What factors might affect the availability of fish to eat for the egret and stork
populations?

Factors such as pollution, the depth at which both birds go down to catch the fish (if you look at experiments populations of fish tend to deplete quicker when in a shallow depth than a deeper depth), another factor would be the use of mammals fishing for food- this factor can range from a bear all the way down to humans taking huge quantities of fish in from the sea.

These are only a few factors I could genuinely think of.

Hopefully this helps :)