On the effect on cod and herring fisheries of retuning the Revised Management Procedure for Minke whaling in the Greater Barents Sea

In a model with 4 species: cod, capelin, herring and minke whales, the fish populations are age and length distributed, while the minke whale is age and sex distributed. The time step is one month, and there are two areas (The Barents Sea and parts of the Norwegian Sea). There is a food-web with minke whales as top predators, consuming herring, capelin and cod according to a non-linear consumption function in available prey abundance. The consumption function for minke whales is roughly estimated. The opportunistic minke whale may forage on plankton and other fish than cod, capelin or herring, and is thus modeled as having carrying capacity and demographic parameters independent of the status of the fish stocks in the model.
The fish-fisheries are managed by fixed VPA-based fishing mortalities (cod and herring) and Captool (capelin), while minke whaling is managed according to the RMP of the IWC. The model is stochastic in fish recruitment and in survey indices for minke whales. The model is simulated over 100 year periods in a number of scenarios spanned by 9 experimental factors. The core of the experimental design is an orthogonal array with 27 points. The primary study variable is the tuning of the RMP, and the response variables are catches and stock sizes of cod, herring and minke whale. The responses are taken as yearly means over the last 90 years of the period.
When the tuning of the RMP is changed from the current level of targeting the final stock at 72% of carrying capacity to 60%, the annual catch of whales increases with some 300 animals, while the annual catch of cod increases with some 0.1 million tons on the average. For herring, no clear main effect was found on catch or mortality rate. The catch of cod is estimated to increase in annual mean with some 6 tons with a mean reduction in the whale stock of one animal. The results concerning the effects on the cod and herring fisheries must be taken as tentative since the ecosystem model used could be improved, and so could the strategies for managing the fisheries.
The study exemplifies how scenario experimentation can be used as a tool for investigating the properties of fishery management regimes.