请教高手们一虚拟句的意思

Low levels of straying

are crucial, since the process

provides a source of novel

genes and a mechanism

      by which a location can be

   repopulated should the fish

there disappear

红字部分的句子该怎么翻译呢？看不动那个should  the fish there disappear。

谢谢NN们指教

Low levels of straying are crucial, since the process provides a source of novel genes and a mechanism by which a location can be repopulated should the fish there disappear。

较低程度上鱼的离群至关重要，因为这个过程提供了新的基因来源和一种机制，通过这种机制，那些鱼群消失的地方能够得以重新繁荣起来。从意思上来讲（语法上略有区别），should=if，不过在这里多少有点even得味道