The level of discarding in the fisheries is not known, and no discards are accounted for in the assessments. Both undersized fish and bycatch of other species can lead to discarding, and also low-paid fish just above the minimum size has been subject to discarding in order to fill the quota with larger and better paid fish (known as highgrading).
Discarding is known to be a (varying) problem, e.g. in the 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 in 1983–2002. The lack of discard estimates leads to less precise and accurate stock assessments. The influence of the fishery on the ecosystem is hence not fully understood. A possible way to estimate values of discarded fish is analysis of landing information (size/weight composition of landings in relation to observe on board fishing vessels). Norway is conducting a pilot project to estimate the discards in some selected fisheries to test and establish methods for estimating discards in all Norwegian fisheries on a routine basis in near future.
Registration of redfish (dominated by S. mentella) taken as bycatch and discarded in the Norwegian shrimp fishery in the Barents Sea since 1984 show that shrimp trawlers removed significant numbers of juvenile redfish during the beginning of the 1980s. This peaked in 1984, when bycatches amounted to about 640 million individuals, a number similar to a good year class of this stock (Figure 18.104.22.168). As sorting grid became mandatory in 1993, bycatches of redfish were reduced drastically during the 1990s. The results also show that closure of areas is necessary to protect the smallest redfish juveniles since these are not sufficiently protected by the sorting grid. The bycatch 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 discards of cod in the shrimp fishery occurred in 1985, 1996–1998. The highest recorded numbers of cod was in 1997 (157 millions). The cod bycatches have declined in recent years (<20 millions). 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). Discards of Greenland halibut in the Barents Sea shrimp fishery have been estimated for the period 2000–2005, and show the highest discard in 2002 and 2000 of about 13 million specimens.
Even if the sorting grid prevents discarding of fish larger than about 18 cm, it becomes obvious that only an effective surveillance and closure of areas for shrimp fishing that can prevent bycatch and discarding of smaller specimens.