As said in the first part, referring to protein in fish nutrition, the relation between protein, fats and carbohydrates is as much important as the quality of them to achieve a good conversion index. Now is the turn to talk about fats.

Lipids (fats) are high-energy nutrients and are about 15% of fish diets. Their main function in the diet are supplying essential fatty acids (EFA) and being fat-soluble vitamins transporters.

Requirements for fish feeding are generally omega 3 and omega 6 fatty acids, this means n-3 and n-6 respectively. These lineal chains can be saturated fatty acids (with no double bonds), polyunsaturated (more than 2 double bonds) or highly unsaturated (more than 4 double bonds). Marine fish oils have a high content (more than 30 %) in omega 3 highly unsaturated fatty acids, so the supply of this fatty acids to the diet is so important, ranging around 0.5-2.0% of dry diet.

FIG I: LINOLENIC ACID (IS ONE OF THE FATTY ACIDS INCLUDED IN THE GROUP OF OMEGA 3 FATTY ACIDS)

The two major essential fatty acids to be apported on diet are eicosapentaenoic acid (EPA: 20:5n-3) and docosahexaenoic acid (DHA:22:6n-3). Freshwater fish do not require the long chain highly unsaturated fatty acids, but often require an 18 carbon n-3 fatty acid, linolenic acid (18:3-n-3), in quantities ranging from 0.5 to 1.5% of dry diet. This fatty acid cannot be produced by freshwater fish and must be supplied in the diet. Many freshwater fish can take this fatty acid, and through enzyme systems elongate (add carbon atoms) to the hydrocarbon chain, and then further desaturate (add double bonds) to this longer hydrocarbon chain. Through these enzyme systems, freshwater fish can manufacture the longer chain such as EPA and DHA which are necessary for other metabolic functions and as cellular membrane components.  As marine fish don’t have this enzymes systems, these fatty acids are required to be added to the food, because is their only source.

Other fish species, such as tilapia, require fatty acids of the n-6 family, while still others, such as carp or eels, require a combination of n-3 and n-6 fatty acids.
Once more the specie has to be known perfectly, to be able to design the optimum feed without wasting resources.

A common mistake is to increase this fat level of 15 % in the dietary formulations to partially substitute part of the protein content to decrease cost. This arrangement will lead to fat deposition in the liver, decreasing the health and market quality of fish if is used as a usual practice.