Early nutrition is crucial for all vertebrates, including fish, having consequences during the larval, but also later in the juveniles stage, in terms of health status, survival, skeletal deformities and growth performance. This is one of the reasons why live feeds such as rotifers and Artemia, tend to be increasingly replaced by inert microfeeds. This has been possible by the progress in quality of commercial microfeeds over the last twenty years, powered by the increasing knowledge on larval nutrition, better production technologies and availability of highly digestible ingredients. High inclusion levels of protein hydrolysates and phospholipids in microfeeds have been key in this endeavor.
Challenges that remain in microfeeds development include a better control of losses of water-soluble nutrients by leaching, in particular in smaller microfeeds for first-feeding larvae. Most marine fish start-feeding with particles around 100 µm, and such a small diameter largely increases the particle surface-to-volume ratio, exposing it to the surrounding water and promoting leaching. For instance, protein losses in microfeeds by leaching may reach up to 50 % in just a few minutes after immersion, reducing nutritional value and water quality. Microencapsulation technologies may allow preserving nutrients inside microfeed particles. Recently, a microencapsulated prototype was shown to be able to reduce 75 % of leaching of a water-soluble protein, while keeping good feed intake and digestibility. However, microencapsulation may also make microfeeds undigestible to the larvae, due the larvae limited digestive capacity.
Fish larvae require diets with high levels of protein, essential fatty acids and micronutrients, and these must be provided by highly digestible ingredients. However, the precise nutritional requirements of fish larvae and suitable complexity of dietary protein are poorly understood. Fish species have different nutritional requirements and high quality ingredients that are suitable for a species, may be not work for other species. Microfeed physical properties such as floatability, sinking speed, dispersion both in tank surface and water column, as also key to success and will likely very with species and larval stages. Moreover, different larval stages are likely to require different microfeed specifications.
The room for improvement in formulation of commercial microfeeds for first-feeding larvae is still very large. But research is still needed on the use of emerging highly digestible ingredients and further testing of microencapsulation technologies. However, even if microfeed quality is expected to increase in coming years as a result of such developments, a strong effort is also needed to improve feeding protocols, both in terms of microfeed quantities and feeding frequencies, as these may have a key role in determining success in larval rearing upon an early weaning. An earlier weaning is expected to bring a higher juvenile quality, leading to shorter production cycles and better fish welfare.