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The goldsinny wrasse (Ctenolabrus rupestris) is a commercially important fish that inhabits coastal areas across the eastern Atlantic. This species moves from a shallow home territory along the coast into deeper waters in the autumn and winter and then returns to that same territory in the spring. Only male goldsinny wrasse exhibit strong territorial behavior, which may manifest as sexual differences in the ability or motivation to return to home territories. The orientation mechanism underlying the homing migration of goldsinny wrasse males and females is unknown. In this study, we hypothesized that goldsinny wrasse use the magnetic field of the Earth to follow a compass‐based path toward their home territory. To test this hypothesis, we collected 50 adult goldsinny wrasse, approximately half males and half females, in a harbor in Austevoll, Norway. Fish were translocated to a magnetoreception laboratory situated north of the site of capture, in which the magnetic field was artificially rotated. In the laboratory, males oriented toward the magnetic south taking a mean direction of 201°, which is the approximate direction that they would have had to take to return to the site at which they were captured. Females oriented in random magnetic directions. There was no difference in swimming kinematics between males and females. These results show that male goldsinny wrasse have a magnetic compass that they could use to maintain site fidelity, an ability that could help them and other coastal fish undertake repeatable short‐range migrations.
Goldsinny wrasse (Ctenolabrus rupestris) have a sex-dependent
magnetic compass for maintaining site fidelity
Alessandro Cresci | Torkel Larsen | Kim T. Halvorsen |
Caroline M. F. Durif | Reidun Bjelland | Howard I. Browman |
Anne Berit Skiftesvik
Ecosystem Acoustics Group, Institute of
Marine Research, Austevoll Research Station,
Storebø, Norway
Alessandro Cresci, Ecosystem Acoustics
Group, Institute of Marine Research, Austevoll
Research Station, Sauganeset 16, N-5392
Storebø, Norway.
Funding information
Norwegian Institute of Marine Research,
Grant/Award Numbers: 15579, 15638, 15655
The goldsinny wrasse (Ctenolabrus rupestris) is a commercially important fish that
inhabits coastal areas across the eastern Atlantic. This species moves from a shal-
low home territory along the coast into deeper waters in the autumn and winter
and then returns to that same territory in the spring. Only male goldsinny wrasse
exhibit strong territorial behavior, which may manifest as sexual differences in the
ability or motivation to return to home territories. The orientation mechanism
underlying the homing migration of goldsinny wrasse males and females is
unknown. In this study, we hypothesized that goldsinny wrasse use the magnetic
field of the Earth to follow a compass-based path toward their home territory. To
test this hypothesis, we collected 50 adult goldsinny wrasse, approximately half
males and half females, in a harbor in Austevoll, Norway. Fish were translocated to
a magnetoreception laboratory situated north of the site of capture, in which the
magnetic field was artificially rotated. In the laboratory, males oriented toward the
magnetic south taking a mean direction of 201, which is the approximate direc-
tion that they would have had to take to return to the site at which they were
captured. Females oriented in random magnetic directions. There was no differ-
ence in swimming kinematics between males and females. These results show that
male goldsinny wrasse have a magnetic compass that they could use to maintain
site fidelity, an ability that could help them and other coastal fish undertake
repeatable short-range migrations.
cleaner fish, homing, magnetic sense, orientation
Wrasse (Labridae family) are coastal fish that are widespread in the
Atlantic, Pacific, and Indian oceans (Helfman et al., 2009). Wrasse
exhibit complex species- and sex-specific social, reproductive, and
small-scale movement behavior (Donaldson, 1995; Hilldén, 1981).
Some species of wrasse undertake facultative parasite-cleaning
behavior when they are near larger fish (Costello & Bjordal, 1990;
Received: 14 September 2021 Revised: 25 November 2021 Accepted: 25 November 2021
DOI: 10.1111/fog.12569
This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any
medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.
© 2021 The Authors. Fisheries Oceanography published by John Wiley & Sons Ltd.
Fisheries Oceanography. 2021;18. 1
Hilldén, 1983). As a result, wild-caught wrasse have been used in
salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss)
aquaculture as cleaner fish to reduce infestations of the copepod
ectoparasite Lepeophtheirus salmonis (Costello, 2009; Costello &
Bjordal, 1990). The demand for wild-caught cleaner fish has driven
the development and expansion of a lucrative commercial fishery
(Halvorsen et al., 2020; Skiftesvik et al., 2014). Among the wrasse
species present in Norway, goldsinny wrasse (Ctenolabrus rupestris)is
one of the most abundant and is widely used as a cleaner fish
(Skiftesvik et al., 2015). Understanding the movement and reproduc-
tive ecology of goldsinny wrasse will inform the management of the
fishery, in particular the practice of fishers translocating and dis-
carding wrasses that are too small to be used as cleaner fish
(Halvorsen et al., 2021).
Goldsinny are among the most territorial of the wrasses, and
there are distinct genetic populations in Europe (Jansson
et al., 2020), as well as along the Norwegian coast (Sundt &
Jørstad, 1998). Goldsinny wrasse typically occupy territories charac-
terized by relatively turbulent water movement (Gjøsaeter, 2010).
This is because wave motion and water circulation influence benthic
algal cover, which is correlated with the availability of shelters
(Gaylord et al., 1994), the benthic invertebrate community, and food
supply (Skiftesvik et al., 2015; Smetacek, 1984). Goldsinny wrasse
also exhibit sex-dependent differences in behavior. The males
occupy a small territory (0.510 m
) in shallow water (050 m) with
a shelter at its center. Females stay near the territory of the male
with which they spawn (Davies & Sheehan, 2019; Hilldén, 1981).
Males continuously patrol their territory to defend it from other
males and almost never leave it, except for brief forays of a few
seconds (Hilldén, 1981).
Goldsinny wrasse vacate their territories in autumn and under-
take small-scale seasonal migrations from summer territories in shal-
low water toward deeper water in the winter (Halvorsen et al., 2021;
Hilldén, 1981; Skiftesvik et al., 2015). This vertical movement is asso-
ciated with seasonal changes in surface water temperature, which is
an important driver of vertical distribution in wrasse species located in
temperate areas such as the Norwegian coast (Freitas et al., 2021). In
situ observations conducted over a period of 3 consecutive years by
scuba divers and using observation rafts show that in the spring, the
males return to the same territory that they left in the autumn
(Hilldén, 1981). Although the general characteristics of their seasonal
movements have been described, the orientation mechanism that
guides goldsinny wrasse to their home territory during migration or
translocation is unknown.
Many species of fish use the Earth's magnetic field as a compass
cue to guide their short- and long-distance movements and migration
(Bottesch et al., 2016; Cresci, Paris, et al., 2019; Durif et al., 2013;
Quinn, 1980). We tested the hypothesis that goldsinny wrasse use a
magnetic compass to guide their return to their home territory during
seasonal migration or translocation and that this ability underlies their
high site fidelity. To explore this hypothesis, we translocated adult
goldsinny wrasse and tested their orientation abilities under artificially
rotated magnetic fields.
2.1 |Experimental animals
Adult goldsinny wrasse (N=50, 1012 cm total length) were col-
lected in a small harbor (60.085 N, 5.261 E) located in the Austevoll
archipelago, Norway (Figure 1). Fish were collected between 19 and
22 October 2020 using standard wrasse pots baited with 4080 g fro-
zen prawns Pandalus borealis (pots were two chambered,
70 40 28 cm, 11 mm mesh size, 60 90 mm elliptical entrances,
12 mm wide escape openings). After capture, fish were maintained in
a submerged net in the same location where they were captured. Fish
were fed with frozen prawns during the period of captivity (minimum
1 day and up to 4 days). Sex was determined visually as males have
distinctive red spots in the abdominal region (Hilldén, 1981). The size
of the fish was measured after they were tested using a measuring
tape inserted into a halved PVC pipe.
2.2 |Compass orientation experiments in the
The main hypothesis tested in this study is that goldsinny wrasse
translocated from their territories to a new, unfamiliar environment
would orient in the direction of their home territory using the mag-
netic field as an orientation cue. In this experiment, the home terri-
tories were located in the harbor where fish were caught, and the fish
were translocated to the magnetic field reception facility (MagLab)
4.5 km north of the harbor (i.e., home territories) (Figure 1). Home ter-
ritories were in a SE direction from the MagLab (142S). Fish were
transported in a 20-L cooler box filled with seawater and dark plastic
sheets, which served as shelter. The cooler was transported by car
(approximately a 10-min drive).
The experiments conducted in the MagLab followed the same
protocol as described in Cresci, Paris, et al. (2017) and Cresci, Paris,
et al. (2019). All tests were conducted during daytime under artificial
The MagLab is designed to study the magnetic orientation of
aquatic animals and to eliminate other possible external cues that
could be used for orientation; for example, the animals are not
exposed to water flows, odor plumes, sunlight, or any celestial cues.
The MagLab is equipped with a triaxial electric coil system
(Figure S1a), designed as described by Merritt et al., 1983, that is con-
nected to a multichannel power supply (max. 3 A). In the laboratory,
the coil system consists of four double wrapped nested electric coils
described in Durif et al., 2013. One was used to cancel out the hori-
zontal component of the ambient field. The other three were used to
produce the artificial magnetic field and to reorient the magnetic
north. The artificial field had the same total intensity and inclination
as the ambient field (48.850 μT and 73, with a deviation of <1).
At the center of the coils, there is a circular tank made of fiber-
glass (diameter, 1.40 m; height, 0.90 m; see Figure S1a) filled with sea-
water, which is pumped from the sea 300 m away. The building (see
Figure S1b,c) is constructed of nonmagnetic material and is far from
any source of magnetic interference (163 m from the nearest electri-
cal disturbance and 365 m from the closest building; Figure S1c). Light
intensity in the tank was low (<0.1 lum/ft
as measured by a HOBO
light sensor) and water temperature ranged between 8 and 9C.
Each wrasse was observed for 20 min, with the first 5 min consid-
ered as an acclimation period. A small cylinder (20 cm diameter) con-
nected to a string that extended to an adjacent room was placed at
the center of the tank. At the beginning of a test, a fish was released
into the small cylinder where it was allowed to habituate. After 5 min,
the cylinder was lifted upward using the line, and the test began. This
protocol was repeated for one fish at a time, and individuals were
tested only once.
Each fish was tested under one of the four simulated magnetic
field conditions, with the magnetic north reoriented to the Earth's
east, south, west, or north (see Figure S2). Each wrasse experienced
only one of these four magnetic conditions. Using this approach, it is
possible to discriminate between magnetic and topographical
orientation cues.
After the experiment was completed, fish were released at the
site where they were captured before the experiment started.
2.3 |Data analysis
The behavior of each fish was determined by analyzing videos col-
lected using a GoPro HERO 7 placed above the tank and looking
downward. Videos were processed using Tracker 5.1.5. (Copyright ©
2020 Douglas Brown, Fish in the videos
were manually tracked, and the fish tracks were used to calculate
swimming kinematics and orientation behavior for each individual.
We tracked the position of each fish, every second, for the 15-min
observation period (900 data points per wrasse), as detailed in
Figure S3. The angle of each position of the fish with respect to the
artificially rotated magnetic north in the laboratory was considered as
a bearing (using the center of the arena as a reference). As the mag-
netic north had a different orientation in the laboratory during each
test, we monitored the direction of the north using an analog com-
pass. If the frequency distribution of the 900 bearings for each fish
was significantly different from uniform (Rayleigh's p< .05), we con-
sidered it as evidence of orientation, and we used the mean of the
900 bearings as the orientation direction of the fish (Cresci, Paris,
et al., 2019; Irisson et al., 2009; Paris et al., 2008).
To test for malefemale differences in orientation, the next step
of the analysis was to evaluate whether the wrasse of each experi-
mental group (females; males) were swimming toward a common ori-
entation direction (Figure S3c). To explore that, we used Rayleigh's
test of uniformity applied to all of the mean individual bearings of all
of the wrasses from each of the experimental groups as data points
(N=19 males; N=22 females).
An ANOVA for circular data was applied to test for influence of
sex on the orientation directions. Malefemale differences in average
and maximum swimming speed and acceleration, and differences in
total distance covered were tested using the nonparametric Wilcoxon
test. Possible confounding effects of body size on speed and accelera-
tion of the fish were assessed with fitting of linear models.
FIGURE 1 Study area. Location of the harbor
with the home territories (green circle) of the
goldsinny wrasse (Ctenolabrus rupestris) used in
the study. The red circle shows the location of the
magnetic laboratory (MagLab) to which the fish
were translocated and in which the experiments
were conducted
The average total length of the males used in this study was 11.9
± 1.2 cm (mean ± SD), which was significantly different, but only
slightly greater than the total length of females (10.6 ± 1.2 cm;
Wilcoxon test, W =518, p-value =.0002). The sex ratio was 46%
males (N=23) and 54% females (N=27). Of the 27 female wrasses
tested, 22 oriented (81%; Rayleigh test of uniformity applied to the
track of each fish; p< .05). This proportion was similar in male fish:
82% showed a significant orientation direction (19 out of 23; p< .05).
Sex had a strong influence on the magnetic field-based orientation of
the fish (circular ANOVA; df =1, F=8.123, p=.007), with the
females that oriented not having a preferred orientation direction
with respect to the magnetic field (N=22, p=.84; Figure 2). How-
ever, on average, males oriented toward the magnetic south (N=19,
mean direction =201,r=.39, p=.05; Figure 2). Among the
orienting males, there was an outlier, as one fish oriented toward the
opposite direction (magnetic northeast) compared to the other males
(Figure 2). Without the outlier, the magnetic orientation of the males
is highly significant toward the south (N=18, mean direction =201,
r=.47, p=.002; Figure 2). A summary of the orientation directions
before correction to the artificially rotated magnetic north is in
Table S1.
Males and females had the same swimming kinematics (Table 1).
The frequency distribution of both swimming speed and acceleration
data had similar shape (Figure 3a,b). The mean speed of the individuals
did not differ between the two groups (Wilcoxon test; W =295,
p=.81), nor did the mean acceleration (W =262, p=.35) (Figure 3).
Furthermore, females and males covered almost the same total dis-
tance during the tests (W =336, p-value =.63) (Figure 4). There was
no influence of total length on mean speed (linear model, F=.29,
p=.56), maximum speed (F=.63, p=.43), mean acceleration
(F=.54, p=.46) or maximum acceleration (F=.25, p=.62).
In this study, male and female goldsinny wrasse were translocated
from their territories to a magnetic laboratory situated to the north,
where their orientation relative to the magnetic field was studied. For
each individually tested fish, the magnetic north in the laboratory was
rotated by 90, and their orientation direction with respect to the
rotated north was assessed. Goldsinny wrasse exhibited sex-
dependent differences in magnetic orientation (Figure 2). Females did
not show any preferred magnetic direction, while males oriented to
the magnetic south (201)the approximate direction of the home
territories from which they were translocated (142).
Several species of both temperate and tropical fish, such as
sockeye salmon (Oncorhynchus nerka) (Quinn, 1980), Atlantic haddock
(Melanogrammus aeglefinus) (Cresci, Paris, et al., 2019), European eels
(Cresci, Durif, et al., 2019; Durif et al., 2013), and cardinal fish
(Ostorhinchus doederleini) (Bottesch et al., 2016), use the magnetic
field as a compass for orientation. However, whether there are sexual
differences in magnetic compass orientation of fish is unknown. Sex
differences in orientation and movement behavior are present in
other animals, such as natterjack toads (Bufo calamita) (Sinsch, 1992)
and blenniid fish (Costa et al., 2011), as well as in humans (Boone
et al., 2018). Only a small number of studies report sex differences in
magnetic field-based orientation behavior. In the fruit fly, Drosophila
melanogaster, males exhibit strong and consistent magnetic compass
response, while females fly in random directions (Phillips &
Sayeed, 1993). In deer mice (Peromyscus maniculatus), males display
better performance in navigation behavior and spatial learning com-
pared to females (Kavaliers et al., 1996), but these differences disap-
pear after a 5-min exposure to weak magnetic fields of 100 μT
(Kavaliers et al., 1996). To the best of our knowledge, sex-dependent
magnetic compass behavior has not been reported in fish.
Magnetic field-based migration behavior has been documented in
many long-distance migrators such as salmon, eels, sharks, and turtles
(Durif et al., 2021; Keller et al., 2021; Lohmann et al., 2007; Putman
et al., 2014). In these species, the benefits associated with an ability
to use a magnetic compass or map to cross hundreds to thousands of
kilometers in pelagic water is clear. However, magnetic orientation
responses are also exhibited by species performing relatively short-
range movements, such as zebrafish, newts, and fruit flies (Cresci, de
Rosa, et al., 2017; Phillips, 1986; Phillips & Sayeed, 1993). For marine
FIGURE 2 Orientation of goldsinny wrasse (Ctenolabrus
rupestris) in a magnetic laboratory. The orientation of female (N=22)
and male (N=19) goldsinny wrasse is presented with respect to the
magnetic north (N) and south (S) in the magnetic laboratory. During
the experiments, the magnetic north in the laboratory was rotated for
each fish (i.e., the magnetic north in the lab had a different direction
for each of the magenta and blue data points). The orientation of each
fish was corrected to the artificially rotated magnetic north in the
laboratory. Each point corresponds to the mean bearing of one
goldsinny wrasse (averaged over 900 data points from the video
tracks; Figure S3). These figures display the mean bearings of the fish
that showed an individual preferred orientation. The black arrow
points towards the mean angle of all the individual bearings. Dashed
gray lines are the 95% confidence intervals around the mean. Absence
of the arrow means that there was no preferred magnetic orientation
short-distance migrants, magnetic field-based orientation could have
several functions: It could help improve accuracy in locating specific
nesting or mating areas; it could serve as a frame of reference in
flowing water when visual landmarks are absent, or it could help pro-
vide the right direction for seasonal migrations such as those under-
taken by goldsinny wrasse. Thus, for goldsinny wrasse, a magnetic
compass could play an important role in guiding their return to their
home territory following overwintering in deeper waters. Goldsinny
wrasse live in shallow water (050 m) mostly in association with rocky
shores and kelp forests (Skiftesvik et al., 2014, 2015), from which they
undertake short-range movements (Hilldén, 1981; Sayer et al., 1993).
Fish perform short-range orientation behavior by using multiple sen-
sory systems, ranging from visual to tactile and olfactory, and by
adopting different orientation strategies (Braithwaite & Burt De
Perera, 2006). Fish use beacons (single landmarks), learn geometric
relationships between the landmarks, and integrate multiple kinds of
spatial information to perform short-range movements (Braithwaite &
Burt De Perera, 2006; Hughes & Blight, 1999). Among these orienta-
tion mechanisms, magnetic orientation could also be used by fish for
short-range migrations, especially when visual cues are unavailable
(Cresci, Paris, et al., 2017).
Both male and female goldsinny wrasse appear to perform short
seasonal migrations, moving to deeper waters in the winter, while
returning to shallow waters in the spring (Halvorsen et al., 2021;
Hilldén, 1981; Sayer et al., 1993; Skiftesvik et al., 2015). Our study
suggests that males, but not females, have a strong motivation and
TABLE 1 Swimming kinematics of male and female goldsinny wrasse (Ctenolabrus rupestris)
Mean speed
Max speed
Mean acceleration
Max acceleration
Total distance
covered (m)
Mean turning angle
Females 4.59 ± 2.77 23.32 ± 14.09 1.27 ± 1.24 12.05 ± 9.26 42.63 ± 20.47 22.87 ± 0.51
Males 4.86 ± 2.91 21.01 ± 11.29 1.44 ± 1.23 10.09 ± 5.60 44.67 ± 20.86 21.20 ± 0.48
Note: Values are reported as mean ± SD.
FIGURE 3 Swimming speed and acceleration of goldsinny wrasse (Ctenolabrus rupestris) males (N=23) and females (N=27). (a,b) The
frequency distributions of swimming speed and acceleration from the video tracks are displayed for males and females. (c,d) Boxplots of
swimming speeds and acceleration (with median, 25th, and 75th percentile)
ability to orient toward their home when displaced. This is consistent
with previous field observations showing that when males and
females are displaced, males return faster to the location from which
they were removed than females (Hilldén, 1981). The ability of males
to follow a magnetic compass direction toward their home territory
likely plays a role in their faster migratory performance compared to
females. It is possible that this sex difference in orientation behavior
reflects a stronger site fidelity of territorial males, who may be moti-
vated to relocate to their former territory and its properties, where
they forage and mate (Hilldén, 1981).
A high-quality territory, which provides shelter and food for
females, can increase the chance of male mating success in fishes
(Hermann et al., 2014), and, under high densities, the number of suit-
able territories may be limited (Warner & Hoffman, 1980). Male
goldsinny wrasse resolve territorial disputes with a distinct behavior
involving boundary displays, mouth fighting, and biting
(Hilldén, 1981). In this context, a magnetic compass could help the
males trace their route back to their home area and reduce the chance
of having to establish new territories, lowering the risk of territorial
disputes. The absence of orientation to the magnetic field in females
requires additional investigation to explore, for example, (i) whether
they lack magnetic field-based orientation entirely or (ii) whether mag-
netic field-based orientation in females is exhibited during the
spawning season but not outside of it.
The male goldsinny wrasse observed in this study had an average
SSW magnetic orientation direction (201). This was the
approximatebut not the exactdirection of their home territories,
which are located 142SE of the MagLab. This difference might be
accounted for by the fact that in the MagLab, wrasse were deprived
of all cues other than the magnetic field. However, other cues could
also play a role, perhaps as a reinforcement of the magnetic compass,
in a more complex and integrative orientation mechanism used for
homing. This could be particularly important at the end of the sea-
sonal migration, when vision and olfaction are likely to be the main
cues allowing males to identify the target territory.
The sex-dependent magnetic compass of goldsinny wrasse is not
associated with differences in swimming performance. The latter did
not vary with sex even though males were slightly, but significantly,
longer than females (in goldsinny wrasse, males reach slightly greater
asymptotic length compared to females) (Olsen et al., 2019). Thus, the
sex difference in magnetic orientation is not an artifact of differential
swimming performance between males and females, but rather is the
manifestation of different choices of orientation direction.
This study provides novel evidence that sex can be an important
factor in magnetic field-based orientation and movement behavior of
coastal fish and that magnetic compass orientation is involved in
short-range movement in coastal waters. Future studies on magnetic
compass orientation should focus on other species of coastal fish, as
this could be an important tool for short- and mid-range movement
behavior and for maintaining site fidelity which has been reported in a
growing number of species.
Thanks to Marina Mihaljevic and Rosa Helena Escobar for help with
some deployments in the laboratory and Tore Hufthamar for support
of the magnetic laboratory. This work was funded by the Norwegian
Institute of Marine Research's project Fine-scale interactions in the
plankton(project # 15579) to HIB and project Wrasse biology and
stock assessmentto ABS (project # 15638). ABS, CMFD, RB, TL, and
KTH were supported by the wrasse project. AC was supported by the
project Assessing the effects of offshore wind on the early life stages
of fishto HIB (project # 15655).
The authors declare no competing interests.
The Austevoll Research station has a permit to operate as a Research
Animal facility for fish (all developmental stages), under Code 93 from
the national Institutional Animal Care and Use Committee (IACUC),
NARA. We did not require specific approval for these experiments
because they are nonintrusive behavioral observations.
A.C. designed the study, collected, analyzed, and interpreted the
data, and wrote the paper; T.L. designed the study and collected
and analyzed the data; K.T.H. designed the study, interpreted the
data, and wrote the paper; C.M.D. designed the study, collected,
and interpreted the data and wrote the paper; R.B. designed the
study and interpreted the data and wrote the paper;
H.I.B. designed the study, collected and interpreted the data, wrote
the paper, and funded the research; A.B.S. designed the study, col-
lected and interpreted the data, wrote the paper, and funded the
FIGURE 4 Total distance covered and mean turning angle of
goldsinny wrasse (Ctenolabrus rupestris). Boxplots of total distance
(meters) covered by males (N=23) and females (N=27) (with
median, 25th, and 75th percentile)
Data are available from the corresponding author upon reasonable
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How to cite this article: Cresci, A., Larsen, T., Halvorsen, K. T.,
Durif, C. M. F., Bjelland, R., Browman, H. I., & Skiftesvik, A. B.
(2021). Goldsinny wrasse (Ctenolabrus rupestris) have a
sex-dependent magnetic compass for maintaining site fidelity.
Fisheries Oceanography,18.
... A female-biased homing behaviour was already observed for swordfish (Muths et al., 2009) and bluefish (Miralles et al., 2014). Male-biased site fidelity was found in a species expressing a strong territoriality (Cresci et al., 2022). For species with a female-biased sexual size dimorphism such as observed for seabass (Saillant et al., 2001), the male-male competition is decreased (Horne et al., 2020), which may explain their lower homing behaviour compared to females. ...
The structure and connectivity of European seabass (Dicentrarchus labrax) populations remain poorly known and ecological evidence is missing to support the current delineation between the northern (southern North Sea, English Channel and Celtic Sea) and southern French stocks (Bay of Biscay). Adult spawning site fidelity and natal homing were analysed by coupling Data Storage Tag (DST) information and otolith microchemistry of recaptured fish to investigate, within the study area, the population structure and connectivity in European seabass. Trajectory reconstructions inferred from DST data were used to assign a spawning area (English Channel or Bay of Biscay) to each spawning winter record. In addition, otolith composition (Mg, P, Mn, Zn, Sr, Ba and δ¹⁸O) was measured in both larvae and adults otolith increments corresponding to a winter spawning event. We built a training dataset using coupled spawning area assignments and otolith elemental signatures (Mg, P, Mn, Zn, Sr and Ba) for winters with DST data. The training dataset was used to calibrate a Random Forest model and assign spawning areas based on otolith winter signatures outside the DST recording period. Results revealed that 64% of the seabass expressed spawning site fidelity. We also found a geographical gradient of site fidelity, with the highest proportions of spawning site fidelity found in seabass tagged at the northern and southern limits of the studied area. Significant ontogenetic effects were observed for trace elements and δ¹⁸O with ratios significantly lower in the larval stage than in the adult stage. These biases and the variability across cohorts prevented us to use the assignment model fitted on adults to study natal homing. At the larval stage, the analysis of spatio-temporal effects on otolith trace elements did not reveal any significant difference between spawning areas. However, the patterns of difference were similar for larval and adult Zn, Sr and Ba between the two spawning areas, suggesting a homing behaviour.
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Anguillid eels grow in freshwater but spawn in the open ocean. The cues that guide eels over long distances to the spawning area are unknown. The Earth's magnetic field can provide directional and positional information and is likely used by catadromous eels during their spawning migration; as magnetosensitivity and compass orientation have been reported in eels. To test whether this is theoretically possible, we compared the migratory routes of five species of temperate eels that undertake long migrations with the geomagnetic field of their distribution/spawning areas. We found that, regardless of the species and although routes are different between life stages, larvae of those species always drift along paths of increasing magnetic inclination and intensity, while adults follow reverse gradients. This is consistent with an imprinting/retracing hypothesis. We propose a general navigation mechanism based on larvae imprinting on a target magnetic intensity (or inclination) at the hatching area and on the intensity (or inclination) gradient during larval drift. Years later, adults retrace the magnetic route by following the gradient of decreasing total intensity (or inclination) values that occurs towards lower latitudes. As they reach the target value, adults switch to compass orientation to stay on the target isoline and reach the spawning area. The proposed mechanism fits for all temperate eels examined. Knowledge about navigational strategies of eels is important to evaluate the effectiveness of management strategies that involve stocking of juveniles displaced from one area to another to rebuild local populations.
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The movement patterns of three commercially important wrasse (Labridae) species inside a small marine protected area (~ 0.15 km²) on the west coast of Norway were analysed over a period of 21 months. The mean distance between capture and recapture locations varied between 10-187 meters and was species and season specific. The extent of movement was not related to body size or sex. These results imply that a network of small strategically located marine protected areas can be used as management tools to protect wrasses from size- and sex-selective fishing mortality. This article is protected by copyright. All rights reserved.
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Small‐scale fisheries (SSFs) tend to target shallow waters, but the depth distributions of coastal fish can vary depending on species, size, and sex. This creates a scope for a form of fishing selectivity that has received limited attention but can have considerable implications for monitoring and management of these fisheries. We conducted a case study on the Norwegian wrasse fishery, a developing SSF in which multiple species are caught in shallow waters (mean depth = 4.5 m) to be used as cleaner fish in aquaculture. Several of these wrasses have life histories and behaviors that are sensitive to selective fishing mortality, such as sexual size dimorphism, paternal care, and sex change. An experimental fishery was undertaken over three sampling periods in 2018. Data on catch, length, and sex of wrasses across a depth gradient (0–18 m) were collected and analyzed. We found that depth distributions were species specific and the vertical overlap with the fishery was high for Corkwing Wrasse Symphodus melops and Ballan Wrasse Labrus bergylta, which were most abundant at depths less than 5 m. Three other wrasse species had invariant or increasing abundance with depth and were therefore less likely to be negatively impacted by this fishery. Body size was positively correlated with depth for these wrasses, and sex ratio became more male biased for the Corkwing Wrasse, the only species that could be visually sexed. This study demonstrates that depth can have strong effects on fishing selectivity at multiple scales and that such knowledge is necessary to develop management strategies that balance fishing mortality sustainably across species, sizes, and sexes. We recommend that management priorities be directed toward the Ballan and Corkwing wrasses—the species having the highest vertical overlap with the fishery. Furthermore, CPUE was strongly affected by seasonality and positively correlated with increasing wave exposure for one of the species. This underscores the general importance of standardizing catch data for several environmental covariates when monitoring species that are affected by SSFs.
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Background: Marine fish populations are often characterized by high levels of gene flow and correspondingly low genetic divergence. This presents a challenge to define management units. Goldsinny wrasse (Ctenolabrus rupestris) is a heavily exploited species due to its importance as a cleaner-fish in commercial salmonid aquaculture. However, at the present, the population genetic structure of this species is still largely unresolved. Here, full-genome sequencing was used to produce the first genomic reference for this species, to study population-genomic divergence among four geographically distinct populations, and, to identify informative SNP markers for future studies. Results: After construction of a de novo assembly, the genome was estimated to be highly polymorphic and of ~600Mbp in size. 33,235 SNPs were thereafter selected to assess genomic diversity and differentiation among four populations collected from Scandinavia, Scotland, and Spain. Global FST among these populations was 0.015-0.092. Approximately 4% of the investigated loci were identified as putative global outliers, and ~ 1% within Scandinavia. SNPs showing large divergence (FST > 0.15) were picked as candidate diagnostic markers for population assignment. One hundred seventy-three of the most diagnostic SNPs between the two Scandinavian populations were validated by genotyping 47 individuals from each end of the species' Scandinavian distribution range. Sixty-nine of these SNPs were significantly (p < 0.05) differentiated (mean FST_173_loci = 0.065, FST_69_loci = 0.140). Using these validated SNPs, individuals were assigned with high probability (≥ 94%) to their populations of origin. Conclusions: Goldsinny wrasse displays a highly polymorphic genome, and substantial population genomic structure. Diversifying selection likely affects population structuring globally and within Scandinavia. The diagnostic loci identified now provide a promising and cost-efficient tool to investigate goldsinny wrasse populations further.
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The European eel (Anguilla anguilla) hatches in the Sargasso Sea and migrates to European and North African freshwater. As glass eels, they reach estuaries where they become pigmented. Glass eels use a tidal phase-dependent magnetic compass for orientation, but whether their magnetic direction is innate or imprinted during migration is unknown. We tested the hypothesis that glass eels imprint their tidal-dependent magnetic compass direction at the estuaries where they recruit. We collected 222 glass eels from estuaries flowing in different cardinal directions in Austevoll, Norway. We observed the orientation of the glass eels in a magnetic laboratory where the magnetic North was rotated. Glass eels oriented towards the magnetic direction of the prevailing tidal current occurring at their recruitment estuary. Glass eels use their magnetic compass to memorize the magnetic direction of tidal flows. This mechanism could help them to maintain their position in an estuary and to migrate upstream.
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Atlantic Haddock (Melanogrammus aeglefinus) is a commercially important species of gadoid fish. In the North Sea, their main spawning areas are located close to the northern continental slope. Eggs and larvae drift with the current across the North Sea. However, fish larvae of many taxa can orient at sea using multiple external cues, including the earth’s magnetic field. In this work, we investigated whether haddock larvae passively drift or orient using the earth’s magnetic field. We observed the behavior of 59 and 102 haddock larvae swimming in a behavioral chamber deployed in the Norwegian North Sea and in a magnetic laboratory, respectively. In both in situ and laboratory settings, where the magnetic field direction was modified, haddock larvae significantly oriented towards the Northwest. We conclude that haddock larvae orientation at sea is guided by a magnetic compass mechanism. These results have implications for retention and dispersal of pelagic haddock larvae.
Migration is common in marine animals,1, 2, 3, 4, 5 and use of the map-like information of Earth’s magnetic field appears to play an important role.²,6, 7, 8, 9 While sharks are iconic migrants10, 11, 12 and well known for their sensitivity to electromagnetic fields,13, 14, 15, 16, 17, 18, 19, 20 whether this ability is used for navigation is unresolved.¹⁴,¹⁷,²¹,²² We conducted magnetic displacement experiments on wild-caught bonnetheads (Sphyrna tiburo) and show that magnetic map cues can elicit homeward orientation. We further show that use of a magnetic map to derive positional information may help explain aspects of the genetic structure of bonnethead populations in the northwest Atlantic.23, 24, 25, 26 These results offer a compelling explanation for the puzzle of how migratory routes and population structure are maintained in marine environments, where few physical barriers limit movements of vagile species. Video abstract Download : Download video (19MB)
Understanding the responses of aquatic animals to temperature variability is essential to predict impacts of future climate change and to inform conservation and management. Most ectotherms such as fish are expected to adjust their behaviour to avoid extreme temperatures and minimize acute changes in body temperature. In coastal Skagerrak, Norway, sea surface temperature (SST) ranges seasonally from 0 to over 20 °C, representing a challenge to the fish community which includes both cold‐, cool‐ and warm‐water affinity species. By acoustically tracking 111 individuals of Atlantic cod (Gadus morhua), pollack (Pollachius pollachius) and ballan wrasse (Labrus bergylta) in 2015 ‐ 2018, we examined how coexisting species within a fish community adjusted their behaviour (i.e. vertical distribution in the water column and habitat selection) to cope with the thermal variation. Mixed‐effect models showed that thermal preference was a main driver of behaviour and habitat use of the fish community in a southern Norwegian fjord. Cod used colder waters, compared with pollack and ballan wrasse. Increases in SST during summer were associated with the use of deeper, colder waters by cod, especially by larger individuals, and conversely with the occupancy of shallower areas by pollack and ballan wrasse. During winter, when SST dropped and the thermal stratification reversed, pollack and ballan wrasse moved to deeper, relatively warmer areas, while cod selected shallower, colder habitats. Though habitat selection was affected by temperature, species‐specific habitat selection was observed even when temperature was similar throughout habitats. This study shows how cohabiting fish species respond to thermal heterogeneity, suggesting that i) temperature regulates the access to the different depths and habitats and ii) behavioural plasticity may be an important factor for coping with temperature variability and potentially for adaptation to climate change.
The spatial extent of animal movement is a key consideration when designing conservation measures, such as marine protected areas. Methods to assess territory size in the marine environment, however, are labour intensive and/or expensive. Here, we explore a novel method to investigate the spatial ecology of territorial fishes by examining their reactions to an artificial light stimulus. During benthic towed video surveys conducted in Lyme Bay, southwest England, several species of wrasse (Labridae) have frequently been observed pursuing a laser projected onto the seabed. While the motivation behind ‘laser‐chasing’ is unclear, we quantified the spatial aspects of this behaviour by comparing chase distance and chase likelihood between and within species, to determine the potential utility of this method for investigating space use and aggression in wild fishes. Cuckoo wrasse (Labrus mixtus) were significantly more likely to display agonistic behaviour towards the laser than Goldsinny wrasse (Ctenolabrus rupestris). Goldsinny wrasse displayed a positive relationship between total length and chase‐distance, but not Cuckoo wrasse. The observed species differences may relate to behavioural factors affecting the motivation behind ‘laser‐chasing’, which is discussed. Chases by the cuckoo wrasse were significantly longer than those by Goldsinny wrasse, and these chase distances were used to estimate theoretical territory sizes for each species. To our knowledge, this is the first study to explore the spatial aspects of the reactions to an artificial stimulus by wild fishes. The potential to develop the method to directly investigate aspects of territoriality and aggression in wild fishes is discussed, including necessary further refinements and testing. Wild wrasses are increasingly exploited in Europe to provide cleaner fish for salmonid aquaculture, and we encourage the development of methods to inform spatial conservation measures for these ubiquitous and iconic species.
Small-bodied wrasse species are important for structuring coastal marine ecosystems but are also increasingly harvested as parasite cleaners on farmed salmon. Identifying management regulations that will support long-term sustainability of wrasse fisheries is challenging, because there is still limited knowledge about the impacts of fisheries on the demography of these intermediate predators in their natural environments. To this end, we studied individual growth histories of goldsinny wrasse (Ctenolabrus rupestris) at a fine spatial scale across replicated marine protected areas (MPAs) and areas open to commercial harvesting on the Norwegian coast. The MPAs were established 1-7 years prior to our sampling. We detected significant fine-scale spatial variation in wrasse asymptotic body size, but found no consistent difference between MPAs and fished areas. Male wrasses reached larger asymptotic body sizes than females, whereas fyke nets captured individuals with larger asymptotic body sizes compared with baited traps. These are the two commonly used gear types in wrasse fisheries. An extended use of baited traps, along with slot-size limits, could therefore aid in protecting large-growing phenotypes such as nest-guarding males.