On the Ocean Conditions tab, select Fishing. Set Net fishing to 50%. Click Advance year 10 times. Now, what changes do you notice on the Coral reef tab?

It went from lots of different fish and even a turtle to only one fish. The diversity and population size drastically changed. There was also some change with the plants/coral but not much.

Explanation:

This is from the Coral Reefs 2 simulator.