Decreasing Selectivity at Large Sizes or Higher Mortality on Fast-Growing Fish?

Two processes may explain observations of fewer large fish than expected in assessment models: dome-shaped selectivity in which the large fish escape capture, or higher mortality on faster growing fish that become selected at an earlier age. Simulation methods are used to explore the confounding between these two processes. The simulation model includes options for time-varying dome-shaped selectivity as well as growth morphs which allow tracking of numbers at age for fish with different growth rates. When time-varying selectivity is estimated, a decrease in the selectivity of large fish over time may be a mis-diagnosis of higher mortality on faster-growing fish.