Quantifying causes of discard variability: an indispensable assistance to discard estimation and a paramount need for policy measures

Fishery-dependent data underpin the scientific advice given to fishery managers. However, discard estimates are often imprecise as a
result of limited sampling coverage. Estimating discard rates from length frequency distributions (LFDs) in commercial catches may
complement information from observer trips. The accuracy of estimates depends greatly on careful investigation of the discard variability.
Here, the impact of three essential factors was quantified for beam-trawl fisheries in the southern North Sea: (i) market prices,
(ii) landings per trip (LPT) limitations, and (iii) selectivity of the commercial fishing gear. Observed discard rates for cod, plaice, sole,
and whiting were compared with estimates based on length frequency data, taking account of the variability attributable to LPT limitations
and market price. Observed discard estimates of cod and whiting differed significantly from LFD-derived estimates because of
highgrading. The results indicate that LFD-derived discard estimates are only reliable if the crucial driving factors are quantified. LFDs
can be collected from research vessels or by fishers in partnership with scientists. Based upon many of these LFDs and the discardvariability
factors identified in observer programmes, discard rates can be estimated better.