Choosing the best sampling unit and auxiliary variable for discards estimations

The effects of two sampling units (haul and trip) and three auxiliary variables (total landings in weight, effort in hours fished and in numbers of fishing trips) commonly used in discard estimations were compared. Considering the sampling unit at haul level will increase variability in the final estimation. In addition, there was a significant linear relationship between discards per trip with both landings and effort, but not at haul level. The different auxiliary variables used to raise discard samples to population levels give significantly different results, with effort in hours fished giving the highest discard estimates. Fishing trips was the recommended variable to estimate discards in six out of ten fleets studied. These results highlight the importance of preliminary studies to determine the methodology to be used in discard estimations, in order to avoid considerably imprecise estimations.