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Fisheries Research 273 (2024) 106906
0165-7836/© 2023 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Anatomical hooking location and bleeding occurrence in northern pike
(Esox lucius) caught in recreational catch-and-release angling in a lake with
reduced prey sh availability
Martin H. Larsen
*
, O. Jonas Palder , Casper Gundelund, Nicolas Azana Schnedler-Meyer ,
Henrik D. Ravn , Christian Skov
Technical University of Denmark, National Institute of Aquatic Resources (DTU Aqua), DK-8600 Silkeborg, Denmark
ARTICLE INFO
Edited by B. Morales-Nin
Keywords:
Hooking injury
Condition factor
Food limitation
Management
ABSTRACT
Catch-and-release (C&R) is a common practice in recreational angling for northern pike (Esox lucius), whereby
the angler releases the sh back into the water after capture with the expectation that it will survive with
negligible stress and physical injuries. This may not always be the case, as hooking in critical anatomical lo-
cations, such as gills, esophagus, and stomach, increases the frequency of bleeding at the hook wound and has
been recognized as a key determinant of post-release mortality in pike. Several factors (e.g. bait type and size,
hook characteristics, and sh length) can inuence the risk of hooking in critical locations, and the sh’s will-
ingness to strike the shing lure/bait may largely depend on previous feeding history and associated motivation
to feed. In this respect, food deprivation and greater feeding motivation can be expected to result in an intensied
response and more forceful attacks on shing lures/baits, which could increase the risk of hooking injuries in
sensitive tissues, as the sh may ingest the hook more deeply. Here, we assess the hooking location and bleeding
occurrence in pike caught on soft plastic shads and baitsh in a shallow eutrophic lake. The prey sh density of
this lake, primarily common roach (Rutilus rutilus), has been substantially reduced by seining as part of a lake
restoration project. Experimental angling sessions took place at the end of 2020 and 2022, approximately 1–2
months and 25–26 months after the initial reduction in prey sh density, respectively. Removal of roach from the
lake was also done regularly by seining after the angling sessions in 2020. Pike angled in 2022 had signicantly
lower body condition than individuals caught in 2020, indicating that mass removal of roach from the lake
resulted in food shortage. The risk of hooking in critical locations (herein dened as hooking in gills and back of
mouth) was not associated to angling year, body condition, or length of the pike. However, across the two an-
gling years, the incidence of hooking in critical locations was higher for soft plastic shads compared to baitsh
(24.2% versus 7.4%). Pike caught on soft plastic shads also bled more frequently than individuals caught on
baitsh (19.7% versus 6.2%), most likely due to the fact that hooking in the gills almost exclusively occurred
with soft plastic shads. Bleeding propensity was also not related to angling year, body condition, or sh length.
Hence, the present study found no evidence that long-term reductions in food availability and resulting declines
in body condition inuence how pike strike and ingest shing lures/baits, at least not in a manner that increases
the risk of hooking injuries in sensitive locations. Instead, bait type seems to play the largest role for the observed
bleeding patterns. We recommend that future studies investigate the potential effects of C&R angling on the post-
release survival of pike with reduced body condition due to limited food availability.
1. Introduction
Recreational angling is widespread and can be a major factor in the
exploitation of aquatic resources (Cooke and Cowx, 2006). Impacts of
recreational angling on the target sh population may manifest as
reduction in total population abundance, age and size truncation, and
even as evolutionary effects through selective harvest (Arlinghaus et al.,
2010; Ayll´
on et al., 2018; Lewin et al., 2006; Post et al., 2002). More-
over, angling often targets predatory sh at the top of the food chain,
which can induce trophic cascades and, ultimately, alter the structure
* Corresponding author.
E-mail address: mhala@aqua.dtu.dk (M.H. Larsen).
Contents lists available at ScienceDirect
Fisheries Research
journal homepage: www.elsevier.com/locate/fishres
https://doi.org/10.1016/j.shres.2023.106906
Received 29 June 2022; Received in revised form 13 July 2023; Accepted 17 November 2023
Fisheries Research 273 (2024) 106906
2
and functioning of aquatic ecosystems (Altieri et al., 2012).
The northern pike (Esox lucius, hereafter termed pike) is a predatory
freshwater sh with a circumpolar distribution in the Northern hemi-
sphere (Craig, 1996). Recreational angling for pike is popular and has
increasingly been implementing both mandatory and voluntary
catch-and-release (C&R), whereby sh are returned to the water shortly
after capture with the presumption that most or all released sh will
survive and contribute to succeeding generations (Cooke et al., 2013;
Jansen et al., 2013; Paukert et al., 2001; Pierce et al., 1995). Based on
these premises, C&R has become a widely adapted practice to aid in the
management and conservation of wild sh populations (Arlinghaus
et al., 2007; Bartholomew and Bohnsack, 2005; Cooke et al., 2016;
Cooke and Suski, 2005). Previous research has shown that hooking
mortality of pike is generally less than 10% in C&R angling (Hühn and
Arlinghaus, 2011; Tomcko, 1997), with several studies reporting mor-
tality rates below 5% (Arlinghaus et al., 2008; Burkholder, 1992; Burr,
1998; Trahan et al., 2021). Among other factors, hooking mortality has
been associated to the intensity of bleeding during capture, and studies
have shown that the anatomical hooking location is an important pre-
dictor of bleeding propensity (Arlinghaus et al., 2008; Stålhammar et al.,
2014). For instance, bleeding at the hook wound is typically more
frequent and severe when pike are hooked in critical locations (e.g. gills
and esophagus) compared to hooking in the outer parts of the mouth
(Burkholder, 1992; Stålhammar et al., 2014).
Several factors, such as bait type and size, angling technique, and
hook characteristics, can inuence hooking location and likelihood of
bleeding in sh (Arlinghaus et al., 2008; Grixti et al., 2007; Lennox et al.,
2015; Stålhammar et al., 2014). While the angler or local management
regulations can control the type of terminal gear and angling technique,
intrinsic factors (e.g. sh size and maturity status) associated with
hooking injury and mortality are largely outside the realm of control-
lable variables in recreational angling (Arlinghaus et al., 2007; Kupar-
inen et al., 2010; Muoneke and Childress, 1994). Hunger can increase
vulnerability to capture by hooks as hungry sh are usually more
motivated to feed and thus easier to capture (Lennox et al., 2017a;
Mogensen et al., 2014; Raat, 1991). In addition, sh often display
enhanced risk-taking behaviour and food searching activity in response
to low prey abundance (Stoner, 2004). Conversely, sh that are satiated
may reduce foraging activity and become more selective of prey items
(Gill, 2003; Stoner, 2004; Turesson et al., 2006), potentially decreasing
capture vulnerability to angling gear. It is also possible that the level of
hunger and associated feeding motivation inuence how individual sh
approach, attack and ingest shing baits/lures (Fern¨
o et al., 1986;
Stoner, 2003). For instance, food deprivation can be expected to result in
an intensied response and more forceful attacks towards shing lur-
es/baits, which could increase the risk of hooking injuries in vital lo-
cations due to the sh swallowing the hook more deeply. In this context,
past studies have shown that food-deprived whiting (Merlangius mer-
langus) and Atlantic cod (Gadus morhua) are more prone to swallow
baited hooks in longline sheries, leading to higher rates of hooking in
the esophagus and stomach (Fern¨
o et al., 1986; Johannessen, 1983).
However, despite food deprivation being a common stressor in wild sh
populations (McCue, 2010), there has been surprisingly little effort to
understand whether and to what extent limited food resources impact
the fate of individual sh in C&R angling.
In this study, we explore the anatomical hooking location and inci-
dence of bleeding in pike caught on soft plastic shads and baitsh. This
research took place in a small eutrophic lake where the density of prey
sh, mainly common roach (Rutilus rutilus, hereafter termed roach), was
gradually reduced by seining as part of a large-scale lake restoration
project since fall 2020. Specically, angling sessions were conducted at
the end of 2020 and 2022, approximately 1–2 months and 25–26 months
after the initial reduction in prey sh density, respectively. Removal of
roach by seining was also performed between the two angling years. We
predicted that the continued mass removal of roach would impact the
condition factor of pike, such that individuals angled in 2022 should
have lower body condition than those caught in 2020 as they were
exposed to reduced food availability for a longer period. Accordingly,
we hypothesized that pike in 2022 would be more motivated to feed and
therefore attack and ingest the soft plastic shads and baitsh with less
caution, which should increase the risk of hooking in critical locations
and bleeding from the hook wound. We also expected higher rates of
hooking injuries (i.e. bleeding) in sensitive tissues among pike caught on
baitsh relative to soft plastic shads, as natural baits tend to hook sh
deeper than articial lures (Arlinghaus et al., 2007; Bartholomew and
Bohnsack, 2005; Hühn and Arlinghaus, 2011). To put our ndings into
context, we compared the rates of hooking in critical locations and
bleeding observed in our study to those found in existing C&R studies on
pike.
2. Methods
2.1. Study area
The study was conducted in Lake Ormstrup (56◦19’34’’ N, 9◦38’21’’
E), which is located in the central part of Jutland, Denmark (Fig. 1). It is
a shallow eutrophic lake (maximum depth: 5.5 m, mean depth: 3.4 m)
with a surface area of 11 ha. Coverage of submerged macrophytes is very
low and dominated by curly-leaf pondweed (Potamogeton crispus). Lake
Ormstrup has a small outlet stream at the western end. The water level in
the stream varies seasonally and part of it occasionally dries out to form
isolated pools during the summer months. The stream passes several
physical barriers that prevent immigration of sh into Lake Ormstrup
from downstream areas. The sh community is dominated by roach and
pike, but the lake also holds European perch (Perca uviatilis), and very
low densities of European eel (Anguilla anguilla) and tench (Tinca tinca).
A mark-recapture analysis, carried out in 2020, showed that the pike
population was dominated by individuals larger than 50 cm in total
length (TL) with an estimated population size of 384 individuals, cor-
responding to 29.5 individuals per ha (95% CI: 25.0–36.2). Due to its
physical characteristics, the lake has a dynamic mixing regime and al-
ternates between stratied periods and mixing events, especially
throughout the summer (Søndergaard et al., 2023). The deterioration of
the lake through eutrophication has been ongoing for decades. Initially,
it was caused by agricultural runoff and later exacerbated by intensive
stocking and subsequent feeding of ducks for hunting. A lake restoration
project was initiated in fall 2020 which included removal of roach and
tench (i.e. biomanipulation) in attempts to improve water quality.
2.2. Prey sh removal
Removals of roach and tench in Lake Ormstrup occurred between
2020 and 2022 by seining during four intensive sampling periods: fall
2020 (September 22-October 8), spring 2021 (April 12–23), fall 2021
(September 6–17) and summer 2022 (June 13-July 1). At each sampling
period, 15–23 seine hauls were undertaken by towing the seine net at
various positions in the lake. The seine net was capable of catching sh
>5–6 cm TL. All captured roach and tench were removed from the lake
and later used for feed in zoos or energy production in biogas plants,
whereas perch and pike were returned to the lake as quickly as possible.
Before being released, pike were measured for total length (cm), and
weight (g) was taken using a wet weight sling and a handheld scale.
Untagged pike were also injected with a 23 mm PIT tag (Texas In-
struments, Plano, Texas, USA) in the musculature immediately below
the dorsal n, allowing for individual identication in case of recapture.
Altogether, 6091 kg of roach were removed by 72 seine hauls be-
tween 2020 and 2022 (Table 1). Based on subsampling, total catches for
different size-classes of roach were as follows: 210 kg (<6 cm TL),
3497 kg (6–12 cm TL), 2101 kg (12–18 cm TL), 283 kg (>18 cm TL).
Total catches of roach varied between 385 and 4129 kg among the seine
sampling periods, and the highest catch occurred in fall 2020. Apart
from roach, 129 kg of tench were removed from the lake. A few days
M.H. Larsen et al.
Fisheries Research 273 (2024) 106906
3
before each seine sampling period, 90–200 roach (above 12 cm TL) were
caught and subsequently tagged with a 23 mm PIT tag in the body cavity
before they were released back into the lake. After each seine haul, all
captured roach larger than 12 cm TL were scanned for the presence of
PIT tags. This allowed us to estimate the proportion of roach that was
removed from the lake by seining through mark-recapture analysis.
Based on the number of recaptured roach, the estimated reduction in
roach density varied from 5.7% to 66.7% among the four seine sampling
periods (Table 1).
2.3. Experimental pike angling
Lake Ormstrup is located on a private property and according to the
landowner, angling on the lake was very rare in the years before this
study. Experimental angling sessions for pike took place at the end of
2020 (November 10-December 17) and 2022 (November 8–30) in boats
throughout the lake. Anglers consisted of sheries biologists and tech-
nicians from DTU Aqua with substantial recreational angling experi-
ence, and they followed the rules of best practice for C&R such as
minimizing ght- and handling times (Brownscombe et al., 2017). In
each boat, 2–4 shing rods were used simultaneously with either arti-
cial lure or natural bait. Articial lure was a soft plastic shad (19 cm
long) shed with a stinger treble hook (size 2 with barbs, Owner
ST-36BC) attached to a steel wire. Roach (about 12–17 cm TL) sourced
from the lake were used as natural bait. The roach were equipped with
one treble hook (size 4 with barbs, Owner ST-36BC) in the snout. A
shing tackle consisting of a oat and a lead weight was attached to the
main shing line, allowing the baitsh (i.e. roach) to be suspended at the
desired depth. Floats were also used as visual bite indicators and the
anglers aimed to set the hook immediately after a strike was detected to
avoid articially increasing the incidence of deep hooking due to the
pike swallowing the baitsh. Soft plastic shads were shed by casting,
whereas baitsh were slowly trolled behind the boat. Our choices of
terminal gear and angling techniques were intended to reect common
tactics used by anglers targeting pike (Arlinghaus et al., 2017; Bursell
and Arlinghaus, 2018). Once hooked, each pike was landed as quickly as
possible, either by hand under the opercular cover or using a rubberized
shing net. Hooks were removed from the pike using pliers.
The anglers recorded total length, weight, bait type, anatomical
hooking location, and bleeding intensity for each capture. Hooking
location was categorized as either non-critical (outer mouth, mouth
corner, foul-hooked outside the mouth) or critical (back of mouth, gills)
using similar criteria to those developed by Arlinghaus et al. (2008).
Bleeding intensity at the hook wound was assigned into one of three
categories: none (i.e. no blood), moderate (i.e. blood is present, but only
single drops), or substantial (i.e. constant ow of blood). Duration of the
ght (i.e. time in seconds from hooking the pike to landing it) was also
recorded for all angled pike in 2022. All pike were scanned for PIT tags
using a handheld reader, and untagged individuals were PIT-tagged as
previously described. Following these procedures, pike were released
back into the lake.
2.4. Literature review on hooking location and bleeding occurrence in
angled pike
To compare the observed frequencies of hooking in critical locations
and bleeding to values from previous C&R studies on pike, we searched
candidate literature using standard search strings such as “pike AND
angling”, “pike AND catch-and-release”, “pike AND hooking”, and “pike
AND bleeding”. Literature searches were done using Clarivate Web of
Science and Scopus database in March 2023. Additional searches were
performed in Google Scholar, which can help to identify non-indexed
literature published as technical reports, academic books, or disserta-
tions (Haddaway et al., 2015). Apart from hooking location and
bleeding occurrence, we also extracted information on other potential
contributing variables, including bait type and size, hook characteris-
tics, pike length, water temperature, and study site.
Fig. 1. Map of Lake Ormstrup, Denmark. The location of the lake is indicated by the black circle in the inset.
Table 1
Overview of removal of common roach (Rutilus rutilus) and tench (Tinca tinca) by seining between 2020 and 2022 in Lake Ormstrup. Number of seine hauls and total
catches (kg) of roach and tench are shown for each of the four sampling periods (fall 2020, spring 2021, fall 2021, summer 2022). The percentages of PIT-tagged roach
(>12 cm TL) recaptured during seining are also shown. Associated values in parentheses represent the number of recaptured and PIT-tagged roach, respectively.
Roach
Number of hauls <6 cm 6–12 cm 12–18 cm >18 cm Sum Tench Recapture rate
Fall 2020 23 33 2707 1243 146 4129 122 66.7 (60/90)
Spring 2021 15 13 330 91 3 437 6 31.8 (42/132)
Fall 2021 17 163 160 706 111 1140 0 37.5 (75/200)
Summer 2022 17 1 300 61 23 385 1 5.7 (10/174)
M.H. Larsen et al.
Fisheries Research 273 (2024) 106906
4
2.5. Data analysis
For all analyses, angling year (2020, 2022), hooking location (non-
critical, critical), bait type (soft plastic shad, baitsh), and seine sam-
pling period (fall 2020, spring 2021, fall 2021, summer 2022) were
treated as categorical variables, while sh length was treated as a
continuous variable. Because very few pike exhibited substantial
bleeding from being hooked (n =2), bleeding intensity was pooled into
presence or absence of blood at the hooking location in the analyses.
Thus, bleeding was considered as a categorical variable with two levels.
Length and weight measurements of individual pike captured during
seining and angling were used to calculate Fulton’s condition factor (K):
K=(weight
lenght3)×100
Subsequently, we used a GLM (generalized linear model) with
Gaussian distribution and identity link function, followed by Tukey’s
HSD multiple comparisons test, to assess differences in mean condition
factor between pike captured during the seine sampling and angling
events. This was done in order to evaluate whether the reduced density
of prey sh resulted in food scarcity. If so, the body condition of pike was
expected to vary among capture events with a decreasing trend over
time. Condition factor was treated as a continuous variable.
A GLM, following a Bernoulli distribution with logit link function,
was applied to determine the effects of angling year, bait type, and sh
length on hooking location. All possible two-way interactions between
these covariates were considered in the full model. Model selection was
performed on the interaction terms by comparing nested models using
the likelihood ratio test (LRT). By this procedure, non-signicant in-
teractions were progressively removed from the model in a backward
fashion (i.e. the model was retted after elimination of the least sig-
nicant interaction term), but all main effects were retained in the nal
model, even if they were non-signicant (Zuur et al., 2013). All inter-
action terms were non-signicant, and the nal model, therefore,
included the main effects of angling year, bait type, and sh length on
hooking location.
A second Bernoulli GLM was used to examine the effects of angling
year, hooking location, and sh length on the risk of bleeding. The full
model included all possible two-way interactions between the cova-
riates, and model selection was performed as described above. Bait type
was not included as a covariate in the model since exploratory data
analyses revealed collinearity between bait type and hooking location.
Consequently, the model was carried out separately for soft plastic shads
and baitsh. For both bait types, the nal model considered the main
effects of angling year, hooking location, and sh length on the proba-
bility of bleeding. However, to compare bleeding propensity between
bait types, we used another Bernoulli GLM with bait type as a covariate
and bleeding as the response variable. Along similar lines, the condition
factor of the angled pike was associated to angling year and therefore
not included as a covariate in the above analyses on hooking location
and bleeding. Instead, we used separate Bernoulli GLMs to quantify the
effects of condition factor on hooking location and bleeding probability.
The relationship of ght time (treated as a continuous variable) with
length and condition factor of the pike angled in 2022 was explored
using Gaussian GLMs with identity link function (ght time was not
recorded for pike angled in 2020). Lastly, Fisher’s exact test was used to
compare the incidence of hooking in critical locations and bleeding
found in our study with those reported in previous C&R studies on pike.
These analyses were conducted separately for pike caught on baitsh
and articial lures.
For individual pike caught and released multiple times within each
angling year (n =11 in 2020 and n =12 in 2022), only data from the
rst capture event was included in the analyses. The reasoning behind
this approach is that the rst capture event might have altered the short-
term behaviour of the pike following release (Arlinghaus et al., 2017,
2009; Baktoft et al., 2013; Stålhammar et al., 2012), which could in-
uence how they subsequently strike and ingest the shing lure/bait.
However, data on individual pike caught in both 2020 and 2022 were
maintained in the analyses, as we judged that the capture and release
events in 2020 would not affect a similar event in 2022.
Assumptions of homoscedasticity and normality were ensured by
graphical inspection of the residuals according to the protocol described
in Zuur et al. (2010). Statistical analyses were performed using R version
4.1.2 (R Core Team, 2021). Signicance level was set at p <0.05. Re-
sults are presented as means ±SD throughout.
3. Results
3.1. Angled pike
Pike were angled at mean water temperatures of 7.0 ±2.1 ◦C (range:
4.5–9.2 ◦C) in 2020 and 8.7 ±2.3 ◦C (range: 5.3–10.7 ◦C) in 2022.
Mean Secchi depth was 2.2 ±0.4 m and 2.3 ±0.7 m during angling in
2020 and 2022, respectively.
In 2020, 79 individual pike were successfully angled (Table 2). Of
these, 68 individuals were caught once (86%), ten individuals twice
(13%), and one individual three times (1%). In 2022, we angled 68 in-
dividual pike of which 56 individuals were caught once (82%), eleven
individuals twice (16%), and one individual three times (2%). Across
bait types, the mean length of angled pike was 72.4 ±6.8 cm (range:
61–88 cm) in 2020 and 67.1 ±6.2 cm (range: 56–82 cm) in 2022.
3.2. Condition factor of pike
The analysis showed an overall difference in mean condition factor
of pike captured during seining and angling (F =59.650, df =5,
p<0.0001). Pike angled in 2022 had signicantly lower condition
factor than those caught by angling in 2020 and seining in fall 2020,
spring 2021, and fall 2021 (Fig. 2). However, the mean condition factor
did not differ between pike caught by seining and angling in 2022.
Furthermore, the condition factor of pike angled in 2020 was higher
than that of conspecics caught by seining in fall 2021 and summer
Table 2
Number, mean total length (cm), and Fulton’s condition factor (K) of northern pike (Esox lucius) caught on soft plastic shads and baitsh in Lake Ormstrup during the
angling sessions in 2020 and 2022. The table also shows the associated percentages of pike bleeding and hooked in non-critical (outer mouth, mouth corner, foul-
hooked outside the mouth) and critical (back of mouth, gills) locations. Values in parentheses are number of pike. Variation in association with mean values is
given as ±SD.
Hooking location
n Total length Condition factor (K) non-critical critical Bleeding
2020
Soft plastic shad 34 74.0 ±6.72 0.66 ±0.08 76.5 (26) 23.5 (8) 23.5 (8)
Baitsh 45 71.2 ±6.72 0.67 ±0.05 91.1 (41) 8.9 (4) 4.4 (2)
2022
Soft plastic shad 32 67.0 ±5.08 0.52 ±0.10 75.0 (24) 25.0 (8) 15.6 (5)
Baitsh 36 67.2 ±7.04 0.55 ±0.10 94.4 (34) 5.6 (2) 8.3 (3)
M.H. Larsen et al.
Fisheries Research 273 (2024) 106906
5
2022, while it was statistically similar to those caught by seining in fall
2020 and spring 2021 (Fig. 2).
3.3. Hooking location and bleeding in angled pike
The majority of pike were hooked in the outer mouth (64.6%) fol-
lowed by mouth corner (18.4%), back of mouth (8.2%), and gills (6.8%)
(Fig. 3). Foul-hooking was rare (2.0%) and only occurred with soft
plastic shads. All instances of foul-hooking were in the lower jaw,
outside the pike’s mouth, and thus considered non-critical. The
probability of hooking in critical locations did not differ between an-
gling years (LRT =0.002, df =1, p =0.968), and was not related to
length of the pike (LRT =0.105, df =1, p =0.746). However, hooking
location differed between bait types as pike caught on soft plastic shads
were more likely to be hooked in critical locations, especially in the gills,
than pike caught on baitsh (LRT =7.989, df =1, p =0.005; Fig. 3,
Table 2). Overall, the incidence of hooking in critical locations was
24.2% and 7.4% for pike caught on soft plastic shads and baitsh,
respectively.
Pike caught on soft plastic shads bled more frequently from the hook
wound than individuals caught on baitsh (LRT =6.273, df =1,
p=0.012; Fig. 3, Table 2). Bleeding occurred in 19.7% of the captures
on soft plastic shads, compared to 6.2% of the pike caught on baitsh.
For both bait types, the probability of bleeding was not affected by an-
gling year (soft plastic shad: LRT =2.029, df =1, p =0.154; baitsh:
LRT =1.699, df =1, p =0.192) or length of the pike (soft plastic shad:
LRT =1.422, df =1, p =0.233; baitsh: LRT =3.696, df =1,
p=0.055). In addition, hooking location was not associated to the
probability of bleeding for pike caught on baitsh (LRT =0.692, df=1,
p=0.405). However, pike hooked in critical locations with soft plastic
shads were more likely to bleed than conspecics hooked in non-critical
locations (LRT =10.818, df =1, p =0.001). Condition factor of pike
was not a signicant predictor of bleeding probability (LRT =0.189, df
=1, p =0.664) or hooking location (LRT =1.230, df =1, p =0.254).
The mean ght time was 51 ±23 s (range: 8–125 s) across bait types for
pike angled in 2022. Fight time was unrelated to length (F =0.451, df =
1, p =0.504) and condition factor (F =2.545, df =1, p =0.115) of the
pike.
3.4. Literature review on hooking location and bleeding occurrence in
angled pike
We identied eight studies on pike that reported hooking location
and/or incidence of bleeding in relation to recreational angling
(Table 3). Together, these studies included a variety of different bait
types and sizes, angling techniques, as well as hook types, sizes, and
numbers. Six studies were carried out in natural freshwater lakes and
two in brackish water in the Baltic Sea. In two of these studies, pike were
captured by ice angling on North American lakes using so-called “tip-
ups”, consisting of a wooden, plastic, or steel frame with a spool of line
that allows the angler to suspend the baited hook at the desired depth
below the ice. The spool is attached to a release mechanism that ips a
ag above the ice once a sh strikes and swims off with the bait, alerting
Fall 2020
Angled 2020
Spring 2021
Fall 2021
Summer 2022
Angled 2022
Condition factor (K)
0.25
0.40
0.55
0.70
0.85
1.00
Seining
Angling
aaa
b
cc
Fig. 2. Boxplot showing Fulton’s condition factor (K) of northern pike (Esox
lucius) caught by seining (fall 2020, spring 2021, fall 2021, summer 2022) and
angling (angled 2020, angled 2022) in Lake Ormstrup. Horizontal lines within
each box represent the median condition factor, ends of boxes represent the
25th and 75th percentiles, whiskers the 10th and 90th percentiles, and open
circles indicate values beyond the 10th and 90th percentiles. Boxes not sharing
the same letter are signicantly different at p <0.05 (Tukey’s HSD multiple
comparisons test). Between 73 and 134 individual pike were captured during
the four seine sampling periods and the mean lengths were as follows: 70.5
±10.0 cm (range: 29–98 cm, n =120) in fall 2020; 71.2 ±8.2 cm (range:
34–98 cm, n =134) in spring 2021; 70.9 ±6.5 cm (range: 58–85 cm, n =82)
in fall 2021; 70.2 ±6.2 cm (range: 57–85 cm, n =73) in summer 2022.
Anatomical hooking location
outer mouth
mouth corner
foul-hooked
back of mouth
gills
0
30
60
90
none moderate substantial
Bleeding intensity:
Soft plastic shad, 2020
outer mouth
mouth corner
foul-hooked
back of mouth
gills
0
30
60
90
outer mouth
mouth corner
foul-hooked
back of mouth
gills
0
30
60
90
outer mouth
mouth corner
foul-hooked
back of mouth
gills
0
30
60
90
Soft plastic shad, 2022 Baitfish, 2020 Baitfish, 2022
Incidence (%)
Fig. 3. Incidence (%) of different bleeding intensities at the hook wound in relation to anatomical hooking location among northern pike (Esox lucius) caught in Lake
Ormstrup on soft plastic shads and baitsh during the angling sessions in 2020 and 2022. Across the two angling years, the incidence of hooking in critical locations
(back of mouth, gills) and bleeding was signicantly higher for pike caught on soft plastic shads relative to those caught on baitsh.
M.H. Larsen et al.
Fisheries Research 273 (2024) 106906
6
Table 3
Variables extracted from studies that reported anatomical hooking location and/or frequency of bleeding in angled northern pike (Esox lucius), including study site and timing, water temperature, sample size (N), sh
length, bait type, and hook characteristics. Studies that did not have the associated data are marked by NA. Length of the pike is reported as total (TL) or fork (FL) length to the nearest cm, and number in brackets indicates
the average length. Single hooks are abbreviated as “Single” and treble hooks as “Treble”. If reported, the number of hooks is also shown. NS denotes that hook number and size were not standardized within articial lures.
Denition of hooking in critical locations: a =gills, b=esophagus, c =stomach, d =aorta, e =back of mouth. Scientic names of baitsh species: common bream (Abramis brama), common roach (Rutilus rutilus),
European perch (Perca uviatilis), golden shiner (Notemigonus crysoleucas), and white sucker (Catostomus commersonii).
Reference Country Study site Study
timing
Water
temperature
(◦C)
N Fish length (cm) Hook Bait type Bleeding
(%)
Hooking in
critical
locations
(%)
Characteristics of hook and bait/lure
Current study Denmark Lake Ormstrup Nov-Dec 4.5–10.7 81 56–88 TL 1 Treble Baitsh 6.2 7.4
a,e
Baitsh (~ 12–17 cm) and soft plastic shads
(19 cm) were equipped with a size 4 and
size 2 treble hook, respectively. Treble
hooks had barbs. Common roach were used
as baitsh.
66 (70) 1 Treble Soft plastic shad 19.7 24.2
a,e
Althoff et al.
(2020)
†
USA Lake Grand Jan 0.9–3.3 65 35–75 TL Single/Treble Baitsh NA 12.3
a,b
Baitsh (~ 10 cm) were mounted with a
treble hook (size 4) or Extra Wide Gap single
hook (size 2/0). Golden shiners were used
as baitsh.
(51)
Arlinghaus
et al.
(2008)*
Germany,
Canada
Lake Kleiner
D¨
ollnsee, Lake
Opinicon
Summer NA 39 25–102 TL 2 Treble Baitsh 22.4 23.1
a,b
Hook numbers or sizes were not
standardized within or across bait types.
Lures were equipped with at least one treble
hook. Some soft plastic shads and jigs were
shed with a single hook and 1–2 treble
hooks. Baitsh had two treble hooks.
Baitsh species were common roach,
European perch and common bream.
35 (51) NS Spinner 11.4
a,b
103 NS Spoon 18.4
a,b
76 NS Soft plastic shad/jig 13.2
a,b
85 NS Wobbler 7.1
a,b
Bursell and
Arlinghaus
(2018)* *
Denmark,
Germany
Baltic Sea Year
round
NA 73 NA 2 Treble Jerkbait/swimbait 13.7 NA Size of all lures was 17 ±3 cm. Lures used
tradition hook mount or release-rig. Lures
with traditional hook mount had large
treble hooks (size 1/0–2/0) and lures with
release-rig had small treble hooks (size
6–4). Treble hooks had barbs.
Burkholder
(1992)
USA George Lake Jun NA 60 27–94 FL 1 Single Spoon 25.0 8.3
a
No information was available for specic
hook sizes. 60 1 Treble (large) Spoon 31.7 6.6
a
62 1 Treble (small) Spoon 62.9 12.9
a
60 2 Treble Wobbler 23.3 1.7
a
DuBois et al.
(1994)
†
USA Long Lake,
Lipsett Lake,
Lake Mendota
Dec-Feb NA 161 33–76 TL
(46)
Treble Baitsh 24.2 17.4
a,b,c
Size of baitsh was 11–19 cm. The study
used barbed treble hooks (size 4).
Information about the number of treble
hooks is not available. Golden shiners and
white suckers were used as baitsh.
Falk and
Gillman
(1975)* **
Canada Beaver Lake,
Stephanie
minesite
Jun-Aug <19 94 37–88 FL Treble Spoon/wobbler/
spinner
42.6 29.0
a,b
The study used different lures (spoon,
wobbler, spinner), equipped with 1–2 treble
hooks (with or without barbs).
(continued on next page)
M.H. Larsen et al.
Fisheries Research 273 (2024) 106906
7
the angler. Then, the angler grabs the line, sets the hook and pulls the
sh by hand through the hole drilled in the ice. Thus, angling with tip-
ups is a passive technique. In the remaining studies, pike were targeted
using standard rod and reel shing gear tted with baitsh or articial
lures (e.g. spoon, spinner, wobbler, soft plastic shad). Articial lures
were either shed by casting and reeling in or trolled behind moving
boats, while baitsh were mainly shed passively. Articial lures and
baitsh were predominantly equipped with one or two barbed treble
hooks (size 6–2/0). However, some of the larger soft plastic shads and
jigs used up to three hooks: one single hook and two treble hooks.
Baitsh species were roach, perch, common bream (Abramis brama),
golden shiner (Notemigonus crysoleucas), and white sucker (Catostomus
commersonii). Based on the accessible data, the size of baitsh ranged
between 9 cm and 23 cm (present study: 12–17 cm), whereas the arti-
cial lures measured from 8 cm to 21 cm (present study: 19 cm).
However, the study by Arlinghaus et al. (2008) also included articial
lures and baitsh smaller than 7.5 cm.
The mean length of the captured pike was reported in ve studies
and ranged from 46 cm to 67 cm TL (present study: 70 cm TL across
angling year and bait type). Across the different types and sizes of
articial lures, bleeding rates varied between 0% and 62.9% with a
grand mean value of 19.4% (present study: 19.7%), and hooking in
critical locations (here dened as gills, esophagus, stomach, aorta, and
back of mouth) ranged between 0% and 29% with a grand mean value of
13.3% (present study: 24.2%). Fisher’s exact test showed that the risk of
hooking the pike in critical locations was signicantly higher for the soft
plastic shads used in the present study relative to the articial lures used
in the existing studies (p =0.017), whereas no difference was found for
bleeding occurrence (p =0.999). For baitsh, the incidence of bleeding
ranged from 16.7% to 24.2% with a grand mean value of 23.7% (present
study 6.2%), while the rates of hooking in critical locations varied from
8.3% to 23.1% with a grand mean value of 15.9% (present study: 7.4%).
The analysis showed that the pike caught on baitsh in our study were
less likely to bleed from the hook wound when compared to those caught
on different species and sizes of baitsh in the previous studies
(p <0.001). However, the risk of hooking in critical locations was not
statistically different (p =0.094).
4. Discussion
This study found no evidence that prolonged declines in prey sh
availability inuence the anatomical hooking location and occurrence of
bleeding from the hook wound in pike caught on soft plastic shads and
baitsh. Mass removals of roach from Lake Ormstrup most likely
resulted in food scarcity as pike angled in 2022 had overall lower body
condition relative to individuals caught in 2020. On this basis, we ex-
pected that pike in 2022 would exhibit higher motivation to feed and
therefore attack and ingest the soft plastic shads and baitsh with less
caution, which could increase the rate of hooking injuries in sensitive
tissues. However, regardless of bait type, the incidence of hooking in
critical locations (i.e. gills and back of mouth) and bleeding was not
statistically different for pike angled in 2020 and 2022. In addition, the
condition factor of pike was not related to hooking location or bleeding
occurrence across the two angling years. These results collectively
suggest that extended periods of food scarcity, and resulting declines in
body condition, do not cause pike to approach, strike, and ingest shing
lures/baits in a manner that increases the risk of hooking-related
bleeding.
Contrary to our expectations, we found that soft plastic shads caused
bleeding more frequently than baitsh, most likely due to their
increased propensity to hook the pike in critical locations, especially in
the gills. Indeed, hooking in the gills was 11.3 times higher for soft
plastic shads than baitsh, and 80.0% of the gill-hooked pike bled across
both bait types. These results also contradict the general notion that
natural baits tend to be ingested more deeply compared to articial
lures, which increases the rates of hooking injuries in sensitive locations
Table 3 (continued )
Reference Country Study site Study
timing
Water
temperature
(◦C)
N Fish length (cm) Hook Bait type Bleeding
(%)
Hooking in
critical
locations
(%)
Characteristics of hook and bait/lure
Klefoth et al.
(2008)* ** *
Germany Lake Kleiner
D¨
ollnsee
May-Oct NA 12 48-49 TL 2 Treble Baitsh 16.7 8.3
a
Hook numbers or sizes were not
standardized within or across bait types.
Lures (8–16 cm) were equipped with at least
one treble hook. Some soft plastic shads had
a single hook and 1–2 treble hooks. Baitsh
(9–23 cm) had two treble hooks. Baitsh
species were common roach, European
perch and common bream.
2 (58) NS Spinner 50.0 0.0
a
1 NS Spoon 0.0 0.0
a
6 NS Soft plastic shad 0.0 0.0
a
1 NS Wobbler 0.0 0.0
a
Stålhammer
et al. (2014)
Sweden Baltic Sea Mar-Jun 2–22 74 39–113 TL 1 Treble Bucktail 11.6 17.6
a,d
Size of all lures was 18 ±3 cm. Lures were
mounted with barbed treble hooks (size 2/
0).
410 (67) 2 Treble Glider 11.5 10.0
a,d
23 2 Treble Softbait 4.3 13.0
a,d
198 1 Treble Spiny 19.1 23.7
a,d
106 2 Treble Tailbait 6.6 7.6
a,d
51 2 Treble Crankbait 7.8 9.8
a,d
†
Pike were caught by ice angling using so-called “tip-ups” (see main text for further details).
*Data for frequencies of bleeding and hooking in critical locations were extracted from Figure 4 and Table 1, respectively, in Arlinghaus et al. (2008). Bleeding frequency was pooled for articial lures (i.e. spinner, spoon,
soft plastic shad, jig, wobbler) and baitsh.
* * Pike were not angled in the protective period (1 April-15 May). Furthermore, bleeding frequency was pooled for articial lures (jerkbait and swimbait) with traditional hook mount and release-rig (see Bursell and
Arlinghaus, 2018 for details). Frequency of bleeding was 33.3% and 4.1% for articial lures with traditional hook mount and release-rig, respectively.
* ** Bleeding frequency was pooled for articial lures with barbed and barbless treble hooks. Frequency of bleeding was 40.0% and 52.6% for articial lures with and without barbed hooks, respectively.
* ** *Data for frequencies of bleeding and hooking in critical locations were extracted from Table 1 in Klefoth et al. (2008), and are estimated for 22 capture events of 16 individual pike.
M.H. Larsen et al.
Fisheries Research 273 (2024) 106906
8
(Arlinghaus et al., 2008, 2007; Bartholomew and Bohnsack, 2005).
Natural bait is typically shed passively with a relatively slacked line,
giving the sh more time to swallow the bait before the angler sets the
hook (Arlinghaus et al., 2007). The striking sh may also recognize the
natural bait as a food source, which is thought to induce deep hooking
(Arlinghaus et al., 2008). On the other hand, articial lures are usually
shed actively on tight lines and the hook is set instantly after a sh
strike, thereby increasing the odds of hooking the sh supercially in the
mouth (Brownscombe et al., 2017). However, in our study, the soft
plastic shads were shed quite passively as they were allowed to sink
towards the bottom before the angler started to retrieve the lure slowly.
Moreover, short pauses were regularly added during the retrieve, such
that the soft plastic shad dropped slightly back towards the bottom. It is
possible that this angling technique enhanced the risk of gill hooking
and associated bleeding, since slower moving lures are likely easier for
the pike to strike efciently and swallow. In any case, the anglers noted
that some of the pike took the soft plastic shad on the fall rather than
during the retrieve. Arlinghaus et al. (2008) also showed that soft plastic
shads were more likely to hook pike in critical locations relative to
spinners and wobblers, presumably because they were retrieved more
passively as well. The authors further suggested that soft plastic shads
resemble natural prey more closely than lures made of wood, hard
plastic, or metal, which could be an additional aspect facilitating deep
hooking. However, another study by Stålhammer et al. (2014) noted
that softbaits (i.e. articial lures made of soft plastic) tended to hook
pike in non-critical locations (lip and palate) and reduced the risk of
bleeding in comparison to other types of similar-sized lures, especially
bucktails (i.e. articial lures with feather or bucktail hair added to the
hook) and spiny (i.e. articial y shed with regular spinning gear).
This discrepancy was probably because the softbaits were retrieved at a
relatively fast and steady pace. Taken collectively, these results suggest
that the angling technique, such as retrieval speed and pattern, with
articial lures may be important for the anatomical hooking location
and severity of tissue injuries.
The incidence of hooking in critical locations and bleeding in pike
caught on baitsh was at the lower end relative to the values reported in
the existing literature. In total, we identied four studies during the
literature search that reported hooking location and/or bleeding
occurrence in pike caught on baitsh (Althoff et al., 2020; Arlinghaus
et al., 2008; DuBois et al., 1994; Klefoth et al., 2008). The shing gear
and angling techniques used in these studies vary from the present study
in ways that could help explain the discrepancies in bleeding rates and
proportion of critically hooked pike. More specically, Arlinghaus et al.
(2008) included smaller baitsh (<7.5 cm) than the present study
(12–17 cm), which could have increased the frequency of deep hooking,
since smaller baits are likely more easily swallowed by pike. Klefoth
et al. (2008) used two treble hooks on their baitsh (one placed in the
dorsal region and one in the pectoral region), whereas the roach were
equipped with one treble hook through the snout in our study. Larger
numbers of hook points could increase the risk of hooking the pike in
sensitive tissues and therefore offer a potential explanation for the
higher bleeding rate reported by Klefoth et al. (2008). In the last two
studies, pike were captured through the ice using tip-ups rigged with
baitsh (Althoff et al., 2020; DuBois et al., 1994). The wait time from a
pike strike until the angler set the hook and started to retrieve the sh
was quite long in both of these studies, on average 89 s and 143 s in the
studies by DuBois et al. (1994) and Althoff et al. (2020), respectively.
This contrasts the angling technique used in our study where the anglers
aimed to set the hook immediately after a bite was detected, which
probably reduced the incidence of hooking in critical tissues and
bleeding from the hook wound.
As noted above, Stålhammer et al. (2014) caught pike on different
types of similar-sized articial lures. The authors found that larger pike
were more likely to exhibit substantial bleeding from the hook wound,
even though the risk of hooking in critical locations was unrelated to sh
size. Based on these results, it was proposed that larger pike fought more
vigorously than smaller individuals, which could have inicted
increased trauma and bleeding of the tissue penetrated by the hook. The
present study could not conrm this pattern as the length of the angled
pike did not inuence ght time or bleeding occurrence regardless of the
bait type used. However, our result aligns with Arlinghaus et al. (2008),
who also observed no association between bleeding and length of pike
caught on baitsh or different types and sizes of articial lures.
Mortality was not included as an endpoint in our study. However, all
angled pike vigorously swam away upon release, except for one indi-
vidual that died shortly after capture in 2022, suggesting an overall low
immediate hooking mortality of 0.7%. The pike that died was not able to
maintain an upright orientation and oated at the water surface after
release. It was hooked in the corner of the mouth using baitsh and
exhibited minor bleeding, i.e. only a few drops of blood were present at
the hook wound. Bleeding was, therefore, an unlikely cause of mortality
in this case. Notably, the body condition of the pike was very low (K =
0.42), belonging to the lowest 10th percentile of all angled individuals.
A capture and release event inevitably causes some level of physiological
disturbance for sh (Arlinghaus et al., 2007; Kieffer, 2000), and in-
dividuals in poor body condition may be more susceptible to such
additional stressors (Cook et al., 2012). For instance, capture and
handling of sh elicit a glucocorticoid stress response of varying
magnitude and can impose signicant disturbances in the metabolic,
acid-based, osmotic and electrolyte balances (Holder et al., 2022;
Kieffer, 2000). This is particularly true if the hooked sh is subjected to
physical exhaustion during the landing process and/or longer durations
of air exposure before release (Arlinghaus et al., 2007). In some cases,
these physiological disturbances can be so severe that the sh fails to
restore homeostasis following capture, ultimately causing death (Fer-
guson and Tufts, 1992; Holder et al., 2022; Wood et al., 1983). While
earlier studies have suggested that pike are relatively resilient to phys-
iological stressors associated to angling (Arlinghaus et al., 2009; Louison
et al., 2017; Schwalme and Mackay, 1985), it is reasonable to assume
that individuals with a very low body condition may be less likely to
recover from such disturbances. Hence, we suppose that the specic pike
most probably died of capture-induced stress. It should also be noted
that even if a sh swims away upon release, this does not necessarily
imply that it will survive the capture event. Numerous studies have re-
ported delayed mortality of captured sh following release across a
diverse range of species, such as Atlantic salmon (Salmo salar) (Lennox
et al., 2017b), largemouth bass (Micropterus salmoides) (Sylvia and
Weber, 2019), bluegill sunsh (Lepomis macrochirus) (Gingerich et al.,
2007), and rainbow trout (Oncorhynchus mykiss) (Schill, 1996).
Post-release delayed mortality has also been observed for pike (DuBois
et al., 1994; Falk and Gillman, 1975), and the closely related muskel-
lunge (Esox masquinongy) (Margenau, 2007). Hence, we recommend
that future studies assess the potential consequences of C&R angling on
the post-release mortality of pike with a decreased body condition due to
food scarcity. In this respect, it would be highly relevant to look beyond
the effects of physical injuries associated to hooking and quantify the
role of physiological disturbances arising from the capture event.
Our study has some other potential limitations that should be
acknowledged. Firstly, angling for pike was not carried out prior to the
rst reduction in prey sh density by seining in fall 2020. It is therefore
plausible that pike already experienced increased hunger and feeding
motivation during the angling sessions in 2020, occurring 1–2 months
after the initial prey sh removal. If so, however, this was not reected
by changes in their body condition as the analysis showed no difference
in condition factor of pike captured by seining in fall 2020 and angling
1–2 months later. It is also important to keep in mind that the density of
roach was further reduced after the angling sessions in 2020, which
probably lead to additional declines in feeding opportunities for pike as
illustrated by the reduction in body condition from fall 2021 onwards.
Consequently, if prey sh availability and the level of hunger were
important drivers for the strike behaviour of pike towards shing lures/
baits, we would have expected to see differences in hooking location and
M.H. Larsen et al.
Fisheries Research 273 (2024) 106906
9
bleeding rates between pike angled in 2020 and 2022, everything else
being equal. Secondly, sex of the pike was not determined in the present
study. Because the energetic needs to gamete production are higher for
female than male pike (Diana, 1983), the behavioural responses to food
shortage may differ between sexes, which could translate into differ-
ences in strike behaviour towards shing lures/baits. For instance, it is
possible that the higher energetic cost of female gonad development and
related food demand may increase the risk of deep hooking, ultimately
leading to an overrepresentation of females among the group of pike
with hooking injuries in vital areas. If this is the case, the sex ratio of pike
could play an important role in determining the consequences of C&R
angling at the population level. Finally, our study only included one type
of articial lure (i.e. soft plastic shad) and one baitsh species (i.e.
roach). This may limit the generalizability of the present results since
bait type and size can affect hooking location and severity of tissue in-
juries in pike as previously noted (Arlinghaus et al., 2008; Stålhammar
et al., 2014), along with other lure/bait characteristics including tackle
conguration as well as hook type, number and size (Burkholder, 1992;
Bursell and Arlinghaus, 2018; Falk and Gillman, 1975).
5. Conclusion
The present results show no evidence that long-term reductions in
prey sh density and related declines in body condition alter how pike
strike and ingest articial lures or natural baits, at least not in a way that
increases the risk of hooking injuries in sensitive tissues. Consistent with
previous research, we found higher bleeding propensity from the hook
wound when the pike were hooked in critical locations, especially in the
gills. Our study also supports the notion that bait type is an important
predictor for hooking location and level of bleeding in angled pike.
However, contrary to our initial expectations, soft plastic shads caused
bleeding more frequently than baitsh, most likely because they had
higher tendency to hook the pike in the gills. It should be emphasized
that the majority (85.0%) of hooking locations encompassed non-critical
areas across bait types, and the proportion of pike exhibiting moderate
(10.9%) and substantial (1.4%) bleeding was relatively low. Nonethe-
less, bleeding from the hook wound has been shown to increase the
mortality of pike following release in previous studies. On top of this, it
is possible that pike with a very low body condition may be more sus-
ceptible to physical injuries and physiological stress associated to an-
gling than conspecics in better condition. Thus, further research is
needed to assess the potential impacts of C&R angling on pike pop-
ulations exposed to prolonged reductions in food availability before
unambiguous recommendations to sustainable sheries management
can be provided.
CRediT authorship contribution statement
Martin H. Larsen: Writing – original draft, Formal analysis, Data
curation, Funding acquisition, Methodology. O. Jonas Palder: Writing –
review & editing, Data curation. Casper Gundelund: Writing – review
& editing. Nicolas Azana Schnedler-Meyer: Writing – review & edit-
ing. Henrik D. Ravn: Writing – review & editing, Data curation.
Christian Skov: Conceptualization, Funding acquisition, Writing – re-
view & editing, Methodology.
Declaration of Competing Interest
The authors declare that they have no known competing nancial
interests or personal relationships that could have appeared to inuence
the work reported in this paper.
Data availability
Data will be made available on request.
Acknowledgements
The present study was nancially supported by the Poul Due Jensen
Foundation and the Danish Rod and Net Fish License Funds. We are
thankful to the landowner for access to the study site. We extend our
gratitude to the technical staff at DTU Aqua, particularly Hans-Jørn
Christensen, Michael Holm, Andreas Svarer and Jeppe Jørgensen, for
providing invaluable help in the eld. Lastly, we wish to thank Bjarke C.
Hopkins for assistance with the systematic literature search, and Lorine
M. Salel for proofreading the nal manuscript.
Ethics
Fish were sampled and tagged in according to the guidelines
described in permission (2017–15-0201–01164) from the Danish
Experimental Animal Committee.
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