The consumption of seafood species for which there is a high consumer demand such as salmon, trout, halibut and eel is increasing and due to this high demand and limited natural stocks, much effort is spent on developing cost effective aquacultural methods of farming such species. A particularly serious problem is to secure a high survival rate of the hatched larvae of the species being cultivated.
Expansion of the aquaculture industry requires that several problems be addressed, one of the most significant being the difficulty of supplying live prey organisms which provide a nutritionally adequate feed for the larvae. Larval fish in the wild consume a mixed population of phytoplankton prey organisms that provide a balanced nutrition. However, collecting phytoplankton in sufficient quantities to meet the demand in aquaculture is not feasible. As an alternative, selected species of prey organisms, in particular rotifers and Artemia species, are presently cultivated and used as feed.
Generally however, such artificially cultivated prey organisms, although they provide adequate amounts of protein and energy, have a lipid composition which is not adequate to cover the requirement for certain HUFAs, in particular DHA and EPA (eicosapentaenoic acid) which are essential for the optimum survival, growth and development of larvae. Specifically, it has been shown that a high content of DHA is required and that the ratio between DHA and EPA In the prey organisms should be at least 1:1 and preferably at least 2:1.
Currently, this problem is being addressed by cultivating the prey organisms in the presence of enrichment compositions permitting the organisms to be enriched in respect of these essential fatty acids. However, presently available commercial compositions for that purpose such as products sold under the tradename Selco (TM) do not meet the above requirements in that the DHA content is relatively low and/or the DHA: EPA ratio is not high enough. Using such compositions Artemia enrichment levels of 3-5% DHA of total lipids have been reported (McEvoy et al. Aquaculture 163 (1998) 237-250), and 12 to 15% survival rates of fish fed such Artemia (McEvoy et al. supra; Navarro et al. J. Fish Biol. 43 (1993) 503-515). ). In this context, survival rates are defined as survival percentage from first feeding through metamorphosis. For cost-effective aquaculture production a larval survival rate of 50% and preferably higher should be obtained.
WO 99/37166 discloses a method for the enrichment of live prey organisms with nutrients essential for fish larvae based on the use of dry soap powders of HUFAs obtained from the waste stream of marine algae oil extraction. Artemia DHA enrichment levels of about 2.7% of dry weight are disclosed, but the use in aquaculture and efficacy with respect to fish larvae survival is not disclosed.
Another material intended for use in aquaculture is described in WO 99/06585. Examples disclose a DHA content of 24 wt %, but the phospholipid content is not disclosed. The material however, contains a high proportion of free fatty acids (about 32-37 wt %) and a high content of non-lipid material which may reduce the lipid uptake efficiency of prey animals. A high content of free fatty acids is generally considered harmful for fish juveniles.
Neither of the two last-mentioned materials is fish-based and they lack many HUFAs found in fish, such as EPA and other n-3 fatty acids, desirable for fish juveniles.
In a recent review by Sargent et al. (Aquaculture 179 (1999) 217-229) it is emphasized that in addition to the requirement in respect of HUFAs, fish larvae have a dietary requirement for phospholipids and it is stressed that the ideal diet for fish larvae is a diet having a composition similar to the yolk of the eggs. According to these authors fish egg yolk contains about 10 wt % (on a dry matter basis) phospholipids which contain about 17 wt % of DHA and about 9 wt % of EPA. These authors conclude in their review that a problem remains with respect to how to construct such a diet on a commercial scale from currently available materials.
It has now been found that it is possible to provide enriched aquacultural prey organisms having, in respect of HUFAs and phospholipids, a composition which is very close to that of fish egg yolk. By using the prey organisms of the invention it is possible to secure optimum survival, growth, pigmentation and morphogenesis of aquatic organism larvae such as halibut larvae. As demonstrated herein, the invention provides much higher survival rates during the larval stage and increased quality parameters than previously disclosed for fish such as Halibut, thus making aquacultural rearing of many high-demand fish species more economical and commercially viable.