Unveiling the Relationship Between Sea Surface Hydrographic Patterns and Tuna Larval Distribution in the Central Mediterranean Sea

Abstract

Thunnus thynnus (Atlantic bluefin tuna, ABT) and other tuna species reproduce in the Mediterranean Sea during the summer period. Despite the Central Mediterranean Sea, the Strait of Sicily in particular, being a key spawning site for many tuna species, little is known on the effects of oceanographic variability on their larval distribution in this area. The abundance and presence-absence of larval specimens for three tuna species (ABT, bullet tuna and albacore) were modeled in order to examine their relationships with environmental factors, by analysing historical in situ information collected during seven annual surveys (2010–2016). The results revealed that most tuna larvae for the three species were found in the easternmost part of the study area, south of Capo Passero. This area is characterized by a stable saline front and warmer nutrient-poor water, and it has different environmental conditions, compared with the surrounding areas. The models used to investigate the presence-absence and abundance of the three species showed that ABT was the most abundant, followed by bullet tuna and albacore. The presence and abundance data collected are comparable with those of other spawning areas in the Mediterranean. Regarding biological and physical parameters, the results suggest that temperature, salinity, and day of the year are the key factors for understanding the ecological mechanisms and geographical distribution of these species in this area. Temperature affects the presence of ABT larvae and salinity, which, with a physical barrier effect, is a key factor for the presence-absence of bullet and albacore and for albacore abundance.

Interannual summer biodiversity changes in ichthyoplankton assemblages of the Strait of Sicily (Central Mediterranean) over the period 2001–2016

“…The observed pattern is further supported by the similarity between the response plots of the two explanatory factors Longitude and Salinity in Figures 4A, B, as it known that surface salinity locally presents a west-east increasing gradient that is induced by the motion of Atlantic Modified Water toward the Eastern Mediterranean basin (Robinson et al, 1999;Garcıá Lafuente et al, 2002). Therefore, the importance of concentration and retention processes locally induced by oceanographic features is confirmed by the present study (Figure 3A), not only in support of larval growth and survival, and related enhanced probability of recruitment success for many fish species as already shown by previous studies in the SoS (Torri et al, 2018;Patti et al, 2020;Russo et al, 2021;Torri et al, 2021;Russo et al, 2022;Cuttitta et al, 2022), but also more in general for sustaining high biodiversity levels (Figure 3B). Actually, frontal structures are generally responsible for plankton aggregation, so supporting the increase in feeding opportunities for fish larvae and inducing favorable conditions for their survival, as already observed in many other marine environments including the Mediterranean Sea (Bakun, 2006;Alemany et al, 2010;Avendaño-Ibarra et al, 2013;Erisman et al, 2017;Acha et al, 2018;Sato et al, 2018;Torri et al, 2018;Patti et al, 2020;Russo et al, 2021;Torri et al, 2021).…”

“…Therefore, the importance of concentration and retention processes locally induced by oceanographic features is confirmed by the present study (Figure 3A), not only in support of larval growth and survival, and related enhanced probability of recruitment success for many fish species as already shown by previous studies in the SoS (Torri et al, 2018;Patti et al, 2020;Russo et al, 2021;Torri et al, 2021;Russo et al, 2022;Cuttitta et al, 2022), but also more in general for sustaining high biodiversity levels (Figure 3B). Actually, frontal structures are generally responsible for plankton aggregation, so supporting the increase in feeding opportunities for fish larvae and inducing favorable conditions for their survival, as already observed in many other marine environments including the Mediterranean Sea (Bakun, 2006;Alemany et al, 2010;Avendaño-Ibarra et al, 2013;Erisman et al, 2017;Acha et al, 2018;Sato et al, 2018;Torri et al, 2018;Patti et al, 2020;Russo et al, 2021;Torri et al, 2021). In turn, this may account for the observed increased diversity in larval fish assemblages associated to spatial and temporal patterns of mesoscale oceanographic structures (Richardson et al, 2010;Rooker et al, 2013;Meinert et al, 2020).…”

“…This region is generally considered oligotrophic; however, the southern coast of Sicily is an upwelling area with relatively high productivity levels (Piccioni et al, 1988;Patti et al, 2010). In addition, local surface circulation and oceanographic features such as gyres and fronts (Robinson et al, 1991;Lermusiaux, 1999;Lermusiaux and Robinson, 2001;Bonanno et al, 2014;Placenti et al, 2022) were demonstrated to have an important role in concentration and retention processes for early stages of many taxa, including important pelagic fish species (Garcıá Lafuente et al, 2002;Falcini et al, 2015;Cuttitta et al, 2016a;Cuttitta et al, 2018;Torri et al, 2018;Falcini et al, 2020;Patti et al, 2020;Russo et al, 2021;Torri et al, 2021). Actually, in the SoS, the surface circulation is dominated by the motion of Atlantic waters, which is known as AIS (Atlantic Ionian Stream; Robinson et al, 1999) off the southern coast of Sicily.…”

Hydrological and Biogeochemical Patterns in the Sicily Channel: New Insights From the Last Decade (2010-2020)

“…These observations suggest that vertical oceanographic structures, such as upwelling processes identified along the Sicilian coast (Lermusiaux and Robinson, 2001;Bignami et al, 2008;Patti et al, 2010;Torri et al, 2018), could play a significant role in the diffusion of nutrients from the lower layers, making them available for the photosynthetic activity with a cascading effect on the marine organisms that in the Central Mediterranean Sea found a suitable habitat for feeding and reproduction (e.g. Bonomo et al, 2018;Patti et al, 2020;Russo et al, 2021). Indeed, it could also be noted that the SC is a place of strong diffusion and dissipation that may favor such vertical transport (Ferron et al, 2017;Vladoiu et al, 2018).…”

A Hierarchical Bayesian Model to Monitor Pelagic Larvae in Response to Environmental Changes

“…These authors have underscored the role of physical forces, such as currents, gyres, and fronts, in concentrating larvae born along the Sicilian coast in a larval retention area positioned to the southeast, precisely in the Capo Passero region. In contrast to the spawning areas situated in the northwestern coastal zone and characterized by elevated chlorophyll values associated with the upwelling phenomenon, the Capo Passero area typically exhibits a relatively lower concentration of food, although characterized by higher larval densities compared to the other areas (Russo et al, 2021;Torri et al, 2023). The model thus encapsulates a noteworthy pattern concerning the distribution of this species in the study area, namely the presence of larval concentration areas characterized by lower food availability compared to other egg deposition areas.…”