In the November 2014 issue of Hatchery International, contributor Erich Luening described some recent work from James Cook University in Townsville, Australia, on the shelf life of cultured micro-algal concentrates. The article also described their potential to replace live micro-algae used to enrich rotifers and other live feeds for marine fish larvae.
In correspondence with Valentin Thépot, one of the scientists involved in this work, Hatchery International has gleaned more details about the benefits of blending micro-algal species when enriching rotifers fed to larval barramundi (Lates calcarifer). Thépot and his colleagues, Drs Arnold Mangott and Igor Pirozzi, have begun to explore the technology’s potential.
Marine fish larvae, unlike many freshwater species, cannot be reared solely on artificial feeds and living zooplankters are essential for their production. Currently little is known regarding the nutritional requirements of marine fish larvae, especially for the exogenous feeding period until weaning onto artificial diets can begin. Common live feeds in commercial marine fish hatcheries include rotifers (Brachionus sp.) and brine shrimp (Artemia sp.), which are respectively fed to larvae at early- (2-30 days post hatch, dph) and later (7-40 dph) larval development stages. Due to their potentially variable nutritional quality, much attention has been paid to the reduction or replacement of live feed for the culture of marine fish larvae, however with limited success.
In the meantime, the enrichment of live feed organisms, generally with micro-algae, is a common and necessary practice to boost their nutritional quality. But given the high cost of on-site micro-algal production, much attention has been allocated to replacing micro-algae with off-the-shelf enrichment products (e.g. marine oil emulsions), which mostly enhance the fatty acid content of the live feed at the expense of other key nutrients (amino acids, pigments, etc.).
The development of cost-effective, off-the-shelf micro-algal pastes has offered farmers an additional option for the enrichment of live feeds. These micro-algal pastes include Nannochloropsis oculata and Chlorella vulgaris. In addition to feeding rotifers and Artemia they can also be used to dose the larval rearing tanks to create “green water”. The cell walls of micro-algae act as capsules that prevent the nutrient-rich contents from leaving the cell and compromising water quality in larval culture- and live-feed tanks.
The green water technique maintains the nutritional quality of rotifers as a feed and provides the larvae with a more shaded environment, which in turn improves feed ingestion, survival and growth. Marine oil emulsions used for the enrichment of live feed will lower water quality, another reason why micro-algae are preferable for inducing “green water.”
Barramundi have been cultured in Australia for more than 30 years. There has been significant improvements in their culture technology, but bottlenecks still remain for the larval phase. These challenges include mortality during the early larval period (2-13 dph) and at metamorphosis (19-25 dph), characterised by the vacuolation of the spine and brain, and high sensitivity to stress.
These symptoms have been associated mainly with deficiencies of essential fatty acids (EFA’s), particularly long chain highly unsaturated fatty acids (HUFAs). Important HUFAs include docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA) and arachidonic acid (ARA). These all have critical roles in the development of the neural - and visual systems, structure and function of cell membranes, and are linked to growth, survival, and stress-resistance in marine fish.
Enriching live feed for barramundi larvae in Australia relies largely on one or other of two micro-algae: N. oculata or C. vulgaris. N. oculata has little or no DHA, but tends to be rich in EPA with a moderate amount of ARA. On the other hand, DHA-enriched C. vulgaris is low in EPA and has no ARA. Considering the fatty acid profiles of these two species one might be tempted to mix them to achieve a balanced diet rather than using them singly. In fact, blending micro-algae is essential for the successful production of many molluscs, crustaceans and echinoderms. However this practice is not common in marine fish hatcheries that use rotifers as the first feed, and is rarely adopted in barramundi hatcheries.
Thépot and his colleagues conducted an experiment at James Cook University to test whether the quality and performance of barramundi larvae (growth, stress-resistance and development) could be improved by feeding rotifers that had been enriched with mixtures of C. vulgaris and N. oculata in comparison to a monospecific diet.
Four groups of 2-dph barramundi larvae were fed for 11 days (until 13 dph) with rotifers enriched with either 50:50 or 75:25 (dry weight) blends of N. oculata and C. vulgaris, or each species singly. DHA, EPA and ARA were used as qualitative proxies of the feed. The four groups of larvae were measured at 2, 6, 10 and 13 dph for total length, eye diameter and tail height at the anal vent.
The larvae were also inspected for the percentage undergoing flexion, the developmental change characterised by the notochord flexing upward. At 13 dph 100 larvae from each group were tested for stress resistance. The stress test consisted of exposing the larvae to the air in a 250 μm net for three minutes and then returning them to a 1-l aerated beaker. After seven minutes, the numbers of dead and live larvae were recorded.
The barramundi larvae fed with the 50:50 enriched rotifer diet outperformed all other treatments, having significantly greater larval length, eye diameter and body depth at 13 dph; trends that were observed as early as 10 dph. Larvae fed the 50:50 enriched rotifer diet also achieved the highest flexion percentage at 10 dph and scored the highest stress-test survival (85%) at 13 dph.
The best performing treatment (50:50) had the second highest level of dietary DHA, which suggests that while DHA is an essential fatty acid for barramundi larvae it is likely that its interactions with other EFA’s and monounsaturated fatty acids also play a critical role in larval development.
Overall, using blends of micro-algae rather than single species for enriching live feeds can improve the balance of dietary fatty acids for barramundi larvae, leading to better growth, faster development and higher stress resistance.
Depending on the micro-algae used and their cost, the efficiency and profitability of barramundi hatcheries could potentially be improved by mixing micro-algal species rather than by using them singly to enrich rotifers.
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– David Scarratt