Post larval development

Before transferring seed outdoors, post-larvae are reared in indoor conditions and fed on cultured microalgae to improve their growth. This is micro-nursery phase, the ultimate step in hatchery/nursery process.

The limiting factors for this stage concern seed quality and technological aspects. Seed quality has been considered as satisfactory when mortalities remain low during this phase. However, poor quality hatchery seed has been suggested as a cause of oyster summer mortality. To refute or confirm this hypothesis, more information is needed on genetics, physiology and immunology. Existing hatchery technology can also pose constraints during the micro-nursery phase as most post-larval production in commercial nurseries is done in down- or up-wellers with a partial exchange of seawater each day, or in flow-through tubes (Helm et al., 2004). Technological progress is expected on continuous phytoplankton culture and on RAS, which will reduce labour and energy expenses and improve seed growth and survival.

1 Genetic diversity

A major characteristic of seed quality that accounts for sustainable development of hatchery propagated stocks is its genetic diversity. In REPROSEED, multiplexed sets of markers will be developed to provide convenient tools to evaluate genetic diversity in hatchery-produced mussels and scallops. Markers are already available for C. gigas.

2 Early reproductive maturation

REPROSEED intends to answer two scientific questions on this life stage.

First, there is a need to understand sex determination of alternate hermaphrodite oysters, like C. gigas, which change sex during life in a seemingly irregular way. Temperature and nutritional factors are known to have some influence on maturation in such gonochoric and hermaphrodite species. Moreover, some cascades of genes show statistically significant results in oyster (C. gigas). This improved understanding of sex determination will allow better control of sex ratio during the conditioning period.

At the same time, the project will study the gametogenesis initiation period, when defence mechanisms are weakened. At this time, young hatchery-reared oysters are kept protected indoors under controlled environmental conditions. Ultimately, avoiding or at least delaying maturation of seed improves its robustness when transferred in the field.

3 Recirculating systems and outdoor continuous cultures

Introduction of RAS technology and the use of different water treatments, such as biofilters and protein skimmers, will provide the opportunity to increase the biomass cultured and improve growth and survival without any considerable increase in the costs.

Two methods are possible for producing outdoor phytoplankton as feed for bivalves: continuous culture, which is highly dependent on environmental conditions and vulnerable to contamination; and batch culture, which is the most frequently used but can require intensive labour. Whatever the culture method, optimal conditions for the most commonly used microalgae are still poorly known. Another technical challenge is to develop artificial diets with a particle size of ±10µ, which will meet the nutritional requirements specific to each species; this could be the best way of decreasing costs compared with producing live algae.

REPROSEED is developing new technology for producing healthy seed at lower cost: a High Density Seed System, RAS recirculating system and mass outdoor production of algae specifically adapted for each of the studied bivalve species.