Factors for the variability of discards: assumptions and field evidence.

Discarding is an issue of increasing concern and there is a growing number of studies aiming at estimating discard amounts and characteristics. However, the sampling design and methods used in these studies generally rely on implicit assumptions. In this perspective, we examine the available evidence in favour of or refuting these assumptions. We find that (i) the assumptions most commonly used for estimating discards, namely that discards are proportional to catch or to effort, are generally not supported by the available evidence, (ii) both environmental conditions and fishing methods influence the amounts and composition of discards, but because of the huge variability, sampling stratification according to these factors might not result in any improvement of the precision of discard estimates, and (iii) many intricate factors can play a role in determining discards in a particular fishery. We conclude that assumptions should be more carefully checked prior to being taken for granted in discard studies and that more studies designed to improve knowledge of the discarding processes are needed. [ABSTRACT FROM AUTHOR]