Bluefin tuna larvae (Photo: IEO/EFE)

Scientists discover that bluefin tuna larvae choose surface layer to develop

•   The temperature of the water is the main variable that determines its distribution.

•   The IEO is leading a work in which Spanish and American scientists have studied the distribution of the larvae of this species in two of its main breeding areas in the world.

Palma (EFE) .- Researchers from the Spanish Institute of Oceanography (IEO) have led a study that has determined that water temperature is the main variable in the distribution of bluefin tuna larvae, so that, in the Mediterranean, they choose the most superficial layer of the sea to develop.

The study, published in the journal "Oceans", compares for the first time the distribution of the larvae of the Atlantic bluefin tuna (Thunnus thynnus) and that of other species of fish at different depths, and the environmental factors that determine it in two of the main breeding areas of this emblematic species: the western Mediterranean and the Gulf of Mexico, the IEO has detailed in a statement.

Above: Map of primary Atlantic bluefin tuna spawning grounds GOM: Gulf of Mexico; MED: Mediterranean Sea; SlopeS: Slope Sea. Study areas are highlighted with squares. Below: Detail of the sampling stations (red dots) 

They have analyzed the influence of temperature and other environmental variables in the main breeding areas of this species.

The work indicates that temperature is one of the main variables that conditions the depth at which these fish larvae develop and is based on data obtained in two twin oceanographic campaigns that were carried out in the western Mediterranean and the Gulf of Mexico in 2012.

Researchers have modeled the relationship between the depths where different communities develop and the environment that characterizes those depths.

Bluefin tuna larva caught on board the BO Ángeles Alvariño. Photo: Itziar Álvarez / IEO

They conclude that, in the Mediterranean, temperature is the main factor that determines the depth structure of fish larval communities, while in the Gulf of Mexico, salinity and fluorescence are those that have a greater relationship with this distribution in the vertical.

In the western Mediterranean, the thermal stratification of the water in summer (the formation of layers of different densities due to the difference in temperature), conditions the distribution of bluefin tuna larvae, which can only live at more than 20ºC. This limits the optimal areas for its development to the first few meters of depth.

On the other hand, in the Gulf of Mexico the water temperatures are higher and there is no such superficial stratification, so the bluefin tuna larvae could find optimal temperature conditions up to 80 meters deep.

The first author of the study, Itziar Álvarez, explains that "the fact that tunas are visual predators causes them to be distributed in areas where light reaches". "On the other hand, the voracity of the bluefin tuna larvae causes the rest of the tuna to descend a little more to continue in the illuminated area but avoiding coexistence with this voracious species", adds the scientist.

The IEO highlights that studies with standardized and comparative methodologies such as this one provide important insights into the larval ecology of tuna and associated species. In addition, they make it possible to differentiate the processes that take place at the local level from the responses at the general level of the entire Atlantic bluefin tuna population, "information that is key to fisheries management".

Researchers from the IEO of the Balearic Islands, Malaga and Gijón have collaborated in the study, with the Oregon Department of Fish and Wellfare, the North American National Office for Oceanic and Atmospheric Administration (NOAA), the Cooperative Institute for Marine and Atmospheric Studies (CIMAS) of the University of Miami, the General Directorate of Fisheries and the Marine Environment of the Government and the Balearic Coastal Observation and Prediction System (SOCIB).

Bibliographic reference: Itziar Álvarez, Leif K. Rasmuson, Trika Gerard, Raúl Laiz Carrión, Manuel Hidalgo, John T. Lamkin, Estrella Malca, Carmen Ferra, Asvin P. Torres, Diego Alvarez Berastegui, Francisco Alemany, José M. Quintanilla, Melissa Martin, José M. Rodríguez and Patricia Reglero, 2021. Influence of the Seasonal Thermocline on the Vertical Distribution of Larval Fish Assemblages Associated with Atlantic Bluefin Tuna Spawning Grounds. Oceans. 2021, 2(1), 64-83.

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