Here's why:
* Limited space: Fish eggs are relatively small, meaning they have limited space to store nutrients.
* Rapid development: Many fish species have eggs that hatch quickly, meaning the embryos don't have much time to rely on their yolk sac for nourishment.
* Environmental dependence: Many fish eggs rely heavily on external factors like plankton and other food sources for survival.
Exceptions: Some fish species, like sharks and rays, have larger eggs with a bigger yolk sac that provides more nutrients for the developing embryo.
Overall, the majority of fish eggs have a relatively small food store, highlighting the importance of external factors for their survival.