Feeds for easy and efficient production of nutritionally optimised rotifers
by Eric Henry, Ph.D. Research Scientist, Reed Mariculture Inc. otifers remain the most widelyused live feed for early stages of larval fish culture, and hatchery success is critically dependent on a reliable supply of healthy, nutritionally-enriched rotifers that can provide the nutrition larvae need to support rapid growth and normal development.
“They are what they eat”
The nutritional value of rotifers depends entirely on the feeds used to produce them. Typically, batch cultures are grown to harvest density using low-cost feeds based on yeast or algae that are easily produced but of limited nutritional value, such as Spirulina, Chlorella, or Nannochloropsis. Some of these feeds are available as dry powders, which can be shipped at a low cost and stored at room temperature. But dry feeds can be laborious to properly re-hydrate and dispense to rotifer cultures, and if the powders are not completely dispersed as un-clumped particles less than 20 micrometers in diameter they cannot be ingested by the rotifers. Uneaten feed is not only wasted, but will foul cultures and promote blooms of bacteria and ciliate protozoa that are present in many rotifer culture systems. Dry feed particles are also subject to rapid leaching of soluble nutritional components before they can be consumed by the rotifers.
“Too much and too late”
When such batch cultures have grown to the required density, the feed must then be switched to a lipid-emulsion “enrichment” feed for a few hours before the rotifers can be fed to larvae, in an effort to compensate for the poor nutritional quality of the grow-out feed. This “gut loading” strategy fills the digestive tracts of the rotifers with the lipid-rich feed, to be delivered to the larvae when the rotifers are consumed. But this conventional enrichment practice can be described as “too much and too late.” Too much, because the extreme lipid content of conventional enrichment feeds is nutritionally unbalanced and is quite stressful to the rotifers, as evidenced by the dramatic rise in oxygen consumption of rotifers during enrichment feeding. Too late, because enrichment feeding at the end of the culture cycle is so brief that only the gut contents of the rotifer are enriched, while the rest of the body of the rotifer is unchanged. Active, clean, healthy rotifers are essential for production of the healthiest larvae, but the stress caused by the extreme lipid content of conventional enrichment feeds weakens the rotifers and reduces their motility.
Lipid emulsions foul the rotifer enrichment tank as well as the rotifers, so some of the enrichment feed is not consumed by the rotifers and is therefore wasted. The emulsion-fouled rotifers then must undergo stressful harvesting and washing procedures before they can be fed to larvae. They suffer additional stress when the popular “cold bank” technique is used (storing rotifers at refrigerator temperature and feeding out to larvae over 18-24 hours). Temperature shocks when enriched rotifers are cold banked, and again when they are transferred to the larval tank, can cause the rotifers to eject their gut contents (and enrichment), fouling the cold storage or larval tank, reducing the nutritional value of the rotifers, and further weakening or killing many before they are fed to larvae. Finally, once the rotifers are transferred to the larval tank, the enriched gut contents diminish as they are used to support rotifer metabolism, or they may be lost as poorly digested feces when transport through the gut accelerates as the rotifers consume “greenwater” algae.
A new paradigm
A much simpler and more effective enrichment strategy is continuous culture “grow-out enrichment” using enriching, whole-algae, liquid feeds, so that the entire body of the rotifer is enriched. The benefits are multiple: • Liquid feeds require no preparation before use, and are ideally suited to automated feeding by dosing pumps. • Liquid feeds made of intact algae cells are not subject to the rapid leaching of nutrients that affects dry feeds when they are re-hydrated. • Rotifers are not stressed, so they retain good health and motility in the larval tank. • The enrichment is in rotifer tissue and is more bioavailable (as phospholipids, organically bound minerals and vitamins, etc.) • Rotifers require no washing, and in some applications can be fed directly to the larval tank, eliminating the labour of harvesting and washing, and avoiding temperature shocks that cause ejection of the gut contents • Rotifers enriched during grow-out retain their enrichment in the larval tank longer and deliver more enrichment to the larvae. RotiGrow Plus and RotiGrow OneStep are concentrated, microalgae-based, liquid rotifer feeds. Subsequent to the microalgal cells being kept hydrated, they retain their full nutritional value. RotiGrow Plus is optimised for fish with moderate DHA and ARA requirements, while RotiGrow OneStep provides still higher levels of enrichment for the most demanding applications. Both RotiGrow feeds provide all these advantages such as fullspectrum enrichment. In addition to optimal highly unsaturated
24 | July 2017 - International Aquafeed