Final answer:
Catfish and carp are found at the mouth of a river due to their adaptation to brackish water, while trout are found nearer the source of the river as they require clean freshwater.
Step-by-step explanation:
Catfish and carp are found at the mouth of a river because they are both adapted to brackish water, which is a mixture of freshwater and saltwater. The mouth of a river is where freshwater from the river meets the saltwater from the ocean, creating a brackish environment. Catfish and carp can tolerate the varying salinity levels in this area.
On the other hand, trout are found nearer the source of the river because they are adapted to pure freshwater environments. Trout require clean, highly oxygenated water with specific temperature conditions. These conditions are more prevalent in the upper reaches of a river where the water is flowing faster and is less polluted.
Learn more about Distribution of fish species in a river system