Date of Award

Winter 11-25-2019

Document Type

Thesis: Open Access

Degree Name

MS Marine Science


Environmental and Ocean Sciences

Committee Chair

Michael D. Scott

Committee Co-Chair

Zhi-Yong Yin

Committee Member

Lisa Ballance


Tuna and dolphins swim together in the waters of the eastern tropical Pacific, and this association has long benefitted tuna fishermen and intrigued scientists. Although the tuna-dolphin association is often referred to as a “mystery,” much is known about the association. Yellowfin tuna are primarily caught with spotted dolphins and, to a lesser extent, spinner dolphins; historically the spotted dolphin has borne the brunt of the bycatch mortality. The tuna-dolphin association is thought to be a product of the distinct oceanography of the ETP: a shallow mixed layer, a thick oxygen minimum zone, and warm surface waters. As the mixed layer deepens, the association begins to break down: first with spinner dolphins, then with spotted dolphins.

Important ecological and management questions remain. What are the effects of season, El Niño Southern Oscillation (ENSO) cycles, and long-term climate change on the association? What will be the future effects on the association with the continued influence of climate change? And how will these changes affect the fishery and dolphin mortality?

We used the IATTC observer data from 1992-2017 for pure herds of spotted dolphins and spinner dolphins (n=201,988 sightings), oceanographic data from global data assimilative models (0.25ox 0.25o resolution), GIS and explanatory and predictive models (R-based Boosted Regression Trees) to understand the tuna-dolphin distribution and dynamics in the ETP. The mixed layer depth and chlorophyll were the most significant factors, and sea surface temperature, temperature at depth, and sea surface height were also important factors. The spatial distribution of the association expanded and contracted with season and ENSO events, overlaid on a long-term expansion caused by climate change. The conditions that promote the tuna-dolphin association are intensifying and the management implications are already apparent: sets on pure spinner dolphin herds have increased, and spinner dolphins have replaced spotted dolphins as the leading component of incidental dolphin mortality.