this strategy will depend on the breeding strategy of the
hatchery operator (number spawners per generation,
outcrossing of lines, etc), the degree of relatedness
among the broodstock sold to the hatchery, and possibly
environmental quality.
The mean inbreeding level of the study population
was low (b10%) throughout the study period and few
families had inbreeding levels greater than 20%. As a
result, IBD at moderate to high levels of inbreeding may
differ from IBD estimates obtained in this study,
although IBD is typically linear (Falconer and Mackay,
1996). The rate of inbreeding accumulation in the study
population was low (b1%/generation) and IBD may be
more severe at higher inbreeding accumulation rates
(Gjerde et al., 1983). Inbreeding accumulation in well
managed breeding programs will likely be similar to the
rate observed in this study. However, inbreeding may
accumulate much faster at commercial hatcheries, where
the mating of unpedigreed stocks is common.
In summary, inbreeding had a small but significant
effect on growth. Growout survival was not affected by
inbreeding, but inbreeding effects could have occurred
prior to stocking (b1–2 g). IBD on survival to TSV–AG
was moderate (8–12%), whereas IBD on survival to
TSV–BG and WSSV appeared to be more severe
(N30%). However, the regression coefficients for TSV–
BG andWSSV were not significant. IBD appeared to be
sensitive to environmental quality, with IBD becoming
more severe as the environmental quality declined.
Clearly, more research is needed to evaluate the effects
of inbreeding on shrimp under diverse stock management
strategies and growout conditions.