17. That means a higher proportion of what is in the sea is being caught, so the real difference between present and past is likely to be worse than the one recorded by changes in catch sizes.

【分析】并列多重复合句。由so连接的两个并列分句。第一个分句中 a higher proportion of what is in the sea is being caught 是省略that的宾语从句，of what is in the sea作proportion的后置定语。第二个分句的主干为 the real difference…is likely to be wortle than...，其中含有比较结构worse than；between present and past和过去分词结构recorded by...分别作difference和the one的后重定语。
【译文】这意味着海洋中还有更大比例的鱼类正在被捕杀，因此现在和以前的数据真正的差别有可能比所记录的捕获量的变化更糟。
【点拨】size在本句中是“规模”的意思。

18. In the early days, too, longlines would have been more saturated with fish. Some individuals would therefore not have been caught, since no baited hooks would have been available to trap them, leading to an underestimate of fish stocks in the past.

【分析】第一个句子是简单句,第二个句子是复合句。两个句子都使用了虚拟语气。第二句中包含since引导的原因状语从句，句末leading...是现在分词结构作伴随状语。
【译文】在早期渔业中，同样的，多钩长线上本可以挂满更多的鱼，因此一些鱼也是可以逃脱的，因为没有带鱼饵的钩去捕获它们，这也导致低估了过去存在的鱼类数量。