Collaborative Research between PhOD and SEFSC Presented at Bluefin Tuna Workshop

Sang-Ki Lee, Yanyun Liu, David Lindo, Francis Bringas, and Gustavo Goni, Barbara Muhling, and John Lamkin

A joint workshop hosted by NOAA and the National Aeronautics and Space Administration (NASA) for bluefin tuna research was held at the University of Miami on December 6-7th. Researchers with AOML’s Physical Oceanography Division (PhOD) presented the results of their collaborative efforts with NOAA’s Southeast Fisheries Science Center (SEFSC), which have focused on the link between the ocean and stock assessment species of relevant commercial importance.

Although the Atlantic bluefin tuna (Thunnus thynnus) is widely distributed, spawning in the western Atlantic has been recorded predominantly in the Gulf of Mexico from April to June. While bluefin tuna can better tolerate colder waters than other tropical tunas, they are adversely affected by warm water (>28°C) and avoid warm features in the Gulf of Mexico such as the Loop Current.

The IPCC-AR4 climate model simulations for the 20th century, mid 21st century, and late 21st century (graphic at right) indicate that the preferred spawning habitat of bluefin tuna may be profoundly reduced in area (yellow/orange) areas on the three maps) due to the impact of anthropogenic global warming (AGW). However, since the IPCC-AR4 models have a very coarse resolution, the simulated changes in the strength, position, and eddy-shedding characteristics of the Loop Current, which are important factors for the upper ocean temperature response to the changing climate, are not realistic. AOML scientists are collaborating with Barbara Muhling and John Lamkin of SEFSC to explore this issue.

Yanyun Liu and Sang-Ki Lee of PhOD presented a regionally-enhanced view of potential future changes in the Gulf of Mexico under the impact of AGW. The highlight of their results is that the Loop Current may be drastically reduced by as much as 20% by the late 21st century, resulting in a mitigation of AGW-induced surface warming, particularly in the northern Gulf of Mexico, a known spawning ground for Atlantic bluefin tuna. AOML researchers will continue their collaboration with SEFSC to further refine these projections using additional data and improved ocean modeling techniques.

David Lindo, Francis Bringas, and Gustavo Goni, all with PhOD, together with Barbara Muhling of SEFSC, also made a presentation to report new results on the application of altimeter satellite-derived observations to bluefin tuna research in the Gulf of Mexico. Given the strong link between temperature and salinity in habitats, the main mesoscale features are related to larvae distribution of six fish species, bluefin tuna among them.

Time series of the northernmost and westernmost locations of the Loop Current and a ring census were obtained from altimetry fields and related with spring larvae distributions from January 1993 to December 2008. Altimetry fields and fishery surveys indicate that when the Loop Current is farther to the south, larvae are more abundant in the eastern Gulf of Mexico than in the central and western portions of the basin.

Analysis made from 23 rings shed by the Loop Current during the study period indicate that warm-core rings shed by the Loop Current are generally less likely to contain larvae. Associations between the inner and outer regions of mesoscale features and larval catches show higher bluefin tuna abundances in the boundaries and frontal areas of anticyclonic and cyclonic regions.

Collaborative work will continue between AOML and SEFSC to analyze the link between ocean features and stock assessment.

IPCC projected changes of bluefin tuna’s spawning habitat in the Gulf of Mexico due to anthropogenic global warming for three time frames: top (late 20th century), (middle) mid 21st century and (bottom) late 21st century. The colors indicate preferred spawning habitat, with yellow/orange for favored locations and blue/purple for non-ideal regions. Liu and Lee’s research indicates that the area of spawning habitat for the bluefin tuna may not be as drastically reduced as in the IPCC results shown in these figures.