因为当时没有（充足的）饵钩来诱他们上钩，所以有些鱼就未被捕捞到，以致低估了过去的鱼藏量。
将这句重新编排会较易理解：
Since no baited hooks would have been available to trap them, therefore some individuals (fish) would not have been caught, leading to an underestimate of fish stocks in the past。
  
这意思是说：
基于延绳钓法（longlines）的局限，假如当时的饵钩是足够的话，有些鱼就没这么容易逃得过去。。。
部份原文如下：
Dr。 Worm acknowledges that these figures are conservative。
   One reason for this is that fishing technology has improved。 Today’s vessels can find their prey using satellites and sonar, which were not available 50 years ago。
   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。
   Furthermore, in the early days of longline fishing, a lot of fish were lost to sharks after they had been hooked。 That is no longer a problem, because there are fewer sharks around now。
  
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