Pelagic predator associations: tuna and dolphins in the eastern tropical Pacific Ocean

Abstract

The association of yellowfin tuna and pantropical spotted dolphins in the eastern tropical Pacific Ocean (ETP) has been exploited by tuna fishermen and has intrigued scientists for decades, yet we still have questions about what the benefits of the association are -whether the association is obligatory or facultative, why the tuna are most often found with spotted dolphins, and why the species associate most strongly in the ETP. We review the hypotheses that have been proposed to explain the bond and present results from 3 studies conducted to address these hypotheses: a simultaneous tracking study of spotted dolphins and yellowfin tuna, a trophic interactions study comparing their prey and daily foraging patterns, and a spatial study of oceanographic features correlated with the tuna-dolphin association. These studies demonstrate that the association is neither permanent nor obligatory and that the benefits of the association are not based on feeding advantages. These studies do support the hypothesis that one or both species reduce the risk of predation by forming large, mixed-species groups. The association is most prevalent where the habitat of the tuna is compressed to the warm, shallow, surface waters of the mixed layer by the oxygen minimum zone, a thick layer of oxygen-poor waters underlying the mixed layer. The association has been observed in other oceans with similar oceanographic conditions, but it is most prevalent and consistent in the ETP, where the oxygen minimum zone is the most hypoxic and extensive in the world.

Predicting Interactions between Common Dolphins and the Pole-and-Line Tuna Fishery in the Azores

“…Association between the two species accounts for over 73% of cetacean-tuna associations recorded in the Azores [42]. As mentioned above, the two species overlap in their seasonal abundance in the area, possibly because they have similar prey preferences and explore the same feeding areas [43, 44]. Higher prey abundances would presumably attract common dolphins and tunas to forage in the same areas.…”

Assessment of eco-labelling schemes for Pacific tuna fisheries

“…There is incidental and generally non-marketable catch (bycatch) of other fish species, turtles, seabirds, and marine mammals [32]. In the eastern Pacific, where their habitat is compressed to warm surface waters [33], yellowfin tuna often school with dolphins; this can be exploited by detecting dolphins at the surface and setting purse seine nets around them: historically, this has resulted in higher dolphin catches in the eastern Pacific than further west [34].…”

Purse-seine vessels as platforms for monitoring the population status of dolphin species in the eastern tropical Pacific Ocean

“…Life history data were collected by NMFS and IATTC observers from the 1970s to the mid1990s (e.g., Perrin et al, 1973;Perrin et al, 1976;Perrin and Henderson, 1984;Perrin and Reilly, 1984;Hohn and Hammond, 1985;Myrick et al, 1986;Perrin and Hohn, 1994;Galván-Magaña, 1999;Olson and Watters, 2003;Scott et al, 2012). Observers recorded information on length, sex, and color phase in spotted dolphins (an indicator of age class: Perrin, 1970) from dolphins that died in purse-seine sets, and collected reproductive organs, stomach contents, and teeth (for estimating age).…”