Why are catfish and carp found at the mouth of a river, while trout are found nearer the source?

The correct reason would be "the levels of dissolved oxygen at the mouth of the river and at the source of the river".

The mouth of the river is the region where it flows into the ocean, lake, another river, reservoir, sea et cetera. The level of dissolved oxygen in this region is too low to support great biodiversity.

Organisms like catfish and carp can survive here due to their low oxygen demand. For example, catfish can survive in water with an oxygen content of 2.5 parts per million.

In contrast, the level of dissolved oxygen is higher at the source of the river. Organisms like trout are found here due to their high oxygen need. For example, rainbow trout require minimum 5-6 parts per million dissolved oxygen for its survival.