Question

Rerouting a river could affect fishing in the river downstream because

Answer 1

Downstream river would have less water, so there would be less fish to catch.

River re-routing is a process wherein the mouth of the river is mouth to another location. Thus, existing mouth of the river will lose its water source and have less water. Fishes in the existing river path will decrease because of the decreased level of water. 

Answer 2

There are many possibilities how rerouting a river could affect fishing in a river. However, from the answers you've provided, less water being available because of the rerouting of a river is likely the most correct answer - rerouting means less water which in effect means less fish to be found.