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.