U. Labarta

The physiological energetics of mussels (Mytilus galloprovincialis Lmk) from different cultivation rafts in the Ria de Arosa (Galicia, N.W. Spain)

Abstract 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 mm 3 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 viriable. 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.

Growth of mussels (Mytilus edulis galloprovincialis) on cultivation rafts: influence of seed source, cultivation site and phytoplankton availability

Mussel seed, obtained from the intertidal zone and permanently immersed collector ropes, was cultivated on ropes suspended from three rafts located at three different sites within the Ria de Arousa (Galicia, NW Spain). Sites were characterized by different levels of phytoplankton availability. The source of seed stock had a marked influence upon subsequent mussel growth; seed originating from collector ropes had higher growth rates than seed collected from intertidal areas and was probably due to the higher condition index and previous adaptation to rope culture conditions (permanent immersion) for these samples. Cultivation site also affected mussel growth; differences in chlorophyll a content and water current speed, which influence phytoplankton availability, were the major factors underlying variation in growth rate and condition index. It is recommended that seed obtained from collector ropes should be used in the commercial exploitation of this species, since it would shorten the total duration of the cultivation process by more than 10%.

The effect of diets of phytoplankton and suspended bottom material on feeding and absorption of raft mussels (Mytilus galloprovincialis Lmk)

Abstract Measurements of feeding rate and absorption efficiency were performed to assess the nutritional value of sedimentary organics combined with phytoplankton for raft mussels ( Mytilus galloprovincialis Lmk) in Ria de Arosa (Galicia, N.W. Spain). Both clearance rate and absorption efficiency reached a maximum on mixed diets in which the proportions of phytoplankton and sediment particles were similar. Consequently, maximum absorption rates were reached on laboratory suspensions that closely resembled features of natural seston from the Ria. Values of absorption efficiency recorded with suspended sediments (around 13%) do not apply to the detrital component of ingestion in mixed diets, leading to the conclusion that a positive effect of phytoplankton on absorption efficiency of sedimentary organics is taking place. On the other hand, comparison of results for mixed and monoalgal diets also suggests that the occurrence of silt in the ingesta enhances absorption of microalgae, possibly through improving mechanical treatment within the stomach. It was concluded that mixing of bottom material with phytoplankton in the water filtered by raft mussels greatly increases growth potential, provided that no reduction of phytoplankton concentration below 40% of particulate volume takes place. Mussels with different locations in the Ria exhibit variable degrees of infection by the parasitic protozoa Marteilia refringens . Concomitant differences in physiological behaviour could be attributed to digestive impairment caused by parasitization.

Effect of microalgal and inert (cornmeal and cornstarch) diets on growth performance and biochemical composition of Ruditapes decussatus seed

Research was carried out into the effect of phytoplankton, cornmeal and cornstarch diets on growth and biochemical composition of the seed of the little-neck clam, Ruditapes decussatus. The seed of R. decussatus, fed on daily rations of Isochrysis galbana (organic weight) of 0.5 and 1% of live weight of the seed, showed an improvement in growth rate when cornstarch, which is 99% carbohydrate, was added to these diets. Thus in the case of a daily ration of 0.5%, daily growth rates increased by between 33.5 and 32.3%, depending on whether we are referring to organic weight, dry weight or live weight, when 1.5% cornstarch was added. In the case of a ration of 1% I. galbana, the addition of another 1% cornstarch lead to an improvement in daily growth rates, depending on the different weight class in question, of between 14.1 and 15.5%. When compared to a daily ration consisting of 2% phytoplankton, which was considered to be the optimal ration for growth in the seed of these clams, the replacement of half the quantity of I. galbana by a quantity of cornstarch of equivalent weight gave a growth rate in terms of organic weight of 87.9% that of the phytoplankton diet, while the rates for dry weight and live weight were 89.6 and 87.9%, respectively. These results improved noticeably when cornmeal, consisting of 10% protein and 90% carbohydrate, was used instead of cornstarch. In the case of a 2% phytoplankton diet, if we substituted an equivalent quantity of cornmeal for 50% of the phytoplankton, the growth rate in organic matter was the same (99.0%) as those for the diet consisting of phytoplankton alone, while growth rates in dry weight and live weight were 6.2 and 5.9% higher, respectively, than those of the phytoplankton diet. It would therefore appear that cornmeal (and to a lesser extent cornstarch) can be successfully used as a partial substitute for phytoplankton in diets for the seed of R. decussatus and its use in hatcheries and nurseries devoted to the culture of this species would lead to a considerable reduction of production costs.