#### Answer

The fish on the line exerts a downward force on the rod. The interaction partner of this force is the upward force exerted on the fish hanging on the line by the rod.
The earth exerts a gravitational force on the rod that is directed downward. The interaction partner of this force is the upward gravitational force exerted on the earth by the rod.
The person exerts an upward force on the rod. The interaction partner of this force is the downward force exerted on the person by the rod.

#### Work Step by Step

When the fish is suspended on the line, there are three forces acting on the rod.
The fish on the line exerts a downward force on the rod. The interaction partner of this force is the upward force exerted on the fish hanging on the line by the rod.
The earth exerts a gravitational force on the rod that is directed downward. The interaction partner of this force is the upward gravitational force exerted on the earth by the rod.
The person exerts an upward force on the rod. The interaction partner of this force is the downward force exerted on the person by the rod.