Including discard data in fisheries stock assessments: Two case studies from south-eastern Australia

Discarding of target species can be substantial in some fisheries. For fisheries managed using Total Allowable Catches, such as Australia's Southern and Eastern Scalefish and Shark Fishery (SESSF), discarding of target species can occur for reasons related to the size of the fish caught, markets, and the amount of quota held by individual quota holders. This paper illustrates how the assessments for two of the species in the SESSF, blue grenadier, Macruronus novaezelandiae, and the western stock of blue warehou, Seriolella brama, are conducted to take account of discards. Discards of blue grenadier are predominantly small fish so the assessment distinguishes between retained and discarded fish, and includes likelihood components related to the weight of the discards and the age-structure of the discards. In contrast, discards of blue warehou are due primarily to quota-related issues, so the assessment adds the discards to the catches and the catch-rate indices, and fits to total catches and catch-rates. Model outputs suggest that strong year-classes can be detected before they enter the fishery by including data on discards in assessments while ignoring such data when conducting assessments can lead to biased assessment outcomes. Several caveats related to the use of data on discards in stock assessments are outlined.