The rate of development during embryogenesis is temperature dependent. The lower the temperature the slower the rate of development. The video clips included here demonstrate some of the early cell divisions occurring between the time of spawning and 2 to 3 hours post-spawning.

The two video clips are of the same material. However, Video 1 is of lower resolution (770 Kb) while Video 2 is of higher resolution (2.78 Mb). Single photographs with descriptions of the changes which occur during egg development may be found in the following "Time after spawning" sections.

Early cell division can be easily distinguished through the hatching envelope under a low magnifying, 10 x 40, dissecting microscope. A suitable unit can be purchased for a few hundred dollars. To determine fertility rates it is critical that eggs be collected at specific times after spawning. If eggs are collected too early after spawning before egg development begins it is not possible to distinguish between fertilised and non-fertilised eggs. If eggs are collected the following morning after spawning the eggs are too developed to be able to accurately distinguish fertile from non-fertile eggs, as the latter also undergo cellular cleavage. The cleavage pattern of unfertilised eggs, however, is asymmetrical. Although this can be distinguished a few hours after spawning, it is extremely difficult to distinguish the morning after a spawning since cell division is so far advanced. By this time the egg is composed of thousands of eggs and distinguishing a symmetrical compared to a non-symmetrical pattern is not possible with any accuracy.

Estimates of fertility rates of eggs collected the morning following spawning by several independent observers are highly variable and unreliable. In contrast, the same observers were able to make consistent and accurate determinations of fertility rates of eggs collected 1-2 hours post-spawning. At the AIMS hatchery the time of spawning of 186 wild broodstock occurred approximately 5 hours after lights are turned off (Figure 2.0). If eggs are collected 1 – 2 hours after spawning they will be between the 2^nd (4-cell stage) to 7^th (128 cell stage) cleavage. Approximately 1.25 hours after spawning the 4-cell stage egg can be easily distinguish from non-fertilised eggs. Non-fertilised eggs have either not begun to divide at all or have undergone 1-2 asymmetric cell cleavages. This pattern clearly differs from the symmetric 4-cell stage of a fertilised egg. A trained technician can score a sub-sample of 100 eggs in 5-15 minutes. This information is useful in determining whether poor hatchings are due to the eggs being unfertilised, and hence there being no embryos to hatch, or due to the embryos dying during development and not hatching as nauplii.

The rate of development during embryogenesis is temperature dependent. The lower the temperature the slower the rate of development. The timing of events described here is typical of those found at a water temperature of 30ºC.