That matters because theory suggests that the maximum sustainable yield that can be cropped from a fishery comes when the biomass of a target species is about 50% of its original levels.

这确有其事，因为理论告诉我们如果一个渔场的总储量如果连起初的最大可承受的捕鱼量的50％都不到，就应该降低捕鱼数量了。