All known species of tuna in the Mediterranean breed at Balearic Sea

For nearly 10 years a team of researchers from the Spanish Institute of Oceanography (IEO), led by Francisco Alemany, has undertaken a project to characterize the spawning habitat for bluefin tuna and other tuna in waters off the Balearic Islands and have recently published an initial summary of findings in the journal Progress in Oceanography.

This work is the result of the analysis of information obtained during five cruises, carried out during the spawning season of this iconic and threatened species in waters of the Balearic Sea. In each of these cruise were covered over 200 sampling stations distributed regularly around the Balearic Sea, performing in all hydrographic profiles, water sampling for the determination of physicochemical parameters at different depths and various plankton hauls, some of them led to the sampling of the smaller fractions (micro and mesozooplankton) and others aimed at the capture of tuna larvae.

The vast amount of data available, both on environmental and biological parameter, such as those resulted of the laborious process of analysis of these planktonic samples, confirm that the Balearic Sea is one of the main spawning areas for bluefin tuna and for other top predator fishes inhabiting the Mediterranean Sea, and it allows to know the factors that determine the spatial distribution of specific spawning areas.

During the cruises the researchers have found larvae of all tuna species recorded in the Mediterranean: bluefin tuna (Thunnus thynnus), albacore (Thunnus alalunga), bullet tuna (Auxis rochei), little tunny (Euthynnus alletteratus), atlantic bonito (Sarda sarda) and even the tropical skipjack tuna (Katsuwonus pelamys).

By analyzing the spatial distribution of bluefin larvae in relation to the hydrographic yearly prevalent conditions, it was observed that tuna larvae were mainly concentrated in areas of water convergence of less saline surface masses of recent Atlantic origin, which reaches the Balearic Islands from the Alboran Sea, and Mediterranean resident surface water masses. Thus, the areas of interaction between Mediterranean and Atlantic waters in the Balearic Sea, seems to meet the most suitable environmental conditions for a successful larval development.

The analysis of the ratio of DNA and RNA and the daily growth of larvae showed a better condition and higher growth rates during the warmest year in the series. This could be the ultimate cause of the relationship between warm periods and greater abundance of the Atlantic bluefin stock reported by various authors.

With this basic information the IEO research teams are proceeding to develop various models that will allow to know, in near real time, the most probable distribution of spawning areas based on the yearly hydrographic scenario and to estimate the survival of successive annual cohorts, trends in spawning stock and even predict the possible evolution in hypothetical future climate scenarios. For this new phase of the project the IEO researchers are collaborating with experts from other institutions like the University of Bergen (Norway), the National Oceanic and Atmospheric Administration (USA), various universities from this country and others Spanish institutions as CSIC.