Warm Sushi?

By Rachel Berman

An ensnared tuna of approximately 600lbs. Credit: Danilo Cedrone

Atlantic bluefin tuna is the largest species of tuna, very valuable in the sushi markets, and a highly migratory species. For all of these reasons, Thunnus thynnus has been a focus of study for some time. One of these scientists is Dr. Barbara Muhling, and she has spent several years studying the bluefin tuna in the Gulf of Mexico. In particular, she is investigating how warming temperatures from climate change could affect its spawning grounds. Dr. Muhling studies the links between environment, climate and the early life ecology of highly migratory fishes, as well as climate change impacts on marine fishes more generally.
The bluefin tuna population has been decreasing since 1950, motivating scientists like Dr. Muhling to look closely at the species. There are two populations of Atlantic bluefin tuna, and they have separate spawning grounds: one in the Gulf of Mexico and one in the Mediterranean Sea. Tagging programs associated with this species follow their extensive migratory pattern back and forth across the Atlantic. Spawning areas are very important to the tuna, and they return to these specific areas to spawn every spring.
The Gulf of Mexico spawning ground has special characteristics that are necessary for the success of bluefin embryos. For example, the embryos require temperatures of at least 75 and no more than 80 degrees Fahrenheit. However, the warmer end of this spectrum is too warm for an adult tuna and can produce a lot of thermal stress for a spawning tuna. The adults therefore need access to cold water for cooling off, and tuna spawning is accompanied by a distinctive “mating dance,” a repetitive swimming behavior only seen during spawning times. The adult fish oscillate between deep, cool waters to cool off and warm surface waters for spawning. This creates a small window of time for spawning, from April to June: there has to be warm water available for the embryos and cool water available for the adult fish.
With an expected rise in temperature by the end of the 21st century, spawning is expected to happen earlier, starting in mid March through late April. “We assume that warmer temperatures in the Gulf of Mexico could trigger earlier spawning. In our historical data, we see that where spring temperatures in the Gulf of Mexico are warmer, there’s a slightly earlier initiation of spawning.”
Though spawn timing may change, Muhling affirms the spawning location is non-negotiable: “The adult bluefin tuna have to be in the Gulf of Mexico to spawn.” Muhling further explains, “If waters get too warm, and suitable temperatures are present much earlier in the year, they may need to migrate earlier to get to the Gulf of Mexico at the best time. We don’t know what their capacity is to do this.”
The future is often filled with uncertainty, and researchers like Barbara Muhling are working hard to better understand how warmer water temperatures will affect bluefin tuna and to inform others of the possible outcomes. There is still much work to be done. “We need many more studies looking at ecology,” she says. “We don’t have enough information yet on how climate change will affect growth and survival of larvae, how it might change what planktonic prey are available for them, and how it might affect juvenile stages.” These are important questions, as the answers will help uncover the future for this unique species that travels incredible distances and creates some of the most prized sushi on earth.
Note: Bluefin tuna are not caught in the scientific surveys currently included in OceanAdapt.
To read more about Atlantic bluefin tuna:
  • Tag-a-giant: a tagging program following bluefin tuna migrations
  • A New York Times article about bluefin tuna
  • The International Commission for the Conservation of Atlantic Tunas (ICCAT)’s plan to revive the dwindling population
  • Block, B. A., Dewar, H., Blackwell, S. B., Williams, T. D., Prince, E. D., Farwell, C. J., … Fudge, D. (2001). Migratory Movements, Depth Preferences, and Thermal Biology of Atlantic Bluefin Tuna. Science , 293(5533 ), 1310–1314. http://doi.org/10.1126/science.1061197
  • Block, B. A., Teo, S. L. H., Walli, A., Boustany, A., Stokesbury, M. J. W., Farwell, C. J., … Williams, T. D. (2005). Electronic tagging and population structure of Atlantic bluefin tuna. Nature, 434(7037), 1121–1127. Retrieved from http://dx.doi.org/10.1038/nature03463
  • Muhling, B. A., Roffer, M. A., Lamkin, J. T., Ingram Jr., G. W., Upton, M. A., Gawlikowski, G., … Richards, W. J. (2012). Overlap between Atlantic bluefin tuna spawning grounds and observed Deepwater Horizon surface oil in the northern Gulf of Mexico. Marine Pollution Bulletin, 64(4), 679–687. http://www.sciencedirect.com/science/article/pii/S0025326X12000574
  • Muhling, B. A., Lee, S.-K., Lamkin, J. T., & Liu, Y. (2011). Predicting the effects of climate change on bluefin tuna (Thunnus thynnus) spawning habitat in the Gulf of Mexico. ICES Journal of Marine Science: Journal Du Conseil . http://doi.org/10.1093/icesjms/fsr008
  • Muhling, B. A., Reglero, P., Ciannelli, L., Alvarez-Berastegui, D., Alemany, F., JT, L., & Roffer M A. (2013). Comparison between environmental characteristics of larval bluefin tuna Thunnus thynnus habitat in the Gulf of Mexico and western Mediterranean Sea . Marine Ecology Progress Series, 486, 257–276. Retrieved from http://www.int-res.com/abstracts/meps/v486/p257-276/
  • Ravier, C. and Fromentin, J.-M. (2004), Are the long-term fluctuations in Atlantic bluefin tuna (Thunnus thynnus) population related to environmental changes?. Fisheries Oceanography, 13: 145–160. http://dx.doi.org/10.1111/j.1365-2419.2004.00284.x