问题描述：
英语翻译
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.
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.
问题解答：
我来补答
翻译：
因此有些鱼（这里的“个体”就是指鱼）没有被捕到——因为（渔线上）没有更多的鱼钩（都被咬住了）,从而导致了对过去的鱼储量的低估.
结构：
Some individuals would therefore not have been caught, 主句,主语是some individuals（有些鱼）；
leading to an underestimate of fish stocks in the past,状语从句,[这些没有被捉到的鱼]导致了对过去鱼储量的低估；
since no baited hooks would have been available to trap them,跟主句是因果关系,这里的them就是指那些鱼（主句主语）.
关于would,下面是我的个人看法：乍一看,句子貌似有would have done的结构,疑似有虚拟结构,但是,结合上下文可以看出,这里没有虚拟条件.其实这是would的一个不是很常用（口语里较多）的一个意思.
下面是American Heritage字典的解释和例句：
8. Used to express repeated or habitual action in the past: “Every morning we would walk in the garden.”
如果还是比较难理解,你可以把这个句子改成现在时,描述现状：
Some individuals therefore have not been caught, since no baited hooks have been available to trap them, leading ... .
所以这里不用去想是否有该捉未捉的问题,would只是用来描述对过去常态发生的动作.
[再补充一下]：
原句在考研英语里出现过,但是原文出处可能是这里：
再问： 这里的would have been available 也不是虚拟吗
再答： 我认为不是，虚拟语气所对应的虚拟条件是条件（状语）从句的一种特例。英文称为Counterfactual Conditional，维基上有下面两个不同时态的例子，分别为陈述和虚拟语气：1. 过去状态：
If Oswald did not shoot Kennedy, then someone else did. [陈述]“如果不是Oswald行刺Kennedy，那么凶手另有他人。”这里可以肯定地是Kennedy被shot了，凶手不确定。
If Oswald had not shot Kennedy, then someone else would have. [虚拟]"若不是Oswald刺杀了Kennedy，别人也会干的。“而事实是，Oswald杀了Kennedy，别人没有杀。这里首先肯定了Oswald杀了Kennedy，再说明也有其他人也想刺杀Kennedy.
2. 现在时的用法：If it is raining, then he is inside. [陈述]”如果外面在下雨，那么他肯定呆在里面。“说话的人不知道是否”外面下雨“，只是给出一个可能性。
If it were raining, then he would be inside. [虚拟]”如果外面正在下雨的话，他就会呆在里面了。“而实际情况是，（说话的人也知道），外面不在下雨，他也不在屋里。
所以虚拟语气的出现都是对已知事实相反的推论。在这里如果是虚拟语气的话，句子应该是：Some individuals would therefore have not been caught, if no baited hooks were available.但是这样的话，句子的意思是”这些鱼其实被捉到了“，这个跟原文的意思不一致。
另外一个判断虚拟语气的办法是看看是否满足(if-then)条件句式，有些从句采用倒装会把if省掉，但是判断的时候可以加入if-then。原文中不是这个意思: 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. 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.
关于longline fishing，看下图（
因此有些鱼（这里的“个体”就是指鱼）没有被捕到——因为（渔线上）没有更多的鱼钩（都被咬住了）,从而导致了对过去的鱼储量的低估.
结构：
Some individuals would therefore not have been caught, 主句,主语是some individuals（有些鱼）；
leading to an underestimate of fish stocks in the past,状语从句,[这些没有被捉到的鱼]导致了对过去鱼储量的低估；
since no baited hooks would have been available to trap them,跟主句是因果关系,这里的them就是指那些鱼（主句主语）.
关于would,下面是我的个人看法：乍一看,句子貌似有would have done的结构,疑似有虚拟结构,但是,结合上下文可以看出,这里没有虚拟条件.其实这是would的一个不是很常用（口语里较多）的一个意思.
下面是American Heritage字典的解释和例句：
8. Used to express repeated or habitual action in the past: “Every morning we would walk in the garden.”
如果还是比较难理解,你可以把这个句子改成现在时,描述现状：
Some individuals therefore have not been caught, since no baited hooks have been available to trap them, leading ... .
所以这里不用去想是否有该捉未捉的问题,would只是用来描述对过去常态发生的动作.
[再补充一下]：
原句在考研英语里出现过,但是原文出处可能是这里：
再问： 这里的would have been available 也不是虚拟吗
再答： 我认为不是，虚拟语气所对应的虚拟条件是条件（状语）从句的一种特例。英文称为Counterfactual Conditional，维基上有下面两个不同时态的例子，分别为陈述和虚拟语气：1. 过去状态：
If Oswald did not shoot Kennedy, then someone else did. [陈述]“如果不是Oswald行刺Kennedy，那么凶手另有他人。”这里可以肯定地是Kennedy被shot了，凶手不确定。
If Oswald had not shot Kennedy, then someone else would have. [虚拟]"若不是Oswald刺杀了Kennedy，别人也会干的。“而事实是，Oswald杀了Kennedy，别人没有杀。这里首先肯定了Oswald杀了Kennedy，再说明也有其他人也想刺杀Kennedy.
2. 现在时的用法：If it is raining, then he is inside. [陈述]”如果外面在下雨，那么他肯定呆在里面。“说话的人不知道是否”外面下雨“，只是给出一个可能性。
If it were raining, then he would be inside. [虚拟]”如果外面正在下雨的话，他就会呆在里面了。“而实际情况是，（说话的人也知道），外面不在下雨，他也不在屋里。
所以虚拟语气的出现都是对已知事实相反的推论。在这里如果是虚拟语气的话，句子应该是：Some individuals would therefore have not been caught, if no baited hooks were available.但是这样的话，句子的意思是”这些鱼其实被捉到了“，这个跟原文的意思不一致。
另外一个判断虚拟语气的办法是看看是否满足(if-then)条件句式，有些从句采用倒装会把if省掉，但是判断的时候可以加入if-then。原文中不是这个意思: 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. 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.
关于longline fishing，看下图（
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