The suitability of the free-living nematode panagrellus redivivus as alternative live food for first feeding fish larvae

Although hobby aquarists have claimed for decades that free-living nematodes are a potential food source for first feeding fish, their use has not become a real alternative to other common live foods due to the lack of suitable mass production technologies. A new technology for mass production of Panagrellus redivivus enables hatchery operators to rely on a cheap, standardized and permanently available live food for first feeding fish larvae. The technology involves the multiplication of nematodes in monoxenic (single microorganism: Saccharomyces cerevisiae) solid culture in autoclavable plastic bags. Different culture media have been tested and found to induce consistently high production rates. Yields of about 400 million nematodes per kg medium have been / can be reached. Mass produced nematodes have been tested on common carp (Cyprinus carpio) and whitefish (Core-gonus lavaretus) larvae which differ in size and the morphology of their alimentary canal. These stu-dies indicated that P. redivivus is readily ingested by first-feeding fish larvae. Different stages of nematode digestion could be observed in transverse sections of fish larvae sampled at regular intervals after feeding. By means of a stable isotope approach the onset of nutrient retention as well as the subsequent assimilation of carbon from the nematodes could be determined precisely for first feeding common carp and whitefish larvae. Larvae fed nematodes showed a high survival rate and could afterwards easily be adapted to a commercial dry diet after, for example, one week of feeding. The fatty acid composition of P. redivivus can be enhanced by enriching the culture medium with essential fatty acids. In this way the nutritional value of mass produced nematodes can be specifically tailored to the particular needs of fish larvae. Due to the small size (length: 50-2000µm) and round, elongated shape as well as the ability to survive for up to 72 hours in sea water, P. redivivus has a high potential as live food organism for small-mouthed marine fish larvae. This book gives an overview of current results which prove the suitability of P. redivivus as alternative live food for first feeding fish larvae. Directions for future research are described.