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Examining the effects of authentic C&R on the reproductive potential of Northern pike



The practice within recreational fisheries to release captured fish back to the wild, known as catch-and-release (C&R), is an increasingly important strategy to protect fish stocks from overexploitation. However, C&R is a stressor and since animal reproduction is particularly sensitive to stress there is reason to suspect that such a practice induces sublethal fitness consequences. Here, we investigated whether and how C&R fishing influenced the reproductive potential in an anadromous population of Northern pike (Esox lucius). First, female pike were exposed to authentic C&R using rod-and-reel fishing in a coastal foraging habitat prior to the spawning period. Next, we observed the migration to the freshwater spawning habitat and compared both the timing of arrival and maturity stage between C&R-treated and control individuals. Finally, to evaluate effects on the quality and viability of eggs we stripped captured control and recaptured C&R-treated females, measured egg dry mass to assess nutrient content, conducted artificial fertilisations and incubated eggs in a controlled laboratory experiment. We found no evidence of C&R causing alterations in either arrival time, maturity stage, or the quality and viability of fertilised eggs. In combination, our results suggest that long-term effects of C&R-induced stress on key reproductive traits of pike, if any, are minor.
Fisheries Research 243 (2021) 106068
Available online 13 July 2021
0165-7836/© 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (
Examining the effects of authentic C&R on the reproductive potential of
Northern pike
Henrik Flink
, Oscar Nordahl
, Marcus Hall
, Anton Rarysson
, Kristofer Bergstr¨
Per Larsson
, Erik Petersson
, Juha Meril¨
, Petter Tibblin
Centre for Ecology and Evolution in Microbial Model Systems (EEMiS), Department of Biology and Environmental Science, Faculty of Health and Life Sciences, Linnaeus
University, SE-39231 Kalmar, Sweden
Swedish University of Agricultural Science, Department of Aquatic Resources, Institute of Freshwater Research, Drottningholm, Sweden
Ecological Genetics Research Unit, Organismal and Evolutionary Biology Research Programme, Faculty of Biological and Environmental Sciences, University of Helsinki,
Helsinki, Finland
Research Division of Ecology and Biodiversity, Faculty of Science, University of Hong Kong, Hong Kong Special Administrative Region
Handled by Dr. Steven X. Cadrin
Recreational shing
The practice within recreational sheries to release captured sh back to the wild, known as catch-and-release
(C&R), is an increasingly important strategy to protect sh stocks from overexploitation. However, C&R is a
stressor and since animal reproduction is particularly sensitive to stress there is reason to suspect that such a
practice induces sublethal tness consequences. Here, we investigated whether and how C&R shing inuenced
the reproductive potential in an anadromous population of Northern pike (Esox lucius). First, female pike were
exposed to authentic C&R using rod-and-reel shing in a coastal foraging habitat prior to the spawning period.
Next, we observed the migration to the freshwater spawning habitat and compared both the timing of arrival and
maturity stage between C&R-treated and control individuals. Finally, to evaluate effects on the quality and
viability of eggs we stripped captured control and recaptured C&R-treated females, measured egg dry mass to
assess nutrient content, conducted articial fertilisations and incubated eggs in a controlled laboratory experi-
ment. We found no evidence of C&R causing alterations in either arrival time, maturity stage, or the quality and
viability of fertilised eggs. In combination, our results suggest that long-term effects of C&R-induced stress on key
reproductive traits of pike, if any, are minor.
1. Introduction
The release of captured individuals within recreational sheries, a
practice known as catch-and-release (hereafter C&R), is becoming
increasingly popular to ensure sustainable use of sh populations and to
preserve the quality of sheries (Arlinghaus et al., 2007; Bartholomew
and Bohnsack, 2005). For C&R to be a sustainable alternative to the
traditional recreational shing (catch-and-kill), it requires sufcient
survival and successful reproduction of released sh. Many studies show
that mortality rates following C&R are generally low (Arlinghaus et al.,
2007; Cooke and Suski, 2005; Ferter et al., 2013). However, the con-
sequences of C&R on reproductive potential have rarely been
A typical C&R event will impose multiple stressors on sh including
hooking-related tissue damage, exercise during landing and exposure to
adverse air temperatures, oxygen deciency and gravity during
handling. These will trigger a full stress-response, similar to exhaustive
exercise, leading to energetic, ionic and hormonal changes (such as
elevated cortisol, lactate and glucose levels) (Arlinghaus et al., 2007).
Research in stress physiology recognise that animal reproduction is
particularly sensitive to stress, an overabundance of cortisol can due to
its anti-developmental, anti-growth and immunosuppressive traits have
negative long-term effects in sh and their progeny (Campbell et al.,
1992; Espmark et al., 2008; Giesing et al., 2011; Schreck et al., 2001).
For the period of gonadal development, stressed sh may have to alter
the allocation of energy between reproduction, maintenance or somatic
growth which can result in delayed ovulation, reduced gamete quality,
lowered progeny survival or, in worst case, complete spawning failure
* Corresponding author at: Centre for Ecology and Evolution in Microbial Model Systems (EEMiS), Department of Biology and Environmental Science, Faculty of
Health and Life Sciences, Linnaeus University, SE-39231 Kalmar, Sweden.
E-mail address: (P. Tibblin).
Contents lists available at ScienceDirect
Fisheries Research
journal homepage:shres.2021.106068
Received 1 March 2021; Received in revised form 2 July 2021; Accepted 6 July 2021
Fisheries Research 243 (2021) 106068
due to inhibited vitellogenesis or egg atresia (Campbell et al., 1992;
Roff, 1982; Schreck et al., 2001; Sopinka et al., 2016). However,
whether C&R-induced stress produce such severe effects in wild sh is
largely unexplored despite its clear implications for management and
sustainable use of sh stocks.
To date, most studies investigating C&R consequences on the
recruitment of wild sh have focused on parental care in black bass
(Micropterus spp.) rather than direct inuences of C&R on gonadal
development and reproduction. When nest-guarding black bass males is
temporarily removed from their nest by C&R it results in nest aban-
donment, brood predation and thereby reduced reproductive output (e.
g. Suski et al., 2003; Steinhart et al., 2004; Stein and Philipp, 2015).
Remaining studies have, with few exceptions, addressed the effects of
C&R on recruitment by studying movement behaviour in salmonids,
subjected to C&R during the stressful and energetically taxing spawning
migration rather than during the foraging season. These studies have
shown ambiguous results revealing either indirect evidence suggesting
impaired reproductive output due to alterations in movement behaviour
(Thorstad et al., 2007; Richard et al., 2014; Havn et al., 2015; Lennox
et al., 2015, 2016, 2017; Twardek et al., 2019; but see Jensen et al.,
2010; Smukall et al., 2019 for examples of no effects) or changes in
migratory timing (Thorstad et al., 2007; Havn et al., 2015; but see
Smukall et al., 2019 for examples of no effects). While there is a small
number of studies that have estimated C&R effects on reproductive
success more directly, results are disparate suggesting either none (Roth
et al., 2019; Smukall et al., 2019) or size dependent negative effects on
reproductive outcome with larger individuals being more susceptible
than smaller ones (Richard et al., 2013).
Here, we explore C&R effects on key reproductive traits in Northern
pike (Esox lucius, hereafter referred to as pike), an important target
species in recreational C&R sheries in the northern hemisphere as well
as an ecological and socioeconomical key species (Ekl¨
of et al., 2020;
Larsson et al., 2015). In contrast to most salmonids, which at time of
spawning migration have completed their reproductive investment
(King et al., 2003), female pike vitellogenesis and oocyte development
continuous up till spawning (Medford and Mackay, 1978). This has
consequences for the putative effects of C&R on reproduction since
stress-induced impairments are expected to compromise the develop-
ment of oocytes during vitellogenesis by energy deciencies and transfer
of stress hormones (Schreck et al., 2001). Further, we chose to assess
effects of authentic C&R in natural settings, rather than simulating C&R
events in controlled environments. While simulation of C&R is a com-
mon method and has some benets, it might obscure potential conse-
quences relevant for the actual implementation of C&R in management.
To the best of our knowledge, this is the rst non-salmonid study to
consider long-term sublethal effects of authentic C&R in natural settings
on migratory timing and reproductive potential. Our approach was to
subject female pikes to authentic C&R using rod-and-reel shing in the
coastal foraging habitat, prior to the spawning period. After this, we
observed the migratory timing to the dened spawning habitat
(wetland) using passive integrated transponder tags (PIT) and compared
both timing of arrival and maturity stage between C&R-treated and
naïve (control) individuals. Finally, to evaluate effects of C&R on the
quality and viability of eggs we stripped gametes from C&R-treated and
control females recaptured in the spawning habitat using fyke-nets. We
assessed the nutrient content in eggs by measuring dry mass and eval-
uated egg viability by conducting articial fertilisations and subsequent
incubation of eggs in a controlled laboratory experiment.
2. Material and methods
2.1. Study species
Pike is a large-bodied, iteroparous and long-lived species, important
as a top predator in freshwater lakes and brackish waters (Donadi et al.,
2017; Ekl¨
of et al., 2020). Anadromous pike in the Baltic Sea migrate to
spawn in freshwater streams and wetlands between February to May,
forming subpopulations geographically separated during early life
stages (Nilsson et al., 2014; Tibblin et al., 2015). Since the 1990s many
Baltic Sea coastal pike populations have declined, essentially due to
recruitment problems, and this has resulted in regulations of recrea-
tional sheries such as shery closure during spawning season, bag-limit
and size window with mandatory C&R (Ljunggren et al., 2010; Nilsson
et al., 2019, 2004). The species serve as an established model organism
in ecology and evolution (Forsman et al., 2015) and its homing behav-
iour support studies of reproductive potential in wild sh by enabling
recapture of ripe individuals prior to spawning (Berggren et al., 2016;
Tibblin et al., 2016a, 2015). Pike is one of the most popular target
species in recreational sheries in Fennoscandia and pike shing during
late winter is particularly popular (Swedish Agency for Marine and
Water Management, 2019). One reason for this is that large numbers of
pike aggregate in shallow waters during winter and thereby become
easily accessible for anglers. Also, this is prior to spawning and the fe-
males are then close to their maximum weight. Pike is considered to be
relatively resilient to C&R shing and previous studies have found low
direct mortality (Arlinghaus et al., 2008, 2009; Baktoft et al., 2013;
Klefoth et al., 2008; Muoneke and Childress, 1994; Stålhammar et al.,
2.2. Study area
We conducted C&R shing in three adjacent bays at the Baltic Sea
shore of ¨
Oland, Sweden (Fig. 1). This area is known to harbour a pop-
ulation of anadromous pike that use the coastal habitat for foraging and,
at time of spawning, migrate through a small stream (~350 m long, <3
m wide and average depth of <50 cm) to reach their spawning habitat:
the wetland Harfj¨
arden (N 564915.6′′; E 164832.7′′). We chose this
study area based on low recreational shing pressure such that pike will
be naïve to prior C&R experience, in combination with extensive
background knowledge on population dynamics and migratory behav-
iour from previous studies (Nordahl et al., 2019; Sunde et al., 2018a,
2018b, 2019). In addition, the small stream allows robust detection of
migrating PIT-tagged sh and can be entirely closed off with a fyke-net.
To conrm our assumption of low recreational shing pressure in the
area we performed continuous monitoring (35 visits) of the study area
between October 2019 to February 2020. No pike shing was observed
in the study area during these visits. Throughout the sampling period,
two temperature loggers (HOBO Pendant) were placed in the wetland at
potential spawning grounds to hourly track the water temperature
(Fig. 2b).
2.3. Catch-and-release treatment
We C&R a total of 87 female pike (total length 66.7 ±9.0 cm, mean ±
s.d.) between 30 October 2019 to 18 February 2020 in 15 separate
shing efforts. We shed either from boat or the shore, by actively
casting with spinning rod and reel, using articial hard- or soft lures. All
sh were captured in depths less than 1.5 m, thus excluding potential
effects of barotrauma. C&R followed the common procedure regarding
landing (Cooke and Suski, 2005); captured pike were landed swiftly
(average landing time was 42 s) in a rubber net and, if possible, handled
under water whilst unhooked with pliers.
Following unhooking, focal sh was handled above water to mimic
the procedure of documentation (photographs and measurements of
length and weight). The handling time was on average 2.5 min, which is
similar (but in the higher end) to previous C&R studies on pike
(Arlinghaus et al., 2009; Dubois et al., 1994). During this time, we also
recorded length (total length, nearest cm) and inserted a PIT-tag (23.1
mm long and 3.85 mm in diameter, model HDX23, Biomark, Boise,
Idaho, USA). To minimise the risk of damaging reproductive organs,
PIT-tags were inserted under the pelvic girdle rather than in the body
cavity, using a sterile needle mounted on an injector. After the marking
H. Flink et al.
Fisheries Research 243 (2021) 106068
procedure all pike were released back to the water. There were cases of
multiple C&R events for individual sh. Eight pikes were subjected to
C&R twice and one pike was C&R three times. Out of the individuals
C&R-treated twice, three were included in the analyses of migration
timing and maturity stage, the rest were not detected at the wetland
(except for one individual partially detected, see below for explanation).
All other analyses were only based on single C&R individuals.
2.4. Swim-through PIT-tag reader to monitor spawning migratory timing
To evaluate inuences of C&R on migratory timing, we placed a PIT-
tag reader station directly downstream from the outlet pool of the
wetland (Fig. 1). The PIT-tag reader was installed February 14, 2020,
but a proportion (n =20) of tagged and C&R-treated females had
already migrated to the wetland or failed to be detected at arrival as
evidenced by their subsequent downstream migration. The PIT-tag
reader was removed April 24, thirteen days after the last upstream
movement had been detected. To estimate timing of arrival of C&R-
treated individuals, we included only females for which we could
conrm an exact date of upstream migration (n =39). The PIT-tag
reader station consisted of two antennas that enabled determination of
migration direction, however due to partial detections of sh (detected
by only one of the two antennas), we know that an additionally 6 C&R-
treated females migrated to the wetland during the spawning season but
the direction of movement could not be determined, and they were
subsequently not included in any comparisons.
Tagging of individuals has been conducted in the study area since
2017. Here, we take advantage of previously tagged female pike, with no
recent (focal year) experience of C&R, to estimate timing of arrival in
control sh (n =32) during 2020.
2.5. Assessment of maturity stage and stripping of gametes
To capture focal sh for assessment of maturity stage and to strip
gametes for egg quality assessment and the articial fertilisation
experiment (see below), we used a stream-wide fyke net that completely
shut-off the outlet pool of the wetland between February 23 to March 27,
2020 (Fig. 1). Since a high proportion of females caught during up-
stream migration were not yet ovulating, the stream-wide fyke net were
only used periodically. Additional fyke nets (n =3) were placed inside
the wetland, where the proportion of ovulating females were higher.
Fyke nets were emptied daily. Maturity stage, categorised as either pre-
spawn condition (not ovulating) or spawn (ovulating)/spent condition,
was assessed for all females by applying a gentle pressure on the
Fig. 1. Study site. Map of the study area at the Baltic Sea shore of ¨
Oland, Sweden. Map includes the three bays in which pike were caught-and-released, the
freshwater wetland to which pike migrate and spawn as well as the outlet pool of the wetland were a swim-through PIT-tag reader was installed.
H. Flink et al.
Fisheries Research 243 (2021) 106068
In total, 10 ovulating C&R-treated females were recaptured and
stripped for eggs. Simultaneously to each recapture, ovulating C&R-
naïve females (to be used as control) as well as C&R-naïve males were
stripped for gametes to produce crossings by articial fertilisation (de-
tails, see below). There was an incident of shortage of ovulating C&R-
naïve females, resulting in one less stripped control female and male.
Before release back to the wetland, all stripped sh were measured for
body length (total length, nearest cm) and, if unmarked, PIT-tagged to
ensure that no sh were stripped twice. Body length was similar in C&R-
treated (n =10, total length 69.3 ±10.0 cm) and control females (n =9,
total length 66.6 ±9.0 cm) stripped for articial fertilisation (two
sample t-test, t
=0.62, p =0.54). The rst batch of eggs from each
female was discarded in order to avoid contact with water and a
resulting premature opening of the micropyle. Eggs were collected in 10
mL Falcon tubes and milt in 1.25 mL Micro tubes that were put on ice
and transported to the laboratory.
2.6. Articial fertilisation experiment and measurements of egg quality
The fertilisation experiment to evaluate effects of C&R on egg
viability was carried out at the Linnaeus University, Kalmar Sweden
between March 2 to April 2 in a temperature-controlled room, set such
that the water (tap water aerated for 48 h) temperature was ~6 C to
mimic natural conditions during spawning according to previous studies
(Sunde et al., 2019). Fertilisation was conducted no more than three
hours after gamete stripping. To account for potential effects of male
sperm quality on fertilisation success and egg viability, milt from each
individual male was used to fertilise eggs from both a C&R-treated and
the respective control female, resulting in 19 crosses. Each cross was
done in two replicates, which were produced by independent articial
fertilisations to reduce effects of random errors on fertilisation and in-
cubation, resulting in a total of 38 experimental units. The fertilisation
process for all experimental units followed the method established in
Sunde et al. (2018, 2019) by placing ~30 eggs (31 ±2 eggs) from the
focal female in a small porcelain bowl, pipetting an excess of milt (~50
l) from the focal male on top and adding 1 mL of water from the
experimental system to the bowl. The gametes were mixed by a gentle
whirl for 2 min, rinsed with water three times to remove excess milt and
then immediately transferred to an 800 mL plastic cup lled with water.
All cups/experimental units were randomly distributed within the room
and two temperature loggers (Hobo Pendant) were placed in unused
cups lled with water to get hourly measurements of water temperature
during the experiment (Fig. 2b). Photographs of each experimental unit
was taken immediately after fertilisation so that the exact number of
eggs at start could be quantied. A partial water exchange (75 % of
volume) was performed once a day across the experimental system by an
automated water drip system and identication and removal of dead
eggs were done daily in each replicate to avoid potential confounding
effects through effects on the water quality. Dead pike eggs become
opaque and are easy to discern, however, unfertilised eggs are discern-
ible rst after ~35 days when incubated in freshwater (Sunde et al.,
2018a). Consequently, it is difcult to visually discriminate between
unfertilised eggs and eggs that die during early development. Viability of
incubated eggs (estimated as still being alive) was thus analysed by
counting viable eggs from a second photograph taken 1021 days post
fertilisation, including also potential unfertilised eggs. Due to the paired
design (a single male fertilising eggs from both control and C&R-treated
females at same occasion/day) as well as replication within family
crossings, this measurement captures the ability of the egg to be fertil-
ised and subsequently develop for a dened period. Although the in-
cubation time until the count of viable eggs varied between families
there was no variation in incubation time between treatments due to the
paired design.
Dry mass per egg, a proxy of nutrient content and egg quality
(Berggren et al., 2016; Murry et al., 2008), was estimated by drying 1 mL
of unfertilised eggs from each female in the fertilisation experiment
overnight at 60 C in a heating cabinet. However, two paired samples,
one from each treatment, failed (resulting in 9 samples from
C&R-treated females and 8 samples from control females). To increase
the sample size and robustness of the comparison, eggs from additional
C&R-naïve females (n =9), sampled concurrently, was added resulting
in a total of 17 control samples. Body length was similar in C&R-treated
(n =9, total length 67.7 ±8.9 cm) and control females (n =17, total
length 68.9 ±9.7 cm) used for estimating egg quality (two sample t-test,
=0.34, p =0.74). Before drying, eggs from each female were spread
Fig. 2. a) Migration timing. Arrival of caught-
and-released and control pike females to the
spawning wetland according to PIT-tag de-
tections. The grey background show 25-75 %
quartile of arrivals. b) Water temperature.
Hourly water temperature in the wetland (grey
line) and in the incubation experiment (black
line) during the study period. The hourly water
temperature for each location is calculated as
an average from two temperature loggers.
Dashed vertical line show the median arrival
date of all females to the wetland.
H. Flink et al.
Fisheries Research 243 (2021) 106068
out in a single layer on an aluminium baking tin and photographed for
quantication of number of eggs (1 mL of eggs amounted to 131 ±22
eggs). Following drying, each sample of eggs was weighed to the nearest
0.1 mg (BL 210 S Analytical balance, Sartorius, Goettingen) and divided
by the number of eggs to calculate dry mass/egg.
2.7. Data analysis and statistics
The software R 4.0.3 (R Core Team, 2020) was used for all statistical
analyses. Figures were prepared in R using the packages ggplot2
(Wickham, 2016) and ggmap (Kahle and Wickham, 2013). Photographs
of eggs for measurements of dry egg mass and egg viability were ana-
lysed using ImageJ software (1.52q) (Schneider et al., 2012). An alpha
level of 0.05 was used for all statistical tests.
Timing of arrival to the wetland was examined using median, 25th
and 75th percentiles of calendar date for C&R-treated and control fe-
males. In addition, a Kolmogorov-Smirnov (KS) test was used to evaluate
if timing of arrival, measured as number of days from start of monitoring
until arrival of focal sh, differed between C&R-treated and control
Contingency tables with Fisher´s exact test for count data were used
to determine whether stage of maturity, dened as either pre-spawn or
spawn/spent, differed between C&R-treated and control females. Ana-
lyses were separated for females caught arriving to the wetland and
females caught inside the wetland.
ANCOVA was used to test the possible effect of C&R treatment on
resource allocation in eggs (dry mass per egg) with female body length
treated as a covariate. The nonsignicant interaction term between
treatment (C&R vs control) and body length was removed from the
model. However, the covariate body length was signicant and thus kept
in the model (ANCOVA, effect of total length: F
=5.09, p =0.03).
The inuence of C&R treatment on egg viability was analysed using a
generalised linear mixed model (GLMM) in the lme4 package with a
binomial t and a logit-link function. The response variable was the
number of viable eggs at the end of incubation out of the total number of
eggs at start of incubation. The treatment of females (C&R vs control)
was treated as a xed factor. To account for replication within family
crossings, individual families were included in the model as a random
effect. Parameter estimates and associated statistical signicance levels
of the random effect are not reported since we were not interested in
quantifying these effects as such.
3. Results
3.1. Migration timing
In total, 60 out of 87 (69 %) C&R-treated females were conrmed by
detection of their PIT-tags to arrive at their spawning habitat in the
wetland. C&R-treated females had their peak arrival, i.e. the time period
when 50 % of the individuals arrived, between February 25 to March 4,
2020 (n =39, median =March 2) (Fig. 2a). In comparison, PIT-tagged
females that had not been subjected to C&R in this study (control fe-
males) had an overlapping peak of distribution at March 15 (n =32,
median =March 4). There was no signicant difference between C&R-
treated and control females in time of arrival (two sample KS-test, D =
0.25, p =0.20).
3.2. Maturity stage
The maturity stage of C&R-treated and control females did neither
differ signicantly at arrival to the wetland (C&R-treated: n =10, 80 %
pre-spawn, control: n =66, 72 % pre-spawn, Fisher´s exact test: odds
ratio =0.67, p =1) nor inside the wetland (C&R-treated: n =14, 7% pre-
spawn, control: n =74, 24 % pre-spawn, Fisher´s exact test: odds ratio:
4.13, p =0.29) (Fig. 3).
3.3. Egg quality
Egg quality, measured as dry mass per egg, of C&R-treated (~2.30
mg, condence interval: 2.142.47 mg) and control female pikes (~2.28
mg, condence interval: 2.162.39 mg) did not differ signicantly
(ANCOVA, effect of C&R: F
=0.09, p =0.76) (Fig. 4a).
3.4. Egg viability
Egg viability was high in both C&R-treated (~93 % viable eggs,
Fig. 3. Maturity stage. Proportion females in pre-spawn and spawn/spent condition, caught either on arrival to the wetland (C&R: n =10, control: n =64) or inside
the wetland (C&R: n =14, control: n =74).
H. Flink et al.
Fisheries Research 243 (2021) 106068
GLMM condence interval: 8996 %) and control females (~94 % viable
eggs, GLMM condence interval: 9097 %). Egg viability did not differ
signicantly between the groups (GLMM, effect of C&R treatment: z =
0.38, p =0.71 (Fig. 4b).
4. Discussion
Fish reproduction is sensitive to stress, accordingly researchers have
advocated C&R sheries to avoid capturing sh immediately before or
during the spawning period, until there is rm evidence that there is no
C&R-induced negative impacts on recruitment success (Arlinghaus
et al., 2007; Cooke and Suski, 2005). Yet, to evaluate reproductive
consequences of C&R-induced stress in natural settings is challenging,
hence the knowledge and understanding on such effects are inadequate
and, adding to the complexity, the few existing studies show disparate
results. Our study provides unique and important knowledge by testing
such long-term effects of authentic C&R in natural settings in pike
during the period of vitellogenesis, whilst oocyte development is sus-
ceptible to stress. Although our results suggest that C&R did not impair
any of the focal components of reproductive potential in pike, continued
cautiousness is required in the implementation of these ndings in
Arrival timing at spawning sites have clear bearings on the tness of
sh by inuencing the survival of both juvenile and adult life-stages
(Einum and Fleming, 2000; Tamario et al., 2019; Tibblin et al.,
2016b). We anticipated C&R to alter arrival time and maturity stage in
anadromous pike due to the anti-developmental and inhibitory repro-
ductive effects of stress hormones. For example, rainbow trout (Onco-
rhynchus mykiss) exposed to acute stress by air exposure have been
shown to delay ovulation several weeks (Campbell et al., 1992) and
exposure to C&R in rainbow trout leads to a reduction in plasma levels of
oestrogens (Pankhurst and Dedualj, 1994). Further, C&R have shown to
induce downstream movement and delay upstream spawning migration
in Atlantic salmon (Salmo salar). However, this is suggested to result
from short-term effects on physical recovery, loss of orientation or
escape behaviour rather than due to suspended gamete development
(Havn et al., 2015; Thorstad et al., 2007). Our results show an over-
lapping peak arrival of C&R-treated and control females, without any
tendency of C&R delaying spawning migration. Similarly, we found no
evidence of delayed ovulation in C&R-treated females. Upon arrival to
the wetland, 80 % of C&R-treated females were in pre-spawn condition,
similar proportion as in control sh. Among sh caught in the spawning
grounds only 7% of C&R-treated sh were found in pre-spawn condi-
tion, the rest were either ovulating or spent, as expected during a normal
spawning event. Our results are in accordance with pond-based studies
on Australian bass (Macquaria novemaculeata) and golden perch (Mac-
quaria ambigua), demonstrating normal gonadal development following
simulated C&R, with exception for harshly angled sh (Hall et al., 2017,
In oviparous (and viviparous) sh, all nutrients essential for
embryonal development must be incorporated in the oocyte before
ovulation, hence nutrient content of an egg together with the genetic
material determines egg quality (Brooks et al., 1997). Rainbow trout
exposed to acute stress experience reduced egg size and viability
(Campbell et al., 1992). However, consequences of C&R-induced stress
on egg nutrient content has prior to our study never been considered.
Our results demonstrate no difference in egg dry mass, a proxy of
nutrient content in pike eggs (Murry et al., 2008), between C&R-treated
and control females. Egg dry mass from C&R-treated individuals was
around 2.142.47 mg per egg (95 % condence interval) compared to
2.162.39 mg in control females. The observed quality of eggs coheres to
the results from our laboratory experiment of fertilised egg viability that
showed over 90 % overall egg viability in both C&R-treated and control
females. Previous studies on the effects of C&R on fertilised egg viability
have been conducted in salmonids using simulated C&R which have
generally demonstrated no impacts of C&R (Booth et al., 1995; Pettit,
1977; Smukall et al., 2019). By using authentic C&R in a natural setting
rather than simulated C&R we provide additional and unique support of
that neither quality nor viability of eggs seem to be signicantly reduced
by C&R.
Admittedly, the lack of signicant statistical effects of C&R on the
recruitment potential of pike, in combination with the focal sample
sizes, require cautious inferences and implementation of the results.
Still, that there were no signicant negative effects in any of the
investigated responses to C&R (arrival timing, maturity stage, egg
quality and egg viability) points to that the negative effects of C&R on
Fig. 4. a) Egg quality. Least square means
(adjusted for female body length effect) of dry
mass per egg from C&R and control females.
Black point represents mean and error bar
represents 95 % condence interval. Note that
the scale on the y-axis does not start at zero. b)
Egg viability. Percentage egg viability for C&R
and control females as predicted from the
GLMM model t, black point represents mean
and error bar represents 95 % condence in-
terval. Note that the scale on the y-axis does not
start at zero.
H. Flink et al.
Fisheries Research 243 (2021) 106068
pike reproduction are minor although negative effects mediated through
other pathways such as atresia cannot be excluded. Moreover, in
experimental studies demonstrating reproductive consequences of
stress, focal sh have been exposed to repeated acute stress (Campbell
et al., 1992; Giesing et al., 2011). In our study only a few sh were
captured multiple times and thus we cannot exclude potential additive
effects of multiple C&R which may occur in intense C&R sheries. In the
same vein, it is important to note that sh handling in this study was
conducted by experts and extreme caution was exercised when handling
the sh. It is possible that different handling methods and/or prolonged
retention time would have visible effects on reproductive traits.
A possible explanation to that C&R did not result in discernible
reproductive impairments could be that female pike is capable of pro-
tecting embryos from negative effects of stress by regulating the transfer
of stress hormones to oocytes during vitellogenesis, as demonstrated in
zebrash (Danio rerio) (Faught et al., 2016). Alternatively, stress
induced by C&R might inuence the trade-off between reproductive
output and somatic growth such that C&R-treated females allocated
energy to uphold their reproductive potential but deprioritised
self-maintenance and growth (Schreck et al., 2001). To some extent this
is supported by a whole-lake C&R experiment that showed reduced
growth rates across seven months in C&R-treated pikes (Klefoth et al.,
2011). However, such negative effects of C&R on the growth rates of
pike could potentially also result in lowered fecundity (Berggren et al.,
2016) which emphasis the need of further studies to disentangle the
effects of C&R on the growth-reproduction trade-off.
From a sheries management point of view, our results represent an
important nding suggesting that C&R do not represent a stressor with
substantial long-term negative consequences on the reproductive po-
tential in pike and subsequent impacts on recruitment and population
dynamics. However, such inference of our results rest upon that the
direct mortality of C&R is low, that negative effects of somatic growth do
not translate into effects on reproduction or that survival in later juve-
nile life-stages is impaired, and, nally, that there are no additive effects
of multiple C&R events or unfavourable handling and must thus be
implemented with caution. While pike has shown to be relatively
resilient to C&R, there is considerable variation among species in the
response to C&R shing (Muoneke and Childress, 1994) that stress for
species-specic management (Cooke and Suski, 2005). As an alternative
to traditional catch-and-kill shing, C&R can play an important role as
management tool to aid sustainable sheries, as shown in wild pop-
ulations of Atlantic salmon where C&R-treated sh contributes signi-
cantly to the total reproductive output (Richard et al., 2013; Thorstad
et al., 2003; Whoriskey et al., 2000). As participation in recreational
shing is popular and the practice of C&R increases in many countries
(Arlinghaus et al., 2007; Bartholomew and Bohnsack, 2005), future
research on this topic is warranted. For successful management, there is
a need to better understand sublethal effects of C&R in general, for
example on reproductive traits not covered here such as total repro-
ductive investment, absolute fecundity, spawning competition, hatching
rate and fry performance. Future research should also address the effects
of C&R on the trade-off between reproduction and somatic growth and
try to disentangle how it changes depending on gonadal maturation in
females during C&R exposure.
To conclude, we found no evidence of that C&R of pike during pre-
spawn season cause alterations upon spawning in either arrival time,
maturity stage, egg quality or egg viability. In combination, our results
suggest that C&R-induced stress do not cause signicant long-term ef-
fects on key reproductive traits. The results are of general interest
because we, unlike related studies, exposed females to C&R during
vitellogenesis while there is an additional demand of energy and oocytes
are susceptible to hormonal impairment. In addition, the study reects
authentic C&R on wild sh in their foraging habitat and evaluates the
consequences by eld observations in the spawning ground as well as in
a controlled laboratory experiment. Although our results are promising
in the context of the sustainability of C&R, there is still large gaps of
knowledge in our understanding of the consequences of C&R on
recruitment, and species- and context-dependence thereof, that needs to
be answered for C&R to be unconditionally implemented in sheries
Ethical approval
All applicable international, national, and/or institutional guidelines
for the care and use of animals were followed. The laboratory was
approved as research facility (Dnr 5.2.1817988/18) and the study was
granted ethical approvals (Dnr 168677-2018; Dnr 19359-2019) by the
Ethical Committee on Animal Experiments, Swedish Board of Agricul-
ture, in Link¨
oping and Stockholm respectively.
CRediT authorship contribution statement
Henrik Flink: Conceptualization, Project administration, Method-
ology, Writing - original draft, Writing - review & editing, Formal
analysis, Visualization, Investigation. Oscar Nordahl: Investigation,
Writing - review & editing. Marcus Hall: Investigation, Writing - review
& editing. Anton Rarysson: Investigation, Writing - review & editing.
Kristofer Bergstr¨
om: Investigation, Writing - review & editing. Per
Larsson: Investigation, Resources, Writing - review & editing. Erik
Petersson: Conceptualization, Writing - review & editing. Juha Meril¨
Conceptualisation, Writing - review & editing. Petter Tibblin:
Conceptualization, Project administration, Resources, Methodology,
Writing - original draft, Writing - review & editing, Funding acquisition,
Supervision, Investigation.
Declaration of Competing Interest
The authors declare that they have no known competing nancial
interests or personal relationships that could have appeared to inuence
the work reported in this paper.
We thank Jasper Münnich, Johanna Sunde, Per Koch-Schmidt,
Robert Franz´
en and Jonas Jakobsson for excellent help in the eld and
the laboratory. We are grateful to the landowners granting us access to
the spawning pike sampling site. We thank two anonymous reviewers
or valuable comments on earlier drafts of the manuscript. This work
was supported by funds kindly provided by the Swedish Research
Council FORMAS(grant 2018-00605 to P.T.) the Crafoord Foundation
(grant 20190636 to P.T.) and G¨
ote Borgstr¨
oms Foundation(grant to P.
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... 16 • 31'18.4"E), and artificially fertilizing the gametes according to an established protocol (Flink et al., 2021; outlined in detail by Sunde et al., 2018). In short, gametes (about 20 ml eggs from females, 1 ml milt from males) were dry stripped from the parental fish and brought to the Linnaeus University laboratory where fertilization were conducted by mixing gametes (each female were fertilized by multiple males to improve fertilization rate) and adding freshwater to induce fertilization. ...
Full-text available
Predator-prey interactions can modify population dynamics and influence ecosystem functioning. An interesting, but less understood, aspect of such species interactions is predator-prey role reversal where a mesopredator can reduce the recruitment success, and ultimately the abundance, of their own predator. Such role reversal is ongoing in the Baltic Sea where increasing abundances of mesopredatory sticklebacks (Gasterosteus aculeatus) have resulted in intense predation on the recruits of their predator the northern pike (Esox lucius). Still, the mechanistic underpinning of this reversal remains poorly understood. Here we investigate the role of chemical communication in this predator-prey role reversal with the aim to understand whether and how sticklebacks use olfactory cues to assess whether pike constitutes a predator or prey. By performing a laboratory preference experiment, we show that chemical communication is indeed used by sticklebacks for assessing the predation risk emanated by adult pike but that it requires additional alarm cues (Schreckstoff) informing that sticklebacks are predated upon. Adult pike kairomones or alarm cues alone did not result in any adaptive response by sticklebacks nor did olfactory cues of pike as prey (pike larvae kairomones and dietary cues of zooplankton). This knowledge contributes to the understanding of predator-prey interactions as well as the dynamics of the shifting coastal fish community in the Baltic Sea. The results also suggest that addition of Schreckstoff to confined pike (and perch) spawning and recruitment habitats may deter sticklebacks from entering which should be further explored by management.
... In 2020, only 15% of the pike caught was landed (Statistics Sweden, 2021). While catch-and-release may have pronounced effects on pike behaviour and growth, survival of pike is high Hühn and Arlinghaus, 2011) and effects on reproductive traits are minor (Flink et al., 2021). Although some negative population-level effects from extensive catch-and-release fisheries can still be expected, this shift in practices has likely led to lower fishing-related mortality. ...
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In the Baltic Sea, the large predatory fish northern pike (Esox lucius L.) is important for both recreational fisheries and ecosystem functioning. As existing fishery-independent surveys do not adequately monitor pike populations, a general lack of knowledge on population status and trends poses challenges for management. Here we use recreational angling data as an alternative method to describe pike population development along the Swedish Baltic Sea coast from 1938 onwards and assess the change over time in potential mortality factors by estimating harvest by fisheries and consumption by large predators. Data from a Swedish national register on trophy-sized pike (>12 kg) showed that numbers and maximum sizes peaked in 1990–1995, after which declines in both metrics are evident. In logbooks from a sport fishing club in the Stockholm archipelago, a simultaneous decrease in maximum sizes of pike was observed, together with a decrease in the total number of pike caught per fishing day. Jointly, these data indicate a decline in the abundance of large pike since around 1990. While commercial pike fisheries in the Baltic Sea have decreased continuously since the 1950s, recreational fishing increased after 1985, when Swedish coastal waters were made open access to anglers. The declines in large pike starting in the 1990s could, thus, have been driven by an increase in mortality from recreational fisheries, which at the time primarily practiced catch and kill. Since the 2000s, bag and slot length limits, spawning closures, and a general increase in catch-and-release fishing has reduced the landings of pike in recreational fisheries. Despite these fisheries regulations and higher release rates the decline in catches of large pike has continued, indicating an effect of other mortality factors. The strong growth of grey seal (Halichoerus grypus) and great cormorant (Phalacrocorax carbo sinensis) populations suggest that predation pressure on pike has increased over time. In the Stockholm archipelago these two predators were estimated to remove 5–18 times (range based on different diet composition estimates) more pike biomass than total fisheries landings in 2014–2017. To improve the situation for northern pike in the Baltic Sea managers need to consider both fisheries restrictions and measures to decrease predation pressure and increase recruitment. Catch data from recreational fisheries may be used to evaluate such management efforts by providing information on the population development of this keystone species.
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Regime shifts in ecosystem structure and processes are typically studied from a temporal perspective. Yet, theory predicts that in large ecosystems with environmental gradients, shifts should start locally and gradually spread through space. Here we empirically document a spatially propagating shift in the trophic structure of a large aquatic ecosystem, from dominance of large predatory fish (perch, pike) to the small prey fish, the three-spined stickleback. Fish surveys in 486 shallow bays along the 1200 km western Baltic Sea coast during 1979–2017 show that the shift started in wave-exposed archipelago areas near the open sea, but gradually spread towards the wave-sheltered mainland coast. Ecosystem surveys in 32 bays in 2014 show that stickleback predation on juvenile predators (predator–prey reversal) generates a feedback mechanism that appears to reinforce the shift. In summary, managers must account for spatial heterogeneity and dispersal to better predict, detect and confront regime shifts within large ecosystems.
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• Aquatic systems often lack physical boundaries for gene flow, but ecological and behavioural barriers can form surprisingly fine spatial scale genetic patterns that challenge traditional, large scale management. To detect fine spatial scale structures, understand sources of intraspecific diversity, and design appropriate management plans requires identification of reproductively isolated units. • This study reports on genetic differentiation in pike (Esox lucius) within a coastal area stretching 55 km from south to north in the central Baltic Sea. Pike is here an economically and ecologically important top predator that has declined in abundance. However, population structures have mostly been studied on large spatial scales, and without considering the potential for genetic divergence between the sympatric anadromous fresh water and the resident brackish water spawning ecotypes. • To this end, 487 individuals from the east coast of Sweden and the island of Öland, representing sympatric anadromous and resident spawning individuals, categorized to ecotype based on spawning location or otolith microchemistry, were genotyped for 10 microsatellites and used to test for divergence between ecotypes. Furthermore, divergence between regions (island/mainland), neighbouring spawning locations (n = 13) and isolation by distance within and between regions were evaluated for the anadromous ecotype. • The results revealed strong genetic differences between regions, between spawning locations separated by as little as 5 km and the first evidence of genetic differentiation between resident and anadromous ecotypes; despite a high dispersal capacity of pike and a high connectivity within the study area. The signatures of isolation by distance indicated that connectivity among populations differed between regions, probably reflecting availability of spawning habitats. • To safeguard against the challenges and uncertainties associated with environmental change, adaptive conservation management should aim to promote high intra‐population functional genetic diversity without compromising the continued integrity and coexistence of the different ecotypes and of locally adapted sub‐populations.
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Background: In the wake of climate change many environments will be exposed to increased and more variable temperatures. Knowledge about how species and populations respond to altered temperature regimes is therefore important to improve projections of how ecosystems will be affected by global warming, and to aid management. We conducted a common garden, split-brood temperature gradient (4.5 °C, 9.7 °C and 12.3 °C) experiment to study the effects of temperature in two populations (10 families from each population) of anadromous pike (Esox lucius) that normally experience different temperatures during spawning. Four offspring performance measures (hatching success, day degrees until hatching, fry survival, and fry body length) were compared between populations and among families. Results: Temperature affected all performance measures in a population-specific manner. Low temperature had a positive effect on the Harfjärden population and a negative effect on the Lervik population. Further, the effects of temperature differed among families within populations. Conclusions: The population-specific responses to temperature indicate genetic differentiation in developmental plasticity between populations, and may reflect an adaptation to low temperature during early fry development in Harfjärden, where the stream leading up to the wetland dries out relatively early in the spring, forcing individuals to spawn early. The family-specific responses to temperature treatment indicate presence of genetic variation for developmental plasticity (G x E) within both populations. Protecting between- and within-population genetic variation for developmental plasticity and high temperature-related adaptive potential of early life history traits will be key to long-term viability and persistence in the face of continued climate change.
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Migration strategies in fishes comprise a rich, ecologically important, and socioeconomically valuable example of biological diversity. The variation and flexibility in migration is evident between and within individuals, populations, and species, and thereby provides a useful model system that continues to inform how ecological and evolutionary processes mold biodiversity and how biological systems respond to environmental heterogeneity and change. Migrating fishes are targeted by commercial and recreational fishing and impact the functioning of aquatic ecosystems. Sadly, many species of migrating fish are under increasing threat by exploitation, pollution, habitat destruction, dispersal barriers, overfishing, and ongoing climate change that brings modified, novel, more variable and extreme conditions and selection regimes. All this calls for protection, sustainable utilization and adaptive management. However, the situation for migrating fishes is complicated further by actions aimed at mitigating the devastating effects of such threats. Changes in river connectivity associated with removal of dispersal barriers such as dams and construction of fishways, together with compensatory breeding, and supplemental stocking can impact on gene flow and selection. How this in turn affects the dynamics, genetic structure, genetic diversity, evolutionary potential, and viability of spawning migrating fish populations remains largely unknown. In this narrative review we describe and discuss patterns, causes, and consequences of variation and flexibility in fish migration that are scientifically interesting and concern key issues within the framework of evolution and maintenance of biological diversity. We showcase how the evolutionary solutions to key questions that define migrating fish—whether or not to migrate, why to migrate, where to migrate, and when to migrate—may depend on individual characteristics and ecological conditions. We explore links between environmental change and migration strategies, and discuss whether and how threats associated with overexploitation, environmental makeovers, and management actions may differently influence vulnerability of individuals, populations, and species depending on the variation and flexibility of their migration strategies. Our goal is to provide a broad overview of knowledge in this emerging area, spur future research, and development of informed management, and ultimately promote sustainable utilization and protection of migrating fish and their ecosystems.
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Many fish populations have experienced declines in recent decades due to anthropogenic disturbances, such as overfishing and habitat exploitation. Despite management actions many populations show a limited capacity to recover. This may be attributed to reversal of predatory‐prey roles, yet empirical evidence to that effect remains scarce. Here we combine field and laboratory studies to investigate the interaction between pike (Esox lucius), a large keystone top predatory fish, and the small‐bodied mesopredatory threespine stickleback (Gasterosteus aculeatus) in the Baltic Sea where pike populations have declined. Our data suggest that stickleback predation on pike larvae depletes a large proportion of the recruitment and influences the size distribution through size‐selective predation, which is corroborated by a gape‐limitation experiment and diet analysis of wild captured sticklebacks. The effects of stickleback predation are present across several populations and years, and our data suggests that early arrival of sticklebacks has stronger effects on juvenile pike survival. Finally, we use data on pike gape‐limitation and the size distribution of sticklebacks to illustrate the process of role reversal. These findings suggest that mesopredator behaviour can reduce recruitment of a top predator species and impair the capacity of populations to recover. This emphasizes predator‐prey role reversal as an important ecological and evolutionary driver that influences the outcome of restoration and management actions. This article is protected by copyright. All rights reserved.
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Steelhead Oncorhynchus mykiss are captured and released during spawning migrations by the commercial, subsistence (Indigenous), and recreational fishing sectors, though the consequences of these fisheries interactions on Steelhead migration are poorly understood. This study evaluated injury, reflex impairment, behaviour, and survival of released wild adult Steelhead following capture in the subsistence dip net, subsistence beach seine, and recreational angling fisheries of the Bulkley River, British Columbia. Wild Steelhead were captured using common handling practices employed in each fishery and were monitored post‐release using radio telemetry. A greater proportion of Steelhead captured by dip net and seine had impaired righting reflexes compared to angled fish, but only fish captured by dip net had notably higher incidence of injury (i.e., net marks, torn fins, flesh wounds, scale loss). Fish captured by dip and seine net had considerably faster peak migration rates (> 4000 m‐day−1) than angled fish (< 1000 m‐day−1) which likely reflects when the Steelhead are encountered during their migration in these fisheries (earlier vs. later stages). Air exposure (15‐74 s) and water temperature (9.2‐15.1 °C) at the time of capture had significant negative relationships with 24‐h fallback behaviour (temperature only), intermediate‐term (10‐20 d after capture) migration rates, and peak migration rates in dip net captured Steelhead. There were no significant effects of capture duration or fish length on injury, righting reflexes, or migratory behaviour. Immediate mortality upon release was rare and occurred in only one fish captured by dip net. The 3‐d survival was 88‐97% for dip net caught Steelhead, 96‐100% for seine caught Steelhead, and 68‐100% for angled Steelhead. Despite inherent differences in timing and location between these fisheries, findings suggest air exposure and water temperature can decrease Steelhead migration rates. Fishers should look for opportunities to avoid or minimize these capture and handling conditions when releasing Steelhead. This article is protected by copyright. All rights reserved.
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Intraspecific genetic admixture occurs when previously separated populations within a species start interbreeding, and it can have either positive, negative, or neutral effects on reproductive performance. As there currently is no reliable predictor for the outcome of admixture, an increased knowledge about admixture effects in different species and populations is important to increase the understanding about what determines the response to admixture. We tested for effects of admixture on F1 offspring quality in three subpopulations of pike (Esox lucius). Gametes were collected in the field, and eggs from each female were experimentally fertilized with milt from a male from each population (one “pure” and two “admixed” treatments). Three offspring quality measures (hatching success, fry survival, and fry length) were determined and compared between (a) pure and admixed population combinations and (b) the sex‐specific treatments within each admixed population combination (based on the origin of the male and female, respectively). The results suggested that although there were no overall effects of admixture on offspring quality, the consequences for a given population combination could be sex‐specific and thus differ depending on which of the parents originated from one or the other population. All offspring quality traits were influenced by both maternal ID and paternal ID. Sex‐ and individual‐specific effects can have implications for dispersal behavior and gene flow between natural populations, and are important to consider in conservation efforts.
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Environmental heterogeneity is a key determinant of genetic and phenotypic diversity. Stable and homogenous environments tends to result in evolution of specialism and local adaptations, while temporally unpredictable environments may maintain a diversity of specialists, promote generalist strategies, or favour diversified bet hedging strategies. We compared salinity tolerance between two anadromous subpopulations of pike (Esox Lucius) that utilize freshwater spawning sites with different salinity regimes. Eggs from each population were artificially fertilized and incubated in a salinity gradient (0, 3, 5, 7, and 9 psu) using a split-brood design. Effects on embryonic development, hatching success, survival of larvae, and fry body length were compared between populations and families. The population naturally spawning in the stable freshwater habitat showed signs of specialization for freshwater spawning. The population exposed to fluctuating selective pressure in a spawning area with occasional brackish water intrusions tolerated higher salinities and displayed considerable variation in reaction norms. Genetic differences and plasticity of salinity tolerance may enable populations to cope with changes in salinity regimes associated with future climate change. That geographically adjacent subpopulations can constitute separate units with different genetic characteristics must be considered in management and conservation efforts to avoid potentially negative effects of genetic admixture on population fitness and persistence.
Catch-and-release angling of salmonids is a widely implemented fisheries management strategy intended to provide recreational fishing opportunities while limiting harvest. Previous research has indicated mortality may be low, but the extent of sublethal impacts, including inability to reach spawning grounds or decreased spawning effectiveness, is relatively under-studied. Sockeye salmon (Oncorhynchus nerka) were subjected to simulated catch-and-release events, with a subset also enduring air exposure, to determine how physiological stress influences the ability of sockeye salmon to reach spawning grounds and the viability of eggs produced. Short-term postrelease mortality was highest for fish that were subjected to air exposure, but once recovered, neither simulated catch-and-release nor air exposure had a major impact on their ability to reach spawning grounds. Egg viability was similar between treatment groups, indicating physiological stress associated with the trials did not have a negative impact on reproduction. These results suggest that handling, especially air exposure, is the primary concern and substantially impacts the short-term postrelease mortality of sockeye salmon, which has important implications for management.