Northwest Fisheries Science Center

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Document Type: Journal Article
Center: NWFSC
Document ID: 4307
Title: Hiding or dead? A computationally efficient model of selective mortality
Author: Ian G. Taylor, R. D. Methot
Publication Year: 2013
Journal: Fisheries Research
Keywords: Stock Synthesis, Rosa Lee's Phenomenon, selective mortality, dome-shaped selectivity,

100 years after Rosa Lee (1912) showed that higher mortality on faster growing fish can alter length-at-age distributions in fish populations, we present a computationally-efficient and parsimonious method for modeling size-selective mortality within a commonly-used assessment model, Stock Synthesis. Stock Synthesis allows the normal distribution of length-at-age to be partitioned into three or five overlapping platoons with slow, medium, or fast growth trajectories.  The platoons are tracked separately in the model, and experience different degrees of size-selective fishing pressure and mortality, but are assumed to be unobservable except through changes in the length distribution. Simulations are used to explore this phenomenon in conjunction with dome-shaped selectivity, an alternative explanation for observing fewer than expected large fish in sampled data, but with very different implications for population productivity. For data simulated both with and without platoons, misspecification of the assumptions about growth are found to bias model results, with selectivity often incorrectly identified as the cause of fewer observations of larger fish. Trends in dome-shaped selectivity were explored as a potential diagnostic of model misspecification.

Theme: Sustaining Marine Ecosystem and Human Health
Foci: Develop methods, technologies, and data integration tools to predict ocean-related public health risks into health early warning and ocean observing systems