Revealing trophic interactions among sympatric odontocetes in an oceanic ecosystem through stable isotope analysis

Abstract Odontocetes are key predators in marine food webs, but knowledge of their habitat use and trophic niche still needs to be improved. In this study, we used carbon (δ 13 C) and nitrogen (δ 15 N) stable isotopes to identify the trophic position, isotopic niche size, foraging habitat, and potential sex‐based differences in the dietary patterns of six odontocete species in the Madeira Archipelago: short‐beaked common dolphin ( Delphinus delphis ), Atlantic spotted dolphin ( Stenella frontalis ), common bottlenose dolphin ( Tursiops truncatus ), false killer whale ( Pseudorca crassidens ), short‐finned pilot whale ( Globicephala macrorhynchus ), and sperm whale ( Physeter macrocephalus ). Results delineated two distinct groups. The first (common dolphin, spotted dolphin, bottlenose dolphin) exhibited lower δ 13 C and δ 15 N values, lower trophic positions, and larger isotopic niches, indicating that these species are generalist predators and rely more on the pelagic food web. The second group (false killer whale, short‐finned pilot whale, sperm whale) displayed higher isotopic values and trophic positions, with smaller isotopic niches, relying more on shallow‐to‐deep mix food webs and displaying a more specialist diet. This distinction was further supported by comparisons with stable isotope values of potential prey species. However, for deep‐diving species such as short‐finned pilot whale and sperm whale, some prey species were not represented in the niche models due to lack of available samples. The isotopic niche overlap was highest for two pairs of species: spotted and bottlenose dolphin, probably related to their generalist diet and temporal co‐occurrence; false killer whale and short‐finned pilot whale, from which no interspecific competition is expected due to differing habitat preferences. No sex‐related differences were found in the five species analyzed for either isotope. This study characterizes the trophic ecology of several oceanic odontocetes in the Eastern North Atlantic and establishes a valuable baseline for future research. Understanding the trophic interactions of these species offers important insights into their ecological roles and the dynamics of the marine ecosystem they inhabit.