Inclusion of discards in stock assessment models

A large portion of the catch in many stocks may comprise discards which need to be accounted for in assessments in order to avoid bias in estimates of fishing mortality, stock biomass and reference points. In age structured assessment models, discards are sometimes treated as a separate fleet or are added to the landings before fitting so that information about discard behavior and sampling error is lost. In this paper an assessment model is developed to describe the discard process with size as a covariate while retaining age structured population dynamics. Discard size selection, high grading and bulk dumping of fish at sea are modelled so that the temporal dynamics of the process can be quantified within the assessment. The model is used to show that discarding practices have changed over time in a range of Northeast Atlantic demersal fish. In some stocks there is a substantial increase in high grading and evidence for bulk discarding which can be related to regulatory measures. The model offers a means of identifying transient effects in the discard process that should be removed from both short-term forecasts and equilibrium reference point calculations