Spare the young fish? Maybe not. An age-structured predator-prey analysis

This study presents a bioeconomic, age-structured, multi-fleet, predator-prey model. By solving the model for a range of scenarios and parameter values, we show that preferred selectivity and optimal harvesting change with the levels of predation and predation-weight conversion rates. While the model reproduces insight from age-structured single-species and biomass predator-prey models, it also shows that positive scaling of age-specific predation coefficients can shift the preferred selection pattern towards smaller predator individuals and increase the overall fishing pressure for the predator. This involves sacrificing utilization of predator growth potential to achieve better utilization of prey growth potential, both at an individual level and at the stock level. In addition, it involves sacrificing predator harvest efficiency to achieve better prey harvest efficiency. The model also shows that positive scaling of predation-weight conversion rates can counteract the abovementioned. To the best of our knowledge, this represents novel findings. The findings are important because they bring awareness to why managers should think twice before changing gear restrictions in direction of targeting bigger fish on basis of single-species analyses, in which selectivity studies are common. Moreover, they display the usefulness and value of age-structured multi-species modeling, which has received limited attention in the bioeconomic literature so far, as opposed to age-structured single-species modeling and biomass multi-species modeling.