Overfishing is defined as human take out more fish from ocean than the fish can naturally replenish.
It is caused by several factors.
First and foremost, overpopulation increases global fish demand for food. Moreover, with higher quality of life and due to the traditional culture such as shark fin soup in China, people are more capable paying for fish and hence expensive fish species are overexploited by fishers to maximize their profit.
Second, to cope with increasing demand, advances in fishing gear technology to capture fish in massive scale within a short period of time within the last 50 years that are also environmentally damaging. In particular, bottom trawling wipes out everything in its path and throws back everything unwanted.
Third, from global perspective, fishing in the open ocean. As no nation has sovereignty there and fish does not belong to any nation, they are reluctant to put any effort to protect the fish. Without restriction on the take, every nation want to take as much advantages as possible. Nations compete for their own interests and conflicts arise for uneven distribution of yield. The resources are overexploited.
Forth, from local perspective, economic and political forces. International cooperation cannot take care of fish resources, therefore exclusive economic zones are set up to request nations
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Because mammals such as dolphins need to surface to breathe, bycatch traps them in the nets and, without contact of air, causes drowning to them. Also, since fish breathe through gills in the water, air exposure when they are caught on deck of fishing fleets leads to death as they are not adapted to breathe in air. Since the limited capacity of fish fleets, unwanted marine animals are discarded back into the sea. They are weakened and injured, thus having poor chance of survival upon return to the