Some good zebrafish facilities routinely collect eggs in the first few hours after the fish are exposed to the light phase of the photocycle. However, some of our best eggs are collected later in the day. When we desire high fertilization rates, we perform the following time consuming procedure. Shortly after the beginning of the light part of her photocycle, the female is placed in a small tank with at least one male. The fish are observed and as soon as breeding activity commences the female is removed from the tank and squeezed for eggs.
Good eggs are slightly granular and yellowish in color.
Although not always completely spherical, the best eggs look full. The best batches contain few, if any, broken eggs and no whitish or withered eggs. The chorion of good eggs can be observed under a stereoscopic dissecting microscope to elevate up away from the plasma membrane when ovarian fluid is diluted with water. An elevated chorion indicates that the egg is no longer capable of being fertilized. Thus, we often test a few eggs from each female by adding water to them.