Combining area closures with catch regulations in fisheries with spatio-temporal variation: Bio-economic implications for the North Sea saithe fishery

Although there exists an EU-Norway long-term management plan for North Sea saithe, including a catch regulation, the spawning stock biomass has declined in the last few years, recruitment has been below average since 2006 and growth rates are low. Moreover, catch rates used as a proxy of stock abundance in stock assessment, are believed to decline much more slowly than the actual stock abundance. Thus, a quota-based system may not be sufficient to sustain the stock. A bio-economic simulation and optimisation model was used to explore how various area closures in combination with the quota-system affect levels of by-catch, net profit of individual fleet segments from different ports, and stock development in that fishery. Tested area closures differed in duration, size and location relative to major ports and to seasonal movement patterns of species. These closures were tested under variable recruitment. Area closures that were covering the seasonal migration route of saithe revealed almost two times greater increases in spawning stock biomass than closures that were not covering the migration route. Even area closures where a high dispersal rate of individuals was assumed resulted in increased spawning stock biomass of saithe. Benefits of the tested area closures were distributed heterogeneously among individual fleet segments. Increases of saithe stock size were offset by increases in cod by-catch. The location of an area closure relative to the home port of fleet segments decided if steaming costs increased and catches decreased.