Answer:
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 :)
