36 MARCH 2016 • WORLD AQUACULTURE • WWW.WAS.ORG In the trial with white seabass, the PARA and MEM diets were marked with the rare earth metal yttrium oxide (Y2O3) as described by Johnson et al. (2009). Larvae with microdiet in the gut were saved and analyzed via inductively coupled plasmaoptical emission spectroscopy (ICP-OES) with a Perkin Elmer Optima 3000 Radial ICP-OES7. Consumption was expressed as µg per larvae. Feeding incidence of white seabass larvae offered MEM was significantly less than that of larvae fed PARA or OTO diets at 37 and 40 dph (Fig. 5). Feed consumption (µg/larvae) also followed a similar pattern with larvae offered MEM with significantly lower feed consumption than the larvae fed PARA at 34, 37, and 40 dph (Fig. 6). Feeding incidence of California yellowtail fed PARA, OTO, and GEMMA diets was not significantly different, although the feeding incidence of fish fed PARA was slightly greater throughout the trial (Fig. 7). Feeding incidence at 19 dph indicated that ingestion of the MEM diet by early-stage white seabass larvae was difficult because the MEM process produced only larger particles (500-800 µm). In contrast, the PARA, OTO, and GEMMA diets encompassed a suitable range of smaller particles (100-500 µm), and feeding incidence numbers were similar among these diets throughout both trials. In addition, when feeding the MEM diet to younger larvae, the sinking rate of the MEM particle was almost twice as fast as the smaller PARA particles, resulting in less exposure time to the larvae and making it more difficult to wean larvae onto the MEM diet. Discussion The development and use of microdiets for fish larvae typically focuses on weaning through co-feeding microdiets with live prey or totally replacing live prey at first feeding (Hamre et al. 2013). Red drum Sciaenops ocellatus, gilthead sea bream Sparus aurata, and European sea bass Dicentrarchus labrax can be reared with commercial diets from a very early age, with some research indicating the possibility of eliminating live feeds from the culture process completely (Fernandez-Diaz and Yufera 1997. Holt 2002, Holt et al. 2011). Cobia Rachycentron canadum and southern flounder Paralichthys lethostigma, however, require a live feeds co-feeding regime. Weaning involves a co-feeding stage and is typically achieved after larvae metamorphose into juveniles (Holt et al. 2011). The process of weaning larval finfish to a microdiet is considered optimized when the duration of the live feed co-feeding period is reduced to the least possible (Parma and Bonaldo 2013). The ability to reduce reliance on live feed organisms with manufactured microdiet feeds during the culture process saves time and money, while providing a high-quality diet to fish larvae (Hamre et al. 2013). Obviously, reliable feeds with appropriate nutritional and physical characteristics are necessary to make this advancement. The selected microdiet must meet the energetic and nutritional requirements of the species for proper larval growth and development (Langdon 2003). The commercial microdiets had greater protein and lower fat levels than the two FIGURE 5. Feeding incidence of white seabass fed one of three weaning diets (PARA, MEM, or OTO). Values sharing uppercase letters are not significantly different (P<0.05). FIGURE 6. Feed consumption (µg/larvae) of white seabass fed one of three weaning diets (PARA, MEM, or OTO). Values sharing uppercase letters are not significantly different (P<0.05). FIGURE 7. Feeding incidence of California yellowtail fed one of three weaning diets (PARA, OTO, or GEMMA).
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