ly unded om he
Eu opean Fishe ies Fund (h p://ec.eu opa.eu/
ishe ies/c p/e /index_en.h m) awa ded o he Finnish
Game and Fishe ies Resea ch Ins i u e by he ELY-
Cen e o Kainuu (#1000862, #1001767) and he Maj
and To Nessling Founda ion (h p://www.nessling. i/
en/, #201200171, 201300044, 201400081,
201500146 o SMO, MK). The unde s had no ole in
s udy design, da a collec ion and analysis, decision o
publish, o p epa a ion o he manusc ip .
by en i onmen al pollu ion. The seal popula ions ha e been eco e ing, bu he p esen num-
be o g ey seals (census size: 30 000 seals) is s ill less han hal and inged seals (13 000 seals)
less han one en h o he es ima ed his o ical abundance a he beginning o he 20
h
cen u y
[11–13]. The eco e y o hese popula ions has led o an inc ease in seal-induced losses o
coas al ishe ies, which has had nega i e e ec s on a i udes owa ds he conse a ion o seal
popula ions [14]. The Bal ic g ey seal in gene al causes mo e losses o ishe ies han he inged
seal [6,15]. Howe e , inc easing numbe s o inged seals a e assumed o cause subs an ial ca ch
losses in he Bo hnian Bay, he no he nmos pa o he Bal ic Sea (Fig 1)[16]. The hun ing o
seals has become a key ac ion o he mi iga ion o damage in he no he n Bal ic Sea [14,17].
In addi ion o hun ing, by-ca ch mo ali y may be ano he signi ican componen o he an-
h opogenic mo ali y o seal popula ions. Es ima ed annual by-ca ch o he Bal ic g ey seal is
in he o de o 2000 animals o mo e [18] bu o inged seals he magni ude is unknown. An
in o ma ion gap ega ding by-ca ch and i s mi iga ion me hods in coas al and small scale ish-
e ies has been ecognized globally [19]. Also in he Bal ic Sea b oade knowledge o by-ca ch is
o g ea impo ance o sus ainable ishe y and popula ion managemen [20].
Conside able e o s ha e been pu in o esea ch on in e ac ion be ween Bal ic g ey seal and
ishe ies [10,14,17,18,21,22] and on he de elopmen o seal-p oo ishing gea . Fo example, ap
ne s wi h modi ied ish chambe s, i.e. pon oon aps, a e epo ed o educe g ey seal-induced
losses o ishe ies and simul aneously o educe by-ca ch [8,23]. Howe e , ela i ely li le a en ion
has been paid o es ima ing and educing he by-ca ch mo ali y o seals in o he s a iona y gea s.
In addi ion, he e is li le in o ma ion on he in e ac ion be ween inged seals and ishe ies, no
only in he Bal ic Sea [24], bu also globally. Me hods o educing in e ac ions o Bal ic inged
seals and coas al ishe y a e o g ea impo ance, as clima e change can be expec ed o u he
h ea en he popula ion g ow h o his ice-b eeding species, especially in he sou he n b eeding
a eas, whe e he s ocks ha e eco e ed slowly, i a all [12]. In his s udy we es ed he e ec i eness
o a new ype o pinniped by-ca ch educ ion de ice o yke ne s, called a seal sock. We also ex-
amined he demog aphic p ope ies, such as age and sex dis ibu ion, o he by-caugh seals.
Me hods
A ound 70% o he Bal ic inged seal popula ion is ound in he Bo hnian Bay (Fig 1), and g ey
seal is also encoun e ed in his egion. Coas al yke ne ishe y in he a ea a ge s whi e ish
(Co egonus la a e us), endace (C.albula), A lan ic salmon (Salmo sala ) and b own ou (S.
u a) in pa icula . Va ious modi ica ions o yke ne s a e used, bu in gene al hey a e la ge,
bo om-ancho ed ishing gea ha consis s o a leade ne and wings guiding he ish h ough
he middle chambe in o he ish chambe . Seals en e ing he chambe s may no ind hei way
ou and d own, as he chambe s a e usually subme ged (app oxima ely 2 m om he su ace).
The seal sock is a by-ca ch educ ion de ice ha is designed o enable seals o ha e access o
he su ace o b ea he while apped inside he yke ne . The sock was ini ially de eloped and
cons uc ed by a Finnish ish ap manu ac u e (Ab Scandi Ne Oy).
We de eloped a modi ica ion o he seal sock, comp ising a cylind ical ne (diame e 0.7 m,
leng h 2.5–4.0 m) made o s ong seal-p oo ne ing (Dyneema, mesh size 30 mm). I was a -
ached o he oo o he ish chambe in a yke ne (Fig 2A). The sock was i ed wi h one o
wo hoops, o keep i s shape, and a small loa . To examine he e ec i eness o he seal sock in
educing by-ca ch, we compa ed he numbe o seals caugh ali e in a se o yke ne s be ween
yea s when he socks we e no used (2008–2010) and when hey we e used (2011–2013). An-
nually, be ween May and Oc obe –No embe , a comme cial ishe man ished wi h 4–6 yke
ne s wi hin he same ishing a ea in he Bo hnian Bay (64°32N, 24°19E, Fig 1). The yke ne s
comp ised a leade ne (mesh size 200–300 mm), wings (60–100 mm) and wo cylind ical
Mi iga ing By-Ca ch o Bal ic Seals
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Compe ing In e es s: The au ho s ha e decla ed
ha no compe ing in e es s exis .
chambe s (30–35 mm, Dyneema, Fig 2B). Fishing e o was de e mined based on he ishe -
man’s logbooks and was ela i ely same be ween bo h pe iods (2106 days o yea s 2008–2010
and 1767 days o yea s 2011–2013).
The ishe man kep a logbook on he numbe , species and cap u e da e o by-caugh seals.
Du ing he es ing o he sock in he yea s 2011–2013, he ishe man also eco ded he sex and
weigh (kg) o age class (ju eniles and adul s) o he by-caugh seals. Resea che s ook pa o
he ield wo k ( agging li e animals and aking samples om dead ones) du ing yea s 2011–
Fig 1. The Bal ic Sea.
doi:10.1371/jou nal.pone.0127510.g001
Mi iga ing By-Ca ch o Bal ic Seals
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2013 and hey o en had possibili y o e i y he eliabili y o he da a. Al hough he ishe men’s
mo i a ion o epo by-ca ch has gene ally been low in Finland [18], he olume o by-ca ch e-
po ed in his s udy is high and compa able be ween he wo pe iods (2008–2010 and 2011–
2013). I is he e o e likely ha he olumes o by-ca ch de ec ed in ou s udy a e no unde -
a ed. Weighed seals we e u he di ided in o age classes on he basis o an age-weigh da abase
(Na u al Resou ces Ins i u e Finland). Weighed inged seals wi h a body weigh >50 kg we e
classi ied as adul s (es ima ed age 4 yea s). G ey seal males wi h a body weigh >92 kg and
emales >65 kg we e ca ego ized as adul s (es ima ed age 5 yea s). We also used he seal
socks o li e-cap u ing inged seals o sa elli e agging (Oksanen e al., unpublished). We es ed
he e ec o he seal sock on su i al wi h Fishe ’s exac es . The e ec o weigh and cap u e
mon h on su i al o inged seals when he sock was in use (2011–2013) was es ed wi h bina y
logis ic eg ession. E ec s o weigh and mon h on he g ey seal su i al we e no es ed due o
he small sample size. We conduc ed s a is ical es ing wi h IBM SPSS S a is ics 20 so wa e.
E hics S a emen
The use o a seal sock and animal handling was pe mi ed by he game au ho i ies (pe mi no.
2011/00082 and 2013/00197) and he Animal Expe imen Boa d o Finland (no. ES AVI/1114/
04.10.03/2011).
Fig 2. A ype o by-ca ch educ ion de ice, a seal sock, is a ached o he ish chambe o a yke ne allowing he seal o ha e access o he su ace.
(A) A side iew o a seal sock and chambe s. (B) A yke ne (seen om abo e) consis s o a leade ne , wings and chambe s.
doi:10.1371/jou nal.pone.0127510.g002
Mi iga ing By-Ca ch o Bal ic Seals
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Resul s
A o al o 135 seals (30 li e and 105 dead) we e caugh (4–6 yke ne s annually) du ing he
s udy yea s in 2008–2013 (S1 Da ase ). O all he by-caugh seals, 103 (76%) we e inged and
32 (24%) g ey seals. Du ing he yea s 2011–2013, 95% o he inged (n = 40) and 67% o he
g ey seals (n = 18) we e ju eniles (Table 1). The mean weigh o he by-caugh and weighed
inged seals (n = 33) was 36 kg (SD 11). The weigh o he inged seals isi ing yke ne s in-
c eased owa ds au umn (linea eg ession, R
2
= 0.469, F = 27.3, p<0.001; Table 1): inged
seals by-caugh and weighed in May (n = 4) had a mean weigh o 19 kg (SD 9) (co esponding
o young-o - he-yea ), whe eas indi iduals by-caugh in Oc obe –No embe (n = 7) weighed
on a e age 44 kg (SD 10). The sex- a io o he inged seals was e en (49% males and 51% e-
males), bu o he g ey seals i was biased owa ds males (71% males and 29% emales;
Table 1). In ac , while he sex dis ibu ion o ju enile g ey seals (n = 12) was e en and hey
we e caugh mos ly du ing summe (8 caugh in May-July), he adul s (n = 6) we e mos ly
males (5 males, 1 unknown) and mos ly cap u ed du ing he au umn (5 caugh in Augus -No-
embe : Table 1).
O e all, he su i al o seals in he yke ne s inc eased when he sock was used (Fishe ’s
exac es , p<0.001). Al oge he 77 dead seals we e by-caugh du ing he yea s when he seal
sock was no used (2008–2010), whe eas 30 li e and 28 dead seals we e caugh when he seal
sock was used (2011–2013) (Fig 3). The seal sock inc eased su i al o inged seals (p<0.001):
o all caugh inged seals (n = 40), 83% ound hei way o he sock and 70% emained ali e.
Howe e , he su i al o g ey seals was no inc eased (p = 0.308): only 17% o he g ey (n = 18)
seals ound way o he sock, and 11% emained ali e. The inged seals ha did no ind hei
way in o he sock we e mos ly small pups (5/7) ha ing a mean weigh o 16 kg (SD 9) and hey
we e mos ly cap u ed in ea ly summe . In ac , cap u e mon h was he only s a is ically signi i-
can p edic o o inged seal su i al in logis ic eg ession (model summa y: χ
2
(1) = 4.210,
p = 0.040, Nagelke ke R
2
= 0.142; pa ame e mon h: p = 0.048, coe icien = –0,405). By con-
as , all he by-caugh adul g ey seals (n = 6) we e caugh in he middle chambe , while 8 ou
o 12 o ju eniles we e ei he in he ish chambe o in he sock. Howe e , only 3 ou o 8 g ey
seals eaching he ish chambe ound hei way in o he sock.
Table 1. Mon hly a ia ion in numbe s, age and sex dis ibu ion and su i al o he by-caugh seals in yke ne s equipped wi h he seal sock.
Mon h Ringed seals G ey seals To al
To al Ju eniles/ adul s Males/ emales Ali e/ dead Mean weigh To al Ju eniles/ adul s Males/ emales Ali e/ dead
May 6 6/0 3/3 2/4 19 ±9 3 3/0 1/2 0/3 9
June 1 1/0 1/0 0/1 na 3 2/1 3/0 0/3 4
July 3 3/0 2/1 2/1 24 ±8 3 3/0 1/2 0/3 6
Aug 4 4/0 2/2 4/0 34 ±8 2 1/1 2/0 0/2 6
Sep 16 15/1 7/9 13/3 38 ±8 4 2/2 4/0 1/3 20
Oc 8 7/1 3/4
a
5/3 45 ±12 1 1/0 0/1 1/0 9
No 2 2/0 1/1 2/0 43 ±1 2 0/2 1/0
a
0/2 4
To al 40 38/2 19/20 28/12 36 ±11 18 12/6 12/5 2/16 58
In yea s 2011–2013, annually 4–6 yke ne s equipped wi h he seal sock we e se ou o fishing in he Bo hnian Bay. Mon hly mean weigh (kg) ±SD o
inged seals is epo ed o a o al o 33 weighed indi iduals.
a
Gende o one seal no eco ded
doi:10.1371/jou nal.pone.0127510. 001
Mi iga ing By-Ca ch o Bal ic Seals
PLOS ONE | DOI:10.1371/jou nal.pone.0127510 May 18, 2015 5/9

Discussion
The seal sock p o ed o be e ec i e in educing he by-ca ch mo ali y o inged seals in yke
ne s, bu i did no pe o m as well wi h g ey seals. Acco ding o ou pilo es , he seal sock
also seemed o wo k well in a so-called bo om yke ne (Flex R, Ab Scandi Ne Oy), which we
es ed du ing he summe o 2013, and all o he cap u ed seals (7 inged seals and 1 g ey seal)
su i ed in he sock. Al hough he use o he seal sock inc eased su i al o inged seals in gen-
e al, small pups caugh du ing ea ly summe su i ed poo ly despi e he sock. On he con a y,
adul g ey seals we e by-caugh only in he middle chambe , and i is likely ha hey could no
each he ish chambe h ough he na ow passage be ween he chambe s (diame e 0.3 m).
To o e come his p oblem, a sock could be inse ed also in o he middle chambe . Howe e ,
only 3 ou o 8 g ey seals ha eached he ish chambe ound hei way in o he sock, which
may indica e beha io al di e ences be ween he species. Ringed seals keep open b ea hing
holes in he sea ice, which may be se e al me e s hick in he A c ic [25], and swimming up-
wa ds in a na ow unnel migh , he e o e, be mo e ypical beha io o hem. In addi ion o e-
ducing by-ca ch, he seal sock has po en ial applica ions in cap u ing seals ali e and emo ing
indi iduals epea edly isi ing he ishing gea . When he seal sock is used only o by-ca ch
mi iga ion, i could ha e an opening a he su ace enabling seals o climb ou o he gea , as ex-
clusion de ices a e usually designed o enable apid exi o he animal om he ishing gea .
Ou esul s illus a e ha he majo i y o seals by-caugh in yke ne s a e young seals o bo h
gende s. This was especially e iden in inged seals, o which 95% we e ju eniles and equally o
bo h gende s, bu also among g ey seals mos o he by-caugh indi iduals we e ju eniles
(67%). The dominance o ju eniles in by-ca ch could o some ex en be explained by hei na-
i e y, as ju eniles o many seal species a e mo e ulne able o being by-caugh [26,27]. Ringed
seal pups a e especially p one o ge caugh in ishing gea jus a e weaning in he ea ly sum-
me [28]. We also obse ed lowe su i al o inged seals du ing ea ly summe compa ed o
au umn, and ou esul s indica ed ha su i al in he seal sock was mo e dependen on nai e y
o he young-o - he-yea han weigh o he seals. Al hough he o e all sex a io o g ey seals
was biased owa ds males, young g ey seals o bo h sexes we e by-caugh especially in sp ing,
and adul males in au umn. A simila empo al end in by-caugh g ey seals has been epo ed
Fig 3. Mon hly a ia ion in he numbe o by-caugh seals in yke ne s (4–6 yke ne s annually). Fi s
ba s: yke ne s wi hou a seal sock (yea s 2008–2010). Second ba s: yke ne s wi h a seal sock (2011–2013).
doi:10.1371/jou nal.pone.0127510.g003
Mi iga ing By-Ca ch o Bal ic Seals
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in p e ious s udies [29,30]. A s ong male dominance has also been obse ed in g ey seals isi -
ing pon oon aps [10,21,22,31].
Ou s udy indica es ha by-ca ch pa icula ly inc eases ju enile mo ali y, which may no
educe popula ion g ow h and iabili y as much as a simila inc ease in adul —especially e-
male—mo ali y would [32]. Ne e heless, by-ca ch mo ali y may cons i u e ano he signi i-
can sou ce o an h opogenic mo ali y in he g ey seal popula ion, whose annual hun ing
p essu e in he Finnish sea a ea was a ound 5% o he o al popula ion [33], and should he e-
o e be aken in o accoun in he popula ion managemen . By-ca ch mo ali y also inc eases
o al mo ali y o he inged seal popula ion and mi iga ing i may become inc easingly impo -
an o popula ion managemen , as he popula ion g ow h o his ice-dependen species is p e-
dic ed o dec ease due o clima e change [12,34].
Knowledge o he by-ca ch mo ali y and i s mi iga ion me hods a e impo an aspec s o
he sus ainable managemen o seal popula ions. Reducing inciden al mo ali y is also becom-
ing inc easingly impo an measu e o he ishe ies managemen o espond o g owing de-
mands o sus ainable sea ood. Se e al ecolabels ake by-ca ch educ ion in o accoun [2]. Fo
example ecolabel o he Ma ine S ewa dship Council (MSC) is g an ed o ishe ies ha ollow
he sus ainable ishe y s anda ds o he MSC. Ou s udy in oduces a no el and inexpensi e
ool, a seal sock, o mi iga ing by-ca ch o seals in coas al yke ne ishe ies. In addi ion, he
seal sock p o ides a p ac ical and e hical me hod o he selec i e emo al o seals epea edly
isi ing yke ne s in a eas whe e high seal abundance causes subs an ial losses o ishe ies. I
can also be used in scien i ic s udies whe e seals need o be cap u ed ali e, o example eleme-
y s udies.
Suppo ing In o ma ion
S1 Da ase . De ails o seals by-caugh in yke ne s (4–6 yke ne s annually) du ing he yea s
2008–2013. The yke ne s we e se ou o ishing wi hou a seal sock in he yea s 2008–2010
and wi h a seal sock in 2011–2013 in he Bo hnian Bay. Species: Phb = Bal ic inged seal, Phoca
hispida bo nica; Hg = g ey seal, Halichoe us g ypus. Loca ion o he seal in a yke ne : sock/
middle chambe / ish chambe / wings. na = no a ailable
(PDF)
Acknowledgmen s
We would like o hank ishe men M. Vii anen, T. Ma inlassi and M. Pos i o hei close co-
ope a ion du ing he s udy. We also wish o hank K. Kyy önen o he yke ne g aphics and L.
Meh ä alo o he s a is ical consul ing. Thanks a e also due o M. Val onen and anonymous
e iewe s o hei aluable commen s on he manusc ip .
Au ho Con ibu ions
Concei ed and designed he expe imen s: SMO MPA JO MK. Pe o med he expe imen s:
SMO MPA JO MK. Analyzed he da a: SMO. Con ibu ed eagen s/ma e ials/analysis ools:
SMO MPA JO MK. W o e he pape : SMO MPA JO MK.
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