B. Patti

The “Mad Sea” Phenomenon in the Strait of Sicily

Sea level observations at Cape Bon, Tunisia, and Mazara del Vallo, Sicily, show that large, coherent oscillations exist across the Strait of Sicily with significant energy bands centered at periods of 35.3, 41.6, 50.6, 75.8, and 134.5 min, whose possible structure is confirmed by a numerical approximation to the gravitational barotropic normal modes with realistic topography. It is observed that these oscillations are related to the passage of synoptic weather systems over the region. An investigation on the configuration, phase velocity, and direction of approach of atmospheric disturbances over the region suggests that the oscillations in the Strait of Sicily could be forced by instabilities that develop in large-scale, low pressure fronts that propagate as pressure gravity waves with an approximate phase speed between 24 and 30 m s21.

Influence of environmental variability on anchovy early life stages (Engraulis encrasicolus) in two different areas of the Central Mediterranean Sea

The survival of early stages of small pelagic fish species (e.g. Engraulis encrasicolus) is highly dependent on environmental conditions in both spawning and nursery areas. Knowing the relationship between the mortality rates of the early stages and the environment may help to study and model recruitment fluctuations. During the summer of 2006, two consecutive oceanographic cruises were carried out in the Central Mediterranean sea (CMED) in two different areas: the western Libyan waters and the Sicilian–Maltese waters. For the first time a nearly synoptic comparison between the two border areas of the CMED is performed. In spite of a higher overall egg density in the Sicilian–Maltese waters, there was a higher density of anchovy larvae in Libyan waters. The comparison between the oceanographic datasets singled out different circulation patterns and different characteristics of water masses, which helped to explain the differences in density of the spawning products in the two areas. A Lagrangian transport model was used to evaluate the effects of major oceanographic features on offshore egg and larval transport. The results of the model simulations and the nutritional conditions, as indicated by lipid, carbohydrate and protein contents, support the hypothesis that the western Libyan waters may represent a more favourable nursery ground compared to the Sicilian–Maltese waters in terms of environmental conditions and food availability.

Acoustically detected pelagic fish community in relation to environmental conditions observed in the Central Mediterranean sea: a comparison of Libyan and Sicilian–Maltese coastal areas

An analysis of the influence of environmental conditions on the pelagic fish community structure and species distribution in two areas of the Central Mediterranean sea, the Sicilian–Maltese and the Libyan continental shelves, is presented. The Libyan waters suffer from the lack of historical information on these species, and a thorough characterisation of the pelagic fish community is missing for the entire study area. In summer 2008, two multidisciplinary surveys permitted for the first time the collection of acoustic, biological, and hydrological data in the Libyan, Maltese, and Italian waters of the Central Mediterranean. Satellite and in situ measurements were used here to describe the environmental conditions characterising the two ecosystems, and to analyse the relationships between environmental factors and pelagic community structure, and pelagic fish biomass. The datasets support the hypothesis of a more favourable feeding ground for pelagic fishes in the southwestern part of the study area, close to the Gulf of Gabes, characterised by a larger continental shelf and a higher productivity than the northern and the eastern sides. Environmental gradients, such as the ones related to temperature and salinity, may influence in different ways the pelagic fish community structure in the two areas. The Libyan waters, where environmental gradients develop longitudinally in about 700xa0nmi, likely produce a better spatially structured fish community. In the Sicilian–Maltese area, characterised by stronger environmental gradients and shorter longitudinal extension (150xa0nmi), such spatial structure is less evident. In this latter area a higher spatial overlap among pelagic species is mainly linked to the limited continental shelf and the spatially compressed environmental gradients.

Anchovy (Engraulis encrasicolus) early life stages in the Central Mediterranean Sea: connectivity issues emerging among adjacent sub-areas across the Strait of Sicily

The combined use of field data on anchovy (Engraulis encrasicolus, Linnaeus, 1758) egg distribution in the Central Mediterranean Sea on both sides of the Strait of Sicily (Sicilian–Maltese and Tunisian waters) and Lagrangian simulations were used to assess the pattern of connectivity between these two sub-areas as a result of spawning activity. The field data were collected during ichthyoplankton surveys carried out in summer 2008 and 2010. The simulation runs showed considerable (up to 20%) rates of particle exchange in both directions (from Tunisian to Sicilian–Maltese waters and vice versa). However, considering the typical high mortality rates of anchovy early stages, the actual larval exchange rates across the Sicily Strait are supposed to be significantly lower (<1%), supporting the hypothesis that the anchovy population sub-units in the Strait of Sicily can be considered as separate fish stocks for the evaluation of their optimum exploitation rates.

Linking surface hydrodynamics to planktonic ecosystem: the case study of the ichthyoplanktonic assemblages in the Central Mediterranean Sea

Oceanographic processes play a key role in influencing the structure of the marine planktonic ecosystems. Taking advantage of the quasi-simultaneous collection of a large ichthyoplanktonic dataset in different regions of the Central Mediterranean Sea (Italian/Maltese, Tunisian and Libyan waters), this study aimed at the identification of the main environmental drivers that control the structure of the larval fish assemblages. Spatial distribution and taxa composition were related to physical forcings (geostrophic currents and wind stress) and environmental conditions (bottom depth, temperature, salinity, chlorophyll-a concentration). ANOSIM and SIMPER identified contribution of fish taxa to the average Bray–Curtis dissimilarity among regions. In Italian and Libyan waters, two assemblages (neritic and oceanic) were identified, while a mixed assemblage characterized only some stations. Two neritic and one oceanic assemblages were discriminated in Tunisian waters. Random Forest classification model highlighted the essential role of the bathymetry, while Lagrangian simulations evidenced the action of the hydrodynamics in mixing neritic and oceanic assemblages in the Italian/Maltese and partially in Libyan waters. These findings highlighted the importance of the multidisciplinary approach and shed light on the potential value of the ichthyoplanktonic surveys for the assessment of the state of the marine ecosystem and the conservation of the fishery resources.