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Supporting Online Material
Calculation of pre-selection fitness function: Maternal fitness is calculated as the product of
fecundity and survival, both of which are functions of egg mass. Empirical data was used to
parameterize the coefficients (Fig. 1) of the maternal fitness function. The optimal egg size was
estimated as the point where the first derivative of the maternal fitness function was equal to
zero (0.16 g). To estimate the fitness cost of the observed decline in egg mass, we calculated
the maternal fitness relationship for the YIAL salmon prior to the study period. We assumed
that the initial (pre-selection) egg mass of 0.27 g was the optimal size in nature and that the
fecundity - egg mass relationship was invariant over the study period, we then solved for the
survival – egg mass relationship by setting the first derivative equal to zero at an egg mass of
0.27 g. The calculated (pre-selection) survival function is;
Survival = 1.485(egg mass)0.923
and the resulting estimated (pre-selection) maternal fitness function is;
Maternal Fitness = 2496(egg mass)0.923 - 4437(egg mass)1.923
These relationships are the same as those derived from the empirical data, except that
the exponent in the survival function increased from 0.405 to a calculated 0.923 (the effect of
this change is to reduce predicted survival and maternal fitness by approximately 55% for an
egg of 0.20 g). Using the estimated pre-selection maternal fitness and survival relationships, the
evolved egg mass of 0.20 g represents a reduction in egg survival of 24% and a reduction in
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maternal fitness of 6%, if the fish were to be released into the wild (the reduction in maternal
fitness is less than the reduction in egg survival due to changes in female fecundity). The
dramatic reduction in predicted offspring survival and maternal fitness for the pre-selection
relationships agree with expectations of lower fitness and survival under natural, or noncultured, conditions.
Figure Caption (Supplemental Material)
Figure S1: Changes in wet body mass (Kg ± 1 SE) over time for female YIAL chinook
salmon. Body mass was measured prior to spawning (i.e. egg removal). No significant
trend in body size was found (regression analysis; P > 0.15).
Figure S2: Changes in female body length (mm; post-orbital to hypural plate) over time
for the four Vancouver Island chinook salmon populations. Robertson Creek and
Nahmint River data are for parents of fish used in hatchery rearing, Nanaimo River data
are from dead fish collected in the river after spawning (M. Folkes, Department of
Fisheries and Oceans, Canada, pers. comm.), and the Quinsam River data are for 5-yearold fish (mean age of maturation) collected in the river (www.dfo-mpo.gc.ca/csas/;
Assessment of Campbell/Quinsam chinook salmon (Oncorhynchus tshawytscha)
2000/151). No significant trend in body length was found for any of the populations
(regression analysis; P > 0.20).