Answer
The line exerts an upward force on the fish. The interaction partner of this force is the downward force exerted on the line by the fish.
The earth exerts a gravitational force on the fish that is directed downward. The interaction partner of this force is the upward gravitational force exerted on the earth by the fish.
Work Step by Step
When the fish is suspended on the line, there are two forces acting on the fish.
The line exerts an upward force on the fish. The interaction partner of this force is the downward force exerted on the line by the fish.
The earth exerts a gravitational force on the fish that is directed downward. The interaction partner of this force is the upward gravitational force exerted on the earth by the fish.
