Introduction
Aquaculture has a long history, but it is only in the late 20th and early 21st centuries that it has reached the point of providing 50% of the world’s fisheries resources (FAO, 2018). As human population increases and aquaculture needs to be more and more effective, there is an on growing interest in Integrated Multi-Trophic Aquaculture (IMTA) which is considered a means for increasing aquaculture sustainability. IMTA is defined as the cultivation of two or more aquatic species from different trophic levels in the same area, imitating the energy flow of natural ecosystems (Chopin et al., 2004). The aim of the present study was to investigate the potential to cultivate bivalve species (Mytilus galloprovincialis, Pinctada imbricata radiata and Pinna nobilis) using ΙΜΤΑ methodology in existing fish farms of Greece and in the Allocated Zones for Aquaculture (AZAs) both existing and under development.
Material and Methods
The present study is based on bibliographic data, satellite data, as well as on data collected in the context of the “PINNA SOS” and IDMA projects. Fish farming sites and AZAs were recorded on a GIS map based on the study of Papageorgiou et al., (2021) and satellite data of chlorophyll-α (Chl-α), sea surface temperature (SST) and salinity (SAL) data from COPERNICUS were downloaded for the same sites. For those environmental variables, the ecological niche of each species was determined using data from the scientific literature and in situ data collected in the context of the above-mentioned research projects. Finding the appropriate value range of the studied variables (Chl-α, SST, SAL and juvenile presence), under which the conditions are more suitable for the growth of M. galloprovincialis, P. radiata and P. nobilis, a bibliographic review was performed focusing on their growth potential for each one of these variables. Therefore, this study identified a range of values where growth is maximized (optimal conditions) and two sets of values where the organism survives with little growth above and below the optimal conditions (experimental study). In the remaining values there is no potential for growth or reproduction. The juvenile presence records were based on spatial distribution data of P. radiata (Katsanevakis et al., 2008) and on existing mussel-farms from which juveniles (M. galloprovincialis and P. nobilis) are released in adjacent sites. Distribution data of natural populations were also used as a potential source of juveniles. Based on the values of each variable, a score was given (0 = unsuitable, 1=medium suitability, 2=high suitability) to determine whether each region was suitable for the growth of each species. The individual scores were then summed to determine the final score in terms of the probability of successful development of multi-trophic aquaculture in the existing units. Technically the minimum score equals 0 and the maximum score equals 10 points. The equation applied was:
SST + SAL + 2x (Chl-α) + 2 x (Juvenile Presence)
Due to the fact that chl-a and juvenile presence are significant parameters for the bivalve growth, they were multiplied by a factor of 2.
Results
From the above results, probability maps of the three studied species were produced, within the AZAs of Greece on an annual time scale. In the case of P. radiata and M. galloprovincialis, the derived maps reflect the current situation in Greek waters, as in situ observations (IDMA project) in most of the studied areas verify the presence probability of the species assessed. P. radiata, as it seems, has no specific ecological requirements conflicting with the existing habitat characteristics and as a result it spreads easily along the entire coastline of Greece. On the other hand, M. galloprovincialis is more sensitive to several environmental changes and thus, as the results of the survey indicated, the probability of its presence is increased in the northern, central and western part of Greece, while in the southeastern part the probability of its presence is almost absent, in other words, in oligotrophic waters. Although the environmental requirements of P. nobilis are not yet clear, it is considered that since it is an endemic fan mussel (i.e., a relative of the mediterranean mussel) (Zotou et al., 2020), it is also sensitive to environmental variables, as is the case with M. galloprovincialis. Thus, the extracted maps were expected to be similar for these two species.
Conclusions
P. radiata is one of a few invasive species that has a high commercial potential in the Mediterranean Sea. Due to its low feeding and ecological requirements, according to the produced maps it can be cultured in oligotrophic fish farms. The main inhibitor seems to be the distribution of natural population as no juveniles are commercially available. On the contrary, juveniles of M. galloprovincialis can easy be bought, however suitable IMTA locations require eutrophic waters, more commonly found in northern Greece. P. nobilis is a more complicated case since in the recent years, due to a mass mortality of the species, no natural population is available, as an infection by Mycobacterium and Haplosporidium pinnae has occurred causing the extinction of the species in a huge proportion throughout the Mediterranean region (PINNA SOS Project). The results obtained represent a theoretical scenario for the potential of growth, in order to find the most suitable IMTA locations, which P. nobilis could use as nursing ground for raising its juveniles. Although P. nobilis prefers Posidonia oceanica meadows for settlement, the environmental condition of an IMTA can cover their needs.
Acknowledgements
This is a part of the project “Innovative Actions for The Monitoring-Recovering –Enhancement of The Natural Recruitment of The Endangered Species (Fan mussel) Pinna nobilis. (MIS 5052394) Funded by the Operational Program for Fisheries & Maritime 2014–2020.
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