Climate Change as Seen Through Atlantic Bluefin Tuna

By Olivia Schuitema, SRC intern

Climate change is the phenomenon in which global temperatures are rising due to an increase in the amount of CO2 in the atmosphere, largely resulting from burning fossil fuels and deforestation. The Earth’s ozone layer acts as a protective “blanket,” trapping warming greenhouse gases, such as CO2, in the Earth’s atmosphere. This “greenhouse effect” also affects the oceans; in the last 45 years, the mean ocean temperature in the upper 300m (where a majority of marine life live) has increased by 0.3°C (Muhling, 2011). Although this change seems small, even minor differences in temperature, salinity, and pH can affect organism and ecosystem success.

As atmospheric CO2 concentration increases, more CO2 dissolves into seawater and results in decreasing amounts of inorganic carbon in the ocean (Fraile, 2016). The inorganic carbon isotope is an important element in recycling nutrients throughout the ocean. Similarly, the ratio of stable oxygen isotopes in seawater can be related to the temperature and salinity of the water (Fraile, 2016), which can affect marine habitats. A recent study aiming to estimate CO2 uptake in the Mediterranean Sea over the past 20 years, suggests that evidence of climate change, spurring changes in the ratios of stable carbon and oxygen isotopes, can be seen in Atlantic bluefin tuna, Thunnus thynnus, otoliths. Otoliths are aragonite structures located in the inner ear of teleost fish that aid in balance and orientation (Fraile, 2016). As bluefin tuna age, they add new layers of aragonite on their otoliths, forming rings. The otolith layers are used similarly to ice cores and tree rings, in which they show environmental conditions, such as the amount of CO2, of the time period. In their first year of life, tuna are highly mobile and remain in surface waters of the Mediterranean Basin. Thus, the carbon and oxygen isotopes accumulated in the first year of life, shown in the central otolith layer (“otolith core”), are likely to reflect the seawater conditions of the Mediterranean Basin (Fraile, 2016).

Figure 1. The rings in fish otoliths can be used to determine fish age, NOAA

Researchers captured tuna of different sizes and measured their fork lengths (from the tip of the snout to the fork of caudal fin) in order to help determine their age. Otoliths were extracted from the tuna and cut in a cross-section to expose the otolith core (Fraile, 2016). Cores were powdered and analyzed for stable carbon and oxygen isotopes. Combining the anatomical age data and the otolith core data, a record of the annual amount of carbon and oxygen isotopes was compiled for the years 1989-2010. It was found that oceanic carbon and oxygen decreased in the studied years, inversely related to increasing atmospheric CO2 (Fraile, 2016). Decreasing amounts of inorganic carbon have negative impacts on biogeochemical cycling (cycling of nutrients, such as carbon, between the sediment, organisms and the water) in the ocean, leading to changes in environmental conditions. These changes could have cascading effects, affecting species on the organismal level and affecting populations throughout entire food chains.

Figure 2. Atlantic Bluefin Tuna, Thunnus thynnus, NOAA

Climate change can also lead to increasing ocean temperatures, which puts stress on marine organisms and causes degradation of marine habitats. Many fish, such as the Atlantic bluefin tuna, undergo physiological stress due to increase in seawater temperature, which impacts swimming abilities, spawning activities, egg hatching and larval growth (Muhling, 2015). A study conducted in the Gulf of Mexico and the Caribbean Sea aimed to gain further insight on the effects of climate change on tuna species through modeling systems (Muhling, 2015). Researchers applied ocean temperature fields with past and present data onto models of suitability (optimal conditions for survival) for the larval and adult stages of skipjack tuna and Atlantic bluefin tuna.

It was found that Atlantic bluefin tuna larva and adult survival decreased with increasing surface temperature (warmest temperatures in water column) and increased at deeper depths with cooler water (Muhling, 2015). This suggests that the temperate Atlantic bluefin tuna prefer to inhabit cooler waters and are negatively affected by warming temperatures. Conversely, tropical skipjack tuna larva and adults had higher survival rates at higher surface temperatures, indicating a preference for warmer temperatures. Future projections were also made, by using current tuna habitat suitability models with projected environmental trends due to climate change. By 2090, waters in the Gulf of Mexico will be highly unsuitable for both adult and larval stages of Atlantic bluefin tuna (Muhling, 2015). On the other hand, skipjack tuna adult and larvae suitability is projected to expand greatly, and possibly expand into bluefin tuna habitat in the future. As seen in this study, climate change can cause unbalanced changes in top predator ocean dynamics; some species like the skipjack tuna, thrive and have the potential to over dominate, while others, like the Atlantic bluefin tuna, are negatively impacted and can have a potentially reduced role in food webs.

Figure 3. Change in sea surface temperature 1901-2015, EPA

Global climatic patterns are also influenced by climate change. With increasing temperatures, there has been an increase in the frequency of droughts and heat waves (ex. California), and similarly, an increase in the number and intensity of hurricanes and tropical storms in the Caribbean (ex. Hurricanes Irma and Maria in 2017). According to a study conducted by analyzing Atlantic bluefin tuna vertical migrations with seasonal environmental conditions, tuna behavior is affected by ocean surface temperature (Bauer, 2017). During average seasonal temperatures, common bluefin tuna behavior involves periods of surfacing. However, data shows unusual deep diving intervals during thermal fronts, due to the increase in water surface temperature (Bauer, 2017).  Researchers hypothesize that increasing numbers of abnormal climate events can greatly affect the behavior of vertical migrators, such as sharks, sailfish and the Atlantic bluefin tuna (Bauer, 2017).

Rising global temperatures, largely due to anthropogenic influences, can cause a wide array of changes in the earth’s climate including extreme weather events, ocean acidification, and sea level rise. Systems in the marine environment, along with commercial and recreational fisheries, will also be adversely affected (Muhling, 2011). The effects of climate change will continue to intensify unless measures are taken to reduce the anthropogenic footprint on the earth.


Bauer, Fromentin, Demarcq, & Bonhommeau. (2017). Habitat use, vertical and horizontal behaviour of Atlantic bluefin tuna (Thunnus thynnus) in the Northwestern Mediterranean Sea in relation to oceanographic conditions. Deep-Sea Research Part II, 141, 248-261.

Fraile, Arrizabalaga, Groeneveld, Kölling, Santos, Macías, . . . Rooker. (2016). The imprint of anthropogenic CO2 emissions on Atlantic bluefin tuna otoliths. Journal of Marine Systems, 158, 26-33.

Muhling, Liu, Lee, Lamkin, Roffer, Muller-Karger, & Walter. (2015). Potential impact of climate change on the Intra-Americas Sea: Part 2. Implications for Atlantic bluefin tuna and skipjack tuna adult and larval habitats. Journal of Marine Systems, 148, 1-13.

Muhling, B. A., Lee, S. K., Liu, Y. T., & Lamkin, J. (2011). Predicting the effects of climate change on bluefin tuna (Thunnus thynnus) spawning habitat in the Gulf of Mexico. ICES Journal of Marine Science, 68(6), 1051-1062.

Atlantic Bluefin Tuna Fisheries: A Case of Mismanagement

By Hanover Matz, RJD Intern

While many fisheries around the world are currently being devastated by the overwhelming power and efficiency of modern fishing fleets, the Atlantic bluefin tuna fishery of the Atlantic and Mediterranean is one that has come to the forefront of marine conservation as an example of mismanagement and overexploitation. The bluefin tuna fishery in the Atlantic has traditionally been divided between the west Atlantic and the east Atlantic and Mediterranean stocks, with disagreements over the divisions of distinct populations (Sumaila and Huang 2012). Figure 1 shows the distribution of bluefin tuna in the Atlantic, with major spawning grounds (dark gray spotted areas) and migration routes (arrows). Tuna fishing in the Mediterranean can be traced back to ancient times, with hand lining and seine fishing practiced by peoples as early as the Phoenicians and the Romans. Fishing practices expanded into trap fishing and beach seine nets between the 16th and 19th centuries, and eventually were replaced by the modern industrial seine and longline fleets of the 20th century (Fromentin and Powers 2005). It is during the late 20th century that major changes in the total catches of bluefin tuna occurred.


Tuna Figure 1

Distribution of Atlantic bluefin tuna fisheries and migration routes (Fromentin and Powers 2005)

Catch data from the 1970s onward shows an increase in total catch beginning in the 1990s. Figure 2 shows bluefin tuna catches in the Atlantic from 1950 based on gear type. Bluefin tuna catches rose from levels between 5,000 to 8,000 tons in the 1970s to 40,000 tons in 1995. The International Commission for the Conservation of Atlantic Tunas (ICCAT) was established in 1969 to oversee the management of bluefin tuna, but this management has faced several issues with regards to limiting the overexploitation of tuna stocks (Sumaila and Huang 2012).  One significant error on the part of ICCAT was the setting of Total Allowable Catches (TAC) above the limits suggested by advisory scientific bodies. Fromentin et al. (2014) describe the various problems that have plagued the management of bluefin tuna by ICCAT. Along with a disregard for recommended scientific limits, tuna stocks have been overfished due to the frequency of Illegal, Unreported, and Unregulated (IUU) fishing. With bluefin tuna fishing occuring over such a large expanse of ocean in the Atlantic alone, crossing waters under the control of various nations and the high seas, it is difficult to effectively enforce management policies. The authors of the 2014 report also identify how uncertainties in stock assessment have contributed to the mismanagement of bluefin tuna.

Tuna Figure 2

Total catch of bluefin tuna in tons by gear type since 1950, showing significant increase since the 1990s (Sumaila and Huang 2012)

Three sources of uncertainty in bluefin tuna have contributed to difficulties in establishing management policies: uncertainity in the biology and populations of tuna, poor quality of data, and errors in the ability of models to predict tuna population dynamics. (Fromentin, Bonhommeau et al. 2014). Given the migratory nature of bluefin tuna and the expanse of ocean which they inhabit, it is difficult to conduct studies on their biology and development. Catch data has also been inaccurate in the past due to the levels of illegal and unreported fishing in the industry. Finally, uncertainties in the models used to predict population dynamics make it difficult for management bodies such as ICCAT to develop effective policies. Bluefin tuna cross the Exclusive Economic Zones (EEZs) of many different countries, contributing to further difficulties in managing fish stocks that may be subjugated to fishing regulations across multiple nations (Sumaila and Huang 2012). While a better understanding of how bluefin tuna populations may overlap and mix has been established in the past decades, more research still needs to be conducted (Fromentin and Powers 2005). Another indicator that Atlantic bluefin tuna stocks have declined is the measurement of spawning stock biomass, the portion of the stock population capable of reproducing. Data since 1970 up to 2005, including both reported and illegal, unreported, and unregulated fishing, shows a decrease in spawning stock biomass by 60% since 1974 (Sumaila and Huang 2012). This means that overfishing may not only be reducing current populations, but hindering their ability to reproduce by depleting the number of reproductive individuals.

In response to increased fishing pressure on bluefin tuna stocks and decreased catches, aquaculture of tuna now occurs in several regions. Figure 3 shows current locations of tuna aquaculture. Starting with the cultivation of Atlantic bluefin tuna in Canada and Pacific bluefin tuna in Japan in the 1960s, farming of tuna has spread to the Mediterranean and Australia. However, most of this farming consists of capturing wild tuna and fattening them in pens for later harvest, while it still remains incredibly difficult and costly to rear tuna from larvae to adults. This method of catching wild tuna in seine nets and fattening them most likely does not help contribute to alleviating fishing pressures on wild stocks (Metian, Pouil et al. 2014)

Tuna Figure 3

Global distribution of bluefin tuna farms (Metian, Pouil et al. 2014)

Given the current level of harvesting, better management of Atlantic bluefin tuna needs to be put in place. The capacities of the purse seine net fleet and longline fleet in the Atlantic already exceed the mean productivty of bluefin tuna (Fromentin and Powers 2005). Even if there are uncertaintities in the measurements of tuna productivity, the status of tuna populations is precarious enough that it would be risky to continue the current fishing effort. Sumalia and Huang (2012) make several policy recommendations to better manage Atlantic bluefin tuna stocks. First, the total allowable catch needs to be reduced to levels as recommended by scientific research. Second, a better detection and penalty system needs to be established in order to reduce illegal fishing. Finally, the establishment of Marine Protected Areas and the listing of Atlantic bluefin tuna as endangered on the Convention for International Trade in Endangered Species (CITES) would afford tuna some protection to allow populations to recover. However, the multinational fishing effort and policy formation process of ICCAT has made it difficult to come to reasonable agreements between nations to manage tuna. To protect this valuable species, action needs to be taken to reduce the current fishing effort and total allowable catch. Better scientific research will provide more effective management tools, but the current advice being given by scientific bodies needs to be headed when establishing catch limits. If Atlantic bluefin tuna stocks are to continue to provide a valuable resource of seafood to world markets, a more sustainable fishery needs to be established.



  1. Fromentin, J.-M., S. Bonhommeau, H. Arrizabalaga and L. T. Kell (2014). “The spectre of uncertainty in management of exploited fish stocks: The illustrative case of Atlantic bluefin tuna.” Marine Policy 47: 8-14.
  2. Fromentin, J.-M. and J. E. Powers (2005). “Atlantic bluefin tuna: population dynamics, ecology, fisheries and management.” Fish and Fisheries 6: 281-306.
  3. Metian, M., S. Pouil, A. Boustany and M. Troell (2014). “Farming of Bluefin Tuna–Reconsidering Global Estimates and Sustainability Concerns.” Reviews in Fisheries Science & Aquaculture 22(3): 184-192.
  4. Sumaila, U. R. and L. Huang (2012). “Managing Bluefin Tuna in the Mediterranean Sea.” Marine Policy 36(2): 502-511.