Strain and maturation effects on female spawning time in diallel ...

<strong>Strain</strong> <strong>and</strong> <strong>maturation</strong> <strong>effects</strong> on female spawning
time in diallel crosses of three strains of rainbow
trout (Oncorhynchus mykiss)
Cheryl D. Quinton*, Laura R. McKay, Ian McMillan
Centre for Genetic Improvement of Livestock, Dept. Animal & Poultry Science, University of Guelph, Guelph,
Ontario N1G 2W1, Canada
Abstract
Received 6 February 2003; received in revised form 12 January 2004; accepted 14 January 2004
Female spawn date was investigated in three diverse strains of rainbow trout (Oncorhynchus
mykiss) <strong>and</strong> two year classes of complete diallel crosses between them. Spawn dates at 3 <strong>and</strong> 4 years of
age were recorded for individual females in two generations. Spawn dates were analysed to find the
<strong>effects</strong> of strain <strong>and</strong> year of <strong>maturation</strong>, <strong>and</strong> to estimate repeatability. <strong>Strain</strong>s had significantly different
spawning seasons, <strong>and</strong> hybrids generally performed intermediate to pure strains, with little evidence
of heterosis. Some differences in performance occurred between reciprocal crosses such that hybrids
spawned more like the strain of their dams. Four-year-old maiden females spawned approximately 18
days later than 4-year-old repeat spawning females. Spawn date repeatability estimates were 0.72 <strong>and</strong>
0.83. The second spawning season occurred 6 to 35 days earlier than the first season, depending on
strain. Spawning time appears to be inherited in an additive manner, so it should be possible to predict
future spawning dates based on past data <strong>and</strong> change spawning time through selection.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Rainbow trout; Oncorhynchus mykiss; Quantitative genetics; Spawning time; Diallel cross; Age at
maturity; Repeatability
1. Introduction
Aquaculture 234 (2004) 99–110
Rainbow trout <strong>and</strong> other salmonids have an annual reproductive cycle, releasing
gametes during certain months of the year which are referred to as the spawning season.
* Corresponding author. Tel.: +1-519-824-4120x56683; fax: +1-519-767-0573.
E-mail address: cquinton@uoguelph.ca (C.D. Quinton).
0044-8486/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.aquaculture.2004.01.016
www.elsevier.com/locate/aqua-online

100
The timing of the spawning season has major consequences on farming operations:
seasonal product availability <strong>and</strong> labour requirement, <strong>and</strong> reduced facility use. The result is
lower production efficiency with consequent profit loss. In order to optimise production, a
producer may want to move the spawning season of part of their broodstock to a different
time of year, or condense the season to a more specific time of year. Either may be
accomplished by using genetic selection methods. By determining the underlying genetic
factors influencing spawning time, the potential for selection as a method to manipulate
the spawning season in rainbow trout can be evaluated.
The genetic factors controlling spawning time have been studied somewhat less than
have those relating to other production traits. Purdom (1993) stated that different strains of
rainbow trout had varying spawning times, indicating some genetic basis for this trait. Gall
<strong>and</strong> Huang (1988b) demonstrated that spawning season was controlled by a strong genetic
component. There is anecdotal evidence that a wide range of spawning times has been
developed by selection (Purdom, 1993) <strong>and</strong> responses to spawn date selection have been
observed (Siitonen <strong>and</strong> Gall, 1989; Sadler et al., 1992). A few studies have attempted to
quantify the variance components of spawning time in salmonids. Heritability <strong>and</strong>
repeatability have been estimated by Gall et al. (1988), Siitonen <strong>and</strong> Gall (1989), Sadler
et al. (1992), Su et al. (1997, 1999), <strong>and</strong> Quinton et al. (2002).
A project is underway to develop a synthetic strain of rainbow trout for aquaculture
which would be characterised by fast growth, reduced precocious <strong>maturation</strong>, <strong>and</strong> a spring
spawning season (McMillan <strong>and</strong> McKay, 1992). Knowledge of the underlying genetics of
these traits is necessary to establish the optimum selection <strong>and</strong> breeding methods required
to develop the strain. To determine genetic factors regulating spawning time, four main
objectives for this study were set: rank three strains of rainbow trout <strong>and</strong> their hybrids
according to spawn date; determine strain interactions that affect spawn date (characterised
by heterosis <strong>and</strong> differences between reciprocal crosses); find <strong>effects</strong> of age of <strong>maturation</strong>
on spawn date; <strong>and</strong> calculate the repeatability <strong>and</strong> shift in time of spawn date from 3 to 4
years of age.
2. Materials <strong>and</strong> methods
2.1. Experimental design
C.D. Quinton et al. / Aquaculture 234 (2004) 99–110
The study was conducted at the Alma Aquaculture Research Station (AARS), University
of Guelph, Guelph, Ontario, Canada. Fertilised eggs from three farmed strains of rainbow
trout were brought to the AARS to form a base population (G0). All G0 fertilisations had
been done between March 14 <strong>and</strong> April 6, 1991. <strong>Strain</strong> OC was an Ontario commercial strain
in which females had been selected for late spawning <strong>and</strong> males had been selected for fast
growth. <strong>Strain</strong> MG was descended from the Ontario Ministry of Natural Resources’
Ganaraska River strain, <strong>and</strong> had undergone two generations of mild selection for faster
growth. <strong>Strain</strong> WC was a Washington State commercial strain. Previous studies have
compared these pure strains for early growth, survival, <strong>and</strong> <strong>maturation</strong> (McMillan <strong>and</strong>
McKay, 1992; McKay <strong>and</strong> McMillan, 1997). Evaluations of mitochondrial DNA <strong>and</strong>
allozyme variation have also been done for the Ontario strains by Ferguson et al. (1993).

At maturity, two sets of complete diallel crosses were made among the strains, forming
a G1 generation: a 94 year class when the parents were 3 years old, <strong>and</strong> a 96 year class
when the parents were 5 years old. All reciprocal crosses were made. For this paper, strain
combination A B denotes dam strain A crossed with sire strain B. There was no time when
dams from all three strains spawned at the same time, so to complete the diallel crosses,
matings were done over a period of several weeks (Table 1). Each strain combination was
made from individual matings of two to five r<strong>and</strong>omly chosen animals from each dam <strong>and</strong>
sire strain. Twenty-eight females <strong>and</strong> 29 males parented the 94 year class. Twenty-nine
females <strong>and</strong> 38 males (of which 5 females <strong>and</strong> 12 males had also been used as parents of
the 94 year class) parented the 96 year class. Individual females were mated once per
season <strong>and</strong> individual males mated up to six times per season.
2.2. Husb<strong>and</strong>ry <strong>and</strong> data collection
C.D. Quinton et al. / Aquaculture 234 (2004) 99–110 101
All three year classes were raised in similar optimal conditions (McMillan <strong>and</strong> McKay,
1992; McKay <strong>and</strong> McMillan, 1997). Water temperature remained constant at 8.5 jC <strong>and</strong>
dissolved oxygen was 96% of saturation. Simulated natural photoperiod was maintained.
Tanks were monitored to maintain consistent levels of biodensity.
For G0, each pure strain was raised separately in replicate tanks until tagging at 25 to
26 months post-fertilisation. For the G1 94 year class, fertilised eggs from individual pair
matings of were pooled into incubator compartments by strain combination within
fertilisation date. For the G1 96 year class, fertilised eggs from individual pair matings
were held in separate incubator compartments, <strong>and</strong> after hatching were pooled into tanks
Table 1
Diallel cross mating schedule, showing fertilisation dates for strain combinations in two G1 year classes. The
number of parent individuals used in each cross is indicated by ‘‘a
number of sires
b’’ where a is the number of dams <strong>and</strong> b is the
Fertilisation date <strong>Strain</strong> combination (top: strain of dam, bottom: strain of sire)
OC OC OC WC WC WC MG MG MG
OC WC MG OC WC MG OC WC MG
94 year class
94/02/10 4 4 4 4 4 4 4 4 4 4 4 4
94/02/24 4 4 4 4 4 4
94/03/24 5 4 5 4 5 4 4 4 4 4 4 4
94/03/30 2 4 2 4 3 4 3 4
94/04/27 2 4 2 4 2 4
96 year class
95/11/30 3 3 3 2 3 4
95/12/07 2 2 2 4 2 2
95/12/21 2 3 4 4
96/01/09 1 3 1 3 3 3 3 3
96/02/13 3 4 1 4
96/02/27 3 3 1 3 3 3 3 3 3 3
96/03/12 2 3 2 3 2 3 4 3 4 3 4 3
96/03/27 4 5 4 5 4 5

102
by strain combination within fertilisation date. At hatching, equal numbers of fry from
each incubator compartment were pooled by strain combination to create the groups
summarised in Table 1. Groups within the 94 <strong>and</strong> 96 year classes were raised in separate
tanks until tagging. The 94 year class was tagged at 16 to 18 months post-fertilisation, <strong>and</strong>
the 96 year class was tagged at 10 to 11 months post-fertilisation. All individuals were
marked with passive integrated transponder (PIT) tags (AVID Identification Systems)
injected in the abdominal cavity. Tagged fish of each year class were pooled <strong>and</strong>
distributed r<strong>and</strong>omly over replicate tanks. Mature fish were housed in 2.0-m semi-square
fibreglass tanks <strong>and</strong> a commercial dry salmonid feed (Martin Feed Mills, Elmira, Ontario)
was dispensed with dem<strong>and</strong> feeders. Males <strong>and</strong> females were housed together.
Under the conditions of this study, females matured at 3 or 4 years of age <strong>and</strong> the
spawning season ran from October until May. Imminent spawning was assessed in
anaesthetised females by colour, look <strong>and</strong> feel of the abdomen, <strong>and</strong> vent extension.
Females judged close to spawning were checked weekly for ripeness. Ripe females (those
from whom eggs were easily stripped) were anaesthetised <strong>and</strong> eggs stripped by applying
gentle manual pressure to the abdomen. Females who had spawned were separated from
the population <strong>and</strong> were not h<strong>and</strong>led for the remainder of the season. All fish were checked
for gamete production once in June or July between 3- <strong>and</strong> 4-year old spawning seasons.
2.3. Statistics
C.D. Quinton et al. / Aquaculture 234 (2004) 99–110
2.3.1. Data
For this study, ‘maiden’ spawners are defined as females spawning for the first time;
‘repeat’ spawners are defined as 4-year-old females spawning for the second time (<strong>and</strong> had
spawned as maidens at 3 years of age). Female spawn dates were recorded for individuals
in each year class at 3 years of age (all 3-year maiden spawners, 3M) <strong>and</strong> at 4 years of age
(4-year repeat spawners, 4R, <strong>and</strong> 4-year maiden spawners, 4M). Spawning date was
defined as the period in days after October 1 (an arbitrary date) that eggs were stripped
from a particular female. Numbers of 3M, 4R, <strong>and</strong> 4M observations used in analyses are
summarised in Table 2.
All statistical analyses were done with SAS Version 8 (n 1999 SAS Institute Inc., Cary,
NC, USA). The Levene test was used to test homogeneity of 3-year-old <strong>and</strong> 4-year-old
spawn date variances across all strain combinations within each year class. Variances were
not significantly different across strain combinations, except for the 94 year class spawn
dates at 4 years of age ( P=0.0193).
2.3.2. <strong>Strain</strong> <strong>and</strong> <strong>maturation</strong> year <strong>effects</strong>
<strong>Strain</strong> <strong>effects</strong> on spawn date at 3 <strong>and</strong> 4 years of age <strong>and</strong> <strong>maturation</strong> year <strong>effects</strong> on
spawn date at 4 years of age were tested with general linear models. G0 <strong>and</strong> G1 were
analysed in separate models because G0 consisted of three pure strains, while G1 consisted
of nine strain combinations. Spawn dates at 3 years of age <strong>and</strong> spawn dates at 4 years of
age were analysed separately. All females who spawned at 3 years of age matured at 3
years of age (therefore there was only one <strong>maturation</strong> year effect level at 3 years of age),
but females that spawned at 4 years may have matured at 3 or 4 years of age (therefore,
there were two <strong>maturation</strong> year effect levels at 4 years of age).

C.D. Quinton et al. / Aquaculture 234 (2004) 99–110 103
Table 2
Numbers of spawn date observations at 3 <strong>and</strong> 4 years of age for 3-year-old maiden spawners (3M), 4-year-old
repeat spawners (4R), <strong>and</strong> 4-year-old maiden spawners (4M) of each strain <strong>and</strong> year class
Generation year <strong>Strain</strong> Spawn dates at 3 years of age Spawn dates at 4 years of age
class
3M 4R a
4M
G0 91 OC 42 35 8
WC 76 67 17
MG 27 22 13
G1 94 OC OC 5 2 2
OC WC 3 2 1
OC MG 2 2 1
WC OC 10 9 3
WC WC 21 20 1
WC MG 11 11 3
MG OC 5 5 5
MG WC 6 5 5
MG MG 5 4 3
96 OC OC 26 18 1
OC WC 23 22 2
OC MG 39 32 3
WC OC 29 25 1
WC WC 23 20 3
WC MG 25 15 2
MG OC 29 25 2
MG WC 27 20 0
MG MG 19 15 7
a All 4R females were 3M females in the previous season.
<strong>Strain</strong> <strong>and</strong> <strong>maturation</strong> year <strong>effects</strong> on G0 3 <strong>and</strong> 4 years of age spawn dates were tested
with Model 1,
yijk ¼ l þ Pi þ Mj þðPMÞij þ eijk
ð1Þ
where yij is the 3-year-old or 4-year-old spawn date for individual k in pure strain i with
<strong>maturation</strong> year j; l is the population mean; Pi is the fixed effect of pure strain i (i=1, 2, 3);
Mj is the fixed effect of <strong>maturation</strong> year j ( j=1, 2); (PM)ij is the interaction of pure strain i
<strong>and</strong> <strong>maturation</strong> year j; <strong>and</strong>eijk is the residual error associated with individual ijk.
<strong>Strain</strong> <strong>and</strong> <strong>maturation</strong> year <strong>effects</strong> on G1 3 <strong>and</strong> 4 years of age spawn dates were tested
with Model 2,
yijklm ¼ l þ Yi þ Dj þ Sk þ Ml þðDSÞ jk þðDMÞ jl þðSMÞ kl þðDSMÞ jkl þ eijklm
ð2Þ
where yijkl is the 3-year old or 4-year old spawn date for individual m from year class i with
dam strain j, sire strain k <strong>and</strong> <strong>maturation</strong> year l; l is the population mean; Yi is the fixed
effect of year class i (i=1, 2); Dj is the fixed effect of the dam strain j ( j=1, 2, 3); Sk is the
fixed effect of the sire strain k (k = 1, 2, 3); (DS)jk is the interaction of the dam strain j <strong>and</strong>
sire strain k; (DM)jl is the interaction of dam strain j <strong>and</strong> <strong>maturation</strong> year l; (SM)kl is the

104
C.D. Quinton et al. / Aquaculture 234 (2004) 99–110
interaction of sire strain k <strong>and</strong> <strong>maturation</strong> year l; (DSM) jkl is the interaction of dam strain j,
sire strain k <strong>and</strong> <strong>maturation</strong> year l; <strong>and</strong> eijklm is the residual error associated with
observation ijklm. A similar analysis was done by Friars et al. (1979) for diallel crosses
of Atlantic salmon.
Least squares means for strain <strong>effects</strong> were estimated from Models 1 <strong>and</strong> 2 <strong>and</strong>
compared with Tukey’s test for pairwise comparisons. Heterosis, defined as a significant
difference between the average spawn date of two pure strains <strong>and</strong> the average spawn date
of their hybrids, was tested with Model 2 contrasts.
Contrasts between G1 reciprocal crosses were inestimable from Model 2, therefore each
strain combination was considered a separate fixed effect. Model 3 contrasted 3 <strong>and</strong> 4
years of age spawn dates among reciprocal crosses,
yijkl ¼ l þ Yi þ Cj þ Mk þðCMÞ jk þ eijkl
where y ijkl is the 3-year old or 4-year old spawn date for individual l in year class i with
strain combination j; C j is the fixed effect of strain combination j ( j=1,..., 9); (CM) jk is the
interaction of strain combination j with <strong>maturation</strong> year k; e ijkl is the residual error
associated with observation ijkl; <strong>and</strong> remaining variables are as described for Model 2.
2.3.3. Repeatability
Repeatability is the correlation between repeated measurements on the same individual,
<strong>and</strong> is used to predict future performance (Falconer <strong>and</strong> Mackay, 1996). In this study,
spawn date repeatability was estimated on 4R females only because these fish had two
spawn date measurements each. The repeatability estimate for each generation was the
partial correlation between spawn dates calculated from analysis of variance. The analysis
removed <strong>effects</strong> of having different strains to find a pooled within-strain correlation value
for each generation. Model 1 was used for G0 <strong>and</strong> Model 2 was used for G1, where the
dependent variable was the 3 or 4 years of age spawning dates <strong>and</strong> other variables were as
previously described.
To predict future performance, it is also practical to know if the entire spawning
season of a population can shift in time over years. The overall time shift in days
between the first <strong>and</strong> the second spawning seasons was examined using general mixed
linear models. This estimated the difference in time, in days, from the first spawning
season to the second spawning season. In this case, all the spawning data were used, but
a r<strong>and</strong>om individual effect was added to distinguish maiden from repeat spawners. In the
first spawning season, all females were maidens, but in the second spawning season,
females could be either repeat or maiden spawners. The spawning season number effect
accounts for permanent environmental <strong>effects</strong> of having spawned previously. Model 4
was used for G0,
yijkl ¼ l þ Pi þ Tj þðPTÞ ij þ Ik þ eijkl
where yijkl is the 3- or 4-year old spawn date for individual k in pure strain i <strong>and</strong>
spawning season j; Tj is the fixed effect of the spawning season number j ( j=1, 2); (PT)ij
is the interaction of pure strain <strong>and</strong> spawning season number; Ik is the r<strong>and</strong>om effect of
ð3Þ
ð4Þ

individual k; e ijkl is the residual error associated with observation ijkl; remaining
variables are as described in Model 1. Model 5 was used for G1,
yijklm ¼ l þ Yi þ Cj þ Tk þðCTÞ jk þ Il þ eijklm
where yijklm is the 3- or 4-year old spawn date for individual l in year class i <strong>and</strong> strain
combination j in spawning season number k; Tk is the fixed effect of the spawning
season number k (k=1, 2); (CT)jk is the interaction of strain combination <strong>and</strong> spawning
season number; Il is the r<strong>and</strong>om effect of individual l; eijklm is the residual error
associated with observation ijklm; remaining variables are as described for Model 3.
3. Results
3.1. <strong>Strain</strong> main <strong>effects</strong>
C.D. Quinton et al. / Aquaculture 234 (2004) 99–110 105
Analysis of variance results for Models 1, 2, <strong>and</strong> 3 are shown in Table 3. Year class was
not significant. None of the interactions were significant in any analysis. Effects of pure
strain, dam strain, <strong>and</strong> sire strain on spawn dates were highly significant. Least squares
means for strain <strong>effects</strong> are summarised in Table 4. All G0 strains had significantly
different mean spawn dates such that on average, OC females spawned first, WC females
Table 3
Results of analysis of variance testing for strain, dam strain, sire strain <strong>and</strong> <strong>maturation</strong> year <strong>effects</strong> on spawning
dates at 3 years of age <strong>and</strong> 4 years of age in generations 0 <strong>and</strong> 1
Model, Effects 3-year old spawn date a
4-year old spawn date
generation
df MS P df MS P
1, G0 Pure strain ( P) 2 54619.0

106
Table 4
Female spawning date least squares means (s.e.) for G0 pure strains <strong>and</strong> G1 dam strains, sire strains, <strong>and</strong> strain
combinations. Spawning dates at 3 <strong>and</strong> 4 years of age (in days from Oct. 1) are in first <strong>and</strong> second rows of each
group, respectively. Differences a between dates in first <strong>and</strong> second spawning seasons are shown in third rows of
each group
G0 Pure strain
OC WC MG
92.1 (3.7) 132.3 (2.8) 172.7 (4.6)
62.8 (6.0) 115.8 (4.1) 162.3 (5.3)
35.8 26.0 17.2
G1 Sire strain Overall dam strain
OC WC MG
Dam strain OC 86.9 (3.7) 92.8 (4.1) 111.5 (3.4) 97.1 (2.3)
77.1 (8.0) 84.2 (7.9) 101.8 (6.9) 87.7 (4.5)
20.6 8.7 13.5
spawned second, <strong>and</strong> MG females spawned last ( P

OC MG spawned approximately 13 days earlier than MG OC ( P=0.0047). At 4 years of
age, OC MG spawned approximately 21 days earlier than MG OC ( P=0.0148).
3.3. Maturation year <strong>effects</strong>
In both generations, at 4 years of age, repeat females spawned significantly earlier
( P

108
Evidence for non-additive genetic <strong>effects</strong> on rainbow trout growth has been found (Gall
<strong>and</strong> Huang, 1988a; Su et al., 1996b; Pante et al., 2001), but at this time no conclusions can
be made about non-additive genetic <strong>effects</strong> on spawning time. In the sole incidence of
heterosis (between WC <strong>and</strong> OC strains at 3 years old), there was only a small difference
between hybrid <strong>and</strong> pure strain performances, <strong>and</strong> other internal factors could have
contributed to this difference. If an individual female must reach some threshold body size
in order to spawn in a given season, those just on this threshold may have delayed oocyte
development <strong>and</strong> ovulation in their maiden season. This delayed spawning would be seen
particularly in the early-spawning OC OC 3 year maiden spawners (as compared with
other strains that spawned later in the year). When size or growth rate was no longer a
factor at 4 years of age, the average spawn date of the hybrids was the same as the average
spawn date of the pure strains.
Inbreeding in the WC or OC strains could also have caused changes in spawn date,
similar to results found by Su et al. (1996a) who reported that inbred females spawned at a
later age. Any inbreeding effect would be removed by crossing with different strains,
therefore resulting in heterosis. There were, however, no heterosis <strong>effects</strong> when these
strains were crossed with MG, so it is unlikely that removal of inbreeding depression
caused the heterosis effect.
There were also differences for spawning performances between reciprocal crosses.
Hybrids’ spawn dates tended to be more similar to that of their dam’s strain than that of their
sire’s strain, as shown by the order of spawning A A, A B, B A, B B in all crosses.
Hybrids from an early-spawning dam strain spawned earlier than those hybrids that had the
same strain as their sire. In species that rear their young, differences between reciprocal
crosses usually represent differences in maternal ability (Van Vleck et al., 1987), however,
rainbow trout do not rear their young. These results suggest that some other maternal factor
affected spawn date. In this case, the spawning date of the dam, which was confounded with
dam strain, could have contributed to differences between reciprocal crosses. No other
studies on spawning time in reciprocal crosses have been published. Further research is
needed to determine if non-additive genetic or maternal <strong>effects</strong> control this trait.
4.3. Maturation year <strong>effects</strong>
At 4 years of age, repeat spawners (females that had matured at 3 years of age, <strong>and</strong> were
spawning for the second time) spawned earlier than maiden spawners. This result may
relate to the previously-discussed idea that spawning at 3 years of age may be delayed in
females that are close to a threshold body size. By the time these females repeat spawning
at 4 years of age, eggs have had more time to ripen, <strong>and</strong> ovulation can occur earlier. It
would have been interesting to observe if this trend continued in subsequent spawning
seasons, but this was beyond the scope of the current project.
4.4. Repeatability
C.D. Quinton et al. / Aquaculture 234 (2004) 99–110
Repeatability estimates of spawn date were high; therefore, females that spawned early
in the season at 3 years of age also tended to spawn early in the season at 4 years of age.
Similarly, fish that spawned late in the season at 3 years of age also tended to spawn late in

the season at 4 years of age. A very similar repeatability estimate has also been found for
the same population with restricted maximum likelihood methods (Quinton et al., 2002).
These values are higher than that found by Sadler et al. (1992), probably due to more
controlled environmental conditions.
Repeatability is considered to be an upper-limit estimate of heritability (Falconer <strong>and</strong>
Mackay, 1996), which has been quantified for spawning time in a few studies. Published
spawning time heritability estimates are generally high (Gall et al., 1988; Siitonen <strong>and</strong>
Gall, 1989; Sadler et al., 1992; Su et al., 1997, 1999; Quinton et al., 2002), but cover a
wide range due to the definition of traits measured <strong>and</strong> differing methods of estimation.
The reproductive season occurred earlier in the year when the populations were older.
This time shift may have been caused by some change in environmental cues, such as
management differences, from one year to the next. It is unlikely, however, that the same
environmental changes occurred at the same ages in all three year classes to cause similar
advances in the spawning season. There may instead be some innate factor, such as a
larger body size, which allowed repeat spawners to spawn earlier than maiden females.
The results of this study indicate that spawning time is mostly inherited in an additive
manner, so this trait is expected to respond to selection. It should be possible to use genetic
selection to develop strains of rainbow trout with different spawning seasons suitable for a
variety of production schemes. Selecting females for spawn date after their first spawning
season could be done in order to condense future spawning seasons to an optimal
production time. Producers that spawn broodstock over multiple seasons should able to
predict individual future spawning dates based on past data, but should be aware that the
entire spawning season may occur earlier for older animals.
Acknowledgements
The authors wish to acknowledge the staff Alma Aquaculture Research Station,
assistance from V.M. Quinton <strong>and</strong> S. M. Moghadasi, <strong>and</strong> contributions by Aqua-Cage
Fisheries <strong>and</strong> Blue Spring Trout Farms. This research was funded by the Ontario Ministry
of Agriculture <strong>and</strong> Food through the Applied Fish Production Research Program.
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