Breeding and getting eggs from the zebrafish is relatively easy. Their eggs are externally fertilized, produced regularly in large numbers, and are non-adhesive. Their embryos develop rapidly, and are clear throughout their development. Their embryos are also smaller than many vertebrate embryos and contain smaller numbers of cells. It is easier to trace the development of individual cells.
Females lay large quantities of eggs .
For many types of genetic analysis you need to look at many different embryos at many different stages to understand what the problem is with a given mutation. The embryos develop outside the mother's body , so you can have easy access to them. In contrast, mouse embryos develop inside the mother, and you have to kill the mother to get at them. This would have to be done at each of the stages of development you want to look at. Once you do this, of course, the embryos die as well as the mother, so you are very limited in the types of experiments you can do.