Oxygen as a constraining factor in egg size evolution in salmonids

Abstract

Differential survival at low oxygen levels has been proposed as a mechanism for maintaining high within-population variability in egg size in fish. Whether low oxygen levels favour large or small eggs, however, is not clear. To address this question, the effects of egg size on metabolic rates, critical dissolved oxygen levels (Pc), and P50oxygen levels of Chinook salmon (Oncorhynchus tshawytscha) embryos and alevins were determined. Embryonic metabolic rate expanded at a slower rate with increasing egg mass (allometric constant (b) = 0.30) than did capsule surface area (b = 0.67), indicating that larger eggs have larger surface areas relative to their metabolic demand for oxygen. A relatively larger area, however, did not translate into significant differences in Pcor P50values at the egg stage. After hatch, metabolic rate expanded at a rate proportional to (egg mass)0.62. Pclevels were significantly higher for alevins from larger eggs for the first but not second half of the alevin stage. Egg size had no significant effect on P50values at any time during the alevin stage. The modest impact of egg size on hypoxic tolerance of developing Chinook suggests that factors other than oxygen are involved in maintaining high within-population variability in egg size.