Various models of foraging theory have been used to predict the optimal foraging behavior of predators. Foraging theories, however, have only taken in account the behavior of an individual predator and its surrounding prey distribution. In nature, several animals gather to feed in an area of high prey density. Encounter with other animals may cause disturbance or increase competition, yet this was never taken in account in prevailing studies. Therefore in this study, we compared the foraging behavior of humpback whales (Megaptera novaeangliae) between conditions with presence or absence of competitors. Seven humpback whales from Skjálfandi bay (66°05’ N, 17°19’ W), Iceland, were tagged in 2013 and 2014 and a total of 82 hours of behavioral data was obtained from seven whales and a total of 45 hours of video data from six of them, which was used to extract the feeding events and estimate the relative prey density and presence or absence of competitors in front of the whales. As a result, humpback whales were maximizing their mean energy intake per unit time when alone; whereas when they encountered other whales or dolphins, they left the patch earlier taking in account the potential reduction of prey.