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 avaliable to trap them, leading to an underestimate of fish stocks in the past.

第一句：would have been more saturated with fish本可以捕获更多的鱼，而实际上捕捞量却没那么多。

第二句：would therefore not have been ....... would have been ..... 并非虚拟语气，而是过去将来完成式，表示从过去某个时间来看将来某时某动作已经完成，即由于带饵鱼钩数量有限，有些鱼未能捕获。

请问虚拟语气和过去将来完成时有什么区别