Bottom-up interactions in age-structured stock assessment and state-space mass-balance modelling

Age-structured stock assessment models are used worldwide to predict the likely impact of changing harvest on future fisheries yield. However, age-structured models ignore the impacts of predator consumption on prey survival (top-down impacts) and prey availability on predator growth (bottom-up impacts), whereas multispecies statistical catch-at-age models often incorporate top-down but not bottom-up impacts. Here, we address this gap by demonstrating a generic approach for including bottom-up interactions in an age-structured statistical model by linking individual growth to population-scale consumption. We specifically extend Ecostate, a recent model that adapts Ecopath/Ecosim dynamics to jointly estimate biological and fishery parameters as well as unexplained process errors. We first add age-structured dynamics for select species using stanzas, i.e., an age-range over which age-structured productivity and consumption match mass-balance constraints. We then incorporate likelihood components representing fit to age-composition and empirical weight-at-age data while also estimating residual variation in larval survival (recruitment deviations) and consumption (weight-at-age deviations). To demonstrate, we fit to abundance-index and age-composition data for two commercial species (Alaska pollock and sablefish) in the Gulf of Alaska, including mass-balance dynamics for its primary energetic supply, and not fitting weight-at-age data so that it can be used for out-of-sample evaluation of model performance. We show that the model can be viewed as a multispecies age-structured model (e.g., estimating adult mortality rates, survey catchability and selectivity, and biomass while tracking cohorts) and as a mass-balance ecosystem model (e.g., estimate trophic position and weight-at-age based on forage consumption). The predicted weight-at-age is weakly correlated with independent measurements for pollock and sablefish, but were improved when we incorporated forage biomass indices. We conclude that bottom-up interactions can be added to age-structured stock assessment models, and can address new questions regarding forage availability on weight-at-age for use in stock assessments. DOI https://doi.org/10.32942/X2R03K Subjects Ecology and Evolutionary Biology, Life Sciences, Marine Biology, Population Biology

Multispecies model, Ecopath with Ecosim, mass balance, state-space model, bottom-up interactions, age-structured dynamics Dates Published: 2025-01-20 11:20 Last Updated: 2026-01-13 20:04 Older Versions License CC-BY Attribution-NonCommercial-ShareAlike 4.0 International Additional Metadata Conflict of interest statement: None Data and Code Availability Statement: Ecostate is publicly available: https://github.com/James-Thorson-NOAA/EcoState Language: English