这一点很重要，因为理论表明，当目标鱼种的生物量为其原始水平的50%左右时，渔场才能达到可收获的最大可持续产量。
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.
相对于只管理目标鱼种，海洋生态系统的管理需要以下几点。
The management of ecosystems as opposed to managing only target species entails.
由于渔具不同的选择性和多鱼种鱼类共栖，人们在对目标种类进行捕捞时，不可避免地产生副渔获物及其抛弃。
Because of the selectivity of fishing gear and mixed distribution of fish species, there will be inevitably by-catches and discards when people catch the target species.
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