21. That matters because theory suggests that the maximum sustainable yields that can be cropped from a fishery comes when the biomass of a target species is almost 50% of its original levels.
【分析】多重复合句。句子主干是 That matters。because 引导原因状语从句，其中又包含 that 引导的从句作 suggests 的宾语。宾语从句主干为 the maximum sustainable yields...comes...，其中又嵌套 that 引导的定语从句修饰主语 yield，另外还包含 when 引导的时间状语从句。
【译文】这一点很重要，因为理论认为目标物种的生物量达到原始基数的50%左右时，从渔场中才能够获得最大的可持续产量。
【点拨】crop 作名词时，意为“庄稼；收成；产量；一群，一批”。作动词时，意为“收获，获得；减短”，此外，还有短语 crop up“出现或发生（尤指意外地）”。

22. But somewhere from the 19th century onward, more artists began seeing happiness as insipid, phony or, worst of all, boring as we went from Wordsworth's daffodils to Baudelaire's flowers of evil.
【分析】复合句。句子主干为 more artists began seeing happiness as...，其中使用了 see...as...结构。句首短语 somewhere from the 19th century onward 作时间状语；为时间状语从句 as we went from...to...。
【译文】但到了19世纪的某个时期，更多的艺术家开始将快乐视为索然无味、矫揉造作——甚至更糟的是——令人厌倦的东西。从华兹华斯的《水仙花》到波德莱尔的《恶之花》就可以表明这一变化。
【点拨】1)insipid “枯燥乏味的”。2)phony“虚伪的，虚假的”。