Early nutrition is crucial in fish as in vertebrates, with imbalances and/or deficiencies having consequences at short-term, impacting larval quality and performance, but also in the long-term up to the juvenile and adult stage, in terms of health status, feed conversion and growth performance.
The gastrointestinal tract of Arapaima gigas larvae is well developed from 6-10 days after hatching, with a stomach with an apparent acididic digestion capacity around 2 cm, what suggests Arapaima larvae may have already the capacity to digest inert microdiets at this size. However, in practice pirarucu larvae are not normally start weaning with success before reaching 4 cm length. In fact, good timing for transition from live to inert feeds is one of the determining factors to increase the survival rate in A. gigas larval rearing. This is surprising compared to farmed marine altricial fish larvae, such as gilthead seabream, where full stomach differentiation and functionality only occurs at 6-7 weeks of age, and being easy to wean them after this.
Even tough A. gigas has one of the highest growth potential among fish, reach up to 10 Kg in one year, growth rates are not particularly high during the larval phase, reaching up to 20%/day. However, growth rates remain high (around 7%/day) throughout the life cycle of A. gigas. Farmed marine altricial fish larvae typically grow 10-20%/day during the larval stage, with the fastest growing species reaching 50%/day. However, juveniles of these species typically growth less than 3%/day. The high growth during the juvenile stage of A. gigas is certainly powered by the high temperatures at which it is farmed, but also by a high conversion efficiency (FCR can be down to 0.6 up to a size of 140 g).
A. gigas have particularly high requirements for protein (and eventually other nutrients) during the juvenile phase, estimated at 57% until 140 g. However, larval protein requirements of A. gigas are possibly not very different from this, and in line with protein requirements of farmed marine fish.
The amino acid (AA) profiles of natural zooplankton and fishmeal-based foods appear to meet A. gigas AA requirements.
Future work should focus on microfeeds allowing for earlier weaning of A. gigas as well as study requirements on phospholipids and fatty acids, even if n-3 highly unsaturated fatty acids do not seem necessary, in contrast to farmed marine carnivorous fish larvae.