|Abstract: ||According to the Food and Agriculture of United Nations, almost 80% of the world fish populations are overexploited, depleted or in a state of collapse. In general the Mediterranean Sea is characterized by the 33% of assessed stocks fully exploited, 50% overexploited and the remaining 17% non-fully exploited. Bottom trawling fleets predominate in many Mediterranean fisheries, being responsible for a high share of total catches and, in many cases, yielding the highest earnings among all the fishing sub-sectors.
The deep-sea fishing began only in the first decades of the last century, as a result of the development of new technologies that made fisheries in deeper waters possible. The main target species of the Mediterranean deep-sea bottom fishery are the red shrimps Aristeus antennatus (Risso, 1816) and Aristaeomorpha foliacea (Risso, 1827), that represent one of the most important commercial species. In general both species coexist in large part of their distribution range, but in the Mediterranean Sea, where the large part of studies have been undertaken, their distribution is patchy and seems to shows an antagonistic longitudinal gradient.
In this context different hypotheses have been formulated by several authors to explain the different distribution observed. Some of them correlated various environmental variables with the presence/absence of these species to explain the spatial distribution and the temporal fluctuations in landings of these two species. All the identified factors seems not to act in synergy in determining the spatio-temporal variation of the species and which is the parameter that has a significant predictive effect is still unclear. Considering the high degree of interaction between marine organisms and the environment, a more holistic approach incorporating interspecific interactions and physical environmental influences would contribute to greater sustainability by reducing the uncertainty in predictions and promoting an ecosystem based management.
In the light of the above, this thesis applies a multi-disciplinary approach to study the dynamic of the Mediterranean deep-water red shrimps. The main purpose of this thesis is to better understand how the abiotic factors could impact on the spatio-temporal distributions of the decapod crustaceans A. antennatus and A. foliacea.
The first part provides information on the abundance and geographical distribution of deep-water red shrimps using 19 years of trawl survey data. Both red shrimps species occur almost exclusively on the “meso-bathyal” stratum (500-800 m). The distribution pattern appears quite different for the two species, and the abundance indexes as well the spatial maps, evidenced local higher abundances (hotspots) where the blue and red shrimp and the giant red shrimp are mainly concentrated. In
general, a very high interannual variability was detected in all areas probably linked with some environmental factors that occurs in Sardinian waters.
In particular the investigation of the hydrographic conditions of the shelf-slope areas, developed in the second part of the thesis, highlight presences of the typical water masses already detected in literature. In our study area the water masses occurring on the continental shelf-slope zones, where the deep water red shrimps are catched, are mainly These water masses overlap and even partially mix each other during their displacement across the Sardinian shelf-slope area according to the general topography of the lands and the bottoms. Results showed that the distribution of the main water masses is comparable to the literature, moreover, the results confirmed that the great variability between basins and year of sampling is clearly associated with waters of very different origin that follow converging routes and mix together in the study area.
The analysis carried out in third part underlined the influence of peculiar areas, such as seamounts, on the structure of the middle-slope assemblages as well as the behavioural rhythms of the deep water red shrimps. Their daily and nocturnal movements from the base of the seamount to the edge of the continental shelf increase the range of their distribution. This results highlights the fact that shrimps and other species in the Baronie seamount, move during a day-night cycle, probably influenced by some parameter indirectly related to their activity. These nektobenthic displacements are known to occur also in the continental shelf and slope, but in the canyon and seamounts they are broadened and can be studied more easily. These displacements could produce a bias in trawl survey data according to the time of day with effects on population assessment and demographic evaluation.
The fourth part of the thesis provided also the identification of the spawning and nursery grounds, and their relationship to major, oceanographic processes The location of the area appears to be correlated with the spatial pattern of the main persistent oceanographic processes, identified in the Sardinia Sea, such as enrichment due the upwelling present in the south west shelf-slope area. Other important areas of aggregation for both juveniles and mature females were found over the bathyal zones in canyon and seamounts environments, where productivity is reported higher than elsewhere. These privileged areas for recruitment, rich in nutrients thanks to the effect of different currents, would seem to guarantee the regeneration of the resource. Finally, the analysis also suggested the existence of a seasonal bathymetric distribution of the nursery areas. The juveniles of A. foliacea are located in the upper part of the continental slope in spring-summer and reach greater depths in autumn. For A. antennatus, for which nursery areas appear only in autumn, an opposite ontogenic migration from deep sea to upper slope, has been noted during the summer.
In the last part of the thesis, the influences of in situ hydrographic variables on the distribution of the Aristeid stocks on the fishing grounds of Sardinia were analysed. The comparison between hydrographic and biological data evidenced that both species seem to be connected to the peculiar features and fate of the LIW (Levantine Intermediate Water) between 400 and 600 m. In fact, in the south-eastern part, the LIW properties predominate and A. foliacea is the main species, while in the western and northern areas of Sardinia the LIW loses intensity in favor of A. antennatus, which prefers colder temperatures. Besides the oceanographic features influencing the distribution pattern of these species, the variation left unexplained by the model may be attributed to different sources of variation such as the presence of geomorphological structures or reproduction and recruitment phenomena.
In conclusion, following a multidisciplinary approach, this thesis tries to give a substantial contribution to the knowledge of the spatio-temporal dynamics of the two Mediterranean deep-water red shrimps. The results highlight how these patterns of distribution strongly correlated not only with the main oceanographic process (LIW properties) but also with local hydrographic conditions. Among these conditions, upwelling events and the particular geomorphology that characterized the Sardinian shelf-slope region seem to be the most important factors. In addition life history traits of these two species linked to a heavy prolonged fisheries exploitation, could determine a masking effect that makes difficult to interpret the whole picture of the distribution of the two species. For this reason, these results should take in to account for the assessment and management of these important resources.|