A study compared paired daytime and nighttime counts of sea trout, lake trout, and rainbow trout made by the same divers in six lakes during September 2015. Overall, they counted 127 trout during the day and 356 trout at night. The researchers speculate that trout counted at night were present during the daytime but were hidden from view. Biologists should consider that trout behavior and susceptibility to being seen might vary a great deal between daytime and nighttime, even during the summer. In some lakes, the majority of trout may not be seen during the daytime. Determine whether the study is an observational study or an experiment. Justify your answer with specific references to the information in the study.