Inclusion of discards in stock assessment models

Research output: Contribution to journalArticle

Abstract

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
LanguageEnglish
JournalFish and Fisheries
Publication statusAccepted/In press - 19 Aug 2019

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stock assessment
Fishes
Selection Bias
Population Dynamics
Oceans and Seas
Biomass
size selection
demersal fish
Mortality
fishing mortality
population dynamics
biomass
sampling
fish

Keywords

  • demersal fish
  • high grading
  • Northeast Atlantic
  • reference points
  • size selection

Cite this

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title = "Inclusion of discards in stock assessment models",
abstract = "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",
keywords = "demersal fish, high grading, Northeast Atlantic, reference points, size selection",
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Inclusion of discards in stock assessment models. / Cook, Robin.

In: Fish and Fisheries, 19.08.2019.

Research output: Contribution to journalArticle

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AU - Cook, Robin

PY - 2019/8/19

Y1 - 2019/8/19

N2 - 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

AB - 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

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KW - high grading

KW - Northeast Atlantic

KW - reference points

KW - size selection

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