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】并列多重復合句。由so連接的兩個并列分句。第一個分句中 a higher proportion of what is in the sea is being caught 是省略that的賓語從句，of what is in the sea作proportion的后置定語。第二個分句的主干為 the real difference…is likely to be wortle than...，其中含有比較結構worse than；between present and past和過去分詞結構recorded by...分別作difference和the one的后重定語。
【譯文】這意味著海洋中還有更大比例的魚類正在被捕殺，因此現在和以前的數據真正的差別有可能比所記錄的捕獲量的變化更糟。
【點撥】size在本句中是“規模”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】第一個句子是簡單句,第二個句子是復合句。兩個句子都使用了虛擬語氣。第二句中包含since引導的原因狀語從句，句末leading...是現在分詞結構作伴隨狀語。
【譯文】在早期漁業中，同樣的，多鉤長線上本可以掛滿更多的魚，因此一些魚也是可以逃脫的，因為沒有帶魚餌的鉤去捕獲它們，這也導致低估了過去存在的魚類數量。