The latitudinal variation in clutch size found in many animal species, including Pacific salmon, has been an enigmatic problem in ecology. We analyze egg number and egg size of 17 populations of coho salmon (Oncorhynchus kisutch) distributed over a latitudinal gradient in North America. These populations have a significant latitudinal increase in their egg number. But this increase is accompanied by a significant latitudinal decrease in their egg size. The total biomass of eggs produced also declines with latitude. Thus, the positive latitudinal trend in egg number cannot be explained by latitudinal variation in the total investment in eggs. This suggests that local optima in egg size may result in latitudinal clines in egg number. This reasoning, that egg number evolves around selection for egg size, is in fact predicted by life history theory and may explain the clutch size patterns observed in many organisms.