Estimating Discards Using Selectivity Data: the Effects of Including Discard Data in Assessments of the Demersal Fisheries in the Irish Sea

Abstract

A model is described to predict the numbers-at-age of fish discarded in fisheries for which only landings-at-age are known. The model assumes a knowledge of the distribution of length within each age group in the population, and uses mesh selection characteristics and an inferred discarding practice, to derive the proportions of each age group discarded and landed. Mesh selectivity and discarding practice, in terms of proportions of the catch discarded and landed, are described using a logistic fit to the proportions of the population entering the net that are caught, discarded and landed. The application of the model is illustrated using data from the mixed demersal fisheries in the Irish Sea, and the effects of including discards in the assessments and on yield-per-recruit are examined. Including discard estimates in the assessment results in increased estimates of stock numbers and increased fishing mortality on age groups that were subject to discarding. As a result the perceived level of exploitation relative to Fmax was altered.