Modeling Discards in Stock Assessments: Red Grouper Epinephelus morio in the US Gulf of Mexico

To be as accurate as possible, stock assessments should account for discard mortality in fisheries if it occurs. Three common approaches to modeling discards in assessments are to lump dead discards with landings, treat dead discards as their own fleet, or link them conversely with landings through use of a retention function. The first approach (lumping) implicitly assumes that the selectivity of landings applies also to discards. In many cases, that assumption is false, for example, if discards comprise smaller fish than do landings. The latter two approaches avoid the assumption by modeling discards explicitly with their own selectivity pattern. Here, we examine these approaches to modeling discards. Using a simulation study, we demonstrate that the two approaches to modeling discards explicitly can provide identical results under both static and time-varying conditions. Then, using a stock assessment case study of red grouper Epinephelus morio in the U.S. Gulf of Mexico, we demonstrate that in practice the approaches to modeling discards can provide different outcomes, with implications for the resultant management advice. We conclude by comparing and contrasting the different approaches, calling for more research to elucidate which approach is most suitable under various sources of error typically encountered in discard data.

Automated summary

To achieve accurate stock assessments, it is important to consider discard mortality in fisheries. There are three common approaches to modeling discards in assessments: lumping them with landings, treating them as their own fleet, or linking them with landings through a retention function. Simulation studies and a case study of red grouper in the U.S. Gulf of Mexico show that modeling discards explicitly can lead to different outcomes and have implications for management advice.