Pierre-Gildas Fleury

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

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.

Synthèse de l'étude et des campagnes à la mer 2011 sur l'exploitation aux vire-lignes des espèces démersales profondes autour de La Réunion

In an effort to implement new fishing activities for the small scale fisheries of Reunion Island, a prospecting cruise targeting deep demersal fish (200 to 600 m) was set up on the west coast of Reunion Island in 2000. This project was carried out by the Regional Fishery Committee and the French Institute for the Exploration of the Sea (Ifremer). Large commercial stocks were identified, and numerous professional or recreational fishermen started employing electric winches to exploit these deep resources. However, the deep demersal species are a fragile resource due mainly to low growth rates and late sexual maturation. Due to the rapid increase in fishing pressure, Ifremer was asked to set up a new study and two scientific campaigns were carried out in 2011 (68 fishing trips being conducted with 559 dropline operations). The first cruise was conducted at the same sites, using the same types of fishing gear and methods used in 2000. A marked drop of greater than 90% was observed in the catches of commercial species, together with a decrease of most fish sizes, attesting the overexploitation of these stocks. Only, catches of the small non-commercial shark (Squalus megalops) remained important (50% in number and weight of total catches). The second cruise extended the prospection area to all waters surrounding Reunion Island . The overexploitation was confirmed in the west of Reunion Island, and in to a lesser degree in the north. Yields were better along the east and the south coasts. Concurrently, the efficiency of 2 hook sizes was compared : small hooks caught significantly more fish but of significantly smaller body sizes, resulting in comparable total weight of fish. We conclude that medium size hooks (i.e. type 7/0) have a significant selectivity on the fish size without loss in the global catch yield. Various types of baits were also tested with the flesh of small local pelagic fish species (Selar crumenophtalmus and Decapterus macarellus) appearing to be the most efficient. This study provides helpful indicators for the management of demersal fish stocks in Reunion island, which have been poorly surveyed and poorly regulated until now. However, the total fishing effort, especially from recreational fishermen, remains the most important factor to be quantified.