Optimal fishery management accounting for variation in natural mortality: the Baltic sprat and herring case

Economic–ecological modelling has received increasing attention in the effort to achieve sustainable fisheries. So far, mainly single‐species models have been used, which do not account for species interaction and/or climate change. However, both of these processes alter the associated natural mortality rates (M) and have the potential to strongly influence stock dynamics. Many traditional economic fishery models have been criticized by biologists, especially if results were gained by rather simple biomass models. Biological assessment models, on the other hand, rarely explicitly take into account economic considerations. To overcome these shortcomings, we have developed an age‐structured, economic–ecological model that accounts for the dominant processes affecting natural mortality. With the goal of ultimately providing the most appropriate management advice for the operating fishery possible, we adopt an economic objective function (present value of resource rents) and determine optimal management. In the Baltic Sea, mortality rates of sprat and young herring are strongly influenced by adult cod stock dynamics via predation. Furthermore, both clupeid stocks show temperature‐dependant stock–recruitment relationships. We simulated stock trajectories for a period of 30 years for two different natural mortality scenarios: (i) high vs. low cod abundance and associated changes in predation mortality M2 and (ii) high vs. low temperature scenarios and associated changes in the stock–recruitment function. By applying the age‐structured ecological–economic model, we derive the optimal management strategy in terms of net present value of resource rents. We compare the relative importance of both processes (i.e. temperature increase via global change and variation in predation pressure via management of the cod stock) on optimal management