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Interspecific interactions can be based on positive or negative outcome. Within antagonist interactions, predation refers to a predator attacking and feeding on a prey while competition is an interaction where individuals compete for a common resource. Worldwide distributions of long-finned pilot whales and killer whales rarely overlap, and they are not known to feed on the same preys. However, in this study, we described the interactions between long-finned pilot whales and killer whales in the Strait of Gibraltar. The former was seen pursuing away the latter in all observations. The main hypotheses for the cause of these interactions are predation or competition. To test both hypotheses, movement patterns and isotopic niches of both species were investigated in the Strait of Gibraltar through satellite tagging and stable isotopes, respectively. Satellite tracks showed no overlap between one tagged pilot whale and one tagged killer whaler's distributions during 21 days. Similarly, Euclidian distances between centroids of Bayesian standard ellipse areas of carbon and nitrogen stable isotopes were significantly different from zero, showing different isotopic niches for each species. This shows that no competition for the resources should exist between both species in the Strait of Gibraltar and that they do not feed on each other, suggesting that the interactions would not be related to predation. A possible historical presence of marine mammal-eating killer whales in the area, today disappeared, could explain the antipredator defense mobbing-like behavior of pilot whales observed in the Strait. [ABSTRACT FROM AUTHOR]