The larval rearing phase in production of aquatic organisms is the most difficult aspect of production once spawning is achieved, especially for new candidate species. Larvae have unique nutritional requirements that are often poorly understood. Nutrition studies on larvae are complicated by ontogeny as it pertains to vision, locomotion, feeding and digestion, immunity, etc. Larvae are fragile and sensitive, and early performance is affected by maternal influences passed down through the egg. Finally, fish larvae are small and can be difficult to observe.
This project was initiated to support marine finfish aquaculture on the West Coast of the United States by developing and applying appropriate larval feeds and feeding techniques that target two selected native marine species, the white seabass (Atractoscion nobilis) and the California yellowtail (Seriola dorsalis). The objectives of this project were to: 1) establish baseline indices for larval seabass and yellowtail using contemporary culture techniques as a benchmark for evaluating the project’s success over time; 2) design, refine and implement methods that allow assessment of larval feed intake and behavior; 3) increase survival and growth of the targeted species during their larval stages through optimization, or nutrient enrichment of cultured live feeds; and 4) increase larval fish survival and growth during weaning from live foods to formulated feeds through the development and/or identification of appropriate formulated microdiets.
Ultimately, this project resulted in refined live feeds feeding practices where unnecessary food items were eliminated (e.g. 1st Instar Artemia) or reduced (e.g. rotifers) in days fed. In the case of seabass, the commercial enrichment was modified, which reduced malformation rates significantly. We successfully applied various tracers to live feeds and microdiets in order to assess feed intake, prey selectivity, and leaching rates of nutrients. We measured sinking rates of various complex feed particle types in order to understand availability to the larvae, and pair with nutrient leakage rate information. We demonstrated that taurine supplemented in the rotifer enrichment yielded larger larvae at the end of that feeding stage and that by using liposomes taurine levels in the rotifers could be increased to levels found in copepods. We developed an open formula reference weaning diet that performed similarly to commercial diets and tested several particle types to learn that particle-assisted rotationally agglomerated (PARA) and larval extruded (LEX) diets worked best. Finally, we showed that feed attractants (glycine, betaine, and alanine; GBA) improved the taste of food particles and significantly increased feeding incidence and gut fullness in fish larvae. The commercial attractant ProMega55 yielded the fastest growing and best surviving larvae when compared to control and GBA treatments, and reduced the weaning time by 10 days.