Simonetto Serra

Harvesting Effects on Paracentrotus lividus Population Structure: A Case Study from Northwestern Sardinia, Italy, before and after the Fishing Season

Abstract PAIS, A.; SERRA, S.; MELONI, G.; SABA, S., and CECCHERELLI, G., 2012. Harvesting effects on Paracentrotus lividus population structure: A case study from northwestern Sardinia, Italy, before and after the fishing season. The edible sea urchin Paracentrotus lividus is commercially exploited in many European countries. In recent years, however, its stocks have shown a large reduction in several regions because of overfishing. This paper reports the effects of harvesting on P. lividus population structure in shallow rocky reefs from a central-western Mediterranean area where this activity is intensively practiced. Density, size (test diam), and biomass of sea urchin were assessed in October 2004 (just before the fishing season) and in May 2005 (immediately after the fishing season) at two locations where intense P. lividus harvest occurs and at two controls within an adjacent Marine Protected Area (MPA) where P. lividus harvest is prohibited. Density, size, and average biomass of P. lividus were significantly higher at the protected areas than at the exploited ones. In particular, large-sized specimens (test diam >5 cm) were notably less abundant at the fished locations; however, very similar results were detected for both the periods examined (i.e., before and after the fishing season), suggesting that effects of pressing activity of harvest through the past years may have produced effects much more evident than those attributable to the single fishing season. These results reveal the existence of a heavy-fishing impact on P. lividus in northwestern Sardinia and the urgent need of regulating its harvesting to prevent severe direct effects on its populations.

On the Movement of the Sea Urchin Paracentrotus lividus Towards Posidonia oceanica Seagrass Patches

ABSTRACT The movement of the sea urchin Paracentrotus lividus from a rocky habitat to patches of Posidonia oceanica was investigated with respect to the size of the sea urchins and their location from the edge of the patch. With this aim, a manipulative experiment was conducted (4 times) at a location where several P. oceanica patches were interspersed on rocky platforms. Each time, after an accurate removal of the sea urchins populating them, 15 of these patches were randomly assigned in sets of 3 to 5 different urchin addition treatments, using groups of 10 large or small P. lividus specimens (test diameter >50 and <30 mm, respectively) positioned at close and far distances (25 and 100 cm, respectively) from the edges of the patches as follows: large-close, large-far, small-close, small-far, and control patches where no urchins were added. The abundance of sea urchins inside the patches was counted after 24 h. Results highlighted significant variability because of the distance from the patches, whereas no significant effect was observed for sea urchin size. These results suggested that: P. lividus specimens close to P. oceanica patches might have a greater probability of reaching them, and that the chance to reach the patch does not depend on the size of sea urchins. In fact, a comparable ability to move towards the patches was evident for different-sized specimens, indicating that migration from one habitat to the other is possible even for small-sized individuals.

Perspectives for development of low impact aquaculture in a Western Mediterranean lagoon: the case of the carpet clam Tapes decussatus

Some 30,000 specimens of the Mediterranean clam Tapes decussatus were suspended in nylon bags of two different mesh sizes and pre-grown in the Calich lagoon (Sardinia, Italy) from March to June 2001. The samples differed in size at the end of the pre-growth stage. They were then sown at a density of 650 specimens m−2 in two stations of the lagoon. The growth rates in the stations were different according to the Von Bertalanffy model. Primary and secondary plankton production was calculated by field measurements as well as by modelling. The results indicate that the Calich lagoon could produce a maximum of 753.25 g m−2 yr −1 WW, with shell, for Tapes decussatus. Our culture experiments lasted 15 months with an estimated mortality of 50% and the yield of T. decussatus was 4.3 times greater than the calculated natural production.

Response of captive seabass and seabream as behavioural indicator in aquaculture

Abstract Welfare of cultivate fish at high-density represents an important concern for modern aquaculture. The behaviour of European seabass (Dicentrarchus labrax) and seabream (Sparus aurata) reared in cages was studied in a fish farm of northern Sardinia (Italy) in autumn 2006 to test whether captive condition had an effect on the movement patterns of these two species. Video images recorded before, during and after the manual feeding distribution allowed us to collect data on different behaviours of captive fish. Thus, behaviours indicating the position of fish in the water column, swimming direction and possible aggressive behaviours (aggression, direction change and collision) showed juveniles and adults of seabass and seabream were overall affected by feeding rhythms and captive overcrowding. Seabream had a major tendency to swim towards the bottom and higher frequency of horizontal swimming and collisions than seabass. The overall behavioural difference between two species was explained in terms of their differences in ecological features in the wild.

Suspended culture of Ostrea edulis in the Calich lagoon (North western Sardinia, Italy): preliminary results

Abstract Suspended culture is a widespread farming method used for many bivalve species such as mussels, oysters and scallops. In the Mediterranean, this technique is mainly practised in lagoons or in sheltered coastal areas using floating lines from which molluscs are suspended in several ways. In this study, the European flat oyster (Ostrea edulis Linné, 1758) was grown in suspended lantern nets in the Calich lagoon (Sardinia, Mediterranean Sea) from March 2004 to March 2005. Two distinct groups of 6 lanterns each were hung to longline ropes near the mouth (station 1) and in the central portion of the lagoon (station 2). In each lantern (diameter=50cm; height=30cm), 90 O. edulis specimens were grown and, in order to ensure good water circulation inside the lantern net, fouling organisms were removed every month. Overall mortality, shell length (anterior-posterior axis), shell width (maximum distance on the lateral axis, between both valves of the closed shell) and total wet weight of a 180 specimen oyster sample (30 from each lantern) were recorded every 2 months at each site. In addition, water temperature, salinity, dissolved oxygen, and pH were monitored monthly by means of a multi-parametric probe at both sites (between 10a.m. and 12p.m.). One-way ANOVA was used to test for differences in oyster final mean morphometric characters recorded at the 2 growing stations. Chi-square test (with Yates correction for continuity) was performed to compare survival rates at the end of the trial. From an initial mean shell length of 49.5±4.6mm, O. edulis growth rate showed a similar trend at both the stations. Nevertheless, ANOVA detected significant differences (F=7.10; p<0.01) in final mean oyster length values (83.7±6.5mm at station 1 vs 81.7±7.6mm at station 2). Significant differences (F=9.74; p<0.01) were also found in final mean oyster width (28.4±3.1mm at station 1 vs 27.5±2.8mm at station 2) and weight (F=4.00; p<0.05) values (91.4±16.7g at station 1 vs 87.7±18.3g at station 2). Moreover, chi-square test revealed a significantly different survival rate (χ2=10.04; p<0.01) between the 2 groups (57.4% at station 1 vs 47.6% at station 2). Water temperature, salinity, dissolved oxygen, and pH monthly values recorded at the 2 growing stations were almost identical. Thus, the observed differences in oyster growth and survival seemed not to be due to the hydrological variables considered. Instead, they could probably be related to mechanical and chemical effects of water renewal by coastal waters, which may have led to different seasonal seston food supplies at the 2 sites. The suspended culture of the European flat oyster described in this paper can increase the mollusc production of the Calich lagoon by growing a valuable bivalve species which is naturally scarce in this biotope. In fact, our preliminary results showed good survival and growth rates of O. edulis especially near the mouth of the lagoon. Furthermore, this farming technique could be a possible source of economic benefits for local fishermen and, above all, a low impact aquacultural activity compatible with the environment.

A Meramod® model approach for the Environmental Impact Assessment (EIA) of the off–shore aquaculture improvement in the Alghero Bay (North western Sardinia, Italy)

Abstract Marine fish farming generates particulate wastes which are dispersed in the sea environment. To deal with this problem, particulate waste dispersion models have been developed to predict the effects of fish cage culture. In this study, we evaluated the seabed deposition of a fish farming facility located in the central western Mediterranean by using the Meramod® model. The objectives where first to assess the actual scenario, and second to forecast the possible impact due to the forthcoming enlargement of the farming area with the addition of new fish cages. By computing the hydrodynamic measurements and the daily amount of feed recorded between July and December 2006, the impact seabed surfaces forecasted by the model increased from 5.6ha in the actual scenario, up to 7.3ha in the future. The model estimated a maximum level of total solid flux deposition of 3,800g/m2bed/year and a maximum level of total carbon flux deposition of 1,350g/m2bed/year for both scenarios. Furthermore, the model predicted that the installation of 4 new fish cages (with an hypothetical mean daily amount of feed of 50kg/cage) will produce a total solid and carbon flux deposition levels ranging 0-400 and 0-150g/m2bed/year respectively, under the new fish cages location.