ENDANGERED SPECIES RESEARCH
Endang Species Res
Vol. 38: 91–99, 2019
h ps://doi.o g/10.3354/es 00939 Published Feb ua y 21
1. INTRODUCTION
Fishe ies inciden al byca ch poses a signi ican
h ea o many ma ine mammal popula ions, espe-
cially wi h he use o passi e gea , such as gillne s,
which ha e con ibu ed o he popula ion declines o
se e al species (Ree es e al. 2013). Addi ionally, he
Endange ed (Sipilä 2016) land-locked Saimaa inged
seal Pusa hispida saimensis is h ea ened by a ious
human-induced isk ac o s, among which inciden al
by ca ch, oge he wi h clima e change, may be e -
ga ded as he mos c i ical sou ces o cu en mo al-
i y (Ko acs e al. 2012, Niemi e al. 2012, 2013, Au ila
e al. 2014, Val onen e al. 2014, Lyy ikäinen e al.
2015, Kunnas an a e al. 2016, Liukkonen e al.
2017, 2018). In pa icula , ju eniles a e ulne able o
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Inciden al byca ch mo ali y and ishing
es ic ions: impac s on ju enile su i al
in he Endange ed Saimaa inged seal
Pusa hispida saimensis
Pekka Jounela1,*, Te o Sipilä2, Jouni Koskela2, Raisa Tiilikainen2, Miina Au ila2,
Ma ja Niemi3, Me i Kunnas an a3,4
1Na u al Resou ces Ins i u e Finland, 20520 Tu ku, Finland
2Me sähalli us, Pa ks & Wildli e Finland, 57130 Sa onlinna, Finland
3Uni e si y o Eas e n Finland, Depa men o En i onmen al and Biological Sciences, 80101 Joensuu, Finland
4Na u al Resou ces Ins i u e Finland, 80110 Joensuu, Finland
ABSTRACT: Inciden al byca ch, mos ly in gillne s used o ec ea ional ishing, is a c i ical mo al-
i y ac o o he Endange ed Saimaa inged seal Pusa hispida saimensis. Byca ch pa icula ly
a ec s ju eniles, and he e o e ishing has been es ic ed by banning gillne s du ing he mos
c i ical ju enile dispe sal pe iod in sp ing. In addi ion, he mos ha m ul gea ypes o inged seals
o all ages a e o bidden yea - ound. The sp ing ime es ic ion a eas ha e inc eased signi ican ly
since he 1990s, now co e ing o e 90% o he pups’ bi h si es. Howe e , he impac s o ishing
es ic ions on inciden al byca ch mo ali y ha e no p e iously been e alua ed s a is ically. The
p esen es ima es o inciden al byca ch le els in ela ion o ishing es ic ions since he 1990s sug-
ges ha he sp ing ime gillne ishing ban esul ed in an inc ease in popula ion size by 20%
be ween 1991 and 2013 (60 indi iduals gi en an a e age es ima ed s ock size o 355 in 2013). In
addi ion, he es ima ed c i ical pe iod o ju enile su i al in ela ion o ishing ope a ions
appea ed o be he i s 15 mo, which is a much longe ime han p e iously expec ed, sugges ing
ha s a iona y gillne ishing du ing la e win e mon hs may cause a seconda y peak in seal mo -
ali y. One unwan ed es ima ed side e ec o he seasonal ban was a sligh ly inc eased inciden al
es ima ed byca ch le el immedia ely a e he end o he sp ing ime ishing es ic ion pe iod. The
es ima ed byca ch peaked in 2000−2005, wi h an a e age o 13.3 ju eniles annually and
dec eased he ea e , mainly in 2009−2013, wi h an a e age o 6.3 ju eniles annually.
KEY WORDS: En anglemen · Gillne s · Machine lea ning · Mo ali y a e · Pinnipeds
O
PEN
PEN
A
CCESS
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Con ibu ion o he Special ‘Ma ine e eb a e byca ch: p oblems and solu ions’
Endang Species Res 38: 91–99, 2019
byca ch mo ali y caused mos ly by gillne s used in
ec ea ional ishing. Va ious ishing es ic ions a e
en o ced in Lake Saimaa: annual gillne ishing p o-
hibi ions a e applied du ing he mos c i ical dispe -
sal pe iod (15 Ap il–30 June) o imp o e he su i al
o he ju eniles as hey dispe se, in addi ion o which
he mos ha m ul gea ypes o seals o all ages ( ish-
bai ed hooks, la ge ish aps wi h closed co e s, wi e
baske ish aps wi h mou hs wide han 150 mm and
a ied es ic ions ela ed o mesh size and line
s eng h o gillne s) ha e been o bidden h oughou
he yea since 1999 (Sipilä 2003). The pa chy a eas
subjec o he sp ing ime gillne ishing ban we e
g adually enla ged du ing 1991−2013, especially in
he cen al b eeding a eas o he seals (Minis y o
he En i onmen 2011). The conse a ion e ec s o
his ban we e e alua ed by compa ing p opo ions
( ela i e sizes) o he banned ishing a ea wi h he
b eeding habi a . In 1991, he ban co e ed less han
20% o he seals’ bi h lai loca ions (Sipilä 2003), and
i was assumed ha by en o cing new p o ec ion
a eas, o e 90% o he bo n pups would be p o ec ed
by ishing es ic ions (Minis y o he En i onmen
2011), al hough he a eas o conce n would s ill no
co e all o he ju enile seals’ po en ial dispe sal pa -
e ns (Niemi e al. 2012, 2013).
Niemi e al. (2013) ound ha he home anges o
Saimaa inged seal ju eniles we e much la ge han
had ea lie been supposed, and hey e i ied ishing-
induced inciden al dea hs o 40% o he adio- agged
pups. This, and he combina ion o an inc eased
numbe o pups being bo n bu he size o he adul
Saimaa inged seal popula ion emaining small,
boos ed a public deba e on he magni ude and
causes o unobse ed dea hs inside and ou side he
ishing es ic ion a eas. Some s akeholde s denied
all unobse ed dea hs, while o he s sugges ed ha
he unobse ed dea hs we e pe haps 3 imes g ea e
in numbe han he obse ed dea hs (Minis y o he
En i onmen 2011). Un o una ely, he e a e no o he
es ima es o unobse ed dea hs han hose p esen ed
by Niemi e al. (2013), and he causes o dea h o he
addi ional obse ed s anded seals ha e emained
unknown. A ew dead, s anded seals a e ound bo h
inside and ou side he es ic ion a eas each yea , bu
i is a ely possible o diagnose he causes o hose
dea hs because o he bad condi ion o he ca casses.
In ecen yea s, ield obse e s ha e also e i ied a
ew inciden al bycaugh indi iduals wi hin he
banned a eas, cas ing doub s on he eal impac o
ishing es ic ions and he biological sus ainabili y o
he popula ion in he ace o such unquan i iable inci-
den al byca ch.
In his wo k, we es ima ed he impac o ishing
es ic ions on inciden al byca ch o Saimaa inged
seals using s a is ical me hods. Ou ools a e pu ely
s a is ical due o he absence o any conc e e es -
ima es o he main pa ame e s equi ed o such
an assessmen , such as causes o dea h, popula ion
size and biological sus ainabili y. Ins ead, an a ay
o machine lea ning echniques was used o com-
pensa e o hese sho comings, wi h hei ad an-
age being hei abili y o ecognize pa e ns (o
dea hs) ha a e oo obscu e o be pe cep ible by
human esea che s employing s anda d s a is ical
ools.
2. MATERIALS AND METHODS
2.1. Da a
Lake Saimaa (61°05−62° 36’ N, 27° 15−30° 00’ E) is
a eshwa e basin ha is app oxima ely 180 km long
and 140 km wide (Fig. 1); he a ea o in e es in his
s udy co e s he cu en dis ibu ion o he Saimaa
inged seal (see Niemi e al. 2012). The mo ali y and
bi h lai da a om 1991−2013 used in he analyses
we e ob ained om he Saimaa inged seal da abase
o Me sähalli us Pa ks & Wildli e Finland (2018). The
mo ali y da a, including cause, loca ion, yea and
age, we e based on 422 Saimaa inged seal ca -
casses. Since no eliable ime se ies exis ega ding
o e all gillne loca ions and ishing e o s, only he
da a based on he causes o seal mo ali y we e used
he e. The causes o dea h we e ei he de e mined by
a wildli e pa hologis o e i ied in he ield (as in he
case o byca ch in gillne s). The causes o dea h we e
classi ied as ollows: (1) s illbo n: pup dead a bi h,
(2) lai dea hs: unweaned pups aged 0−2 mo, (3) ish-
ing: bycaugh seals aged 0−35 y and (4) o he s:
weaned seals olde han 2 mo ha died o na u al o
unknown causes. The ages o he seals (excluding
pups wi h lanugo hai ) we e de e mined by coun ing
he cemen um laye s in he lowe canine ee h (S ew-
a e al. 1996).
The bi h a e da a om 1991−2013 included he
numbe s o Saimaa inged seals bo n in he yea s
conce ned (N = 1104). Since inged seals gi e bi h o
a single pup in a subni ean lai , he numbe s o bi h
lai s can be used o es ima e pup p oduc ion ( o
de ails, see Sipilä e al. 1990, Sipilä 2003). Such da a
we e collec ed du ing annual Saimaa inged seal
snow lai censuses conduc ed h oughou he spe-
cies’ dis ibu ion a ea in la e win e o ea ly sp ing
om 1991−2013.
92
Jounela e al.: Byca ch mo ali y o Saimaa inged seals
2.2. S a is ical me hods
2.2.1. Model s uc u e
Es ima ion o he o al numbe o dea hs began
wi h he o al numbe o annually bo n pups and
ended when he su i o s eached 30 y o age.
The e was only 1 e y old seal (>25 y , max. 35 y ).
The es ima ion o he o al numbe o dea hs by cause
o dea h in yea yis di ided as ollows: (1) es ima ion
o he o al mo ali y a e om age 0 o age 30 y in
yea yand (2) es ima ion o he causes o dea h a
gi en ages in yea y. In ciden al byca ch is assumed o
occu only in a eas wi hou ishing es ic ions, while
o he causes o dea h (s ill bo n, lai dea hs, o he s)
a e assumed o occu bo h inside and ou side he
ishing es ic ion a ea.
2.2.2. Causes o dea h es ima ion
The causes o dea h pa e ns we e es i-
ma ed using a andom o es (RF) model
(B eiman 2001). RF consis s o building an
ensemble ( o es ) classi ie o decision
ees p oduced om bagging (boo s ap
agg ega ed; see B eiman 1996) and a an-
domized a ian o he ee induc ion algo-
i hm. In ecological s udies, RF has been
applied o species dis ibu ion models
(SDMs), e.g. o species conse a ion and
biodi e si y managemen pu poses, and in
he con ex o clima e change (Guo e al.
2015). One gene al eason o using en -
semble SDMs has been o educe unce -
ain y and s abili y in p edic ions, espe-
cially when compa ed wi h a single SDM,
such as a gene alized linea model o a
gene alized addi i e model (Ma mion e
al. 2009a,b, G enouille e al. 2011, Guo e
al. 2015). Die e ich (2000) iden i ied s a is-
ical, compu a ional and ep esen a ional
easons, explaining why ensembles o en
wo k be e han a single model. Fi s ,
when he lea ning se is oo small (as in ou
s udy), a lea ning algo i hm can ypically
ind se e al models ( unc ions) in he
hypo hesis space and, by a e aging se -
e al models, can educe he isk o choos-
ing he w ong hypo hesis. Second, an en -
semble made o indi idual models buil
om many di e en s a ing poin s may
p o ide a be e app oxima ion o he ue
unknown unc ion han one using any o
he single models. By combining se e al
models in an ensemble, i may be possible o expand
he space o ep esen able unc ions and ob ain a
be e model o he ue unc ion. Consequen ly,
among 179 classi ie s a ising om 17 amilies (dis-
c iminan analysis, Bayesian, neu al ne wo ks, sup-
po ec o machines, decision ees, ule-based clas-
si ie s, boos ing, bagging, s acking, RFs and o he
ensembles, gene alized linea models, nea es neigh-
bou s, pa ial leas squa es and p incipal componen
eg ession, logis ic and mul inomial eg ession, mul-
iple adap i e eg ession splines and o he me hods),
he bes classi ie s we e di e en e sions o RF (Fe -
nandez-Delgado e al. 2014).
The p edic o s o cause o dea h dis ibu ions we e
la i ude, longi ude, seal age (in mon hs) and yea .
The pa ame e s o he RF model (speci ically, numbe
o ees: 10−1500; con idence le el used o pes-
93
Fig. 1. A ea o Lake Saimaa, Finland, inhabi ed by Saimaa inged seals,
wi h loca ions o hei bi h lai s (main b eeding a eas) and co e age o
sp ing ime ishing es ic ions in 2013
Endang Species Res 38: 91–99, 2019
simis ic e o calcula ion o p uning, i.e. emo al o
ee sec ions o imp o e p edic i e accu acy by he
educ ion o o e i ing: 0.05−0.5; maximal dep h:
5−30; and c i e ion o selec ing a ibu es and nume -
ical spli s: accu acy, gain a io, Gini index, in o ma-
ion gain) we e op imized using a sequen ial ( e -
pea ed) g id sea ch. Tha is, he op imiza ion was
epea ed nume ous imes using di e en sub- anges
o he numbe o ees and weaked using he e -
maining pa ame e s un il a pla eau was ound in he
s a is ical pe o mance (accu acy) beyond which
mo e ees did no make any di e ence, and he pe -
o mance sco e e en showed a small dip.
The s a is ical pe o mance o he RF model was
e alua ed using 10- old c oss- alida ion (Koha i
1995), which means ha he da a we e andomly spli
in o k(10) olds using s a i ied sampling, and each
ime k−1 olds we e used o aining and he e -
maining 1 old o es ing. The model wi h ce ain
pa ame e alues ha es ed bes o e he 10 non-
o e lapping es olds was hen selec ed. Finally, he
bes model wi h he bes pa ame e s was applied o
all da a. In gene al, he aim o 10- old c oss- alida-
ion is o a oid o e - i ing and o choose a model ha
is nei he oo simple no oo complex. I gua ds
agains es ing hypo heses sugges ed by he da a
(called ‘Type III e o s’) in cases whe e u he sam-
ples a e cos ly o impossible o collec . A model ha
is oo simple usually has high bias and low a iance,
while a model ha is oo complex usually has low
bias and high a iance. Tha is, inding he bes
model amoun s o making an app op ia e ade-o
be ween bias and a iance, i.e. inding he bes
model ollows he ‘no ee lunch’ heo em (Wolpe &
Mac eady 1997).
2.2.3. Mo ali y a e es ima ion
The sample size o ju enile seals
(aged 0−15 mo) was much highe (5−15
dea hs annually) han ha o he olde
adul seals (0−5 dea hs annually), so i
was possible o es ima e an annual mo -
ali y a e o he ju eniles, bu no o
he olde seals. The annual mo ali y
pa e ns we e ecognized using a gene -
alized eg ession neu al ne wo k (GRNN;
Spech 1991). Fi s , he alues o he
p e dic o s o mo ali y (age and yea )
we e s anda dized be ween 0 and 1.
Then, log(ln + 1) ans o med numbe s
(dead ju eniles) in each age class (in
mon hs) in yea ywe e assessed. Finally, mon hly
mo ali y a es we e es ima ed om he (nega i e)
eg ession slope pa ame e s o he GRNN eg ession
es ima e.
The mo ali y a e o he adul seals aged 1−30 y
was es ima ed using log- ans o med age equency
dis ibu ions (also e e ed o as he ‘ca ch cu e’
me hod; Hilbo n & Wal e s 1992). Ini ially, he num-
be o seals in each age class was de e mined, and
hen he numbe s we e log(ln + 1) ans o med;
he ea e , o al mo ali y was calcula ed as he neg-
a i e slope o he eg ession. Finally, he e o o he
mo ali y es ima es was calcula ed as he e o o he
slope o he eg ession.
In he adul mo ali y a e es ima ion, we used
Bayesian linea eg ession wi h unin o ma i e p io s,
i.e. we applied equal p obabili y o each possible
alue o he p io mo ali y a e, which means ha we
did no assume any pa icula (unknown) mo ali y
le el a p io i be o e pe o ming he mo ali y analy-
sis. Fu he mo e, we assumed a cons an annual mo -
ali y a e o he adul seal age g oups o e he yea s
because he annual sample size o adul seals was
ei he 0 o ex emely small (0−5 annually).
3. RESULTS
3.1. Inciden al byca ch and he impac s o ishing
es ic ion a eas
The sp ing ime (15 Ap il−30 June) ishing es ic-
ion a eas ha e inc eased signi ican ly in size and
co e age since hey we e es ablished in 1982, and
94
Fig. 2. Annual sizes o he sp ing ime ishing es ic ion a eas and es ima ed
p opo ions (%) o Saimaa inged seal bi h si es loca ed wi hin hem in
1991−2013
Jounela e al.: Byca ch mo ali y o Saimaa inged seals
he inc ease has had an e ec on ju enile su i al.
The a eas co e ed 18% o he bi h si es in 1991,
whe eas in 2013, he co e age was o e 90% (Fig. 2).
Al hough he i s es ic ions imposed o limi by -
ca ch mo ali y we e es ablished as ea ly as 1982
(Sipilä e al. 1990), sys ema ic es ic ions ha e been
implemen ed on a la ge scale (applying o bo h he
season o he yea and he gea ype) only since 1999.
In 2010, a majo inc ease in he sp ing ime o al ish-
ing es ic ion a ea was en o ced in he main b eed-
ing a eas o he seals, pa ly due o an inc ease in he
dis ibu ion o he ju eniles and o he inged seal
popula ion in gene al (Fig. 2).
In 1991, he sp ing ime ishing e -
s ic ion a eas had only a small posi-
i e in luence on ju enile su i al,
whe e as in 2013, he in luence o he
enla ged o al es ic ion a ea was
mo e signi ican (Fig. 3). Howe e , an
es ima ed side e ec o he es ic ion
a eas in 2013 was a sligh inc ease in
es ima ed inciden al byca ch le els
a e ishing began (on 1 July), when
he ju eniles we e 4 mo old (Fig. 3).
Tha is, a highe es ima ed numbe o
su i o s on 30 June led o a highe
es ima ed inciden al byca ch a e
ishing began on 1 July. The es ima ed
by ca ch peaked in 2000− 2005, wi h an
a e age o 13.3 ju eniles annually,
and was highes in 2004. The en -
la ged ishing es ic ion a eas in
2010− 2013 e ec i ely e duced he by -
ca ch o seals 0−4 mo old (Fig. 4).
The conse a ion impac o he
sp ing ime ishing es ic ions in -
c eased signi ican ly h oughou he
pe iod conce ned (Fig. 5). In he
pe iod 1991− 2013, he annual impac
o he es ic ion a eas peaked a a
ju enile age o 2.5− 3 mo. The conse -
a ion impac o he ishing es ic ion
a eas in e ms o he inciden al by -
ca ch was smalle a age 3.5 mo han
a 2.5−3 mo, and was minimal a 0−
1 mo (Fig. 5).
I he sp ing ime ishing es ic ion
a eas co e ed all o he inged seals’
bi h si es h oughou he pe iod con-
ce ned, i is es ima ed ha he num -
be o ju eniles su i ing as a esul o
ishing es ic ion a eas would ha e
inc eased o an a e age o ~7.8 in -
di iduals annually (a e age slope pa ame e ,
Fig. 6) compa ed o a si ua ion wi h no es ic ion
a eas a all. As he es ic ion co e age was less han
100% (Fig. 2), only an a e age o 2.1 ju eniles we e
es ima ed o ha e been sa ed annually by his man-
agemen measu e.
Du ing 1990−1995, he es ima ed a e age annual
byca ch o ju eniles (0−15 mo old) was 7.9 ( ange
6.4−10.6) indi iduals, and du ing 2009−2013, i was
6.3 ( ange 4.4−8) indi iduals. The es ima ed numbe
o seals inciden ally bycaugh du ing 2009−2013 was
app oxima ely hal o ha in he ea ly 2000s. How-
95
Fig. 3. Es ima ed annual ins an aneous byca ch o Saimaa inged seals (indi-
iduals) wi h and wi hou ishing es ic ion a eas in (A) 1991 and (B) 2013.
When ishing began in 2013, i.e. when he ju enile seals we e 4 mo old, he
g ey line is highe han he black do ed line, indica ing a sligh ly inc eased
byca ch
Fig. 4. Es ima ed byca ch o ju enile Saimaa inged seals aged 0−15 mo in
1991, 2004, 2010 and 2013. No e he peak in 2004, a e which he enla ged
ishing es ic ion a eas e ec i ely educed he byca ch o seals aged 0−4 mo

Endang Species Res 38: 91–99, 2019
e e , he obse ed byca ch mo ali y does no show
he same pa e n as he es ima ed mo ali y, as i has
emained a app oxima ely he same le el h ough-
ou he in e al 1991−2013 (Fig. 7).
3.2. Popula ion size
The ishing es ic ion a eas induced an inc ease in
he popula ion o he Saimaa inged seal. The cumu-
la i e inc easing e ec o he sp ing ime ishing
es ic ions on he popula ion size o he Saimaa
inged seal du ing 1991−2013 was 47 ju eniles + 13
adul s (20% inc ease in popula ion size om 295
seals in 1991 o 355 seals in 2013, Fig. 8). The es i-
ma ed popula ion size is an unce ain pa ame e ha
a ies be ween 2 ex emes, he es ima ed minimum
and maximum dis ibu ions (Fig. 9).
4. DISCUSSION
Ou esul s show he e ec i eness o he enla ged
sp ing ime ishing es ic ion a eas in educing by -
ca ch numbe s o Saimaa inged seal ju eniles. Ou
analyses show ha he es ima ed byca ch numbe s in
ecen yea s we e app oxima ely hal hose in he
ea ly 2000s, whe eas he obse ed byca ch numbe s
o ju eniles (Fig. 7) emained app oxima ely he
same, and i appea s ha he es ic ions did no ha e
any impac on he su i al o ju eniles. Howe e , i
is also he unobse ed (no epo ed, i.e. es ima ed
minus obse ed, Fig. 7) byca ch numbe s ha ha e
dec eased in ecen yea s. This shows he e ec i e-
ness o es ic ions in sa egua ding ju eniles (Fig. 5),
which nea ly doubled he popula ion size (seal num-
be s be o e bi h) du ing 1991−2013. I can be ex -
pec ed ha wi hou he es ic ions, he inciden al
byca ch in ecen yea s would ha e been app oxi-
ma ely double. Simila ly, implemen a ion o he ish-
ing es ic ions in 1991− 2013 o e an a ea co e ing
all o he bi h si es could po en ially ha e inc eased
ju enile su i al, wi h an a e age o ~7.8 indi iduals
es ima ed o ha e been sa ed annually. In p ac ice,
howe e , he e se e co e age was mode a e, and
he e o e he popula ion o 355 seals a he end o he
pe iod s udied he e included an es ima ed 60 indi-
iduals mo e han he e would ha e been wi hou
he ishing es ic ions (295 seals, Fig. 9).
Ou esul s con i m ea lie obse a ions o he mos
c i ical pe iod o ju enile su i al (Sipilä 2003, Min-
is y o he En i onmen 2011, Niemi e al. 2013, Au -
ila e al. 2014) and show ha he impac o ishing
es ic ions on ju enile su i al begins a he age o
1.5 mo and is less ma ked a age 3.5 mo han a 2.5−
3 mo, which oughly co esponds o he ime in e al
15 May−15 July. Howe e , he da a also indica e ha
he es ima ed c i ical pe iod o ju enile su i al, as
a as ishing p ac ices a e conce ned, ex ends o e
he i s 15 mo, as seen in he seconda y peak in
annual byca ch mo ali y a e he end o he sp ing-
ime ishing es ic ion pe iod. Ou analyses clea ly
poin o his seconda y peak in mo ali y and show
96
Fig. 5. Es ima ed ins an aneous sp ing ime ishing es ic-
ion-induced Saimaa inged seal su i o s (a e age numbe
o indi iduals) a gi en ages (in mon hs) in 1991−2013. The
numbe o su i o s peaked in 2012. The impac o he ish-
ing es ic ions on he su i al o he ju eniles s a ed o in-
c ease a he age o 1.5 mo
Fig. 6. Sp ing ime ishing es ic ion-induced Saimaa inged
seal su i o s (aged 0−4 mo) in ela ion o he co e age o
bi h si es (%) in 1991−2013. The eg ession slope pa ame-
e (7.8258) sugges s ha 100% es ic ion co e age s. 0%
co e age could po en ially inc ease he numbe o su i o s
by ~7.8 indi iduals (a e age annual es ima e)
Jounela e al.: Byca ch mo ali y o Saimaa inged seals
ha he es ima ed byca ch peaked in 2004 and de -
c eased he ea e o e he in e al 2009−2013 o an
a e age o 6.3 indi iduals annually. Ou es ima es
a e also suppo ed by ecen eco ds, which indica e
inc eased byca ch mo ali y du ing ha ime (Me -
sähalli us Pa ks and Wildli e Finland 2018). This sec-
ond mo ali y peak is ypically caused by gillne ish-
ing du ing he au umn and win e . Al hough ou
esul s indica e ela i ely low byca ch numbe s a he
end o he pe iod conce ned (2009−2013, see Fig. 7),
his could be an unde es ima e due o noise o con-
cep d i (a change in s a is ical p ope ies o he a -
ge a iable). Some algo i hms may o e eac o
noise, e oneously in e p e ing i as concep d i ,
while o he s may be highly obus wi h ega d o
noise, adjus ing o he changes oo slowly (Tsymbal
2004). The po en ially ele a ed byca ch numbe s
ha e also been obse ed in mo ali y s a is ics in
ecen yea s (Me sähalli us Pa ks and Wildli e Fin-
land 2018).
This is he i s s udy o show he hidden byca ch
mo ali y (see Fig. 7) in he Saimaa inged seal popu-
la ion, as specula ed ea lie (Sipilä 2003, Minis y o
he En i onmen 2011). One o he main assump ions
he e is ha he cause o dea h assessmen assumes
ha he p obabili y o epo ing is equal be ween all
causes, which may no hold ue. In July, o example,
he e a e nume ous people p esen (summe co age
owne s and ec ea ional ishe men) who a e likely o
epo inciden al byca ch dea hs mo e o en han in
win e , when he ha sh condi ions a ac ewe ish-
e men. Howe e , he numbe o ne s can also be con-
side able du ing ha ime because gillne ishing in
Lake Saimaa is ocussed especially on he pike pe ch
Sande luciope ca, which is one o he mos aluable
species bo h comme cially and ec ea ionally. In
addi ion, he willingness o epo inciden al byca ch
dea hs may a y be ween seasons, and hence he
es ima ed inciden al byca ch pa e ns p esen ed he e
97
Fig. 7. Obse ed and es ima ed byca ch mo ali y o ju enile
Saimaa inged seals a age 0−15 mo. Obse ed and es i-
ma ed bycaugh indi iduals and hei p opo ions (%) o
pups bo n annually in 1991−2013
Fig. 8. Es ima ed a e age Saimaa inged seal popula ion
sizes in 1991−2013, wi h cumula i e ishing es ic ion im-
pac s ( ), wi h non-cumula i e (single-yea ) es ic ion e -
ec s (m) and wi h no ishing es ic ion impac s (j)
Fig. 9. Two es ima ed Saimaa inged seal popula ion size
p obabili y dis ibu ions in 2013, wi h 2 unce ain impac s o
he ishing es ic ion a eas. The maximum dis ibu ion
(black solid line) assumes emo al o all (obse ed and hid-
den) inciden al byca ch wi hin he ishing es ic ion a eas
in all yea s, and he minimum dis ibu ion (black dashed
line) assumes ha he ishing es ic ion a eas had a ze o
impac on ishing byca ch inside ishing es ic ion a eas
Endang Species Res 38: 91–99, 2019
should be conside ed ‘bes -guess’ es ima es (app ox-
ima ions) o he inciden al byca ch mo ali y. The e
has also been deba e abou he accep abili y o ish-
ing es ic ions o local people (Ra amäki & Salmi
2015), which may also ha e caused he willingness o
ishe men o epo inciden al byca ch o a y be -
ween he yea s o e he whole pe iod 1991−2013.
The esul s and ends o he p esen s udy, howe e ,
do no signi ican ly de ia e om hose epo ed ea -
lie , and hence, he o e all eliabili y o he assess-
men was conside ed plausible.
The na ionally accep ed biological e e ence poin
o he size o he Saimaa inged seal popula ion is
ha i should each 400 adul seals by 2025 (Minis y
o he En i onmen 2011). This has been o icially de -
ined as an ‘in e media e goal’ and would be achie ed
by allowing he popula ion o g ow by app oxima ely
3−4% y −1. This e e ence poin in eg a es he knowl-
edge and opinions o a ious s akeholde g oups, bu
i emains an open ques ion as o which compu a-
ional e e ence poin me hod would be mos app o-
p ia e o he Saimaa inged seal. Consequen ly,
he e ha e been di icul ies in jus i ying he main e-
nance o easonably conse a i e ishing es ic ions
ha bo h p o ec he Saimaa inged seals and enable
local small-scale subsis ence ishing (Ra amäki &
Salmi 2015). In addi ion, along wi h inciden al by -
ca ch mo ali y, an inc easing h ea o popula ion
eco e y om he mid-2000s onwa d has been he
high pe ina al mo ali y caused by he lack o ade-
qua e snow co e du ing he b eeding season in
Feb ua y−Ma ch (Au ila e al. 2014). Addi ionally, a
sligh ly less acknowledged bu po en ially se e e
h ea has been he apid inc ease in he numbe o
summe co ages and associa ed ac i i ies, which has
s eadily educed he po en ial b eeding g ounds
along he sho es o he Saimaa lake sys em (Liukko-
nen e al. 2017). Coping wi h hese h ea s may p o e
o be a ela i ely complica ed ma e . Ne e heless,
he indings p esen ed he e do show ha educing
inciden al byca ch mo ali y by ex ending he ishing
es ic ions has had subs an ial long- e m e ec s on
he g ow h a e o he Endange ed Saimaa inged
seal popula ion.
Ju enile su i al o he Saimaa inged seal is a c i -
ical elemen o success ul conse a ion. Pe ina al
mo ali y ypically a ies be ween 10 and 20% (Au -
ila 2015), and a e weaning, byca ch is he domi-
nan mo ali y ac o o ju eniles (Kokko e al. 1999,
Sipilä 2003, Niemi e al. 2013). In he u u e, pe ina al
mo ali y is likely o inc ease due o human dis u -
bance (Liukkonen e al. 2017) and changing clima e
(Au ila 2015); he e o e, mi iga ion o byca ch, a
mos acu e mo ali y ac o , is highligh ed in conse -
a ion s a egies, and ishing closu es ha e been
es ablished. Howe e , o be e ec i e, ishing es ic-
ions should be p olonged a leas o he end o July
o educe he byca ch mo ali y peak in summe . In
addi ion, ou esul s on hidden mo ali y indica e ha
he obse ance o ishing egula ions s ill equi es
bo h educa ion and pa olling.
Acknowledgemen s. We hank Ma ja Isomu su o he Finnish
Food Sa e y Au ho i y o pe o ming he seal ca cass nec op-
sies. This wo k was unded by he EU Li e + Sa egua ding
Saimaa inged seal p ojec LIFE12NAT/ FI/ 000367.
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Edi o ial esponsibili y: Rebecca Lewison,
San Diego, Cali o nia, USA
Submi ed: Ap il 25, 2018; Accep ed: Janua y 7, 2019
P oo s ecei ed om au ho (s): Feb ua y 7, 2019