Unwanted fish discarded by commercial fishing operations are a loss in terms of a rich source of dietary protein as well as the stocks of those species that, even if they have low market value, may nonetheless be vital components of the marine ecosystem.

An FAO study estimated that the annual discards from global marine capture fisheries between 2010 and 2014 was 9.1 million tonnes (95% CI: 6.7 – 16.1 million tonnes) which represent 10.8% (10.1% –11.5%) of the annual average catch of 2010 to 2014. About 46 percent (4.2 million tonnes) of total annual discards were from bottom trawls that included otter trawls, shrimp trawls, pair bottom trawls, twin otter trawls and beam trawls.

Bycatch is a generic term referring to catch that is incidental to the target species. It is often associated with trawl fisheries but can be a feature of other fisheries.  The term is sometimes used interchangeably for the unwanted portion of the catch that is discarded and can refer to the less desirable fish that are landed (e.g. low value/trash fish).

Some management measures also limit how much can be landed in a day or in a trip for a species. Total allowable catches (TACs) are designed to limit the quantity of fish to be harvested, usually on an annual basis. This can result in undesirable outcomes, such as high-grading – or the selection of more valuable fish for sale – and increased discarding.