Essai d'évaluation du stock de merlu du Golfe de Gascogne et du plateau celtique

An assessment of the hake stock exploited in the Bay of Biscay and in-the Celtic Sea has been attempted. Schaefer's and Beverton-Holt's methods have been used. Due to the lack or the inadequacy of our data, a number of assumptions have been made which restrict the validity of the conclusions. Nevertheless, both methods agree in showing that a substancial increase in production (50-80 p. 100, as an order of magnitude) could be achieved by a strong reduction of fishing effort (40-60 p. 100). A still larger increase in yield p. recruit could be achieved by a mesh increase, up to 150 mm at leas

Quelques évolutions récentes des relations entre archéologie, forêt et environnement

National audienc

Provisioning of mussel seed and its efficient use in culture

Mussel culture largely depends on seed and feed from the natural environment. This paper focusses on seed provisioning and efficient use of these resources in mussel production. Approaches and technologies for seed supply and efficient use of seed in mussel production are described for the different culture techniques. This includes potential interactions and conflicts with the natural environment. Three methods are used to provide seed: wild harvest, use of suspended collectors and hatchery production. Harvest of wild seed from seaweed (in New Zealand) or natural beds is still a major source for culture in some areas, costs are low but provisioning is often unreliable. Most research concerning spat collection deals with comparison of different types of suspended collectors, settlement cues and problems with biofouling. Hatchery seed is more expensive, but hatcheries provide the opportunity for selective breeding and triploid production giving the product an added value. The challenge is to bring hatchery production costs more in line with the actual sale value of mussel seed. Monitoring genetic diversity can give insight in whether collector seed or hatchery seed growth and survival is negatively affected by reduced diversity. Grow-out occurs in bottom culture, bouchot culture and off-bottom longline and raft culture. In bottom-culture, the focus is on developing better seeding techniques, predator control and optimizing culture practices such as timing of relay, substrate use and harvest. For bouchot culture, technical developments are directed to mechanical methods to increase efficiency in size grading, restocking, harvesting and processing. Innovation in growing-out techniques for longline and raft culture are directed towards the investigation of optimal stocking densities, and on material type and configuration of farms. Production efficiency increases from bottom culture to bouchot culture, to rope and raft culture and are related to the sources of mortality and differences in growth rate. Growth rate of mussels is higher in off bottom culture than in on bottom culture and higher when submerged than in intertidal. Mussels from the Perna genus are found to have a higher growth rate but a lower production efficiency than mussels from the Mytilus genus. Efficient use of seed in mussel culture should aim at a reduction of mussel losses and an increase in growth rates. Important tools are adjusting seeding densities in relation to system design, reducing seeding stress, predator control and applying thinning out or relay