Question

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

Explanation:

Fish distribution in a river is related to water conditions. At the mouth, water is warmer, slower - flowing, and has more sediment and nutrients. Catfish and carp are adapted to these conditions. Near the source, water is colder, faster - flowing, and cleaner. Trout are adapted to cold, oxygen - rich water.

Answer:

Catfish and carp are found at the river mouth because the water there is warmer, slower - flowing, and has more sediment and nutrients which they are adapted to. Trout are found near the source as they are adapted to cold, fast - flowing, and clean water.