I think it is because in the ocean the plankton produces most of the oxygen which is used by aquatic plants and plankton is a primary producer known as autotrops.
Over utilized plants and animals are those plants and animals which are almost going out of existence or are already extincts as a result of over use by humans. Examples of over utilized animals include: West African Black Rhinoceros, Passenger pigeon, Sea mink, Tecopa pupfish, Pyrenean ibex and Tasmanian tiger.
