In a variable interval, the time at which reinforcement or reward is gained occurs at an unpredictable time. That is the animal gets a reinforcement on the grounds of an unpredictable time period.
An example of a variable interval is the catching of fish whereby the fisher catches fishes at an unpredictable time throughout the day.
The result of a variable interval is a controlled but stable response rate. In the case in the question, constant monitoring of the fishing device is important
