270 
BULLETIN OF THE BUREAU OF FISHERIES 
Wilier (1929) suggested that selective destruction by carnivorous forms might 
affect the sex ratio in the kleine Marane ( Coregonus albula ). In the younger groups 
the heavier, less agile female would fall prey to predators more readily than the 
males. In the older groups the heavier females would be protected by virtue of their 
size as the predatory fish would select the slenderer males as prey of more suitable 
proportions. Such an assumption does not serve to explain the change with increase 
in age in the sex ratio of the four populations of the present investigation. Though 
the Trout Lake stock shows the most rapid shift toward a preponderance of females, 
the growth of the two sexes in this lake is the same both with respect to length and 
weight. Furthermore, the greatest changes in sex ratio occur between age groups 
that show only slight differences in average length and weight. The situation in 
Muskellunge Lake is similar to that in Trout Lake. In Silver Lake the males tend 
to be slightly heavier than females of the same age while in Clear Lake the females 
are distinctly heavier than the males. Yet in both populations, particularly in the 
Clear Lake cisco, females are relatively more abundant than males in the older age 
groups. 
In view of the fact that the greatest progressive change in the sex ratio with 
increase in age occurs in the population in which there is least reason to expect selec- 
tion on the basis of sex by the carnivorous forms which prey on that population, it 
must be concluded that the differential mortality upon which the changing sex ratios 
depend is not the result of selection by predators. The differential mortality of the 
two sexes is probably the result of basic differences in their innate physiological 
mechanisms, differences that vary in their manifestations from one population to 
another, and whose effects appear to be associated to some extent with the growth 
rate of each particular population. 
The above suggestion of an explanation of the differential mortality of the sexes 
of the cisco is in agreement with the conclusions of Geiser (1923, 1924, a, b,) 25 who 
held that females are inherently better fitted than males to survive adverse environ- 
mental conditions. The explanation of the sex ratio on these grounds assumes par- 
ticular interest when it is considered in relationship to the correspondence between 
sex ratio and growth rates in the four cisco populations of this study. It was pointed 
out (p. 269) that in these populations slow growth is associated with a relative paucity 
of males. It appears logical to assume that the environmental factors that 
doubtless contributed to the observed differences in growth rate may at the same 
time have produced corresponding differences in the sex ratios of these same popula- 
tions. Thus the very slow growth of the Trout Lake cisco may be associated with 
very adverse environmental conditions that also produce an exceptionally high 
mortality among the less viable males. This excessive mortality of males as compared 
to females would produce the observed rapid change with increased age in the sex 
ratio and the relatively high abundance of females in the population as a whole. 
Similarly, the better growth in Muskellunge Lake and Silver Lake may be taken to 
represent conditions less adverse than those in Trout Lake. In these less adverse 
environments of Muskellunge and Silver Lakes the mortality of males as compared 
to females is less than in Trout Lake, and females are relatively less abundant in 
the populations as a whole. Finally, the favorable conditions that permit such 
excellent growth in the Clear Lake cisco are reflected in the slowly changing sex ratio 
and the almost equal representation of the two sexes in that population. 
28 In these papers Geiser included comprehensive reviews of the literature on the subjects of sex ratios and differential mortality 
of sexes, not only in the fishes but in other animal groups as well 
