Comments on incongruous formulations in the SAM (state-space assessment model) model and consequences for fish stock assessment

The stock assessment model SAM (state-space assessment model) is an important tool for analysing fishery data and is used for abundance estimation of several different fish stocks. We demonstrate incongruous formulations in the equations that relate the catch data to fish abundance, and in the fishery and natural mortality rates. We present example results to show how these formulations introduce bias, and compound the uncertainty in model estimates. Finally, we provide corrections to the system equations to remedy the drawback.