The relationship between food availability and growth in Mytilus galloprovincialis in the open sea (southern Mediterranean)

With the aim of gathering information about the possibility of culturing mussels (Mytilus galloprovincialis) in a south Mediterranean oligotrophic area, different lots of mussels were placed in culture at depths of -5 m and -15 m and their growth monitored on a monthly basis. Temperature and salinity were measured in situ and water samples were collected at different depths each month. Total suspended matter (TSM) and its inorganic (ISM) and organic (OSM) fractions were analysed by gravimetry and loss on ignition. Photosynthetic pigments (chlorophyll-a and phaeopigments), particulate organic carbon (POC) and nitrogen (PON), particulate carbohydrate (CHO), protein (PRT) and lipid concentration (LIP) were also measured. The chlorophyll-a concentrations highlighted the high degree of oligotrophy of the study site. Moreover, the inorganic fraction of total seston, which exceeded the organic fraction throughout the study period, highlighted the importance of the allochthonous input of suspended particles. Two main phytoplankton abundance peaks were observed, in spring and autumn. These peaks were mirrored by the biochemical composition of the biopolymeric fraction of particulate organic matter (POM, the sum of PRT, CHO and LIP concentrations). The relatively high values of the POC:PON ratio indicated that the major fraction of particulate organic matter in the study area was of detrital origin. A clear dilution effect on the organic matter, caused by high concentrations of suspended inorganic material, was also revealed by the LPOM/TSM ratio, used as a qualitative food index. The mussels were found to activate physiological compensatory mechanisms in order to maintain a constant absorption rate of organic matter from the total available seston. In this case study, the mussels survived in an environment in which the quantities of available food were frequently time-varied. The mussels placed in culture as juveniles (total length = 11.20 +/- 4.02 mm) reached a length of approximately 40 mm after 12 months, while the mussels placed in culture as sub-adults (total length = 43.16 +/- 7.5 mm) reached the commercial size of about 60 mm in the same time interval. The sub-adult mussels spawned in autumn and spring, indicating that they acclimatised well, despite the high degree of oligotrophy of the water.