- In the wild, marine fish larvae feed on copepods and other planktonic organisms.
Copepods are often considered the "gold standard" for meeting the nutritional needs of cultured marine fish larvae; however, in captivity, marine fish larvae are generally fed cultured live prey, i.e. rotifers (Brachionus plicatilis) and/or Artemia sp., until they can be weaned onto artificial diets, which typically coincides with the end of the larval phase. Concentrations of certain water-soluble nutrients in rotifers and Artemia, notably taurine, may not be sufficient to meet the nutritional demands of some species of marine fish larvae. The delivery of low-molecular weight, water-soluble substances to aquatic organisms is challenging because these substances are rapidly lost from food particles when suspended in water, referred to as nutrient leaching. One solution to this problem is to encapsulate water-soluble substances within synthetic microparticles that are specifically designed to reduce nutrient leaching. Alternatively, water-soluble substances may be dissolved in the culture water of these organisms facilitating nutrient absorption or ingestion via drinking, hereafter referred
to as the "dissolved method" of nutrient enrichment. In Chapter 2, I provide an
overview of enrichment methods for cultured live prey and put particular emphasis on
the enrichment of water-soluble compounds. In Chapters 3, 4 and 5, my co-authors
and I evaluated the use of synthetic microparticles, liposomes and wax spray beads,
for increasing the water-soluble nutrient concentrations (enrichment) of rotifers and Artemia with the intent to increase their nutritional quality for marine fish larvae.
Furthermore, we compared microparticulate enrichment with dissolved methods
whereby the nutrients were dissolved in the culture medium. Microparticle-enriched live prey were evaluated by: 1) using fluorescent techniques to investigate the ingestion and digestion of microparticles by live prey and 2) determining the growth performance of two species of marine fish larvae, northern rock sole (Lepidopsetta
polyxystra) and California yellowtail (CYT: Seriola lalandi) in response to different
enrichment methods for the water-soluble nutrient, taurine. Taurine was used as a
model nutrient because taurine concentrations are low in cultured live prey, especially
when compared to copepods, and because its was believed to be a growth-limiting
nutrient for marine fish larvae. Taurine concentrations in rotifers and Artemia
enriched with taurine-containing microparticles could be increased to levels reported
in copepods. In Chapters 3 and 4, we found that northern rock sole larvae grew
significantly larger, were more developed and had higher whole body taurine
concentrations when fed rotifers enriched with taurine-containing microparticles
when compared to control treatments. In Chapter 5, we found that CYT larvae fed
taurine enriched rotifers showed increased growth (final dry weights) and had higher whole body taurine concentrations when compared to larvae fed unenriched rotifers but these differences were not apparent in the subsequent Artemia phase, suggesting that unenriched Artemia had sufficient taurine concentrations to allow compensatory growth of CYT. Differences and similarities are discussed between the efficacy of liposomes and wax spray beads for rotifer enrichment and subsequent nutritional effects on fish larvae. Ultimately, this research provides the first evidence of positive growth effects of liposome- and wax spray bead-enriched live prey on marine fish larvae as a result of enhanced delivery of water-soluble nutrients. The outcomes of these findings suggest that microparticles may be used as a research tool to assess the water-soluble nutrient deficiencies of cultured live prey for marine fish larvae.
Furthermore, with further development, these methods may have application for
commercial hatcheries and could result in improved production efficiencies and cost savings for these operations