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

IPCC-AR4 climate model simulations forced by future greenhouse warming project that the upper ocean in the GoM may increase by more than 2°C by the end of the 21C, and thus suggest that the GoM may become a unsuitable habitat for bluefin tuna spawning due to anthropogenic global warming (AGW). However, since the IPCC-AR4 models have a very coarse resolution (typically around 100km), the simulated changes in the strength, position and eddy-shedding characteristics of the Loop Current (LC), which are important factors for the upper ocean temperature response to the changing climate, are far from realistic.

Therefore, in this study, the potential impact of future AGW on the GoM is examined by using a downscaled high-resolution ocean model constrained with the surface forcing fields, initial and boundary conditions obtained from the IPCC-AR4 models. The high-resolution model results indicate that the GoM is warmed everywhere, but the spatial pattern of the warming is quite different from that of the IPCC-AR4 models. In particular, the SST increase in the high-resolution model is much less especially in the northern GoM away from the Florida west coast. A potential cause for this difference may be the weakening of the LC and the associated reduction in the warm water transport through the Yucatan Channel, which are not well simulated in the IPCC-AR4 models. The simulated volume transport across the Yucatan Channel is reduced by 20 - 25% during the 21st century, consistent with a similar rate of reduction in the Atlantic meridional overturning circulation (AMOC). Further surface mixed layer heat budget analysis indicates that the reduced LC and the associated weakening of the warm LC eddy have a cooling impact in the GoM, particularly in the northern GoM. Therefore, the northern GoM is characterized as the region of minimal warming. Low-resolution models, such as the IPCC-AR4 models, underestimate the reduction of the LC and its cooling effect, thus fail to simulate the reduced warming feature in the northern GoM.

In summary, the LC may slow down as much as 25% 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. This research indicates that the area of spawning habitat for the bluefin tuna may not be as drastically reduced as in the IPCC-AR4 projected. AOML researchers will continue their collaboration with SEFSC to further refine these projections using additional data and improved ocean modeling techniques.

Figure 1. IPCC-projected bluefin tuna spawning habitat extent in March, April, May and June (left to right) for the late 20th century (top), 2045-2055 (middle), and 2085−2095 (bottom). Habitat suitability is measured by probability of larval occurrence, and is using interpolated temperature data from an ensemble of 19 IPCC-AR4 climate models under A1B scenario.

Figure 2. The bluefin tuna spawning habitat extent in March, April, May and June (left to right) for the late 20th century (top), 2045-2055 (middle), and 2085−2095 (bottom). Habitat suitability is measured by probability of larval occurrence, and is using temperature data from the downscaling model.

Publications

Muhling, B. A., S.-K. Lee, J. T. Lamkin and Y. Liu, 2011. Predicting the Effects of Climate Change on Bluefin Tuna (Thunnus thynnus) Spawning habitat in the Gulf of Mexico. ICES Journal of Marine Science, doi:10.1093/icesjms/fsr008.