A preliminary study of the behaviour and vitality of reseeded juvenile great scallops, of three sizes in three seasons

Abstract

In order to have a better understanding of recessing in great scallop, Pecten maximus and consequently the causes of mortality at reseeding, this study has monitored, at different seasons, the dispersion and recessing of different sizes of juveniles (about 15, 30 and 45 mm, called ‘small’, ‘medium’ and ‘large’) after seeding. Moreover, the aim was to see when small spat (15 mm) could be seeded, and thus reduce the costs of intermediate culture. Three monitoring approaches were used together: (1) continual observations by remote video camera, of a defined area (less than 1 m2) containing 10 scallops from each size group; (2) daily monitoring of behaviour with divers along three bottom lines, with 20 × 1 m2 plots each and nine marked scallops per plot; and (3) the biochemical content of the muscle: adenylic energetic charge and storage of energy reserves (glucides, proteins, lipids). The video monitoring identified but did not quantify predator behaviour, particularly at night. The role and behaviour of spiny crab, Maia squinado, and of small predators has clearly been shown, such as: (a) small crustaceans, Inachus sp., breaking the edges of scallop valves; and (b) small gobies, Pomatoschistus pictus, pecking the tentacles of the scallop mantle. For the monitoring by divers, filtering appeared much too difficult to look at for it was very disturbed by divers, and anyway the resumption of filtering came immediately after seeding. On the other hand, diver monitoring of dispersal and recessing was quite easy to do with a minimum of practice. On the basis of dispersal, the best seasons for seeding appear to be spring or summer. In autumn, two-thirds of ‘small’ and ‘medium’ juveniles are missing 3 days after seeding, but we could not observe whether they had been eaten by predators or had just moved and recessed farther. There was no experiment in winter owing to adverse conditions for scallop seedings. Biochemical analyses confirmed the unsuitability of autumn for scallop seeding, because of very low glucide content in this season. The adenylic energetic charge in the smooth part of the muscle showed that stress before seeding (aerial exposure, handling), and post-seeding behaviour (swimming, recessing) have a high energetic cost for scallops. In summer and autumn, 3 days after seeding, none of the three size batches recovered their initial vitality.