Endogenous versus Exogenous Natural Mortality and Weight in Bioeconomic Models

Beyond age‐structured single‐species management: Optimal harvest selectivity in the face of predator–prey interactions

“…Starting with zero predation and zero weight-conversion, the plots show that it is most beneficial to apply the "Target big" selectivity mode. This result reflects the classical finding in the single-species literature; spare the young fish and target the older and bigger ones (Bang & Steinshamn, 2022;Diekert et al, 2010;Helgesen et al, 2018;Reed, 1980;Skonhoft et al, 2012). However, when predation increases (right-wards movement on the axis labeled "Level of predation," which is defined by the scaling factor SC P , which along with the baseline predation coefficients determine the scaled predation coefficients in Equation 13, as illustrated in Figure 1), the net benefit of applying the "Target big" selectivity mode shrinks relative to the alternative "Target small" mode, where younger and smaller predator individuals are also targeted (ref.…”

“…The natural mortality rates are considered exogenous and constant. Bang and Steinshamn (2022) show that assumptions of exogenous natural mortality rates can lead to significant overestimation of the biological and economic potential of fish stocks, which makes it clear why such factors should be considered endogenously in models that are intended for applied and practical analysis. However, the focus in this study is more conceptual and theoretical than practical, and we choose to treat these factors exogenously to allow clear focus on the objectives of this study, which encompass the effects of predation, weight‐conversion, and relative prices on optimal harvesting schemes and preferred selectivity.…”

“…In relation to numerical specifications, it should be noted that these are only partly inspired by cod and capelin. Some of the numerical values, like, base weights and predator recruitment, are set based on Bang and Steinshamn (2022) and ICES (2021). Other values, like those pertaining to predation and weight-conversion, are based on reason for conceptual and illustrative purposes.…”

“…Optimal age‐ and size‐specific harvest selectivity is an important topic in bioeconomic age‐structured fisheries research. However, the bulk of these studies are primarily centered on single‐species analysis (Bang & Steinshamn, 2022; Diekert, 2013; Diekert et al, 2010; Helgesen et al, 2018; Reed, 1980; Skonhoft et al, 2012; Tahvonen et al, 2013, just to mention a few). Multispecies considerations have, however, been notably absent from this discourse.…”

“…In single‐species studies, a common finding is that it is best to leave younger fish for future harvest (Bang & Steinshamn, 2022; Clark, 2010; Diekert, 2013; Diekert et al, 2010; Helgesen et al, 2018; Reed, 1980; Zimmermann et al, 2011; among others). This makes the most of the growth potential of individual fish and the whole stock.…”