LATITUDINAL CLINES - A TRADE-OFF BETWEEN EGG NUMBER AND SIZE IN PACIFIC SALMON

Analyzed egg number and egg size of 17 populations of coho salmon Oncorhynchus kisutch distributed over a latitudinal gradient in North America. A significant latitudinal increase in egg number is accompanied by a significant latitudinal decrease in egg size. 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. -from Authors