The effects of seasonal variation, El Niño-Southern Oscillation events, and climate change on the tuna-dolphin association in the eastern tropical Pacific Ocean

Tuna and dolphins swim together in the waters of the eastern tropical Pacific, and this association has long benefitted tuna fishermen and intrigued scientists. 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. We know the primary promoter of the tuna-dolphin association is the unique ETP oceanography: 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 1995-2016 for pure herds of spotted dolphins and spinner dolphins (n=201,988 sightings), oceanographic data from global data assimilative models (0.25ºx 0.25 ºresolution), GIS and explanatory and predictive models (R-based GAMs and Boosted Regression Trees) to understand the tuna-dolphin distribution and dynamics in the ETP. The mixed layer depth had the most influence on the distribution of the association, but Chl, SST_100m and the ONI Index were also significant 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.