Overfishing. Many species of fish will not reach the sizes that they have been known to reach in years past which then puts a stress on the smaller fish to try and reproduce or can't reproduce at all because they are not mature yet. Some species have slot sizes to allow them to grow but not every one does. Overfishing then causes top down effects that influence the rest of the ecosystem. Taking out one species will affect the populations of many other different species including coral, and plankton (oxygen). Overall, it puts a lot of stress on the ocean.