Every year, millions of tons of fish are accidentally caught and discarded. A staggering 100 million sharks are considered such "by catch" each year, often the result of net entanglement. Bycatch not only affects marine ecosystems but also the millions of people who rely on healthy fish stocks for food and employment.

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According to the latest estimates of the UN Food and Agriculture Organisation 7.3 million tonnes of bycatch are discarded annually by the worlds fisheries.

Most fisheries are unselective to some degree, in that they incidentally catch other species along with their target catch during the process of fishing. This non-target catch is known as "bycatch".

Bycatch species tend to be associated with certain fisheries (for example sea turtles with tropical shrimp trawl fisheries). Nevertheless bycatch, in terms of diversity of species caught and ratios of bycatch to target catch, can vary significantly over the geographic region and over time.

Bycatch reflects variations in marine communities, types of fishing methods used, and changes in the target catch of fisheries. For example, in some parts of Asia, former shrimp trawl fisheries have become "multi-species" fisheries where a much greater proportion of the catch is used.

In some cases bycatch is predictable and straightforward to control, and is therefore easier to avoid. In other cases, bycatch is unpredictable and very difficult for fishermen to control. In the latter, bycatch reduction is harder to achieve.

Bycatch also varies according to its impacts. These are not always immediately evident. For example, even a low rate of bycatch can have an ecological impact if fishing effort is high and bycatch species are vulnerable to over-fishing, for example if they are species that are long-lived and slow to reproduce, such as sharks and marine turtles.

A significant proportion of the world's bycatch is discarded back into the ocean.