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

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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