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| Professor Dominique Bureau |
For many years, most aquaculture feeds were formulated with fish oil(s) as the main lipid source. For a long period, these n-3 PUFA-rich lipid sources were competitively priced compared to other lipid source and availability was rarely an issue. Fish oil production reached its peak at roughly 1 million metric tonne (mmt) and demand, both from the aquaculture feed market and the pharmaceutical industry, has increased very steadily since then. The increased demand and fluctuations in production volumes have led to a volatile and expensive fish oil market. This has forced manufacturers to greatly limit inclusion of fish oil in their feed formulations and rely on a different, more economical, lipid sources.
There has been much research and exchange about the use of plant (vegetable) oils in feeds for different aquaculture species. Studies have demonstrated plant oils can be used to provide a large proportion of the total lipids of the diet, without affecting performance of the animals, as long as the nutritional requirements, including n-3 PUFA requirements, are met. With this information, feed formulators have started using plant oils widely and significant levels of plant oils are now used in a very large proportion of aquaculture feeds produced around the world. These lipids are, nevertheless, expensive commodities.
Approximately 12 mmt of terrestrial animal fats are manufactured every year around the world and these lipid sources are generally more economical than plant oils. They have been staples in feed formulations for terrestrial animal feeds for many decades. Their use in aquaculture feeds has been highly limited for various reasons but they deserve more attention today. I am often struck by some of the misconceptions about the nutritive value of animal fats that are prevalent in the field of aquaculture nutrition today. I feel that some of the views need to be revised.
There is a relatively solid body of evidence showing that these lipids are safe and cost-effective lipid sources for fish feeds. Certain animal fats (e.g. poultry fat/oil) have actually found wide use with significant success in commercial salmon and trout feeds in the Americas for about two decades.
A number of early studies suggested that terrestrial animal fats were very poorly digestible to fish, notably cold water fish species, and feeds containing certain amount of animal fat did not support optimal growth performance. Some of these early studies, including a digestibility trial carried out in our fish nutrition research facilities at the University of Guelph, Canada back in the 1980s, have really made a lasting impression on many stakeholders of the industry. Numerous other studies, including several recent one have shown that animal fats are actually very well digested and utilized by many fish species, including rainbow trout reared in cold water. While it seems clear that the apparent digestibility of lipid can be negatively correlated to dietary inclusion level of saturated fatty acids (SFA), results from some studies do not always appear to support this conclusion.
So what is the source of discrepancy between studies?
A few years ago, my colleague Katheline Hua and I carried out a comprehensive assessment of the effect of dietary fatty acids composition, lipid level and water temperature on digestibility of lipids in fish using a nutritional modeling approach. The results from the meta-analysis of data from 16 studies with rainbow trout and Atlantic salmon indicated that variations in apparent digestibility of dietary lipid can be primarily explained by the proportion of SFA in the total fatty acids. A broken line analysis of the data from these studies suggested that SFA can be incorporated in diets at levels below 23 percent of total fatty acids without negatively affecting lipid digestibility. When SFA exceeded 23 percent of the total fatty acids, the apparent digestibility of lipids decreases by 1.5 percent for every 1 percent increase in SFA content of the diet.
The results of a multiple regression analysis of data from the same 16 studies suggested that the apparent digestibility of different types of fatty acids differ significantly and the digestibility of SFA cannot be assumed to be additive when estimating the digestible lipid content of fish feeds. The analysis suggested that monounsaturated fatty acids (MUFA), polyunsaturated fatty acids (PUFA) and increased water temperature have a positive effect on the digestibility of SFA. On the basis of the results from the multiple repression analysis, we suggested the following model for predicting the digestible lipid content of fish feeds on the basis of the SFA, MUFA and PUFA content of the diet and the water temperature:
Digestible lipid content (% of diet) = 0.45 SFA - 0.08 SFA2 + 0.86 MUFA + 0.94 PUFA + 0.03 SFA*MUFA + 0.04 SFA*PUFA + 0.03 temperature*SFA.
Comparison of model prediction with data from independent studies suggested that it accurately predicted the digestible lipid content of diets containing a combination of lipid sources with varying dietary lipid content fed to rainbow trout and Atlantic salmon reared at different water temperatures. The model also accurately predicted digestible lipid content of diets for several warm and cold water fish species. We concluded that this model could be a very simple practical tool for fish feed formulators wanting to explore the cost-effectiveness of different lipid sources.
Of interest in this model (multiple regression equation) is the ‘positive’ effect of PUFA and MUFA on digestibility of SFA. The ‘synergistic effect’ of PUFA on the digestibility of SFA is a well-described phenomenon in poultry. It was demonstrated many years ago (1962 to be precise) that lipid sources rich in SFA, when used alone in the diet, are poorly digested by poultry.
However, combining equal amount of a lipid source rich in SFA (e.g. tallow) and a lipid source rich in PUFA (e.g. soya oil) generally results in metabolizable energy (ME) value for the blended fat that is greater than the average of the two lipid sources, hence the term ‘synergistic effect’.
It is clear that animal fats generally cannot be used as the sole or major lipid source in the diet of most fish species. Feed formulators should ensure that diets are formulated to contain sufficient amount of MUFA and PUFA to facilitate the digestion of SFA and to meet the essential fatty acid requirements of the animal. It is my experience that good quality animal fats can be used in many cases up to about 40 of total lipids in many types of fish feeds.
Agree or disagree? Any feedback? Please don't hesitate to contact me at dbureau@uoguelph.ca
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