Marine wax esters as feed source for farmed fish.

The increase in carnivorous aquaculture is exhausting the world supplies of fish oils and new lipid sources are required. Vegetable oils can be used, but the fatty acids in salmon flesh in particular will change to a "plant" like composition with low cont ents of the marine fatty acids EPA/DHA. These are strongly related to the health benefits of eating fish, and is a major marketing argument. The only marine oils available in large amounts are found at lower tropic levels and possibly mesopelagic fish lay ers. However, much of these lipids are in the form of wax esters and not triacylglycerols as in fish oils. Wax esters are toxic to humans in large amounts, but may be well utilized by fish. If wax esters can totally replace fish oils, vast amounts can be extracted from the seas off Norway, and "unhealthy" fat can be converted into "healthy" fat. It may also offer good usage of unexploited wax ester rich mesopelagic fish species that is inaccessible for human consumption. The proposal outlines several stra tegies to obtain better understanding of the suitability of wax esters in fish diets. The main objectives are growth studies in salmon and cod with the aim of assessing the digestibility and utilisation of the esters and their effect on product quality an d composition, and indices of fish health and welfare. Studies will be performed in small and large fish in order to determine developmental effects on the utilisation. Secondly, because wax esters are likely to be slowly digested that may limit digestibi lity under altered environmental conditions. Accordingly, a limited amounts of short term feeding studies will also be performed at different environmental tempratures. Finally, some preliminary studies may be performed on the effect of wax esters on abso rption of other nutrients eg pigments, utilising individual salmon equipped with dorsal aortal cannulas. Included in these trials will be the effect of feed additives that may modulate intestinal micellar properties (emulsifiers and enzymes) and thereby t he rate of lipolysis and intestinal transport of wax esters. In cod larvae, a micro injection technique combined with a novel digestibility tracer technique will be tested. These studies will however only be evaluated as preliminary and introductory due t o financial limitations.