Fish and shellfish are two valuable resources of the ocean. The health of the ocean ecosystem, however, is in danger as a result of over fishing. Because nobody owns the ocean's resources, they can easily be used for profit by anyone and everyone. When a fishery first opens, it may be fished by a small group of people. These first individuals usually experience good catches and profits. As news of the new fishery spreads, the area may be flooded with more people catching fish. But they catch fewer and fewer fish. If the fishing goes unchecked, a collapse of the fish population will occur. In other words, the total number of fish that can be caught each year is exceeded. This level is called the maximum sustainable yield. Most fisheries in the world today are being fished close to or above their maximum sustainable yield. The California sardine fishery, the Newfoundland cod fishery, and the king crab fisheries of the Bering Sea are examples of the tragedy of over fishing.
Tropical reefs are being exploited with the use of dynamite and spearfishing. Dynamite fishing kills many fish which are not brought in by the fishers because the fish sink to the bottom and are thus wasted. Dynamite also destroys reefs and other natural sites. Many fisheries are now managed to avoid over fishing, but management is too often ineffective as it is expensive and difficult to assess the number, size, and age of the fishery stock.