STUDY OF LARGE PELAGIC FISH LIFE-HISTORY TRAITS IN THE CENTRAL MEDITERRANEAN SEA AND PROJECTION UNDER FUTURE CLIMATE CHANGE SCENARIOS

One of the most sensitive life periods for each species is the first life stages (larval and juvenile). The survival rates of pelagic fish larvae in these phases are usually very low and subject to fluctuations, consequently affecting the recruitment of new generations. From historical Tuna landing data, the stock's abundance is fluctuating, especially for Atlantic Bluefin Tuna (ABT) (Thunnus thynnus, Linnaeus, 1758) Eastern stock. It is the largest Mediterranean Tuna and one of the most expensive species in the world. Some studies hypothesized that the fluctuations are dictated by environmental factors, particularly in the early life stages. Therefore, it appears necessary to better understand the reproductive biology, structure of the reproductive population, and the recruitment processes related to the environmental drivers of the Mediterranean Sea. In fact, it is a crucial area for maintaining ABT at a global level. It is essential for the other two species too, which share time and space during spawning in summer and their first life stages: ABT, Albacore Tuna (Thunnus alalunga, Bonnaterre, 1788), and Bullet Tuna (Auxis rochei, Risso, 1810). They carry out an essential ecological role as top predators, are fisheries targets globally, and have a fundamental influence on the structure and function of marine communities. Therefore, their sustainable management is essential and depends on a better understanding of the fish ecology, especially early life-history (LH) traits. Despite the Strait of Sicily (Central Mediterranean Sea) being an important spawning site for Tuna species, little is known about the relationship between the environmental variables and their larval spatial distribution in this area. Using historical larval collections from seven yearly surveys, in-situ environmental measurements, and satellite data, in this thesis, the interaction between ecological conditions and Tuna LH traits, horizontal distribution and spatial structure of the three Tuna species at the larval stage has been modelled. A multidisciplinary approach involving lagrangian simulations, species distribution modelling, and morphometric analysis has been used to verify the influence of environmental conditions on the distribution pattern of Tuna spawning and its effects on larval development and, consequently, on their survival probability. Finally, the potential impacts of the identified main driver have been investigated through forecasting models to determine the Tuna larval habitat patterns under possible future climate change scenarios. Understanding what influences the growth and survival of individuals among large pelagic fish populations, particularly during the larval stage, is crucial because these factors will eventually determine recruitment variability and the number of survivors that reach the adult stage. This work aims to improve knowledge of the growth and survival of Tuna larval stages, casting the complex system of the Central Mediterranean Sea in a new light. A site where upwelling, gyres, and fronts play a key role and observations could indicate a meta-population structure in Tuna populations. Chapter 1 provides an introduction with a background on the topic of study. Chapter 2 is a work published during my PhD candidate career. It is the basis for the characterization and knowledge of the central Mediterranean larval habitat. The abundance and presence-absence of larval specimens for the three Tuna species were modelled to examine their relationships with environmental factors. The results suggest that temperature, salinity, and day of the year are the key factors for understanding these species' ecological mechanisms and geographical distribution in this area. In Chapter 3, we look back in time and evaluate the conditions experienced by larvae, starting from their hatching to the point at which we found them. In this way, it was possible to better know the larval habitat and the spawning areas selected by the adults. We hypothesized that the environments experienced along the passive transport period would affect the development of these organisms. It seems helpful to describe not the difference in morphometric measurement but an early or late development. This work also highlighted different spatial patterns between the species. Finally, in Chapter 4, predictive models have been structured to model recruitment under different future climate change scenarios. Thanks to these models, it was possible to verify how the proportion of species could change. Despite the potential habitat seeming to expand, the abundances do not always follow the same trend. The results of this study can help manage future adult stocks and understand recruitment patterns under different future climate change scenarios. In fact, thermal sensitivity is a fundamental physiological attribute and one of the main reasons for induced changes in natural communities.