Despite more than 20 years of international research and much progress in the development of early weaning diets, for most cultured marine fish species there are still no suitable artificial diets available that can be used at first-feeding without a substantial reduction in both survival and growth rates. Total replacement of live feeds remains one of the most important and challenging aspirations of aquaculture research.
The main difficulty appears to stem from the poorly differentiated digestive system of most marine fish larvae and their inability to digest and assimilate formulated feeds. Indeed, one of the characteristic features of fish larvae is, with the exception of the salmonids and cichlids, the lack of a functional stomach. While this does not appear to affect the digestion of lipids, which involves hydrolysis to fatty acids and monoglycerides in a similar fashion to adult fish, the absence of a functional stomach has greater consequence for the digestion of proteins.
Without the preparatory action of pepsin and hydrochloric acid, protein digestion occurs at a much lower rate, and as a result of the action of the pancreatic enzymes trypsin and chymotrypsin and the intestinal aminopeptidases. In many marine species these digestive proteases are not produced in any significant amount until several days after the commencement of active feeding. It is therefore not surprising that weaning with a conventional formulated diet is frequently successful only after the development of a functional stomach.
The reason why marine fish larvae have not evolved a more powerful digestive system is simply because they have not needed to. Their natural diet of zooplankton are rich in highly digestible nutrients, in particular free amino acids and free fatty acids, which are maintained at high levels in their body fluids to counteract the dehydrating effect of the surrounding sea-water (15 per cent of the dry weight of Artemia nauplii is FAA).
Zooplankton also contain large quantities of digestive enzymes, required for the digestion of the highly indigestible phytoplankton on which they feed. Once ingested by a fish larva, these enzymes assist in the digestion of the prey from the inside out. Research at CEFAS, has shown that trypsin originating from ingested Artemia can account for approximately 50 per cent of the total trypsin activity in four day-old sole larvae. This situation is comparable to that found in other species, including turbot.
To accommodate the apparent dependence on dietary digestive enzymes, one approach being examined is to incorporate digestive enzymes in the formulation of artificial diets. Protein-cross-linked microencapsulated feeds, originally developed by D.A. Jones of the University of Wales Bangor for crustacean larvae, were produced at CEFAS containing a mixture of proteolytic enzymes from adult cod (Cryotin AE Icebiotech, Iceland) and tested on sole larvae. The microcapsules, which are normally indigestible to fish larvae, were visibly found to break down in the gut. The larvae however did not grow well on this diet, probably because of low ingestion rates (see bellow).
An alternative approach is to make formulated diets more digestible by increasing their content in free amino acids, peptides and soluble protein. Experiments at CEFAS have shown that adding hydrolysed fish protein concentrate improve the weaning survival of juvenile Dover sole, and similar results have been reported in the literature for sea bass.
This approach does however lead to reduced diet stability, due to leaching of the soluble component. This becomes a major problem in small diameter diets, around or below 100 micrometers, where the high surface to volume ratio can cause the soluble component to leach out in a matter of seconds after immersion in water. The diet not only loses its nutritional value but also provides substrates for bacterial growth.
Another area of research which has received comparatively little attention, but which is increasingly being recognized as critical to the successful development of early weaning diets, is the mechanism underlying the feeding behaviour of fish larvae. Several recent reports have highlighted the importance of specific visual and chemical cues in larval feeding behaviour.
A new collaborative research programme, funded by MAFF, between the Conwy Laboratory of CEFAS and the University of Wales Bangor, will focus on the feeding behaviour of turbot, sole and cod larvae, and will aim to quantify the effect of visual, olfactory and gustatory stimuli on the successive stages of feeding, namely; search, encounter, pursuit, attack, capture, ingestion. The project will use a novel spectrofluorometric method to quatify ingestion rates, as well as specialised video techniques to analyse the effect of diet characteristics on feeding behaviour.
(excerpts from article by Dr. O. Day in Fish Farmer, Vol. 21, No. 1, January/February 1998)