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.

【翻译】这很重要，因为理论表明，当目标物种的生物量保持在原来水平的50%左右时，从渔场中可获得的最大可持续的产量才能实现。

【分析】这是一个含有原因状语从句的主从复合句。

1. 主句：That matters 这很重要。

2. 原因从句：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.

3. 分析because从句：theory （主语）suggests（谓语）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（宾语从句）。

4. 分析that宾语从句：在该从句中，主要成分是：the maximum sustainable yield（主语）comes（谓语）。

细分：the maximum sustainable yield （主语） that can be cropped from a fishery （定语从句）comes （谓语）when the biomass of a target species is about 50% of its original levels（时间状语从句）

【注意】英汉翻译看颜色对应部分。