如何理解句中的三个would (not) have done结构

问题描述

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.

请问：如何理解句中的三个 would (not) have done 结构？

老师答疑

周老师：

译文：在早期时期，也是因为缺乏长线多钩工具而（未）能抓更多的鱼，因而很多鱼没有被抓到。由于没有更多的钓钩用来抓捕很多鱼土幸免被抓。这些都导致对过去鱼的总量低估