The level of discarding in fisheries is not estimated, and discards are not accounted for in stock assessments. Both undersized fish and by-catch of other species can lead to discarding; fish of legal size but low market value are also subject to discarding to fill the quota with larger and more valuable species (known as high-grading).
Discarding is known to be a (varying) problem, e.g., in haddock fisheries where discards are highly related to the abundance of haddock close to, but below, the minimum legal catch size. Dingsør (2001) estimated discards in the commercial trawl fishery for Northeast Arctic cod during 1946-1998, and the effects on the assessment. Sokolov (2004) estimated cod discard in the
Russian bottom trawl fishery in the Barents Sea during 1983-2002. The lack of discard estimates leads to less precise and accurate stock assessments. The influence of the fishery on the ecosystem is, therefore, not fully understood. A possible way to estimate discard is through analysis of landing information (size/weight composition of landings relative to observations made onboard fishing vessels). Norway is conducting a pilot project to estimate discards in selected fisheries to test and establish methods for estimating discards in all Norwegian fisheries on a routine basis.
Since 1984, documentation of redfish (mainly S. mentella) taken as bycatch and then discarded in the Norwegian shrimp fishery, shows that shrimp trawlers removed significant numbers of juvenile redfish during the early 1980’s. This discarding peaked in 1984, when by-catch amounted to about 640 million individuals; a number comparable to a good year class in this stock (Figure 18.104.22.168). After sorting grids became mandatory in 1993, by-catch of redfish was reduced dramatically. It was also shown that areal closure are necessary to protect small juvenile redfish, since they are not sufficiently protected by sorting grids. The by-catch and discard of cod consists mainly of 1- and 2-year-olds, but is generally small compared to other reported sources of mortality like catches, discards in the groundfish fisheries and cannibalism.
Noticeable discard of cod in the shrimp fishery occurred in 1985 and 1996-1998. The highest recorded number of discarded cod was 157 million in in 1997. Cod by-catch has declined in recent years (< 20 million). Discards of haddock in the Barents Sea shrimp fishery have been estimated for the period 2000-2005, and show the highest discard in 2007-2008 (about 200 millions). Discard of Greenland halibut in the Barents Sea shrimp fishery was estimated for the 2000-2005 period; highest discard occurred in 2002 and 2000 (approximately 13 million specimens).
Even if the sorting grid prevents discarding of fish larger than 18 cm, it becomes obvious that only effective surveillance and closing areas for shrimp fishing can prevent bycatch and discard of smaller specimens.