What effect does overfishing have on estuaries? what effect does overfishing have on oceans?

Overfishing occurs when more fish are caught than the population can replace through natural reproduction. Gathering as many fish as possible may seem like a profitable practice, but overfishing has serious consequences. The results not only affect the balance of life in the oceans, but also the social and economic well-being of the coastal communities who depend on fish for their way of life.
Billions of people rely on fish for protein, and fishing is the principal livelihood for millions of people around the world. For centuries, our seas and oceans have been considered a limitless bounty of food. However, increasing fishing efforts over the last 50 years as well as unsustainable fishing practices are pushing many fish stocks to the point of collapse.
More than 85 percent of the world's fisheries have been pushed to or beyond their biological limits and are in need of strict management plans to restore them. Several important commercial fish populations (such as Atlantic bluefin tuna) have declined to the point where their survival as a species is threatened. Target fishing of top predators, such as tuna and groupers, is changing marine communities, which lead to an abundance of smaller marine species, such as sardines and anchovies.