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Climate change is rapidly transforming the conditions of marine ecosystems. Rising water temperatures and shifts in food availability are altering the distribution of many fish species, forcing them to reorganize their migratory routes and feeding areas.
Among them is the Atlantic bluefin tuna, one of the most iconic and valuable species in the world’s oceans. Although this fish is known for its remarkable ability to tolerate a wide range of temperatures, its distribution and migrations are closely linked to the presence of prey. When ocean conditions change, its movements change as well.
Understanding how this species responds to new environmental conditions is key to ensuring its sustainable management and long-term conservation.
Despite being one of the most studied marine species, important questions remain about the population structure of Atlantic bluefin tuna.
Traditionally, two major stocks or populations are recognised:
Eastern stock, mainly associated with the Mediterranean Sea and the eastern Atlantic.
Western stock, which mainly spawns in the Gulf of Mexico and nearby waters such as the recently identified Slope Sea.
However, recent research suggests that the reality may be more complex. Some of the open questions include:
Do tuna from both stocks mix only in the Atlantic, or do they also interbreed with each other?
Are there individuals that remain more or less resident in certain regions?
Or do most maintain highly migratory behaviour?
Electronic tagging studies have made it possible to track the movements of some individuals in great detail. However, the number of tagged fish is still relatively small, making it difficult to extrapolate these results to the entire population.
To address this challenge, researchers are turning to a surprising tool: otoliths.
For scientists, however, they are much more than that: they act as true chemical archives.
As the fish grows, otoliths accumulate successive layers of calcium carbonate. These layers record chemical information about the environment in which the fish has lived, in much the same way that tree rings record environmental conditions over time.
By analysing their isotopic composition, scientists can reconstruct key aspects of a fish’s life, such as:
The water temperatures it has experienced
Changes in its metabolism
Differences in the ocean environments it has inhabited
The analysis of two stable isotopes that form the calcium carbonate (CaCO₃) of otoliths provides valuable information about the species’ behaviour.
In inorganic carbonate (such as that found in otoliths), the oxygen isotopic ratio (the relationship between ¹⁸O and ¹⁶O) varies depending on temperature and the isotopic composition of seawater. This allows δ¹⁸O to be used as a proxy to infer the temperature at which each layer formed.
In the case of Atlantic bluefin tuna, this method has already been experimentally validated. This makes it possible to reconstruct the individual thermal history of each fish and understand the environments it has experienced throughout its life.
The analysis of δ¹³C provides complementary information because it reflects:
The metabolic rate of the fish and, therefore,
Differences in feeding
By combining both indicators, researchers can better understand how temperature and fish physiology interact. This helps identify possible thermal limitations and, consequently, interpret migratory behaviour more accurately.
Mugitun is a project funded by the Spanish General Secretariat for Fisheries that aims to generate key information for the sustainable management of the species.
Specifically, the project seeks to:
The first analyses already point to some interesting conclusions.
Based on the δ¹⁸O and δ¹³C values recorded in the otoliths, researchers have identified clear differences in the environmental and physiological histories of the individuals analysed. This suggests the existence of multiple migratory strategies within the population. Some appear to travel long distances, while others follow calmer routines and move within more limited areas.
Although the current dataset is still relatively small, these preliminary results suggest the coexistence of different migratory strategies within the population, adding further complexity to the ecology of Atlantic bluefin tuna.
The next step will be to expand the number of specimens analysed in order to determine which strategies are most common. But that is not all: researchers also want to find out whether these movement patterns are related to where the fish are caught. In other words, whether tuna arriving at the almadrabas at the entrance to the Mediterranean, those caught inside the Mediterranean, or those appearing in northern waters such as Norway show similar proportions of these strategies—or whether each region hosts groups of tuna that behave differently.
