THE PHYSIOLOGICAL ENERGETICS OF MUSSELS (MYTILUS-GALLOPROVINCIALIS LMK) FROM DIFFERENT CULTIVATION RAFTS IN THE RIA-DE-AROSA (GALICIA, NW SPAIN)

Physiological components of growth (clearance and ingestion rates, absorption efficiencies and metabolic rates) were determined in specimens of mussels (Mytilus galloprovincialis) from several cultivation rafts in the Ria de Arosa (Galicia, Spain). Choice of raft location within the estuary was intended to provide an oceanic-terrestrial gradient of conditions. Experiments were performed directly on the raft, under natural conditions of food availability. Water was pumped to feeding trays from two different points (front and back) to account for the variability in seston concentration associated with the filtering activity of the hanging culture. Food concentration was found to be highly variable (0.35 to 1.01 mg POM l-1), the main differences being accounted for by site characteristics. Conversely, food quality (here defined as mg POM per mm3 of packed particles) experienced minor variations (0.59 to 0.71), with the sole exception of the innermost site studied (quality 0.43). Absorption efficiencies were clearly dependent on food quality and could be adequately modelled as an exponential function of that variable. The scope for growth (SFG) measured at the front of the raft was consistently higher than at the back and confirmed empirical evidence on growth rates. This result does not merely reflect improved feeding conditions in the input area but associated effects of increased clearance rates and absorption efficiencies exhibited by mussels from the front of the raft. On the other hand, raft positioning within the estuary appears to induce noticeable differences in the feeding behaviour of mussels, as evidenced by higher clearance rates determined in more oceanic areas.