Overfishing and it's impact

A team of scientists from Canada and Australia have partaken in a study of coral reefs.  And what they discovered was that the increase in commercial shark fishing has caused a decline in the health of coral reefs.  The only explanation for how this has happened is: Sharks (Tertiary consumer) would keep control of the mid-level consumer population, who took care of the herbivorous creatures that took care of the reef.  But as the shark population declined the mid-level consumer population increased and took out more of the herbivorous creatures.  Thus leaving a coral reef with nothing to keep it clean.  Scientists are hoping to block off coral reefs in hope that it will allow the shark population to recover and help the coral reef to recover from coral bleaching and large cyclones that have been occurring due to climate change.

http://www.sciencedaily.com/releases/2013/09/130918180429.htm