Tuna and dolphins swim together in the waters of the eastern tropical Pacific (ETP) and this association has long benefitted tuna fishermen and intrigued scientists. Despite decades of observations and scientific study, researchers still have questions about how the species benefit from the association, whether the association is obligatory, why yellowfin tuna are most often found with spotted dolphins, and why the species associate most strongly in the ETP.
In a new paper published in the journal Marine Ecology Progress Series (Scott MD, Chivers SJ, Olson RJ, Fiedler PC, Holland K. 2012. Pelagic predator associations: tuna and dolphins in the eastern tropical Pacific Ocean. MEPS 458:283-302<http://www.int-res.com/abstracts/meps/v458/p283-302/>), we review the hypotheses that have been proposed to explain the tuna-dolphin bond and present results from three 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 (mainly by large sharks) by forming large, mixed-species groups. This association is most prevalent in the ETP due to its distinctive oceanographic conditions. Underlying the warm, surface waters (the mixed layer) is a thick layer of oxygen-poor waters (the oxygen minimum zone). Both dolphins and tuna are thus tied to the mixed layer: the dolphins because they must regularly return to the surface to breathe and the yellowfin tuna because they cannot spend much time in the deeper oxygen-poor waters. The oxygen minimum zone in the ETP is the most hypoxic and extensive in the world. Similar oceanographic conditions exist in a few areas in other oceans, and, in these areas, the association has been observed as well. The paper is Open Access and can be downloaded without cost. Inquiries to the authors can be directed to: Michael Scott Inter-American Tropical Tuna Commission 8604 La Jolla Shores Dr. La Jolla, CA 92037 (858) 546-7045 [email protected]<mailto:[email protected]>
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