Reduced anti-predator responses in multi-generational hybrids of farmed and wild Atlantic salmon (Salmo salar L.)

Abstract

Cultured organisms undergo genetically-based behavioural changes that may reduce their ability to survive in the wild. This
has raised concerns that interbreeding between escaped cultured and wild organisms will generate hybrids exhibiting maladaptive
behaviours which may ultimately reduce the fitness of the wild counterpart. We compared anti-predator responses in Atlantic
salmon (Salmo salar) from two wild North American populations, the major farmed strain used in regional aquaculture, and their wild-farmed hybrids
(F1, F2, and wild backcross). Anti-predator responses of fry (age 0+parr) were measured under common environmental conditions, using
a model of a natural predator (belted kingfisher, Ceryle alcyon). Farmed fry exhibited significantly reduced anti-predator responses relative to fry from both wild populations. The anti-predator
responses of wild-farmed hybrid fry were intermediate to those of the parental populations (pure farmed or wild). The magnitude
by which wild-farmed hybrids differed in anti-predator responses from pure wild fish also depended on the wild population.
These results suggest that: (1) the observed behavioural differences have a genetic basis; (2) wild-farmed hybrids have, on
average, reduced anti-predator responses relative to wild fish; and that (3) the effects of wild-farmed interbreeding on anti-predator
responses will differ between wild populations. Our study is consistent with the general hypothesis that continual farmed-wild
interbreeding may have detrimental effects on the fitness of wild organisms.

KeywordsAquaculture-F1
-F2
-Backcross-Escape-Risk assessment-Outbreeding depression

Full text

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RESEARCH ARTICLE
Reduced anti-predator responses in multi-generational hybrids
of farmed and wild Atlantic salmon (Salmo salar L.)
Aimee Lee S. Houde Æ Æ Dylan J. Fraser Æ Æ
Jeffrey A. Hutchings
Received: 6 October 2008/Accepted: 6 March 2009/Published online: 20 March 2009
? Springer Science+Business Media B.V. 2009
Abstract
behavioural changes that may reduce their ability to sur-
vive in the wild. This has raised concerns that interbreeding
between escaped cultured and wild organisms will generate
hybrids exhibiting maladaptive behaviours which may
ultimately reduce the fitness of the wild counterpart. We
compared anti-predator responses in Atlantic salmon
(Salmo salar) from two wild North American populations,
the major farmed strain used in regional aquaculture, and
their wild-farmed hybrids (F1, F2, and wild backcross).
Anti-predator responses of fry (age 0? parr) were mea-
sured under common environmental conditions, using a
model of a natural predator (belted kingfisher, Ceryle
alcyon). Farmed fry exhibited significantly reduced anti-
predator responses relative to fry from both wild popula-
tions. The anti-predator responses of wild-farmed hybrid
fry were intermediate to those of the parental populations
(pure farmed or wild). The magnitude by which wild-
farmed hybrids differed in anti-predator responses from
pure wild fish also depended on the wild population. These
results suggest that: (1) the observed behavioural differ-
ences have a genetic basis; (2) wild-farmed hybrids have,
on average, reduced anti-predator responses relative to wild
fish; and that (3) the effects of wild-farmed interbreeding
on anti-predator responses will differ between wild popu-
lations. Our study is consistent with the general hypothesis
that continual farmed-wild interbreeding may have detri-
mental effects on the fitness of wild organisms.
Cultured organisms undergo genetically-based
Keywords
Escape ? Risk assessment ? Outbreeding depression
Aquaculture ? F1? F2? Backcross ?
Introduction
Raising organisms in artificial environments (e.g. captivity)
over several generations is believed to cause genetically-
based behavioural changes that deviate from those
expressed in the natural environment (Price 1997; Frank-
ham 2008). Such behavioural changes occur when the
natural and artificial environments differ in their selective
pressures of certain behaviours (e.g. increased tameness
towards humans or domestication; Hosey 1997).
Organisms that are cultured for economic benefit are
exposed to a variety of selective pressures in aquaculture
that differ from those experienced in their natural envi-
ronment. In fishes, as in many captive-bred populations,
there is intentional selection for commercially beneficial
traits, such as faster growth rates and delayed maturity, in
aquaculture programmes (Gjedrem et al. 1988; Glebe
1998). The aquaculture environment itself may act as an
inadvertent selective force, affecting the behaviour of
farmed individuals (Gross 1998). Behavioural adaptations
to the artificial environment (e.g. changes in aggression)
may be beneficial to living in the aquaculture environment,
but are more likely to lead to a loss of local adaptation (c.f.
Taylor 1991; Garcia de Leaniz et al. 2007) in the natural
environment (Fleming et al. 1994; Verspoor 1998; Jonsson
and Jonsson 2006).
Farmed Atlantic salmon (Salmo salar) often escape
from sea cages and have the potential to enter rivers and
interbreed with wild salmon. While the reproductive per-
formance (reflected, for example, by mate and territory
acquisition) of farmed salmon can be inferior to that of
A. L. S. Houde (&) ? D. J. Fraser ? J. A. Hutchings
Department of Biology, Dalhousie University, Halifax,
NS B3H 4J1, Canada
e-mail: aimee.lee.houde@dal.ca
123
Conserv Genet (2010) 11:785–794
DOI 10.1007/s10592-009-9892-2

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wild fish (Fleming et al. 1996), there are exceptions (Ga-
rant et al. 2003; Weir et al. 2005). Furthermore, numbers of
escaped farmed salmon within rivers are sufficiently high
that interbreeding between farmed and wild salmon
(hereafter, wild-farmed interbreeding) may be widespread.
Morris et al. (2008), for example, reported that farmed
Atlantic salmon have been detected in 87% of monitored
rivers within a 300 km radius of most aquaculture activity
in eastern North America.
Differences between farmed salmon and their wild
counterparts are often sufficient to raise concerns that wild-
farmed interbreeding may have detrimental ecological
effects on wild populations (McGinnity et al. 2003; Jons-
son and Jonsson 2006). The wild-farmed hybrid offspring
may, for example, harbour maladapted traits that were
inherited from the farmed parent which may lead to a lower
fitness in the natural environment (Fleming et al. 2000;
McGinnity et al. 2003; Fraser et al. 2008). This reduction
in fitness is thought to have its greatest effect on small or
declining wild populations, leading to a decrease in pop-
ulation growth rate and, possibly, local extirpation
(Hutchings 1991; Hindar et al. 2006).
One means by which fitness can be reduced is through a
reduction of anti-predator responses. Several studies have
shown that farmed salmon exhibit reduced anti-predator
responses relative to wild fish (Johnsson and Abrahams
1991; Einum and Fleming 1997; Fleming and Einum
1997). The anti-predator response in these studies was
measured as the time it took for an early-life history salmon
to resume foraging after a simulated predator attack. A
faster time to resume foraging can be interpreted as an
increased willingness to expose oneself to a predator
(Johnsson and Abrahams 1991). The reduced response
within groups of farmed salmon may occur for several
reasons (Fleming and Einum 1997). First, culture envi-
ronments experience little natural predation and the anti-
predator response may be lost over time. Furthermore, the
farmed fish experience considerably higher densities than
those experienced in the wild, thus favouring increased
aggression and reduced caution. Farmed salmon may also
produce more growth hormone relative to wild salmon (e.g.
Fleming and Einum 1997; Fleming et al. 2002), potentially
increasing the need to forage to a greater degree to meet
higher energy demands.
Limited research has examined the extent to which anti-
predator responses differ among populations, or among
multi-generational wild-farmed hybrid crosses (i.e. F2or
greater generations; Einum and Fleming 1997; Fleming
and Einum 1997; Tymchuk et al. 2006). Furthermore,
despite extensive evidence that farmed salmon are escaping
in eastern North America (Morris et al. 2008), that aqua-
culture salmon production continues to increase in the
region, and that many of the wild populations located near
these farms are endangered or otherwise declining
(COSEWIC 2006), no studies have examined the anti-
predator responses of farmed and wild salmon in the
region. Evaluations of the differential effects of multi-
generational interbreeding between escaped farm salmon
and divergent wild populations are critical for assessing the
overall risks posed to wild salmon on species-wide scales
(Hutchings and Fraser 2008).
Our study’s objectives were thus to test three hypothe-
ses: (1) that wild salmon had greater anti-predator
responses relative to farmed salmon; (2) that two genera-
tions of interbreeding between farmed salmon and wild
salmon resulted in reduced anti-predator responses in
hybrid salmon relative to wild salmon; (3) that the degree
of reduction in anti-predator responses of hybrids relative
to wild salmon differed between two different river
populations.
Materials and methods
Generation of the crosses
Parental fish were obtained from two wild populations from
Nova Scotia, Canada: Stewiacke River, in the Inner Bay of
Fundy (45? 80N; 63? 220W), and Tusket River, from the
Southern Upland region (43? 510N; 65? 580W). Stewiacke
River parents were captured as wild parr and raised in
captivity until maturity; Tusket River parents were cap-
tured as adults. The farmed strain was originally derived
from the Saint John River, New Brunswick (45? 150N; 66?
30W), and had experienced four generations in aquaculture
at the time our study was initiated in 2001 (Glebe 1998).
Wild Stewiacke (STEW), wild Tusket (TUSK), farmed
(FARM), and F1hybrids (F1STEW-FARM and F1TUSK-
FARM: wild 9 farmed) crosses were generated in
November and December of 2001 at the Aquatron Facility,
Dalhousie University (Fig. 1). These crosses were raised
under common environmental conditions until maturity to
ensure that any behavioural differences observed during
the experiment would be attributable to genetic differences
and not to environmental differences (Fig. 1).
Additional crosses were generated in November and
December 2005, using the 2001 crosses as parents (Fig. 1;
Table 1). Prior to generating the 2005 crosses, all adults
from the 2001 crosses were individually tagged and fin
clipped. DNA was extracted from the fin clips and used to
identify the genotypes at five polymorphic microsatellite
loci. The genotype information was used for parentage
assignments and the assignments were used to avoid inbred
matings (i.e. full-sib, half-sib or cousin matings) in the
generation of the 2005 crosses. The 2005 crosses permitted
the generation of multigenerational hybrids, i.e. F2hybrids
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(F2STEW-FARM and F2TUSK-FARM: F1hybrid 9 F1
hybrid), and wild backcrosses (BC1 STEW-FARM and
BC1TUSK-FARM = wild 9 F1hybrid) and thus multi-
generationalcomparisons
between farmed and both wild populations (Fig. 1). Each
consisted of 9–23 chiefly full-sibling families (Table 1),
except for TUSK and both F2hybrids in which each female
was crossed to two or three different males. The same
parents were used in the generation of multiple crosses.
This was necessary to control for female and male repre-
sentation when comparing trait means between crosses, as
parental effects (especially maternal effects) are prevalent
in Atlantic salmon (Green 2009).
In May 2005, due to space limitations, all families were
pooled within each cross and 500 fish randomly selected
from each cross pool were then transferred to one of four or
five replicate 100 l tanks for a total of 41 tanks, such that
inanti-predator responses
tanks had even densities and even numbers of individuals
per family within each cross. Fry were periodically culled
at random as they outgrew the space. Fish densities were
maintained at equal levels among tanks until the time at
which our behavioural comparisons were made.
Predator tank and predator model design
A predator tank was constructed to measure the anti-
predator response of salmon fry (age 0? parr, 113–
197 days post yolk absorption; Fig. 2). The predator tank
consisted of four testing
36 cm 9 50 cm. Given that fry generally prefer to main-
tain a position on gravel (Gibson 1993), each section
contained a gravel patch (3–10 mm particle size) in addi-
tion to a hiding area (i.e. a refuge) in one of the corners
constructed out of a piece of ABS pipe that had been cut in
half. Fry tend to prefer mean water velocities of 10–
40 cm s-1and depth between 20–40 cm in Nova Scotia
and New Brunswick rivers (Morantz et al. 1987). Attempts
were made to design the predator tank with these charac-
teristics but, due to logistic constraints, our water velocity
and depth were 11 cm s-1and 11 cm, respectively. A
spray bar was positioned at the bottom of the section where
it dispensed water directly over the gravel patch and any
excess water overflowed an 11 cm wall of opaque acrylic.
A cardboard barrier with viewing slits was erected around
the tank to minimize any visual disturbance of the fry.
An aerial predator model was used because aerial pre-
dators have been shown to induce a greater anti-predator
response than in-stream predators (Gotceitas and Godin
1993). A belted kingfisher (Ceryle alcyon) was selected
because it is a natural predator of salmon fry in Nova
sections,each measuring
Fig. 1 Source populations, crosses generated in 2001 and 2005, and
rearing conditions of 2001 crosses that were used as parents in the
generation of crosses in 2005 for the present study
Table 1 Experimental crosses generated in 2005 and the number of families per cross
CrossNumber of families Number of femalesNumber of males
STEW
BC1STEW-FARMa
F1STEW-FARMb
F2STEW-FARM
TUSK
BC1TUSK-FARMc
F1TUSK-FARMd
F2TUSK-FARM
FARM
999
191313
1513 14
23711
1249
1210 12
1189
1478
1188
STEW, Stewiacke; TUSK, Tusket; BC1, wild backcross; F1, F1hybrid; F2, F2hybrid
aDerived from a mixture of the same F1STEW-FARM$/# used to generate F2hybrid families and mainly different STEW $/# (than in parental
STEW crosses)
bDerived from the a mixture of the same FARM$/# (n = 8/5) and STEW$/# (n = 5/9) used to generate parental cross-types
cDerived from an even mixture of the same TUSK$/# and F1TUSK-FARM$/# used to generate TUSK and F2hybrid families
dDerived from the same FARM$ and TUSK# used to generate parental cross-types
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Scotia (Cairns 1998). The dimensions and colours for the
belted kingfisher model were acquired from preserved
specimens at the Natural History Museum of Nova Scotia.
The final model was 30 cm 9 37 cm and was constructed
out of 20 gauge galvanized wire and papier ma ˆche ´. The
model was painted to resemble a female belted kingfisher,
using acrylic paint, and covered in several coats of interior
varnish to provide the model limited water resistance.
Measurement and analysis of the anti-predator
responses
Between September and November 2006, 20 fry were
randomly chosen from pooled family tanks (four to five
random fry from each of the four or five replicate pooled
family tanks) of each cross (total N = 180) to assess their
anti-predator responses. Again, there were several inde-
pendent families within a cross, and the fry used were a
random subsample of the equally pooled families, thus the
lack of independence should be minor. Each fry was
measured (nearest mm) and weighed (nearest 0.01 g)
before being placed into one of the four testing sections of
the predator tank. Observations of fry for each cross were
distributed evenly among the four testing sections. Each fry
was allowed to acclimatize to the tank for 24 h with 10 h of
light and no food. Before the fry was ‘attacked’ with the
kingfisher, the position of the fry (on the gravel patch,
partially inside the refuge, or completely inside the refuge)
was noted.
The fry was then ‘attacked’ (i.e. lunging the beak of the
model kingfisher towards the fry) until it entered the refuge.
The operator of the kingfisher model was hidden behind the
cardboard barrier to limit the exposure of the fry to the
operator. The number of attacks required before fry entered
the refuge was recorded and the timer was started once the
fry entered the refuge. Food pellets (N = 2–4) were drop-
ped into the tank every minute where they circulated in the
water and were visible to the fry in the refuge.
The refuge time was recorded when the fry completely
left the refuge to forage or to return to the gravel patch.
That is, the refuge time was calculated as the time from the
entry into the refuge to the complete exit of the refuge (c.f.
Einum and Fleming 1997; Fleming and Einum 1997). On
occasion, fry would only partially emerge from the refuge
such that only their heads were visible. This period of
partial emergence was termed ‘wait time’. Fry that did not
exit the refuge after 20 min had their refuge time recorded
as 20 min.
All statistical analyses comparing the crosses for the fry
position before the kingfisher attacks, number of kingfisher
attacks, refuge time, and wait time were performed in R
2.7.0. (available at http://www.r-project.org/). Statistical
significance was set at the a = 0.05 level. Joint-scale
analysis (described in Lynch and Walsh 1998) was also
used to test the null hypotheses of additive (A) and additive-
dominance (AD) inheritance of anti-predator responses
between crosses in each wild-farmed comparison. A like-
lihood ratio test subtracting the A test statistic (v2) from the
AD test statistic was then used to determine which model
best fit the data. Epistatic effects could not be estimated
using joint-scale analysis because of the limited number of
degrees of freedom imposed by the number of crosses.
Results
Fry position in the predator tank prior to the kingfisher
attack
Prior to attack by the kingfisher model, fry were either
positioned on the gravel patch, partially inside the refuge,
or completely inside the refuge. There were no significant
differences between crosses for the number of fry posi-
tioned completely inside the refuge (Kruskal–Wallis
one-way ANOVA, P = 0.434), thus the number of fry
positioned completely inside the refuge and the number of
fry positioned partially inside the refuge were pooled
together. There were significant differences in the propor-
tion of the fry positioned on the gravel patch and the pooled
observations (completely/partially inside the refuge) for all
crosses except for STEW and BC1 STEW-FARM fry
(Fig. 3).
Number of kingfisher attacks
All fry eventually entered the refuge after being attacked
by the kingfisher (mean number of attacks ± 1SD = 2.89
Fig. 2 Predator tank design for measuring the anti-predator response
of fry. Arrows represented the direction of water flow A gravel patch,
B ABS refuge, C spray bar at the bottom of the tank, D acrylic wall
and the water over flow area
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± 2.60). There was no relationship between cross and the
number of kingfisher attacks required for the fry to enter
the refuge (Kruskal–Wallis one-way ANOVA, P = 0.687).
Refuge time and wait time
The shortest mean refuge time and second lowest variance
was observed for the farmed fry (Fig. 4). Conversely, the
longest mean refuge times, coupled with high variances,
were observed for the wild fry in both populations. The
hybrid crosses were intermediate in refuge time and asso-
ciated variance to those of the parental populations (pure
farmed or wild). There were significant differences in the
median and mean refuge times between the crosses
(Table 2). The ranks from shortest to longest mean refuge
time for the Stewiacke-farmed crosses were: FARM\F2
STEW-FARM\BC1STEW-FARM\F1STEW-FARM
\STEW. The ranks from shortest to longest mean refuge
time for the Tusket-farmed crosses were: FARM \ F1
TUSK-FARM\F2TUSK-FARM\BC1TUSK-FARM\
TUSK. Excluding the farmed data, there was also a mar-
ginally significant population 9 cross interaction in refuge
time (Factorial ANOVA, P = 0.092). In addition, there
were no statistically significant differences among the
coefficients of variation (CVs) for the Stewiacke-farmed
crosses (Homogeneity of CV test, Zar 1999, P = 0.269)
and Tusket-farmed crosses (P = 0.342).
In both populations, there was a trend for a reduced
mean refuge time in hybrid crosses relative to the wild
crosses as the amount of farmed genes in the hybrid
crosses increased.Accordingly,
revealed that the refuge time anti-predator responses in
both wild-farmed comparisons fit an additive inheritance
model (Stewiacke-farmed: v2
joint-scaleanalysis
A¼ 0:455, df = 3, P = 0.929;
Fig. 3 Fry positions in the
predator tank prior to the
kingfisher model attacks. Open
bars are the pooled observations
for the positions of completely
inside the refuge and partially
inside the refuge. Filled bars are
the observations of the position
on the gravel patch.
Percentages reflect the amount
of farmed genes in the crosses.
*Identify the crosses exhibiting
significant differences between
the two pooled and gravel patch
positions, using two-sided tests
for the equality of proportions
Fig. 4 Boxplots of refuge time
for the various fry crosses. Box
area is proportional to the
dispersion in the data. The bold
line in the box represents the
median and the star represents
the mean. Dots represent
outliers that were included in
analyses. The upper and lower
horizontal line connected to the
box by dashed lines represent
the largest and smallest
observation, respectively, that
was not an outlier. Percentages
represent the amount of farmed
genes in the crosses
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Tusket-farmed: v2
of dominance inheritance did not significantly increase the
fit in either comparison (Stewiacke-farmed: v2
0:004, df = 1, P = 0.951; Tusket-farmed: v2
0:210, df = 1, P = 0.647).
The number of fry that were observed with a wait time
was not significantly different among the crosses (Kruskal–
Wallis one-way ANOVA, P = 0.434). Fry that did not
have an observation of wait time were excluded from
analyses comparing the wait
(remaining N = 120). There were significant differences in
the median and mean wait times between the Stewiacke-
farmed crosses and Tusket-farmed crosses (Table 2).
However, after excluding the farmed data, there was no
significant population 9 cross interaction (Factorial
ANOVA, P = 0.551). Furthermore, there were no statis-
tically significant differences among the CVs between the
Stewiacke-farmed crosses (Homogeneity of CV test,
P = 0.349) and Tusket-farmed crosses (P = 0.842).
A¼ 0:258, df = 3, P = 0.968). Inclusion
A? v2
A? v2
AD¼
AD¼
timesamong crosses
The relationship between wait time and cross was
similar to that between refuge time and cross, i.e. an
increase in the amount of farmed genes in a cross was
associated with shorter mean wait time in the hybrid
crosses relative to wild fish for both wild populations
(Fig. 5). Accordingly, joint-scale analysis revealed that the
wait time anti-predator responses in both wild-farmed
comparisons fit an additive inheritance model (Stewiacke-
farmed: v2
v2
inheritance did not significantly increase the fit in either
comparison (Stewiacke-farmed: v2
1, P = 0.843; Tusket-farmed: v2
P = 0.985).
There was no relationship between the number of
kingfisher attacks and refuge time or wait time for any of
the crosses (Table 3). That is, a higher number of king-
fisher attacks was not associated with a heightened anti-
predator response (i.e. an increase in either refuge or wait
A¼ 0:152, df = 3, P = 0.985; Tusket-farmed:
A¼ 0:135, df =3, P = 0.987). Inclusion of dominance
A? v2
AD¼ 0:0003, df = 1,
AD¼ 0:039, df =
A? v2
Table 2 Non-parametric and parametric one-way ANOVAs with associated post-hoc tests
Cross comparison Kruskal–Wallis one-way ANOVAOne-way ANOVA
Refuge time Wait timeRefuge timeWait time
Stewiacke-farmed comparisons0.001*0.010*0.001*0.005*
STEW–BC1STEW-FARM
STEW–F1STEW-FARM
STEW–F2STEW-FARM
STEW–FARM
0.4750.930 0.3930.867
0.9870.998 0.9510.955
0.016* 0.869 0.023*0.821
0.005*0.006*0.002*0.004*
BC1STEW-FARM–F1STEW-FARM
BC1STEW-FARM–F2STEW-FARM
BC1STEW-FARM–FARM
F1STEW-FARM–F2STEW-FARM
F1STEW-FARM–FARM
F2STEW-FARM–FARM
Tusket-farmed comparisons
0.7930.9890.8300.999
0.6141.000 0.694 1.000
0.404 0.144 0.270 0.035*
0.074 0.9660.1390.997
0.029*0.026* 0.023*0.024*
0.9980.209 0.9530.045*
0.010*0.016* 0.012*0.008*
TUSK–BC1TUSK-FARM
TUSK–F1TUSK-FARM
TUSK–F2TUSK-FARM
TUSK–FARM
1.000 0.9571.000 0.999
0.275 0.951 0.2350.797
0.432 0.767 0.4890.622
0.029*0.045*0.030*0.028*
BC1TUSK-FARM–F1TUSK-FARM
BC1TUSK-FARM–F2TUSK-FARM
BC1TUSK-FARM–FARM
F1TUSK-FARM–F2TUSK-FARM
F1TUSK-FARM–FARM
F2TUSK-FARM–FARM
0.2550.6840.251 0.564
0.4080.4170.5120.362
0.026*0.010*0.033*0.006*
0.9980.993 0.990 0.998
0.899 0.3740.899 0.221
0.7690.6860.656 0.345
Non-parametric Kruskal–Wallis one-way ANOVAs with associated Nemenyi–Damico–Wolfe–Dunn pair-wise tests of significant median dif-
ferences and parametric one-way ANOVA with associated Tukey honest significant differences test of significant mean differences. Data were
natural log transformed to increase normality for the parametric one-way ANOVA. Pair-wise statistical tests employed multiple comparison P
value adjustments
* Identify the tests exhibiting significant differences at the a = 0.05 level
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time). In addition, the difference between refuge time and
wait time did not differ significantly between crosses
(Kruskal–Wallis one-way ANOVA, P = 0.464). That is,
certain crosses were not waiting significantly longer at the
edge of the refuge before completely emerging in com-
parison to other crosses. None of the variables measured
(fry position, refuge time, wait time, and number of king-
fisher attacks) was correlated with fry length or mass
(Table 4). There were also no tank effects on any of the
variables measured (Table 4).
Discussion
Differences in anti-predator responses between wild
and farmed fish
Farmed fry emerged from their refuges earlier than those
from both wild populations. The same trend was observed
for wait time, thus suggesting a genetic link between wait
time and refuge time. If the refuge time of the wild fry
reflects an adaptive response, then the reduction in refuge
time, as observed for the hybrids, may translate to a
reduction in hybrid fitness. The behavioural differences
between farmed and wild salmon were most likely genet-
ically-based because the fish had been raised in a common
environment and there was no correlation between any of
the behaviours and length or mass. However, the reduced
refuge time response in farmed fry could be related to the
different ancestry of the fry and not necessarily to artificial
or inadvertent selection. That is, our study did not test
directly for the effects of ancestry versus the effects of
domestication because wild Saint John River salmon were
not tested. Alternatively, some studies have suggested that
fish that produce more growth hormone (and have a faster
growth rate as a consequence) have a more reduced anti-
predator response because they have a higher need to for-
age and may be more willing to take risks (e.g. Johnsson
and Bjo ¨rnsson 1994; Johnsson et al. 1996; Fleming and
Einum 1997). Nevertheless, differences in refuge time
between farmed and wild fry in the present study could not
be explained by differences in growth rate. Notably, wild
Tusket fry and parr grew marginally faster than farmed fry
and parr (Lawlor 2003; D. J. Fraser and J. A. Hutchings,
Fig. 5 Boxplots of wait time
for the various fry crosses. The
symbols are the same as those
described in the caption for
Fig. 4
Table 3 Spearman’s rank correlation P values between the number
of kingfisher attacks and time variables
CrossRefuge timeWait time
STEW0.4910.261
BC1STEW-FARM
F1STEW-FARM
F2STEW-FARM
TUSK
0.768 0.852
0.2350.956
0.5920.374
0.2720.151
BC1TUSK-FARM
F1TUSK-FARM
F2TUSK-FARM
FARM
0.4690.962
0.8380.782
0.1400.625
0.7000.192
Critical P value after Bonferroni correction was 0.006
Table 4 Spearman’s rank correlation P values between selected
variables and length and mass and nested one-way ANOVAs for tank
effects
VariableLengthMass Tank
Fry position0.9490.945 0.646
Refuge time0.1570.0710.079
Wait time0.229 0.161 0.342
Number of kingfisher attacks0.084 0.1890.693
Critical P value after Bonferroni correction was 0.013
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unpublished data), yet the former had longer refuge times.
In addition, the wild populations did not differ in their anti-
predator responses in terms of refuge time, wait time, and
coefficient of variation suggesting that, at least at the
geographic scale examined in this study, they exhibited
similar anti-predator responses. Collectively, these results
suggested that selection may have contributed to a reduc-
tionofanti-predatorresponses
environment.
in theaquaculture
Conserved aspects of the anti-predator responses
Salmon fry from different crosses did not differ signifi-
cantly in the number of kingfisher attacks it took for them
to retreat to the refuge nor in the number of observations of
wait times. Furthermore, the difference between wait time
and refuge time did not differ among crosses. That is, wild
fry did not spend a longer period of time at the edge of the
refuge before emerging than the other crosses. These
results suggest that some wild elements of the anti-predator
response, such as the initial reaction time to the predator
and the time taken to observe one’s surroundings before
leaving a refuge, have been retained in farmed and hybrid
salmon, despite the potential for artificial or inadvertent
selection in captivity to select for more ‘risk taking’
behaviours. Interestingly, using the same methodology,
Einum and Fleming (1997) and Fleming and Einum (1997)
also documented a lack of differentiation in the time it took
population crosses to enter a refuge (wild, farmed, and F1
hybrids; Imsa and Lone River wild populations from
Norway).
Effects of wild-farmed hybridization on anti-predator
responses
In both wild-farmed comparisons, hybridization generated
F1and F2hybrids that exhibited intermediate (i.e. additive)
refuge and wait times relative to parental populations.
These results were consistent with those reported from the
two other studies that have compared salmonid refuge
times between wild-farmed hybrids and their parental
populations (Einum and Fleming 1997; Tymchuk et al.
2006). In addition, the coefficients of variation as a stan-
dardized measure of dispersion did not differ among the
crosses. Thus, despite changes to the mean anti-predator
response in hybrids, we found no evidence that hybrid-
ization increased or decreased the dispersion around the
means. Variation may have increased if there was increased
non-additive genetic variation in the crosses between
populations without fixed allelic differences or if there was
the recombination of genotypes in the crosses; variation
may have decreased if the crosses had been between pop-
ulations with fixed allelic differences (Edmands 1999).
We did, however, find some evidence, albeit limited,
that the magnitude of the effects of wild-farmed inter-
breeding on anti-predator responses differed between wild
populations for refuge time. That is, of the two wild-farmed
comparisons, the differences in average refuge times were
greatest between Stewiacke versus Stewiacke-farmed
hybrids. Although our data suggested a strong additive
component to the inheritance of anti-predator responses,
the failure to reject an additive model could also be
attributed to the large variance in the data. In addition,
epistasis could not be tested for directly due to the low
number of crosses which limited the number of degrees of
freedom.
There were also some other nuances in the effects of
hybridization on fry behaviour between the wild popula-
tions. Notably, wild Stewiacke and BC1Stewiacke fry did
not exhibit a strong preference for remaining on the gravel
patch during the acclimatization period relative to wild
Tusket fry or other crosses. This was somewhat unexpected
given that fry have been reported to prefer to maintain a
position on gravel (Gibson 1993). Although it is interesting
to speculate that the differences in preference for holding
on gravel patches might reflect adaptive differentiation
between Tusket and Stewiacke juvenile environments (c.f.
Taylor 1991; Garcia de Leaniz et al. 2007), we are unaware
of any major differences in their habitats that might
account for the differences observed here.
Possible consequences to wild fish populations
Wild-farmed hybrids exhibited reduced anti-predator
responses relative to wild fry. As a consequence, hybrid fry
may be more at risk of predation in the wild, contributing
to a reduction in mean fitness in the wild population,
especially if the rate at which wild-farmed hybridization
occurs is considerable. Fleming et al. (2000) estimated a
70% early survival of F1farmed-wild hybrid Atlantic sal-
mon relative to wild salmon in a native river. The authors
attributed reduced anti-predator responses as one possible
reason for the lower survival of hybrids (c.f. Einum and
Fleming 1997; Fleming and Einum 1997). Conversely,
McGinnity et al. (2004) found that, over several genera-
tions of juvenile hatchery-rearing, hatchery salmon from
the same genetic origin as wild salmon did not exhibit
differential adult survival relative to wild fish. This was
despite genetic changes in the hatchery fish in growth, age
of smolting, and other life history aspects. In addition, the
hatchery salmon program in McGinnity et al. (2004) did
not intentionally select for economically beneficial traits,
such as increase growth rate and delayed maturity, as in
farmed salmon programs. Nevertheless, the hatchery-
reared salmon had been exposed to natural predators as
post-juveniles each generation, where as farmed fishes
792Conserv Genet (2010) 11:785–794
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spend their entire life-cycle in captivity and are not
exposed to predators. Furthermore, farmed salmon are not
exposed to natural selection via the exposure to predators
before they reproduce, and this could be an explanation for
the reduced anti-predator responses observed in our farmed
fish that were raised in captivity for several generations.
A loss of hybrid fry through predation could affect wild
populations indirectly through a reduction in overall pop-
ulation size (Einum and Fleming 1997). It could also
directly affect wild populations if the increased catchability
of hybrid fry by predators attracts more predators into the
area. Furthermore, with a trend of less variation in the
behavioural template than wild fish, hybrids would be
expected to have, on average, a reduced capacity to adapt
to the changes in the environment relative to wild fry
(Freeman and Herron 2004).
With an increasing number of generations of artificial
selection in the aquaculture environment, it is also expected
that farmed fish will become more genetically and
behaviourally distinct from wild populations (Gross 1998;
Verspoor 1998; Hutchings and Fraser 2008). Given that the
farmed Saint John River strain is now in its sixth generation
of aquaculture (Quinton et al. 2005), but was only in its
fourth generation of aquaculture in this study, it is expected
that the hybridization between escaped farmed salmon
currently used in aquaculture and wild salmon in Eastern
Canada may result in an even greater reduction of anti-
predator responses and, thus, fitness in wild-farmed hybrids.
Our results contribute to the growing number of studies
that have documented genetically-based behavioural dif-
ferences in farmed and wild-farmed hybrids relative to wild
fish. The present study is distinct, however, in illustrating
the importance of examining multiple wild populations for
risk assessment. The effects of multi-generational wild-
farmed hybridization varied somewhat for different wild
populations. Greater reduction of anti-predator responses in
wild-farmed hybrids relative to the wild state for some wild
populations may pose a greater risk to the persistence and
recovery of these populations relative to wild populations
with lesser degrees of reduction in such responses. Perhaps
more importantly, the effects of hybridization on anti-
predator responses were consistently in a direction that is
likely to be disadvantageous in the natural environment for
both wild populations.
Acknowledgments
Sciences and Engineering Research Council (Canada) through an
undergraduate student research award to ASH, a post-doctoral fel-
lowship to DJF, and a Strategic Grant to JAH. ASH was also
supported by an Atlantic Salmon Federation Olin Fellowship. We
thank at Dalhousie University J. Eddington (Aquatron facility), M.
Merrimen, R. Myers, K. Tae, L. Weir, and P. Debes. We also thank P.
Amiro (Department of Fisheries and Oceans Nova Scotia Region),
J.-G. Godin (Carleton University, Ottawa), A. Hebda (Natural History
Museum of Nova Scotia), A. D-S. Houde (Saint Mary’s University,
The work was supported by the Natural
Halifax), the constructive comments of two anonymous reviewers,
and E. Anderson. Petrie the belted kingfisher.
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