Challenges related to delivery of water soluble nutrients to marine fish larvae. Evaluation of changes in nutritional quality due to production process and leaching from larval diets -with emphasis on protein quality

Marine fish larvae fed formulated diets have suppressed growth and survival compared to larvae fed live feed for the first weeks. Live feed is successfully used in the aquaculture industry, although there are difficulties delivering controllable concentrations of several nutritional compounds. In research, the use of formulated diets is therefore essential to accomplish proper nutritional dose response studies. The focus of this work was to study the properties of a protein cross-linked diet and a heat coagulated diet used for nutritional studies of marine fish larvae. The ability to deliver water soluble nutrients and changes in protein quality due to production processes and exposure to leaching was emphasized. A pancreatic protein in vitro digestibility method, simulating stomachless fish larvae, was used to investigate the digestibility of various live feeds and feed ingredients. The same in vitro model was also used to study the effect of the production processes and the effect of increased inclusion of hydrolyzed protein in compound diets. Both diets showed substantial changes in protein quality due to the production process and exposure to leaching. The protein cross-linked capsules had a nearly complete loss of watersoluble nitrogen (N) during cross-linking and the following washing steps and more than 90 % loss of other water-soluble micronutrients. The protein cross-linking led to a 25 % reduction in in vitro protein digestibility. A large fraction of the soluble N in the feed ingredients was made insoluble by heat denaturation during production of the heat coagulated diet, but the concentrations of peptides and free amino acids (FAA) were not influenced. However, after exposure to leaching for 6 min most of the soluble N fraction was lost and there were no significant difference in concentrations of soluble N between diets with increasing concentration of hydrolyzed protein ranging from 0 % to 45 % of total protein. There were no significant differences in in vitro digestibility between the four diets with increasing concentration of hydrolyzed protein. However, the leached diets showed significantly reduced digestibility compared to the diets that had not been exposed to leaching. In conclusion, neither the protein cross-linked nor the heat coagulated diet may be suitable for the delivery of water-soluble nutrients to marine fish larvae. The protein in vitro digestibility of the protein cross-linked diet, heat coagulated diets and a commercial larval diet (53 – 73 %) were lower than frozen live feed (84 – 87 %). The digestibility of the soluble N fractions was similar for the marine meals and the live feed and higher than the respective insoluble protein fractions. However, the live feed contained 54-67 % soluble N in comparison to the marine meals that only contained 11-17 % soluble N. In the search for other possible diets to deliver soluble nutrients, lipid spray beads (LSB) were investigated. LSB, as part of complex particles, has been an interesting candidate for delivering soluble nutrients with a high retention efficiency, although the fatty acid profile of the LSB have made them inappropriate when delivered in large quantities. LSB with an improved fatty acid profile were developed by inclusion of high concentrations of phospholipids. Due to the hydrophilic behavior of phospholipids, the LSB dispersed in water and could therefore be used to deliver water soluble micronutrients to live feed. The riboflavin content of Artemia was increased from 55 ± 0.6 mg kg-1 (dw) to 329 ± 62 mg kg-1 (dw) after 1 h enrichment. There is still a severe leaching rate of highly soluble nutrients and the LSB might therefore not be able to deliver nutrients needed in large quantities such as FAA and peptides. However, for nutrients needed in small quantities, the LSB seems to be a promising tool.