Time-varying natural mortality in fisheries stock assessment models: Identifying a default approach

Kelli F. Johnson, Cole C. Monnahan, Carey R. McGilliard, Katyana A. Vert-Pre, Sean C. Anderson, Curry J. Cunningham, Felipe Hurtado-Ferro, Roberto R. Licandeo, Melissa L. Muradian, Kotaro Ono, Cody S. Szuwalski, Juan L. Valero, Athol R. Whitten, A. E. Punt

Research output: Contribution to journalArticlepeer-review

82 Scopus citations


A typical assumption used in most fishery stock assessments is that natural mortality (M) is constant across time and age. However, M is rarely constant in reality as a result of the combined impacts of exploitation history, predation, environmental factors, and physiological trade-offs. Misspecification or poor estimation of M can lead to bias in quantities estimated using stock assessment methods, potentially resulting in biased estimates of fishery reference points and catch limits, with the magnitude of bias being influenced by life history and trends in fishing mortality. Monte Carlo simulations were used to evaluate the ability of statistical age-structured population models to estimate spawning-stock biomass, fishing mortality, and total allowable catch when the true M was age-invariant, but time-varying. Configurations of the stock assessment method, implemented in Stock Synthesis, included a single age- and time-invariant M parameter, specified at one of the three levels (high, medium, and low) or an estimated M. The min-max (i.e. most robust) approach to specifying M when it is thought to vary across time was to estimate M. The least robust approach for most scenarios examined was to fix M at a high value, suggesting that the consequences of misspecifying M are asymmetric.

Original languageEnglish (US)
Pages (from-to)137-150
Number of pages14
JournalICES Journal of Marine Science
Issue number1
StatePublished - Nov 4 2014
Externally publishedYes


  • Stock Synthesis
  • model misspecification
  • natural mortality
  • population models
  • reference points
  • simulation
  • time-varying

ASJC Scopus subject areas

  • Oceanography
  • Ecology, Evolution, Behavior and Systematics
  • Aquatic Science
  • Ecology


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