ICES Journal of Marine Science, 63: 246e252 (2006)
doi:10.1016/j.icesjms.2005.11.007
Body weight, sexual maturity, and spinal deformity in strains
and families of Atlantic cod (Gadus morhua) at two years of
age at different locations along the Norwegian coast
K. Kolstad, I. Thorland, T. Refstie, and B. Gjerde
Kolstad, K., Thorland, I., Refstie, T., and Gjerde, B. 2006. Body weight, sexual maturity,
and spinal deformity in strains and families of Atlantic cod (Gadus morhua) at two years
of age at different locations along the Norwegian coast. e ICES Journal of Marine Science,
63: 246e252.
Body weight, occurrence of spinal deformity, and sexual maturity were recorded in 51 fullsib families of the strains coastal and Arctic cod at two years of age. The family groups
were located at three sites along the Norwegian coast including Hordaland, Møre and
Romsdal (M&R), and Nordland County to detect genetic variation in important production
traits and to investigate interactions between genetic composition and production environment. Body weight varied among locations partly owing to different production conditions.
There were also large differences among the locations with respect to spinal deformity.
M&R had the highest occurrence of spinal deformity ( p < 0.001). Comparison of sexual
maturity among the locations was made difficult owing to the different ways the trait was
recorded. Only small differences were found between coastal and Arctic cod in spinal deformity ( p < 0.05 in Hordaland) and sexual maturity ( p ¼ 0.06 in M&R), while no differences were found for body weight. Heritability estimates for body weight (0.51), spinal
deformity (0.27), and sexual maturity (0.21) indicate the potential for improvement of all
three traits by selective breeding using a family-based selection programme. Final recordings at the end of the growing period will provide further information. Genetic correlations
estimated between weight and occurrence of spinal deformity (rg ¼ 0.50) suggest that caution be used when selecting for growth, and that a need exists for including spinal deformity
in the selection index. No significant correlations were found between these two traits and
the incidence of sexual maturity.
Ó 2005 International Council for the Exploration of the Sea. Published by Elsevier Ltd. All rights reserved.
Keywords: Arctic cod, Atlantic cod, body weight, coastal cod, genetic variation, selective
breeding programme, sexual maturity, spinal deformity.
Received 21 September 2004; accepted 15 November 2005.
K. Kolstad, I. Thorland, T. Refstie, and B. Gjerde: AKVAFORSK Genetics Centre AS,
N-6600 Sunndalsøra, Norway. Correspondence to K. Kolstad: tel: þ47 64948095; fax:
þ47 64949502; e-mail: [email protected].
Introduction
The dramatic increase in the interest in intensive production
of Atlantic cod (Gadus morhua) during the previous two
decades has been driven mainly by market considerations
(Tilseth, 1990; Rosenlund and Skretting, 2006). However,
there are challenges related to making production cost efficient, including the genetic aspects, which have been shown
to have great potential for increasing production efficiency
in aquaculture (Gjedrem, 1997, 1998). Before the goal of
a selective breeding programme is chosen, an investigation
must be made of the magnitude of the genetic variation for
important economic traits among, as well as within, available Atlantic cod populations. Some studies present estimates of genetic variation based on differences in
population level to estimate important economic traits in
1054-3139/$30.00
juvenile Atlantic cod (Gamble and Houde, 1984; Svåsand
et al., 1994; van der Meeren et al., 1994; Otterlei et al.,
1999; Suthers et al., 1999; van der Meeren and Jørstad,
2001; Gjerde et al., 2004). Studies also compare growth
performance of Norwegian coastal cod and Northeast
(NE) Arctic cod until first sexual maturity at two years of
age (Svåsand et al., 1996). Only the study of Gjerde
et al. (2004) presents estimates of genetic variation and heritabilities based on full- and half-sib family recordings.
That study found significant heritability estimates for juvenile growth but not for survival.
A strain-specific breeding programme for coastal and
Arctic cod may be needed in Norway. To determine this,
one needs to know whether coastal and Arctic cod differ
in important traits like growth, time to attain sexual
maturity, and flesh/taste quality. The work of van der
Ó 2005 International Council for the Exploration of the Sea. Published by Elsevier Ltd. All rights reserved.
Body weight, sexual maturity, and spinal deformity in strains and families of Atlantic cod
Meeren and Jørstad (2001) on Norwegian coastal and Arctic
cod did not indicate local adaptation by the two populations.
In other studies on Norwegian stocks (Otterlei et al., 1999),
coastal cod grew better than Arcto-Norwegian, in contrast to
the hypothesis of faster growth for northern than for southern stocks (Conover et al., 1997). Brown et al. (2003) suggest this may be because the growing season or mean overall
temperature does not differ much along the coast of Norway.
Also, there may be additional marketing issues with Arctic cod suggesting that separate selective breeding programs
for these two strains may be required. That coastal cod actually contain several distinct sub-populations along the coast
and in the fjords, and that there may be a mixing of fish
among coastal and Arctic cod, indicate that the debate
over one population of coastal and one population of Arctic
cod may be oversimplifying the situation. Therefore, finding
differences between coastal and Arctic cod may be difficult
and will depend on valid verification of the two populations.
The existing method of genotyping for the scnDNA pantophysin locus can be used for this verification. This locus
has two common alleles, Pan IA and Pan IB, the former dominant in the Norwegian coastal cod population, and the latter
dominant in the NE Arctic cod population (Fevolden and
Pogson, 1997). In Norwegian coastal cod, the frequency of
the Pan IA allele increases from north to south along the Norwegian coast (Fevolden and Sarvas, 2001).
The magnitude of the interaction between genotype and
environment (G E) should also be quantified to clarify
the need for more than one selective breeding programme.
Further information is needed to determine whether Arctic
cod will perform better than coastal cod in northern regions,
while the situation may be reversed in the south. If this
proves correct, these two populations may be kept separate
and bred for performance in different regions and for different markets. Moreover, if performance in south vs. north results in re-ranking of family groups, location-specific
selective breeding programmes will need to be considered.
Rearing several full- and half-sib families of each cod population under the same environmental conditions makes
a full quantitative genetic study possible.
The aim of this study was to investigate genetic differences for age-corrected weight, sexual maturity, and spinal
deformity between coastal and Arctic cod to determine if
more than one selective breeding programme for cod is
needed in Norway. Further, the aim was to conduct quantitative genetic analysis of the traits to evaluate the potential
for achieving genetic progress in a selective breeding
programme.
Material and methods
Cod families
In 2002 using paired mating, 51 cod families were produced from wild-caught broodstock from different locations
along the Norwegian coast. Maternal and a few paternal
247
half-sib families were obtained, after the first mating was
successful, by replacing a parent with a different one in
some of the paired mating tanks (Gjerde et al., 2004), providing a 1:2 nested design. As described in detail in Gjerde
et al. (2004), broodstock originated from two regions. Region 1 broodstock represent the Norwegian coastal cod
population, and originated from six sub-populations. Region 2 broodstock represent the Northeast Arctic cod population. This was verified by genotyping all full-sib
families for the scnDNA pantophysin locus. This locus
has two common alleles, Pan IA and Pan IB, the former
dominant in the Norwegian coastal cod population and
the latter dominant in the NE Arctic cod population
(Fevolden and Pogson, 1997). Full-sibs were genotyped because tissue samples for genotyping were not obtained from
the parent fish. The genotyping, however, did show that
both populations, in particular the Region 2 broodstock,
contained individuals carrying both alleles, indicating little
expected genetic differences between the populations. Consequently, there may be no reason from a genetic point of
view to separate the Atlantic cod from these regions into
two populations.
In October 2002, at approximately seven months of age,
all families were PIT-tagged. Soon after, they were transferred to different locations along the Norwegian coast,
including Real Seafood in Hordaland County (approximately
61(N), Akvaforsk research station in Møre and Romsdal
(M&R) County (approximately 63(N), and Gildeskål in
Nordland County (approximately 67(N), for the purpose
of detecting interactions between genetic constitution and
production environment. In Hordaland, the rearing system
is land-based, while the fish are kept in sea cages in M&R
and Nordland. The units in Hordaland and Nordland
represented commercial production environments.
At the time of recording, there was an average of 30 fish
per family in M&R, 13 fish per family in Nordland, and 17
fish per family in Hordaland, resulting in a total number of
3071 fish. It can be shown that, when the value of T ¼ nN
is limited by the size of the experiment, where N is the
number of family groups, and n is the family size, the sampling variance of the intra-class correlation (t) is minimal
when n ¼ 1/t, approximately (Falconer, 1996). According
to this, family size in the present study should be sufficient for traits with heritabilities of approximately 0.1 and
more.
Recordings
Traits recorded included body weight, occurrence of spinal
deformity, and sexual maturity at two years of age. Body
weight was corrected for age by including age in the statistical model when body weight was the dependent variable.
Spinal deformity and sexual maturity were recorded as categorical traits for each individual (0 or 1).
The phenomenon of spinal deformity was observed by
experienced staff and defined as fish with a diverged shape
248
K. Kolstad et al.
of vertebral column that included three conditions: cyphosis, lordosis, and scoliosis. To avoid the influence of subjective recording of this trait, the spinal deformity was
expressed as ‘‘present’’ or ‘‘non-present’’. Any attempt at
making a graded scale for this trait would have increased
the subjectivity of the recording significantly. An indication
of sexual maturity in both sexes was a swollen abdomen.
When the abdomen was squeezed, eggs and milt would
be expressed from the females and males, respectively, permitting determination of sex.
Recording times varied among the locations: November
2003, March 2004, and April 2004 in Hordaland, Nordland,
and M&R, respectively. Sexual maturity was recorded in
different ways at the three locations. In Nordland and
M&R, it was recorded simultaneously with weight recordings, while in Hordaland it was recorded repeatedly during
March/April 2004. Fish were grouped into three classes:
male, female, and sex undetermined. Owing to variable
quality and methods of recording occurrences of sexual maturity at different locations, recordings of this variable from
M&R were excluded from the genetic analysis.
Environmental conditions
The production environment varied among locations. For
example, location-specific light and temperature regimes
at the different geographic locations provided one source
of variation. Further, one location used land-based rearing
systems. There were also differences in mortality and
escapement, resulting in different fish densities during the
growout period. The experiment’s design provided no possibility to estimate the effects of each of these conditions. In
the genetic analysis, the sum of these effects will represent
the sum of all environmental effects at each location.
Because all family groups are represented at all locations,
a sound genetic analysis can be conducted. Interaction
between genetic composition and the environment can
also be estimated. This interaction will provide information
about sensitivity to environmental effects in general,
although not for each factor. Re-ranking of strains or
families in different environments will suggest a detectable
sensitivity and the need for strain- or environmental-specific
breeding programmes.
Statistical analyses
A general linear model based on a factorial design was used
to estimate differences within and interactions among
strains and locations for the traits under investigation.
The fixed effect of sex, strain, location, and interaction between strain and location was included in the model (Model
1). Age at tagging nested within location was included as
a covariate in the model to correct for variation in age at
the time of recording. Families and strains could then be
compared, independent of age at each location. However,
comparison of location will be confounded with the effect
of age in addition to the other environmental factors previously mentioned. The error term including the individual
random error was the only random effect in the model.
The sexual maturation trait only included records from Hordaland and Nordland in the analysis, as records from M&R
were considered uninformative.
For genetic analysis of body weight, spinal deformity,
and sexual maturity, estimates of the fixed effects and variance components for the random effects in the model were
obtained using a mixed linear animal model (DMU; Madsen
and Jensen, 2002). In matrix notation, the model (Model 2)
can be written as y ¼ Xb þ Z1 a þ Z2 f þ e, where y is the
vector of individual body weights, spinal deformity, or
sexual maturity, b is a vector of fixed effects (i.e. the location, sex, and the covariate age at tagging nested within location), a is the vector of random additive genetic effect of
individual animals, f is the vector of random effects common to full-sibs caused by factors other than additive genetics (i.e. environmental effect caused by the separate rearing
of each full-sib family until tagging (tank effect), non-additive genetic effects, and maternal effects), and e is the vector
of individual random error effects. X, Z1, and Z2 are known
design matrices assigning observations to levels of b, a, and
f, respectively. The parents of the full-sib families were assumed to be unrelated, but the additive genetic relationship
matrix among all their offspring, i.e. full- and half-sib relations, was taken into account. The fixed effect of sex was
not included in the model when sexual maturity was analysed. Ten families were excluded from the data so as to obtain the best possible hierarchic structure and raise the
quality of the genetic analysis.
Heritabilities were estimated using single trait analysis,
while correlations were estimated using a multi-trait analysis based on Model 2. The estimated heritabilities are assumed to be narrow sense heritabilities when based on an
animal model and a hierarchic mating design. The assumption of homogeneous variance within subgroups was tested
and found to be true.
Results
Table 1 presents descriptive statistics for each of the three
locations. The traits showed homogenous variation at each
location. Both cage rearing systems lost a large proportion
of the fish mainly as a result of escapement during the period from tagging and transferral to each location to the
time of recording. Thus, differences in density will be included in the overall environmental effect as well. The dates
on which fish escaped from the cages during this period are
not available. Mean body weights at the three locations differed, in part, because of the time when measurements were
taken and the rearing conditions (Figure 1). The overall
mean body weight at two years of age was 1073 g. Overall,
mature females were heavier than immature fish, sexes
pooled ( p < 0.01) (Table 2). In Nordland, mature females
were significantly heavier than mature males and immature
Body weight, sexual maturity, and spinal deformity in strains and families of Atlantic cod
249
Table 1. Descriptive statistics of recordings of family groups of Atlantic cod at tagging and at two years of age. s.d.: standard deviation.
Nordland (n ¼ 698)
Recording
Mean
Number at tagging
Number at recording
Weight at tagging (g)
Age at tagging
Age at recording (d)
Body weight (g)
Sexual maturity (%)
Spinal deformity (%)
1 230
698
26.8
207.5
808
929
65
28
M&R (n ¼ 1518)
s.d.
Mean
9.2
9.0
9.0
268
48
45
2 150
1 518
26.0
205.9
794
1 209
5
74
Hordaland (n ¼ 834)
s.d.
Mean
s.d.
9.5
9.0
9.0
353
21
44
864
834/576*
27.3
203.7
630
677
88
68
9.6
8.7
8.7
217
32
47
*Number when sexual maturation was recorded.
cod ( p < 0.05), while in Hordaland the immature cod
weighed less than mature cod ( p < 0.001). Those with spinal deformities in Hordaland weighed more than those
without deformities ( p < 0.001).
Figures 1e3 present body weight, sexual maturity, and
spinal deformity of coastal and Arctic cod at two years of
age, respectively. There were significant differences among
the three locations ( p < 0.001) in the occurrence of spinal
deformities, with M&R having the highest and Nordland
the lowest proportion (Table 2 and Figure 3). The higher
body weight in M&R may have caused the higher proportion of spinal deformity. This is partially confirmed by
the higher average body weight of individuals with spinal
deformity and by the fact that mature females in Hordaland
had a higher occurrence of spinal deformity than immature
fish ( p < 0.05) (Table 2).
No significant differences were found among coastal and
Arctic cod in body weight at any of the three locations. In
M&R, the difference in proportion of fish sexually mature
between coastal and Arctic cod was close to significant
( p ¼ 0.06). Arctic cod showed a significantly greater
1400
Body weight (g)
1200
Coastal
Arctic
1000
800
600
400
200
0
Nordland
M&R
Hordaland
Location
Figure 1. Least square means and standard error of body weight
(g) for coastal and Arctic cod at two years of age at three locations.
occurrence of spinal deformity in Hordaland ( p < 0.05),
while no differences were found in this trait among the
strains at the other two locations.
Table 3 presents estimated heritabilities for the traits recorded at two years of age. High and significant heritabilities are found for body weight. The overall heritability of
0.51 is a strong indication that a large part of the observed
variation in body weight is the result of genetic variation.
This supports a large potential for genetic gain if this trait
is included in a systematic selection programme. The heritabilities estimated for spinal deformity and sexual maturity
are significant and of intermediate magnitude, suggesting
that genetic gain for these traits will also be possible in
a selective breeding programme.
A significant genetic correlation ( p < 0.001) was found
between body weight and deformity (Table 4). No significant correlations were found among these two traits and
incidence of sexual maturity (Table 4).
Discussion
As a result of our method of recording the incidence of sexual maturity, this variable may not be consistently represented at the three locations. The seasonal timing of
maturation will depend, in part, on their latitudinal distribution (see also Otterå et al., 2006). A low degree of sexual
maturity in M&R is a result of the fact that recording
was done at the end of the season. An earlier recording
most likely would have resulted in a greater proportion of
mature fish. Consequently, to properly examine this trait,
the seasonality of gonad development must be well defined
at all locations. As this can vary between years, the breeding populations should be given close attention as the mating season approaches. Repeated recordings are preferable
because the timing of spawning varies as well. This was
done in Hordaland. Thus, estimates based on these recordings can be considered quite reliable. Using more advanced
methods, such as biopsy or ultrasonography, will also improve estimates of staging gamete development and permit
250
K. Kolstad et al.
Table 2. Least square means (LSM) and standard error (s.e.) for body weight (g) and spinal deformity (frequency) of sexually mature and
immature Atlantic cod at two years of age and at three locations.
LSM (s.e.)
Trait
Nordland
M&R
Hordaland
Overall
Body weight
Sexually mature
Males
Females
Immature
906.0 (26.8)b
972.2 (24.5)a
911.2 (18.5)b
1 203.2 (48.5)
1 204.4 (50.0)
1 203.9 (8.7)
724.7 (18.9)a
750.8 (21.7)a
619.4 (17.7)b
942.9 (19.6)ab
973.9 (19.9)a
910.8 (14.5)b
Spinal deformity
With
Without
922.9 (22.5)
933.5 (14.6)
1 215.2 (26.9)
1 190.1 (24.3)
722.0 (20.8)a
671.4 (21.3)b
953.3 (13.0)
931.7 (12.4)
Spinal deformity
Sexually mature
Males
Females
Immature
0.33 (0.03)
0.26 (0.04)
0.26 (0.02)
0.68 (0.07)
0.69 (0.07)
0.74 (0.01)
0.53 (0.03)ab
0.59 (0.03)a
0.43 (0.06)b
0.51 (0.03)
0.51 (0.03)
0.48 (0.06)
Different letters in superscript within a column and trait indicate significant differences at a ¼ 0.05 significance level.
identification of immature fish by sex. In the present analysis, a relatively large proportion of cod were categorized
as immature. The quality of the comparison of mature vs.
immature fish was considered satisfactory. Efficient and accurate methods are required when large numbers of fish are
processed for a variety of traits. However, greater control
over the recording times of sexual maturity at each location
would improve the present study significantly. Estimating
genetic correlations for incidence of maturity measured
with the present method and more advanced methods is
also recommended.
The present results indicate no important strain differences at two years of age. These findings indicate that there
may be no important genetic differences between what is
considered to be two different types of Atlantic cod. The
similarity of coastal and Arctic cod was also confirmed
by genotyping for the scnDNA pantophysin locus, revealing that the two populations are partially mixed. As the genotyping was done on only 35 of the 51 families, genotyping
of all families is planned. A confirmation of the existing results is expected. Otterlei et al. (1999) reported better
growth in coastal than in Arctic cod. As the coastal cod
population consists of a group of several sub-populations,
it may be difficult to compare findings of different studies
accurately as the genetic composition of the populations
will vary among studies. The importance of the present
study is in understanding the genetic basis for establishing
the selective breeding programme for the population under
investigation consisting of coastal and Arctic cod. Although
the strains differ somewhat in spinal deformity and sexual
0,8
1
Proportion with spinal defomity
Proportion sexually mature
1
Coastal
Arctic
0,6
0,4
0,2
0
Nordland
M&R
Hordaland
Location
Figure 2. Least square means and standard error of proportion sexually mature for coastal and Arctic cod at two years of age at three
locations.
0,8
Coastal
Arctic
0,6
0,4
0,2
0
Nordland
M&R
Hordaland
Location
Figure 3. Least square means and standard error of proportion with
deformity for coastal and Arctic cod at two years of age at three
locations.
Body weight, sexual maturity, and spinal deformity in strains and families of Atlantic cod
Table 3. Estimated heritabilities (s.e.) for each trait overall and at
each location.
Overall
Nordland
M&R
Hordaland
Body weight
0.51 (0.10) 0.47 (0.13) 0.54 (0.10) 0.68 (0.12)
Spinal deformity 0.27 (0.07) 0.26 (0.09) 0.38 (0.09) 0.18 (0.09)
Sexual maturity 0.21 (0.04)* 0.29 (0.10)
0.16 (0.08)
*Nordland and Hordaland. Maturity data for M&R were not used
owing to low frequency and late recording.
maturity in Hordaland and M&R, respectively, this is not an
argument for establishing two breeding populations because both of these traits will be important in breeding.
However, there may be market reasons for establishing
strain-specific breeding programmes, as already suggested.
If all the 51 families can be considered as one breeding
population with one defined breeding goal, this will result
in maximum genetic gain compared with splitting the population into several subgroups according to strain or specific local environmental conditions, as a larger breeding
population increases selection intensity and reduces inbreeding. Even though keeping many strains may maintain
genetic variation, it is not recommended in an efficient
breeding programme, where the use of individual tagging
and pedigree information are used both to achieve genetic
progress and to keep inbreeding under control. Escapement
of cultivated cod, however, may be one good reason for
running location-specific breeding programmes, particularly
when generations of selective breeding create differences
between wild and cultivated cod (Bekkevold et al., 2006).
The high heritabilities found for body weight at two
years of age correspond well with those found by Gjerde
et al. (2004) in the same population at the age of tagging
(h2 ¼ 0.52). These estimates anticipate a large potential
for genetic gain if this trait is included in a systematic selection programme. The recordings at the end of the growing period will further address this. It is expected, however,
that recordings at two years of age will be highly correlated
with recordings at harvest. Much focus has been placed on
environmental factors that result in spinal deformities
(Brown et al., 2003; Hattori et al., 2004). The estimated
heritabilities in the present study show that there might
be important genetic factors influencing this trait. There
Table 4. Estimated genetic (above diagonal) and phenotypic (below
diagonal) correlations between weight, maturation rate, and spinal
deformity for Nordland and Hordaland. Standard errors are given in
parenthesis.
Body weight Spinal deformity Sexual maturity
Body weight
Spinal deformity
Sexual maturity
0.50 (0.13)
0.01
0.06
0.02
0.25 (0.25)
0 (0.28)
251
will be somewhat of a confounding effect between genetics
and environment early in life since cod were reared together
only after tagging, resulting in a common environmental effect. However, there are a considerable number of withinfamily variations, reflecting the presence of non-confounded
genetic variation. Gjerde et al. (2005) found similar results
for Atlantic salmon (Salmo salar). The magnitude of the estimated heritabilities shown in that study for vertebral deformity was determined by a substantial additive genetic
component. The heritability estimates varied from 0 to
0.36 for the 4 year classes of Atlantic salmon studied. An
earlier study of McKay and Gjerde (1986) showed similar
results.
The positive genetic correlations between spinal deformity and body weight and between body weight and sexual
maturity in the present study represent challenges for a selective breeding programme. If not scrutinized carefully, an
unwanted change in spinal deformity and sexual maturity
will occur when selecting for increased body weight, assuming that this relationship exists at the end of the growth
period. In Atlantic salmon, the genetic correlation between
deformity and body weight is negative (McKay and Gjerde,
1986; Gjerde et al., 2005), while in farm animals, rapidly
growing individuals are often at a higher risk of developing
skeletal anomalies in addition to reproductive and metabolic
problems (Olsson, 1978; Rauw et al., 1998), indicating
that fast growth should be recognized as a risk factor.
The final verification of the present results will be done
when results from the final recordings at market size are
available.
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