WWW.WAS.ORG • WORLD AQUACULTURE • SEPTEMBER 2013 23 Microbial interference is often cited as the source of problems. Harmful bacteria, not necessarily virulent pathogens, that develop during live food production (microalgae, rotifers, Artemia) are carried along with live food into larval rearing tanks where they can interfere with the sensitive physiological development of larvae, especially that of digestive and immune systems. This study of hostmicrobe interactions requires a different approach from the former “black box” research approach that did not allow the development of reproducible results (Fig. 15). Interactions between host, environment and microbial communities are just too numerous and complex to unravel under in vivo conditions. Similar to work performed with terrestrial animals (mice, pigs and sheep) farmed in bacteriafree conditions, first results with aquatic species prove that much fundamental progress can be made when working with so-called gnotobiotic model systems. To illustrate, European seabass larvae were cultured under bacteria-free conditions for 12 days and fed with bacteriafree, live brine shrimp Artemia nauplii (Fig. 16). Under so-called optimal commercial conditions, the classical greenwater approach, unpredictable mortalities occur in the first weeks of larval development and result in large variation in performance. In contrast, bacteria-free conditions are much more stable and result in consistent and very high survival rates throughout the first weeks of larval development. Exposure of larvae to stressful conditions, such as intermittent cycles of light and dark, can result in extra mortalities. Larvae kept under bacteria-free conditions display minimal effects to this kind of stressor. It could be that, at mouth opening, bacteria enter the intestine and activate the wrong triggers, resulting in a negative effect on the physiological condition of larvae. These are valuable empirical observations that can now be further analyzed with molecular tools to unravel gene expression in developing larvae exposed to well-defined conditions. A multidisciplinary research approach for the study of the possible effects of the magic greenwater technique in fish and shellfish larviculture, as outlined in Figure 17, is needed in different areas of aquaculture research. Only then we can gain more basic insights of underlying biological mechanisms to allow the development of appropriate solutions for application under practical farming conditions. The ultimate aim should be to develop new hatchery practices (e.g., with regard to microbial steering) and apply innovative products (e.g., substrate for specific bacteria or signal molecules to disrupt their virulence triggers) that ultimately result in more costeffective hatchery production of certified seed with improved characteristics. Priority 3 – Species Selection. We need more proactive attention for species selection. We should not aim to increase the number of species farmed but rather be more selective in identifying suitable species for mass markets, such as Pangasius, and niche species catering to local markets where value-added products might be in good demand. Working with a more limited number of species should also allow more focus in research. Priority 4 – Selective Breeding. Selective breeding is another area where we are far behind terrestrial plant and animal agriculture. There is great need to invest in breeding research for key aquatic species. For example, good progress has been made with selective breeding of white shrimp in recent years. Many of the tools that have been developed in plant genomic (CONTINUED ON PAGE 22) FROM TOP TO BOTTOM. FIGURE 14. Priorities for future aquaculture (from Plenary Lecture by Patrick Sorgeloos “Resources, technologies and services for future aquaculture: a needs assessment for sustainable development” at the Global Conference on Aquaculture, 22–25 September 2010, Phuket, Thailand). FIGURE 15. Gnotobiotic culture systems for the study of host-microbial interactions (courtesy Peter Bossier, Laboratory of Aquaculture, Ghent University, 2010). FIGURE 16. Survival of European sea bass larvae in gnotobiotic versus standard larviculture control systems in dark and light-stressed conditions (courtesy Peter Bossier and Kristof Dierckens, Laboratory of Aquaculture, Ghent University, 2010 and Dierckens et al. 2009).
RkJQdWJsaXNoZXIy MjExNDY=