This paper presents the simulator SHADYS (‘simulator halieutique de dynamiques spatiales’). SHADYS is a tool devoted to the representation of spatial processes involved in fisheries dynamics. It puts together three fundamental entities in an explicit spatio-temporal way: the environment, the fish populations under consideration and the fishing fleet. It is based on simple, realistic and well identified mechanisms (density-dependent habitat selection, advection and diffusion of a fish population, fishermen search strategy, etc.) implemented by interfacing numerical models with a GIS (geographic information system). As a virtual laboratory, SHADYS allows experiments to be performed in a theoretical world, such experiments being generally impossible to perform in reality. As an illustration, different scenarios of a marine refugia-based management have been explored using the SHADYS simulator. Different simulations are conducted to assess the short-term effects on yield of a marine protected area. It is shown that for diffusive or migratory species, the yield per recruit as a function of the protected surface can reach a maximum. Under this condition, the concept of ‘space overfishing’ is meaningful. Protected areas then behave like ‘sources’ and exploited areas like ‘sinks’. For resident populations, the larger the protected area is, the lower the catches per recruit are. Regarding the spatial distribution of the fishing effort, it is shown that if it is spatially distributed in order to maximize catches, then fishing boats will slowly tend to be distributed all along the boundaries of the protected area.

The present paper suggests a method for estimating the fishing power of vessels and for analysing fleet dynamics. The approach is based on quantification of stocks catchability (q), derived from fishing mortality coefficients (F) as calculated by virtual population analysis. Catchabilities for each harvested stock are thus estimated relatively to the fishing effort (fn) of each vessel, according to the equation: q = (F/fn). A linear model is then fitted to these catchabilities. The model allows the identification and quantification of trends in average mortality rates per fishing hour for each stock. Under some assumptions, trends are interpreted as variations in the overall fishing power of each fleet. The approach is applied to three industrial and semi-industrial fleets of Brittany (Lorient, Concarneau and Douarnenez) and to the main gadoid stocks they exploit off the west coast of Scotland (ICES area VIa), and in the Celtic Sea (ICES area VIIf,g,h) for Concarneau. Results show large variations in fishing power. Particularly, a marked increase trend in the fishing power exerted on saithe (Pollachius virens) is highlighted for the three fleets, over the period 1983-1989. These variations can be explained by the redirection of fishing strategies, which may occur on a large scale. Thus, we show how the collapse of saithe stock at first led the three fleets to intensify the harvesting of saithe, and from 1989 on, to adopt different strategies. The possible causes of the observed dynamics are discussed, as well as their consequences for fisheries management. In particular, the relevance of direct control of fishing effort as a regulatory tool is questioned.

There are currently at least six commercial fisheries harvesting six different species of euphausiid, or krill: Antarctic krill (Euphausia superba), fished in the Antarctic; North Pacific krill (Euphausia pacifica), fished off Japan and off western Canada; Euphausia nana, fished off the coast of Japan; Thysanoessa inermis, fished off the coast of Japan and off eastern Canada; and Thysanoessa raschii and Meganyctiphanes norvegica which have been experimentally harvested off eastern Canada. The current world catch of all species of krill is over 150 000 tonnes per annum but few fisheries are being exploited to their maximum theoretical potential. The size of the world krill harvest is currently limited by lack of demand, although some fisheries are being deliberately managed at low levels because of ecological concerns. We have outlined the history of these krill fisheries to determine where there are common trends and issues which will affect their future development. Krill products are currently mostly used for the aquaculture and sport fishing market but considerable effort has also been put into developing products for human consumption, partiularly from Antarctic krill. The future development of krill fisheries is examined in the light of information on trends in krill products which include pharmaceutical and industrial uses in addition to nutritional products. Because of the central ecological role of krill in many marine ecosystems, the subject of krill harvesting is a sensitive issue and krill fisheries require careful management. This requirement has spawned an innovative international management regime in the Antarctic - the Convention for the Conservation of Antarctic Marine Living Resources (CCAMLR) - which has developed procedures for managing the harvest of Antarctic krill that may be applicable to other fisheries. The common problems of managing krill fisheries are outlined particularly those relating to the role of krill in many coastal ecosystems, and as prey for many other species which are commercially fished.

The French oyster farming industry relies almost exclusively on juveniles collected from the natural environment; the supply of spat produced by hatcheries is low, about 10 % of the industry's requirements. Development through selective breeding of oyster stocks which are better suited for aquaculture purposes, is likely to reverse this tendency since only hatcheries will be able to supply such animals. Scallop farming, which at present is poorly developed in France, relies exclusively on hatchery produced spat. Although hatchery technology is constantly being improved, significant production problems remain which must be solved before hatcheries become a major supplier of juveniles for the industry. This paper describes the present state of hatchery technology in France based on experimental results obtained with the great scallop Pecten maximus and the Pacific oyster Crassostrea gigas, over the past ten years. Compared to the great scallop, the Pacific oyster has higher D larva yields (60 % for C. gigas vs. 30 % for P. maximus) and a faster larval growth rate (10 μm·d−1 for C. gigas vs. 5 μm·d−1 for P. maximus). However, Pacific oysters have a greater heterogeneity during larval development and higher mortality rates (40 % for C. gigas vs. 25 % for P. maximus) which produce lower overall yields of pediveliger larvae ready-to-set for Pacific oysters (15 %) compared to the great scallop (30 %). Development of continuous larval and post-larval culture methods along with development of continuous phytoplankton technology offers one of the most promising methods to improve molluscan hatchery techniques.

Le banc ostréicole de Ronce (175 ha) au sud du bassin deMarennes-Oléron (France) constitue un site atelier expérimental pourl'étude des mortalités estivales de l'huître creuse Crassostreagigas. En 1997, la croissance, la maturité sexuelle, la survie etl'environnement des élevages à plat et sur tables sont comparés surquinze sites expérimentaux. En 7 mois, le taux de mortalité desélevages à plat atteint 23−33 % contre 8−19 % pour les élevages surtables. La mortalité varie avec la saison. Elle s'intensifie avec latempérature, en période de pré-ponte et de catabolisme duglycogène. Des modèles statistiques de production de chair sèche del'huître sont établis selon la latitude de la longitude des sites. Cesmodèles (en poids sec) sont orientés nord - nord-ouest et montrent que ladisponibilité alimentaire pour les cheptels d'huîtres, diminueprogressivement du large vers le fond de la baie. Les rendements biologiques (poidstotal), sont compris entre 85 et 2 %, respectivement pour des sites sur tables à25 et 50 % d'exondation. Pour les élevages à plat, une perte de productionsupérieure à 10 % est enregistrée dans certains parcs du sud dubassin. Les meilleurs sites, à 50 % de production (poids total), sontsitués dans la partie est - nord-est du banc de Ronce.

Techniques were developed for labelling prey, and for gut content analysis in fish larvae. Labelling consisted of a short feeding period of the prey (rotifer and Artemia metanauplii) on black-drawing ink before supplying them to the larvae. Prey labelling was rapid and remained stable for at least 3 h when transferred into seawater. In addition, a method was tested to facilitate the observation of larval gut contents. The method, based on Dawson's technique, involved fixation, removal of lipids and clearance of turbot (Psetta maxima) larvae, which become transparent and ready for gut content analysis. This treatment did not interfere with the black ink label of the ingested prey.

Observations were made in Senegal between 1997−1998 on the length of embryonic development in Octopus vulgaris. The data were from eggs which had been laid by two females in the open sea and fourteen females in concrete tanks. The observations in natural surroundings are the first concerning this species. Hatching occurred after 15−42 d at 27 °C, 29−49 d at 22−23 °C, 57−65 d at 21 °C and 80−87 d at 17 °C. These values are higher than the few data previously recorded in European aquariums, particularly for temperatures under 20 °C.