Coexisting dolphin species do not always engage in food rivalry

In a groundbreaking study, researchers from the Federal University of Rio Grande, Brazil, collaborated with scientists from the NIOZ to investigate the trophic levels of various toothed whale species in the Southwest Atlantic Ocean. The research, published in the journal Environmental Research, utilised a novel method involving stable nitrogen isotopes of the amino acid phenylalanine.

Phenylalanine, a building block of every living cell, passes through the food web almost unchanged up to the level of top predators. By analysing the nitrogen isotopes in skin tissue samples from sperm whales, orca, and eight dolphin species, the researchers were able to determine their positions in the food chain.

The amino acid isotope method solves a problem that has long plagued marine researchers: determining the isotope level of the original food in marine environments. However, the accuracy of this method requires knowing the isotope level of the original food, which can be difficult to determine and may vary in marine environments. To overcome this challenge, the scientists use the nitrogen isotope value of amino acids to determine the base value of nitrogen in the environment, correcting for unknown living environments.

The study found some surprising results. For instance, the rough-toothed dolphin Steno bredanensis was found to have a lower trophic level than previously determined, contradicting the belief that it is the top predator of the area. On the other hand, the Atlantic spotted dolphin Stenella frontalis and the common bottlenose dolphin Tursiops truncatus, which hunt at the same depth, have different prey, indicating they have found a way to live together without competing for food.

Moreover, the study also found no overlap in food among three of the dolphin species, contrary to previous research. This suggests that these species may have more specialised diets than previously thought, potentially leading to less competition for resources.

The method of using nitrogen isotopes to determine trophic levels involves heavier atoms being taken up into an animal's body tissue more often as the animal is higher in the food web. The study's findings contribute to a better understanding of the food competition and preferences of toothed whale species in the Southwestern Atlantic Ocean.

This study is an improvement of more commonly used isotope analysis, as it uses nitrogen in free amino acids instead of in the tissue as a whole. This approach provides a more accurate and precise measurement of the animals' trophic levels.

Little is known about the food and food search behaviour of toothed whales, with most historical knowledge derived from the stomach content of dead specimens. This study provides valuable insights into the diets of these marine mammals, shedding light on their ecological roles and interactions in the Southwestern Atlantic Ocean.

The study was published in the journal Environmental Research with the DOI 10.1016/j.envres.2021.111610. The research project involved Genyffer Troina from the Federal University of Rio Grande, Brazil, who worked on her samples with Marcel van der Meer and Philip Riekenberg at the NIOZ. The exact names of the other scientific researchers involved in the study of stable nitrogen isotopes of amino acids in whales in the Southwest Atlantic region are not provided in the available information.