Time required for yolk coagulation in pink salmon and steelhead eggs exposed to mechanical shock

Incubation success in management studies and environmental assessments is often applied as a measure of natural spawning productivity as based on the live: dead ratios of excavated eggs. If sampling occurs too early in the development of the embryo, eggs will die from the sampling procedure and could be mistaken for incubation mortality if the yolk coagulates rapidly after experiencing lethal shock. In this study we examined how long it takes for the yolk of fertilized eggs of pink salmon Oncorhynchus gorbuscha and steel-head O. mykiss to coagulate after experiencing lethal mechanical shock, as it applies to that question. Tests were conducted at 12degreesC from 24 and 48 h (12-24 degree-days) postfertilization to the eyed stage of developing embryos (156 degree-days for steelhead and 223 degree-days for pink salmon). Results demonstrated that yolk coagulation was visible within seconds after shocking, involving as much as 50% of the yolk material within 5 min of shocking. We conclude, therefore, that mortality of eggs can be readily detected within 5 min of experiencing significant shock, regardless of the shock intensity. Yolk coagulation in eggs shocked during excavation of recently prepared pink salmon or steelhead redds occurs rapidly enough to be mistaken for natural incubation mortality. To avoid such potential bias in redd surveys of salmonid incubation success, sampling should be delayed until embryo development from the eggs of the latest spawners has progressed to the eyed stage. This should take approximately 30 d for pink salmon in fall temperatures and 23 d for steelhead in spring temperatures.