Despite increasing interest in the culture of cold-water marine fish, a satisfactory inert feed for the larvae of these species has not been developed. As a consequence, the majority of information on the nutritional requirements of the larval stages has been inferred from studies on juvenile or adult fish or derived from manipulations of the nutrient contents of live feeds. Although the latter technique has highlighted the importance of long-chain (n-3) polyunsaturated fatty acids (PUFAs), it is insufficiently precise to define the requirements for individual fatty acids.
In this study, the (n-3) PUFA nutrition of the larvae of the herring, Clupea harengus L. was studied. Firstly, the ability of the larvae to elongate and desaturate the fatty acids contained in two strains of brine shrimp, Artemia, was considered. Secondly, the feasibility of rearing herring larvae on an inert, microencapsulated diet was considered. Although it was concluded that long-term rearing trials using this method were not viable at the time, larvae weaned onto the inert feed, from live feed, achieved positive growth. Finally, a novel method for incorporating radiolabelled fatty acids, as metabolic tracers, into herring larvae was developed. The viability of using this system for studying the ability of larvae to elongate and desaturate the fatty acids 18:3(n-3) and 20:5(n-3) was demonstrated.
The results from all the trials indicated that a high rate of elongation and desaturation of 18:3(n-3) does not occur in herring larvae. Although some ability to elongate 20:5(n-3) to 22:5(n-3) was demonstrated, significant desaturation of this fatty acid to 22:6(n-3) did not occur. It is suggested that the rate of bioconversion of 20:5(n-3) to 22:6(n-3) is too low to be of nutritional significance.