Endogenous versus Exogenous Natural Mortality and Weight in Bioeconomic Models

We present an age-structured multi-fleet model with cannibalism mortality and endogenous weight at age. Using the model and three simplified versions, we show that assumptions of exogenous natural mortality and weight can lead to significant underestimation of optimal fishing mortality in both maximum sustainable yield (MSY) and maximum economic yield (MEY) scenarios for long-lived cannibalistic fish such as the Northeast Arctic cod. In addition, we show that the harvest, spawning stock biomass (SSB), and net present value (NPV) levels associated with optimal exploitation rates increase significantly with assumptions of exogenous natural mortality and weight. The underestimation of optimal fishing mortality, and the corresponding overestimation of SSB and NPV, is more significant in MSY than MEY scenarios. Meanwhile, the overestimation of harvest is more significant in MEY than MSY scenarios. The study also confirms that the Northeast Arctic cod fishery can achieve higher sustainable yield and NPV by changing the fleet composition and target reference points.