A study compared paired daytime and nighttime counts of electric eel, peacock eel, and black spotted eel made by the same divers in seven lakes during June 2015. Overall, they counted 126 eels during the day and 291 eels at night. The researchers speculate that eels counted at night were present during the daytime, but were hidden from view. Biologists should consider that eel 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 eels 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.