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We estimated the effect of the gill‐net fisheries targeted at whitefish (Coregonus sp.) on anadromous sea trout, Salmo trutta, in the Gulf of Bothnia, Baltic Sea using separate data for fish species. The analysis of sea trout captures was based on tagging and recapture data collected in 1998–2011, while whitefish data were derived from individual samples of commercial fisheries from the same period. The mesh sizes used in gill‐net fishing and the seasonal and temporal distributions of recaptured sea trout and sampled whitefish were compared in the northern and southern Gulf of Bothnia. The trout had typically spent 1–2 years at sea, and they were mainly immature with a median body length of 40–43 cm at the time of recapture in gill nets. Despite the increase in the minimum permitted landing size from 40 to 50 cm in 2008, the median length of recaptured trout remained unchanged during the study period. Most (59%) of the gillnetted trout were caught in the southern Gulf of Bothnia in gill nets with mesh sizes of 40–45 mm, which were also used in the whitefish fishery (72%). In the northern Gulf of Bothnia, nets with a smaller mesh size of 25–39 mm took 83% of the whitefish catch and 39% from recaptured trout. In both areas, the overlap in mesh sizes used to gill‐net catch whitefish and sea trout increased during the study period. There were clear seasonal and areal differences in the relative probability of sea trout being captured in gill nets, suggesting that carefully tailored spatial and temporal restrictions on gill‐net fisheries could provide a tool to protect young sea trout without causing intolerable difficulties for the fisheries targeting other species.
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J Appl Ichthyol. 201 8;1–8 .  wileyonlinelibrary.com/journal/jai  
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© 2018 Black well Verlag GmbH
1 | INTRODUCTION
The coast of the Gulf of Bothnia (GoB) is a feeding and reproduc‐
tion area for whitefish (Coregonus sp), perch (Perca fluviatilis), and
pikeperch (Sander lucioperca), which are intensively targeted by both
commercial and recreational fisheries (Ådjers et al., 2006; Lehtonen,
Hansson, & Winkler, 1996; Olsson, Florin, Mo, Aho, & Ryman, 2012;
Veneranta, Hudd, & Vanhatalo, 2013). For fisheries management,
the coexistence of several important species in the same area poses
a persistent problem, as gill nets, the most commonly used gear,
are unselective with respect to fish species (Graham, Ferro, Karp, &
MacMullen, 2007; Lehtonen & Suuronen, 2004). Safeguarding the
endangered (ICES, 2012; Whitlock et al., 2016) sea‐run brown trout
(Salmo trutta) from being caught as by‐catch in the gill‐net fishery
(Jutila, Saura, Kallio‐Nyberg, Huhmarniemi, & Romakkaniemi, 2006;
Kallio‐Nyberg, Saloniemi, Jutila, & Saura, 2007) has proven espe‐
cially difficult.
Sea trout spend their juvenile years in freshwater, migrate to sea,
and return after one or several sea years to their home river to over
winter or spawn (Jonsson & Jonsson, 2006; 2009). Local sea trout tend
Received:16November2017 
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  Revised:5J une2018 
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  Accepted:28June2018
DOI: 10.1111/jai.13771
ORIGINAL ARTICLE
Anadromous trout threatened by whitefish gill‐net fisheries in
the northern Baltic Sea
Irma Kallio‐Nyberg1| Lari Veneranta1| Irma Saloniemi2| Matti Salminen3
1Natural Resources Institute Finland (Luke),
Vaasa, Finland
2Department of Biology, University of Turku,
Turku, Finland
3Natural Resources Institute Finland (Luke),
Helsinki, Finland
Correspondence
Irma Kallio‐Nyberg, Natural Resources
Institute Finland (LUKE), Vaasa, Finland.
Email: irma.kallio‐nyberg@luke.fi
Funding information
Luke; Strategic Research Council of
the Academy of Finland; National Data
Collection Programme under Council
Regulation (EC) N° 199/2008; SmartSea
project; Strategic Research Council;
Academy of Finland
Abstract
We estimated the effect of the gill‐net fisheries targeted at whitefish (Coregonus sp.)
on anadromous sea trout, Salmo trutta, in the Gulf of Bothnia, Baltic Sea using sepa‐
rate data for fish species. The analysis of sea trout captures was based on tagging and
recapture data collected in 1998–2011, while whitefish data were derived from indi‐
vidual samples of commercial fisheries from the same period. The mesh sizes used in
gill‐net fishing and the seasonal and temporal distributions of recaptured sea trout
and sampled whitefish were compared in the northern and southern Gulf of Bothnia.
The trout had typically spent 1–2 years at sea, and they were mainly immature with a
median body length of 40–43 cm at the time of recapture in gill nets. Despite the
increase intheminimumpermittedlanding sizefrom40to50cmin2008,theme
dian length of recaptured trout remained unchanged during the study period. Most
(59%)ofthegillnettedtroutwerecaughtinthesouthernGulfofBothniaingillnets
withmeshsizesof40–45mm,whichwerealsousedinthewhitefishfishery(72%).In
thenorthernGulfofBothnia,netswithasmallermeshsizeof25–39mmtook83%
ofthewhitefishcatchand39%fromrecapturedtrout.Inbothareas,theoverlapin
mesh sizes used to gill‐net catch whitefish and sea trout increased during the study
period. There were clear seasonal and areal differences in the relative probability of
sea trout being captured in gill nets, suggesting that carefully tailored spatial and
temporal restrictions on gill‐net fisheries could provide a tool to protect young sea
trout without causing intolerable difficulties for the fisheries targeting other
species.
KEYWORDS
Baltic Sea, gill‐net fishing, sea trout, spatial distribution, tag, whitefish
2 
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   KALLIO‐N YBERG Et A L.
to stray less than transplanted populations (Jonsson & Jonsson, 2014).
Feeding ar eas at sea are usually near the home river, but som e individu
als may migrate over longer distances (Kallio‐Nyberg, Saura, & Ahlfors,
2002; Kallio‐Nyberg, Veneranta, Saloniemi, Jutila, & Pakarinen, 2017).
All the remaining natural sea trout stocks in Finnish rivers are critically
endangered (ICES, 2012). At present, the majority of sea trout smolts
leaving Finnish rivers are hatchery reared and released (Koljonen,
Gross, & Koskiniemi, 2014), which has been carried out both for en
hancement of natural stocks and for compensatory fishery purposes
defined in legislation. During the 2000s, about one million sea trout
smolts have annually been released into Finnish coastal rivers of the
Baltic Sea (ICES, 2012). Some sea trout recruit to the fishery as posts
molts during their first year at sea, but the rest are mainly caught
during their second sea year (Jutila et al., 2006). For tagged sea trout
smolts released in 1980–2010, the mean length at recapture was
47 cm and 39 cm in the Bothnian Sea and Bothnian Bay, respectively
(Kallio‐Nyberg,Saloniemi,&Jutila,2015),whichisbelowtheminimum
landingsize(50cm)forstockedseatrout.Themostaccuratestatistics
on sea trout catches from the Finnish side of the GoB are available
from the records of professional gillnet fishermen. The annual com
mercial sea trout catch in GoB in Finnish side in the 20 00s ranged from
15–55ton nes (ICES , 2016).T he recreat ional sea tro ut catch may be 
even larger in the GoB (Jutila et al., 2006).
The main target of the gill‐net fishery in the GoB is whitefish
(Coregonus sp.) (Anonymous, 2016). The catch consists of two white
fish ecotypes: anadromous river‐spawning and sea‐spawning white
fish (Heikinheimo & Mikkola, 2004; Olsson et al., 2012; Säisä et al.,
2008). At sea, the river‐spawning ecotype undertakes a long feeding
migration extending from the rivers of the northern Bothnian Bay to
the sout hern GoB (Aron suu & Huhmarniemi , 2004; Leskelä , Jokikokko,
Huhmarniemi, Siira, & Savolainen, 2004). The more local sea‐spawning
whitefish spawn in the shallow sea areas along the coast (Veneranta et
al., 2013). The fisheries for whitefish are also supported by substantial
annual stocking of larval and one‐summer‐old whitefish (Jokikokko &
Huhmarniemi, 2014). Whitefish start to recruit to the fishery when
they are three years old and approximately 34 cm in length (Leskelä
et al., 2004). In 2013, the minimum gill‐net mesh size in the whitefish
fisheries of the GoB was raised from 38 mm to 40–43 mm.
In this study, two large datasets for the GoB organized accord
ing to ICES r ectangl es (50×50km) were com bined: tag re covery
data on the gillnetted sea trout and whitefish catch sample data
from the commercial whitefish gill‐net catch in 1998–2011. Both
the non‐commercial and commercial fishermen returned tags of sea
trout used in the study (1809 tag returns). In addition, we also used
commercial fisheries catch data registered in Fisheries Statistics
(FS) (Anonymous, 2016; Fisheries statistics Finland 2014) by com
paring validity of our catch sample dat a (see Supporting Information
Appendix S1) The aim was to find measures to minimize the nega
tive effects on sea trout of gill‐net fishing targeted at whitefish. The
spatial, seasonal, and temporal similarity in gill‐net fishing and mesh
sizes of gill nets for the two fish species was assumed to increase
the probability of sea trout being caught as by‐catch of the white
fish gill‐net fisheries. Local or seasonal differences between the
gill‐net fisheries for whitefish and sea trout were analyzed to find
ways to combine the needs of large‐scale coastal fisheries and the
protection of endangered sea trout stocks at sea. The total closure
of gill‐net fishing is not feasible for economic reasons, but the local
or temporal regulation of gill‐net fishing might be possible. Because
the datasets for sea trout and whitefish were separate, the by‐catch
proportion of sea trout in the whitefish sample could not be ex
amined, and we therefore estimated the relative probability of sea
trout occurring as by‐catch in the whitefish fisheries.
2 | MATERIAL AND METHODS
Tagged sea trout smolts were released in 1998–2010 into six Finnish
rivers in the GoB and the tags of recaptured fish were returned by
fishermen to the Natural Resources Institute Finland. The dataset in
cludes 1809 tag returns with information on the capture site and fish
ing gear (Table 1). The rivers represent all important release sites for
sea trout on the Finnish coast of the Bothnian Bay (BB) and Bothnian
Sea (BS) (Figure 1). The mixed stock whitefish data were collected in
1998–2011 from commercial whitefish fisheries in the GoB as a part
of the EU Data Collection Framework (DCF). The data include individ
ual measurements for whitefish, the catch area, and precise informa
tion on the gear used in whitefish fishing. DCF data does’ not include
information on possible by‐caught sea trout or any other fish species.
Whitefish fishing is extensive in the GoB. The commercial catch data
were collected from fishermen and registered in Fisheries Statistics
(FS) (Anonymous, 2016; Fisheries statistics Finland 2014). The data
include information on the fishing effort and whitefish catch in kilo
gram sp erICE Ss tat is tic alre c tan gl ea ndge art ype .A lt ogether,80 %of
whitefish in the dataset were caught using gill nets and rest with trap
nets.Thegill‐netmeshsizesarereportedinclasses(<36,34–45mm)
in FS data, and we thus used DCF data to obtain more precise in
formation on the mesh sizes used in whitefish fishing. We compared
the seasonal and spatial distribution of DCF data with FS to ensure
that the DCF data could be used to indicate the spatial and temporal
fishing pattern in the GoB (see Supporting Information Appendix S1).
The sea trout tag recovery data were combined with the white
fish DCF catch data. There were eight common catch rectangles
fo rb othsp e ci esi nt heB B( rec t ang les2, 6,7,11,12 ,15,16, an d19)
and nine in the BS (22, 23, 24, 27, 28, 32, 37, 42, and 47) (Figure 1).
All returns of tags from gillnetted sea trout in the BB and BS with
the reported mesh size were analyzed.Approximately 5% of sea
trout recaptures were in rectangles that had no commercial white
fish samples, and these were rejected from the data, as were re
captures outside Finnish coastal waters (1%). The geographical
distribution of tag recoveries was illustrated as a point density
map (Figure 1), and the methodology is explained in Kallio‐Nyberg
et al. (2017). Sea trout mainly migrate to feed in coastal waters.
They prefer coastal waters near the release site, but they may
migrate to feed 100 to 200 km to the north or south in the GoB
(Kallio‐Nyberg et al., 2017). The fishing effort with gill nets is high
in all ICES rectangles in Finnish coastal waters in which both fish
    
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 3
KALL IO‐NY BERG Et AL .
species of this study were captured (Figure 1). Because the same
gill nets take both sea trout and whitefish, an increase in the fish
ing effort is likely to increase the catch of both fish species.
The median mesh size and length of the sea trout were recorded
for each sea area, and the differences between areas were analyzed
using the Kruskal‐Wallis test (SAS NPAR1WAY; SAS Institute, 2012).
In addition, the dependence of the catch length of trout on the gill
net mesh size was analyzed using linear regression (SAS REG).
The distribution of trout and whitefish catches between gill‐
net mesh‐size classes (25–39mm, 40–45mm, 46–84mm) was
compared separately for the BB and BS using χ2 tests (SAS FREQ).
The mesh si ze class 40–45mm was narrow b ecause gi ll nets with
mesh sizes in this range dominated the data. The samples from the
BB and the BS were analyzed separately due to differences in the
growth rate and gill‐netfishing(Kallio‐Nyberget al., 2015;Leskelä,
Jokikokko, & Huhmarniemi, 2002).
Linear trends (year between 1990 and 2011 as the continuous
variable) in the changes in gill‐net mesh sizes (continuous) were
examined for both fish species in the different parts of the GoB
(BB, BS) (SAS MIXED). As the mesh size of 40 mm was both in
practice and according to the analysis relevant for different types
of fishery, changes in the mesh size (with classes 26–40 mm and
41–84 mm) were explained by years (1998–2011, continuous vari
able) in a log‐linear model (Proc SAS GENMOD with logit link and
a binomial distribution) (as in Saloniemi, Jokikokko, Kallio‐Nyberg,
Jutila, & Pasanen, 2004). The models were separately constructed
for BS and BB data. The sample sizes for whitefish and sea trout,
respectively, were 8,648 and4,462 in the BS and 7,075and874
in the BB.
The by‐catch was modelled by calculating the relative probabil
ity of catching trout instead of whitefish. The probabilit y (response 0
for whitefish and 1 for sea trout) was analyzed using SAS GENMOD,
as above. Separate models were constructed for the sea areas BB
(model 1) and B S (model 2). The pred ictors used in t he model were the
meshsizeofgillnets(continuousvariable;25–85mm),twotimepe
riods(1998–2004vs.20 05–2011),andtheseason(spring=March–
May; summer=June–August; autumn=September–November;
winter=December–Februar y). The time period (2005–2011 vs.
1998–2004) was used to analyze temporal changes in gill‐net fish
ing. As sea trout recaptures and the whitefish catch were recorded
in separate datasets, the numbers and proportions of these fish spe
cies are not comparable in the models. The result of the analysis is
not dependent on the fishing effort. Sample sizes for whitefish and
sea trout, respectively, were 12 638 and 471 in the BS and 7,494
and453 intheBB.Theseasonaldistributionsofgill‐netfishing for
both species with the same samples as in the model above are also
presented in the Supporting Information Appendix S1.
To determine whether gill‐net fishing (i.e. the catch distribu
tion across different gears and seasons) near the rivers and their
estuaries where sea trout are released differs compared to other
sea areas, χ2 tests (SAS FREQ) were applied. The analysis was con‐
ducted separately for each fish species and the estuaries of the sea
trout rivers[BB:rectangle 7 (Iijoki,Oulujoki)and 15(Lestijoki); BS:
32 (Isojoki) and 37 (Merikarvianjoki)]. Seasonal differences (spring,
summer, autumn, winter) were compared separately for the two sea
areas (within the BB or BS). The samples taken with a mesh size of
25–84mmintheBSandBBinthoserectangleswherebothspecies
were captured were included. The distributions and samples are also
presented in the Supporting Information Appendix.
3 | RESULTS
3.1 | Gill nets, mesh size, and catch size
Most of the tagged sea trout were recaptured in the gill‐net
fisher y in both the Bot hnian Sea (85%) and t he Bothnian Bay
Fish
species,
sea area
Sample Mesh‐size classes of gill net s
Mesh size of
gill nets
n
25–39 mm
(%)
40–45 mm
(%)
46–84 mm
(%) Md (q1‐q3)
Sea trout,
BB
455 39 49 12 40(35–45)***
Whitefish,
BB
7,502 83 17 02 8 (27–3 8)
Sea trout,
BS
472 1 72 26 45(42–50)***
Whitefish,
BS
12,628 31 59 10 40 (38–42)
Note. The sea trout were tagged fish recaptured in gill nets, while the individual whitefish data were
a sample from the commercial gill‐net catch. The sample size (n), the percentage (%)ofindividuals
caught in the three mesh‐size classes, and the median (Md) mesh size with the 1st (q1) and 3rd (q3)
quantiles are presented. The sample is the same for both the separate mesh‐size classes and the
median mesh size. The differences in median mesh sizes between species were analyzed using the
Kruskal‐Wallis test.
***Significance: p < 0.001.
TABLE 1 Mesh sizes used in the
gill‐net fishery for sea trout and whitefish
in the ICES statistical rectangles in the
Bothnian Bay (BB) and Bothnian Sea (BS)
during 1998–2011 (Figure 1)
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   KALLIO‐N YBERG Et A L.
(66%),andthemedian sea age (yearsat sea)forgill‐nettedsea
trout was 2 (quartiles: 1–2; n=637)and1(1–2;n=699),respec
tively. The sea trout were recaptured in Finnish coastal waters,
where the number of the gill‐net days in whitefish fishing was
high (Figure 1).
The median mesh size of gill nets was larger in the Bothnian
Sea(45mm)than in the BB (40mm;Kruskal‐Wallistest: p < 0.001)
(Table 1). The median catch length and weight of sea trout caught
with gill nets were lower (K‐W: p=0.001) in the BB [4 0(35–45)
cm; 0.65(0.45–0 .94) kg, n=365] than in the BS [46 (40–51) cm;
1.05(0.72–1.50)kg,n=334].Theminimumlandingsizeforseatrout
wasraisedfrom40to50cmin2008,butthe lengthoftrout inthe
gill‐net catch did not change between the year periods 1998–2007
and 2008–2011 in any of the sea areas. The median mesh size of
the gill nets that took sea trout decreased from 1998–2007 to
2008–2011 in the BB [1998–2007:43 (range 38–45) mm , n=235
and2008–2011:40(range35–45)mm,n=143,K‐W:p=0.021]and
intheBS[45(range 42–45)mm, n=247and2008–2011:43(range
40–45)mm,n=103,K‐W:p = 0.014].
The mesh size (27–70 mm included) of gill nets explained only
12% of the length of sea trout when all recoveries in the GoB
were inclu ded (linear regr ession: lengt h=219.7+4.9 x mesh size,
F1,709=95.6,p < 0.001). The catch length of sea trout increased as a
function of increasing mesh size in BB and BS data (MIXED model,
type 3 test: area: p=0.005;mesh size,p < 0.001; Figure 2). To sup‐
port the present legal MLS (the minimum landing size) of wild trout
(60cm),aminimummeshsizeof75mmwouldbeneededinfishing
according to the regression model (Figure 2).
3.2 | Gears in the combined sea trout and
whitefish samples
Most of the sea trout (gill net, trap net, rod and line, other gears: BB:
66%,31%,3%,0%;totaln=949andBS:85%,9%,5%,1%;n=717)
FIGURE 1 The spatial distribution of tag recovery densities (recoveries/km2 in a radius of 10 km) for sea trout age groups 1–3 in gill‐net
fishing in the Gulf of Bothnia (on left side), and the annual average number of gill‐net days for whitefish fishing per ICES statistical rectangles
in the northern Baltic Sea (on the right). ICES subdivisions 30–31 (Bothnian Sea, Bothnian Bay) are shown together with ICES statistical
rectangles(BothnianBay:2(nearRiverKemijoki),6,7(Iijoki,Oulujoki),11,12,15(Lestijoki),16,and19;BothnianSea:22,23,24,27,28,32
(Isojoki), 37 (Merikarvianjoki), 42, and 47) near the sea trout rivers
    
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KALL IO‐NY BERG Et AL .
andthewhitefish (BB: 56%,39%,5%,0%;n=13615andBS:89%,
11%,0%,0%;n=14207)werecaughtby gill netsandcoastal trap
nets in the same fishing rectangles in the GoB, and the proportion
of trout taken with rod and line and other gears was minor, account‐
ingfor only 0%–6%of thecatch. Gillnets werethemainmeansfor
catching s ea trout (90% of tag re coveries, n=670) and whitef ish
(89%ofsamples,n=14207)intheBS(gillvs.trapnet,χ2 test: df=1,
χ2=1.5,p=0.227).IntheBB,ahigherpropor tionofseatrout(68%,
n=919)comparedtowhitefish(59%,n=12983)werecaughtingill
nets when only gill and trap catch proportions were compared (gill
vs. trap net, χ2 test: df=1, χ2= 30.1, p=0.0 01).Inbothsea areas,
recoveriesofseatrout tags(89% intheBBand95%intheBS) and
indi vid ualwhitefishdata(99%–100%)camefromt hes ameICESrec
tangles included to the study (See rectangles in Figure 1).
The distribution of the whitefish and sea trout samples was
comparedbetweenthreemesh‐sizeclasses(25–39mm,40–45mm,
46–84mm). IntheBB,themajority ofwhitefish (83%)werecaught
withsmall‐meshgillnets (25–39mm),whereashalfof theseatrout
(49%)werecaughtbygillnets with a larger mesh size (40–45mm)
(Table 1, χ2 test: df=1,χ2=497.5,p=0.001).Incontrast, intheBS,
gill net s with median m esh sizes of 40– 45mm were the mos t im
portantgearforbothsea trout(72%)and whitefish(59%),whereas
smaller‐mesh nets (25–39mm) were common for w hitefish (31%)
butuncommonforrecapturedtrout(1%),andlarger‐meshnets(46–
84mm)werecommonforseatrout(26%)butlesscommonforgill
nettedwhitefish(10%)(Table1,χ2 test: df=1,χ2=574.9,p=0.001).
In the BS, the mean mesh size capturing sea trout in the tag re
covery data decreased from 48 mm to 44 mm between 1998–2011
(linear regression: b=−0.18, p=0.013, n=484), while the mesh
size remained unchanged in the BB (linear regression: b=−0.08,
p=0.274, n=472) (Figure 3a). The mean mesh size capturing
whitefish also decreased in the BS during this time period (linear
regression:b=−0.19,p < 0.001, n=12,638), but therewasaslight
increaseintheBB(linearregression:b=+0.09,p < 0.001, n=7,502)
(Figure 3a).
The effect of a small mesh size (26–40 mm) compared to a larger
mesh size (41–84 mm) was examined with a log‐linear model for the
relative probability of catching trout instead of whitefish. In the BS,
the mesh size used to catch whitefish and sea trout differed over
years1998–2011(catchyear*fishspeciesinteraction:Type3analy
sis: p < 0.001). The use of smaller‐mesh gill nets increased the white‐
fish catch in the BS without affecting trout (fish species: p = 0.001,
Year: p=0.759)(Figure3b).IntheBB,theprobabilityoftroutbeing
caught with a small‐mesh net increased from 0.4 to 0.6 over the
study period (year: p=0.002;fishspecies:p < 0.001; interaction: p
< 0.001) (Figure 3b).
The relative probability of catching sea trout instead of whitefish
in the gill‐net fisheries increased as a function of increasing mesh
size for both the BB and the BS. The relative probability of catching
seatroutalsoincreasedduringthelatertimeperiod(20 05–2011vs.
1998–2004). Furthermore, the relative probability differed between
FIGURE 2 Predicted catch length of sea trout in relation
to the mesh size of gill nets in the Bothnian Bay and Bothnian
Sea in 1989–2010. Catch length was log‐transformed and back‐
transformed in the graph
300
400
500
600
700
25 30 35 40 45 50 55 60 65 70 75
Catch length (mm)
Mesh size (mm)
BB
BS
FIGURE 3 The mesh size of gill nets in relation to catch years
(1998–2011) for sea trout and whitefish according to a linear
model (a) and the probability for using gill nets with a mesh size
of 26–40 mm instead of gill nets with a mesh size of 41–84 mm in
relation to catch years (b) (1998–2011) in the Bothnian Sea (BS) and
Bothnian Bay (BB) in the common catch areas for both whitefish
and sea trout (Figure 1). The models for the southern and northern
Gulf of Bothnia are separate and constructed for the years 1998–
2011, but drawn in the same figure for some years
30
35
40
45
50
1998 2000 2002 2004 2006 2008 2010
Mesh size of gill net (mm)
Catch year
Sea trout_BS Whitefish_BS
Sea trout_BB Whitefish_BB
a)
(a)
(b)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1998 2000 2002 2004 2006 2008 2010
Probability
Catch year
Whitefish_BS Sea trout_BS
Whitefish_BB Sea trout_BB
6 
|
   KALLIO‐N YBERG Et A L.
seasons (Figure 4): In winter and spring months, there was a higher
probability of catching sea trout than whitefish when the mesh size
exceeded 35–40mm in theBB and 45–50mm in the in BS. Inau
tumn months, by comparison, the relative probability of catching sea
trout in the BS was low for all mesh sizes due to intensive fishing for
whitefish (Figure 4). Because the fish species were obtained from
different samples, the proportions of whitefish and sea trout in the
samples are not the proportions in the catches.
3.3 | Gill‐net fishing near sea trout rivers
Sea trout were caught in gill‐net fishing near the home estuaries in
allseasons,butmostlyinautumnintheBB(rectangles7,15;spring,
summer, autumn, winter: 12%, 27%, 48%, 13%; n=288) and in
spring in the BS (rectangles32,37;seasons:43%, 23%,12%, 22%;
n=242)(Figure1).Whitefishwerealsocaught ingillnetsinallsea
sonsnear the sameestuaries,butmostlyinautumnintheBB(7%,
27%, 61%, 5%; n=4,626) and summ er in the BS (26%, 4 8%, 15%,
11%; n=2,051).Seasonally,gill‐netfishingofseatroutdif fered be
tween the estuaries and other sea areas in the BB and the BS, but
especially between the fish species caught in the same sea areas (BB;
7,1 5 :χ2=45.9;p < 0.001) (see Supporting Information Appendix S1).
The trout catch was highest in spring while the whitefish catch was
highest in summer or autumn in the BS (see Supporting Information
Appendix S1).
The distribution sea trout recaptures between gill‐net mesh‐
size classes (25–39mm, 40–45mm, 46–84mm) did not differ
between the statistical rectangles located in large estuaries (7,
15:37%, 49%, 14%; n=289) and tho se in other sea ar eas (38%,
53%, 9%, n=202) in the BB (7,15 & other:χ2=1.9;p>0.05). In
theBS,bothspecies were most oftencaughtwith40–45mm gill
nets(58%–79%).IntheBS,theestuariesofRiverIsojokiandRiver
Merikarvianjoki had similar distributions of catches between mesh
sizes for both species (32, 37: χ2=0.2; p>0.05; trout: 2%,66%,
33% and whi tefish: 2%, 67%, 31%) (see S upportin g Information
Appendix S1).
4 | DISCUSSION
Our results indicate that sea trout were mostly recaptured close
to their home estuaries at a size below the legal minimum landing
size and during the first or second year after their smolt migration
or release from the hatcheries. The main gear in which trout were
captured was gill nets with mesh sizes similar to those used to cap‐
ture whitefish, indicating the by‐catch role of sea trout in the gill‐net
fishery. This study suggests that it is possible to spatially and tem
porally regulate gill‐net fishing in the GoB so that the proportion of
young and endangered sea trout in the gill‐net catch would be lower.
Especially the protection of the feeding sea trout near their home
river is important, that sea trout are able to return migrate to their
home river and spawn (Kallio‐Nyberg et al., 2017). Jutila et al. (20 06),
and Whitlock et al. (2016) have also reported a high mortality of ju‐
venile trout in gill nets and considered this as one of the main causes
of the critically endangered state of the remaining trout stocks in the
Gulf of Bothnia.
In 1998–2010, the annual gill‐net and trap‐net catches of white
fishi ncommerci alfishingintheGoBwer e52 0and1 55tons,respec
tively, while the catch of sea trout during the same time period was
29 tons in gill nets and 21 tons in trap nets (Official Statistics Finland
2015;Anonymous, 2016). Thesefigures indicate thatsea troutac
countforabout5%ofthecombinedtroutandwhitefishc atches.As
only trout exceeding the MLS (the minimum landing size) are likely to
be reported, the actual proportion of sea trout in the catch is prob‐
ably somewhat higher. In the tagging data, sea trout were marked
with a Carlin or T‐anchor type external tag. Potentially, the tag might
increase the probability of sea trout being caught in gill‐net fishing,
as it makes enmeshing easier. In contrast, only a small proportion of
tags are returned, and the return rate in relation to the used gear is
unknown. Tagging may also have an impact on the growth of fish, al‐
though no systematic differences between untagged and tagged fish
have been observed. We consider the results to provide a represen‐
tative picture of the by‐catch of sea trout in whitefish gill‐net fishing.
The mesh size of the gill nets determines both the size of the fish and
the spec ies composition o f the total catche s. The most comm on mesh sizes
usedincommercial coastalgill‐net fishingwere36–45mm(Anonymous,
2016), and the same range of mesh sizes was also favored in the recre
ational f isheries. In sea t rout fisheries , however, all mesh sizes up to 70 mm
mainly catch undersized individuals. The gill‐net catch of the other fish
species(Lehtonen&Jo kikokko,1995;Lehtonenetal.,1993)islowincom
parison to the catch of whitefish in the Bothnian Sea and Bothnian Bay,
especially in cold water periods (Veneranta, Pakarinen, Jokikokko, Kallio‐
Nyberg, & Harjunpää, 2017). The increased use of smaller mesh sizes in
the BS for migratory whitefish is a significant trend. The mesh size, how
eve r,exp la in sonl y12%oft he le ng thofther ec apturedse at ro ut .T hus,the
gill‐net fishery as such is effective in catching sea trout.
FIGURE 4 Relative probability of catching a sea trout (1) instead
of whitefish by using different gill‐net mesh sizes in Bothnia Bay
and Bothnia Sea in two periods and seasonally, when separate data
set of fish species are combined. The year periods are 1998–2004
(=4)or2005–2011(=5)andseasonsare(Sp=March–May;
Su=June–August;Au=September–November;Wi=December–
February). See fishing areas included in Figure 1 (Bothnian Bay:
rectangles2,6,7,11,12,15,16,and19;BothnianSea:22,23,24,
28, 32, 37, 42, and 47)
–0.2
0
0.2
0.4
0.6
0.8
1
1.2
25 30 35 40 45 50 55 60 65 70 75 80 25 30 35 40 45 50 55 60 65 70 75 80
Sea trout probability as by-catch
Mesh size (mm)
Sp-4 Su-4 Au-4 Wi-4
Sp-5 Su-5 Au-5 Wi-5
Bothnian SeaBothnian Bay
    
|
 7
KALL IO‐NY BERG Et AL .
In the BB, the mesh size of gill nets that capture sea trout has
slightly decreased. The fishing of small‐sized sea‐spawning white‐
fish during the spawning period has probably decreased due to seal
pressure, and this has compensated for the change to smaller mesh
sizes in the anadromous whitefish fishery. The efficiency of the MLS
as a management method largely depends on the type and nature
of the fisheries. In coastal trap netting and rod fishing, undersized
trout might survive if handled and released with care (Bartholomew
& Bohnsa ck, 2005; Meka, 2 004). In gill‐net f ishing, however, the
survival rates of entangled and released fish are likely to be low, es‐
pecially if fishing pressure is high (Veneranta et al., 2017; Whitlock
et al., 2016). As a consequence, changes in the MLS are unlikely to
have any effect on the mean size of sea trout captured by gill‐net
fisheries that are targeted at whitefish and other coastal species. In
the Finnish coastal area, almost all tag recoveries came from coastal
fi s her i e s,w h ile r ive rcat c hes a cco u nte dfo r les s tha n 5% oft h ere c ov
eries (Kallio‐Nyberg et al., 2017). The slight increase of gill net mesh
size would decrease whitefish catches but still catch small sea trout.
Thus,onlyamarkedincreaseinthemeshsize,largerthan75mm,or
strict fishing restrictions on gill‐net fisheries would allow sea trout to
reproduce in their home rivers in the Finnish coastal area.
The high gill‐net fishing pressure on immature fish at sea has led
to long‐term overfishing, which has severely endangered the growth
and reproduction of the remaining sea trout stocks, as natural re
production has ceased in many rivers (Jutila et al., 2006). Presently,
the sea trout stocks are only maintained by large‐scale stocking of
hatchery‐reared smolts (1.1 million smolts in total released into the
GoB in 2012; ICES, 2013). The intensive gill‐net fishery with small
mesh sizes has also reduced the growth of whitefish (Heikinheimo &
Mikkola, 2004; Jokikokko & Huhmarniemi, 2014).
Due to the critically endangered status of the wild sea trout stocks,
the fishing pressure with gill nets should be reduced, especially in the
coastal areas of the GoB, but also elsewhere, as earlier recommended by
Jutila et al. (2006). The fishing of young undersized sea trout that have
not yet spawned has the most critical effect. It is uncer tain whether sea
trout enmeshed in gill nets can be released undamaged. To reduce the
fishing pressure and avoid the fishing of undersized sea trout, restric
tions on the mesh sizes of gill nets should be implemented, at least in
the mostcritical locations andperiods. Onlymesh sizesof65mm or
larger wou ld ensure that most s ea trout exceed the ML S of 60 cm (Jut ila
et al., 2006). However, this approach in general would cause a collapse
in the profitability of the coastal fisheries for other commercial species.
Spatial or temporal restrictions, whereby the gill‐net fishery near river
mouths or estuaries is totally or periodically prohibited, would be one
solution to spare sea trout. Marine protected areas (MPAs) or no‐take
areas of this type have already been established to protect essential
fish habitats (Degerman, Andersson, Häggström, & Persson, 2011;
Murray et al., 1999). To protect sea trout, the extent of closed areas
should be defined individually for each trout river and estuary based
on migration data and the geography of the estuary (Kallio‐Nyberg et
al., 2002, 2017 ), following the geographical and biological limits. This
type of ecosystem approach should be implemented in management to
conserve critical fish stocks, as well as to enable profitable fisheries (Le
Pape, Delavenne, & Vaz, 2014). The management measures presented
here are rather similar to those suggested earlier in Finland by Jutila et
al. (2006), in Sweden by D egerman, Leonardsson, an d Lundqvist (2012),
and for all the northern parts of the Baltic Sea by ICES (e.g. ICES , 2013).
However, a lack of action is likely to lead to the extinction of wild sea
trout stocks in Finnish waters.
ACKNOWLEDGEMENTS
We thank the staff of the Natural Resources Institute Finland
(Luke), especially Hannu Harjunpää and Alpo Huhmarniemi for
their substantial contribution to the whitefish data collection, the
staff in the tagging office for their major efforts in running the tag
ging program, and the Finnish Data Collection team for its consid
erable work in collecting whitefish data according to the EU Data
Collection Framework in Finland. The present study was partly fi
nanced by the SmartSea project, funded by the Strategic Research
Council of the Academy of Finland, partly by the National Data
Collec tion Programme un der Council Regulat ion (EC) N° 199/2008 ,
and partly by Luke. Dr. Roy Siddall revised the English language.
ORCID
Irma Kallio‐Nyberg http://orcid.org/0000‐00019149‐4461
Lari Veneranta http://orcid.org/0000‐0001‐5074‐0822
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SUPPORTING INFORMATION
Additional supporting information may be found online in the
Supporting Information section at the end of the article.
How to cite this article: Kallio‐Nyberg I, Veneranta L,
Saloniemi I, Salminen M, . Anadromous trout threatened by
whitefish gill‐net fisheries in the northern Baltic Sea. J Appl
Ichthyol. 2018;00:1–8. https://doi.org/10.1111/jai.13771
Appendix
Anadromous trout threatened by whitefish gill-net fisheries in the northern
Baltic Sea
VALIDITY OF THE COMPARISON OF DATA COLLECTION FRAMEWORK AND FINNISH STATISTICS DATA
The comparison of commercial whitefish catch statistics (Finnish Statistics, FS) with the EU Data
Collection Framework (DCF) whitefish samples indicates that the DCF is a representative sample of
the Finnish whitefish fishery.
In the southern GoB (BS), the maximum difference in seasonal distributions (spring, summer,
autumn, winter) between the DCF sample and the catch statistics was 3.3% in autumn (² = 0.68, df
= 3, p = ns), while in the northern GoB (BB), the maximum difference in autumn was 4.5% (² = 0.74,
df = 3, p = ns). The differences in seasonal proportions during spring, summer, and winter were
minor (Figure 1).
The whitefish gill-net effort (FS) and whitefish samples caught with trap nets or gill nets had a
similar spatial distribution in the northern GoB (Spearman correlation: r = 0.74, p = 0.037, n = 8
rectangles) (Figure 2). The spatial distribution of effort differed between whitefish caught with nets
(r = 0.63, p = 0.091, n = 8) and sea trout caught with all gears (r = 0.54, p = 0.160) or only nets (r =
0.38, p = 0.352). Both the effort and number of sea trout caught with nets was high in rectangles 11
(0.26; trout: 0.13) and 15 (0.20; trout: 0.40). In the southern GoB, the distribution of the whitefish
gill-net effort between the nine rectangles differed from that for sea trout with all gears (r = 0.502, p
= 0.168, n = 9) and with nets (r = 0.517, p = 0.154), and differed from the distribution of gillnetted
whitefish (r = 0.117, p = 0.765) (Figure 2). The release sites of sea trout were located in rectangles 37
and 32.
FIGURE 1. Validity of the EU Data Collection Framework (DCF) data for whitefish as a measure of the
actual official catch statistics (FS) for whitefish. The seasonal distribution (%) of the whitefish
fisheries catch according to the DCF data (open symbol, broken line) and official Fisheries Statistics
for Finland (continuous line) in the Bothnian Sea (BS) and Bothnian Bay (BB). The distribution for the
DCF is calculated from whitefish individuals and the official catch is the weight of captured whitefish
in kilograms from 19982011.
0
10
20
30
40
50
60
Spring Summer Autumn Winter
Proportion (%)
Season
DCF_BB DCF_BS FS_BB FS_BS
FIGURE 2. The spatial distribution of whitefish gill-net effort (Effort), the whitefish catch sample with
all gears (Whitefish-all) or only nets (Whitefish-net) and sea trout tag recoveries with all gears (Sea
trout-all) and nets only (Sea trout-net) in the northern and southern Gulf of Bothnia from 1998
2011. The locations of the ICES rectangles (numbers on x-axis) are indicated in Figure 1 of the study
(main text).
Northern area
Southern area
FIGURE. 3. Median distance (with upper quartile = q3) from the release river of sea trout recaptures
in age classes (sea years) 13 or older (4 = sea age 47) in the Gulf of Bothnia. Gill-net catches in
19982010 are presented for five rivers [Rivers Merikarvianjoki , Isojoki , Lestijoki , Oulujoki, and
Kemijoki]. The migration distances are based on the distance between the release site and the tag
recovery site. The median and its upper quartile are shown with same symbols as the river, for
example a solid triangle (median for Merikarvianjoki) and open triangle (upper quartile for
Merikarvianjoki).
0
50
100
150
200
250
1234
Distance (km)
Sea age (year)
Merikarvianjoki q3
Isojoki q3
Lestijoki q3
Oulujoki q3
Kemijoki q3
FIGURE 4. Seasonal distribution of the proportions of whitefish and sea trout caught in gill-net
fishing in the periods 19982004 and 20052011 in those ICES statistical rectangles of the Bothnian
Bay or Bothnian Sea where both species were caught (see Figure 1). Sample sizes are presented
above the columns, and the median mesh size of gill nets is shown for each season within the sea
area and fish species (numbers within columns).
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
1998-
2004 2005-
2011 1998-
2004 2005-
2011 1998-
2004 2005-
2011 1998-
2004 2005-
2011
Whitefish Sea trout Whitefish Sea trout
Bothnian Bay Bothnian Sea
Proportion
Catch area / Species / Catch year
Winter
Autumn
Summer
Spring
n=4291
3211
201
252
8286
4352
234
237
27
28
33
28
28
45
41
35
38
35
_27
45
40
45
40
40
40
45
45
42
40
43
43
40
40
43
45
50
45
38
40
27
TABLE 1. Sea trout and whitefish in gill-net fishing near the sea trout river estuaries (ICES rectangles:
7, 15, 32 and 37; Figure 1) and other areas and the distribution of the catch according to season
(spring, summer, autumn, winter) and the mesh-size classes (mm) of gill nets set in the Bothnian Bay
(BB) and Bothnian Sea (BS) during 19982011. The differences in seasonal or mesh-size distributions
of the catch between areas within fish species (distributions compared between areas marked with
superscripts as follows: 1&2, 3&4, 5&6, 7&8) were compared by using the χ2 test (Significance: p).
Species
Sea
Area
Spring ;
Summer;
Autumn;
Winter
(%)
Sample
(n)
χ2 test
season
Mesh size
(mm): 2539;
4045; 4684
(%)
Sample
(n)
χ2 test
mesh size
Trout
BB
7, 151
12; 27; 48; 13
288
1&2 8.8 p =
0.032
37; 49; 14
289
1.9 p = 0.375
Trout
BB
Other2
20; 31; 38; 11
201
38; 53; 9
202
Whitefish
BB
7, 153
7; 27; 61; 5
4626
3&4 301.9 p <
0.001
87; 12; 0
4626
131.5 p < 0.001
Whitefish
BB
Other4
7; 41; 52; 0
3074
78; 12; 0
3074
Trout
BS
32, 375
43; 23; 12; 22
242
5&6 12.9 p =
0.005
2; 66; 33
243
9.6 p = 0.008
Trout
BS
Other6
44; 16; 23; 17
238
1; 79; 20
238
Whitefish
BS
32, 377
26; 48; 15 ; 11
2051
7&8 411.1 p <
0.001
2; 67; 31
2051
1758.7 p <
0.001
Whitefish
BS
Other8
20; 30; 36; 14
10587
36; 58; 6
10587
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