Aquaculture view
Part of the IAF editorial panel, Dom has a PhD in Nutritional Sciences from the University of Guelph, Canada.
Today he teaches various undergraduate and graduate courses on animal nutrition and agriculture at the University of Guelph. Between 2007 and 2009, he coordinated the “Paris Semester”, a study abroad program for undergraduate students at the University of Guelph.
He serves on a number of international committees, including the US National Research Council Committee on Nutrient Requirements of Fish and Shrimp.
See all of the Aquaculture view columns here.
May - June 2013
Nutrient requirement: elusive concept
In a context of significant competition and low profit margins, aquaculture feed manufacturers are required to formulate to increasing lower or narrower nutritional specifications (‘specs’) in order to minimize feed cost. Decreasing specs for certain nutrients (e.g. lysine, methionine, DHA, available phosphorus) can significantly reduce the cost of feeds. However, at the same time, feed manufacturers must be sure that their feeds can sustain high growth, feed efficiency, health, and product quality of the animals at the farm. Consequently, formulating cost-effective aquaculture feeds can be a very delicate balancing act that requires accurate and precise information on the nutritive value of feed ingredients (an issue that has been the focus of some of my past columns) and on the nutrient requirements of animals.Significant efforts have been invested over the past six decades on the definition of the nutrient requirements of numerous fish and crustacean species and the body of knowledge is growing significantly every year. Reviews of the literature and nutritional recommendations are provided on a relatively regular basis by different groups of researchers or committee of experts.
Not blindly relying on published estimates
The relatively new NRC (2011) 'Nutrient' Requirements of Fish and Shrimp and other reference documents are providing feed manufacturers with a reasonably good basis for the formulation of feeds meeting of requirements of many of the commercially important aquaculture species. However, through frequent discussions with different stakeholders of the aquaculture feed industry I have come to realise that these estimates of requirements are too often taken at face value and/or misunderstood. I believe that we should never blindly rely on published estimates of nutrient requirement, even from highly authoritative document. Feed formulators should dig in the primary research literature for the real data and develop their own opinion. Feed manufacturers should also focus a significant part of their R&D efforts toward verifying the adequacy and suitability of their nutritional specifications.
Nutrient requirement not cast in stone
Estimates of requirements are generally derived from studies with young fish fed diets containing purified and chemically defined ingredients that are highly digestible and, generally, represent minimum nutrient concentrations required for maximizing performance of these young animals under laboratory conditions. While this type of approach and definition of 'requirement' may sound relatively simple and straightforward, reality is a lot more complicated. Significant differences may exist in the experimental conditions (diet composition, experimental design, duration of study, fish trains, life stages), measured parameters (live weight gain, protein gain, enzyme activity, body stores, histological changes), performance achieved (growth rate, feed efficiency), and methods of analysis of the results for 'similar' studies. Consequently, very different 'estimates' of requirement can be derived from similar studies. Moreover, the same dataset (e.g. data from one single study) can also be interpreted in very different ways through the use of different mathematical models to analyse data or by simply putting more emphasis on different parameters (body stores vs live weigh gain vs enzyme activity). Defining a nutrient requirement value is not a straightforward thing.
It must be recognised that published estimates of nutrient requirement are derived from consensus among 'experts' and are thus very much products of opinion and not some sort of unchallengeable truth. It must also be recognised that requirements are probably moving targets and that pinning down one 'unique' and 'true' value is probably wishful thinking. However, how requirements evolve with changes in the genetic, weight, growth rate or feed conversion achieved, or health status of the animal is something that, in my humble opinion, has not been adequately studied for aquaculture species.
I strongly believe that the mode of expression of requirement is an issue that has not received sufficient attention. There are numerous diverging opinions with regards to appropriate modes of expression of essential nutrient requirements. It is especially the case for essential amino acid (EAA) for which very different modes of expression of requirement are used, often interchangeably, in the literature. These different modes of expression are based on different, often diametrically opposed, assumptions. In practice, the use of different modes of expression of EAA requirement can often result in dramatically different nutritional recommendations. Individual EAA levels deemed adequate may be very different depending on the mode of expression adopted and the composition of the diet formulated. This is a significant issue since feeds for a given species are formulated to widely different protein, lipid, starch, and digestible energy levels. The root cause of these conflicting views is our limited understanding of how endogenous and dietary factors affect EAA utilization and requirements of fish.
Finally, requirements are somewhat ideal biological values and it is also important to consider a reasonable safety margin to account for potentially lower digestibility or bioavailability of nutrients in practical ingredients, for losses of nutrients during manufacturing and storage of the feed, and for potential 'changes' in nutrient requirements imposed by various environmental or endogenous factors. What represents a reasonable safety margin is again something up for discussion.
Not enough emphasis on commercially important species
Aquaculture nutrition is a dynamic field of research. However, the number of fish and crustacean species studied is staggering and this leads to dilution of research efforts. Globally, there is need for significant improvements in the focus of nutritional studies, and the scope and quality of the experimental efforts invested in the definition of essential nutrient requirements of commercially important species. It would be recommendable to increasingly focus the research efforts on the 15 or so fish and crustacean species (e.g., Chinese carps, Indian major carps, Nile tilapia, Pangasid catfish, Atlantic salmon, Pacific white leg shrimp, etc.) that represent the bulk of the global farmed fish and crustacean production.
Any feedback? Do you agree or disagree?
Any suggestions for future topics?
Let me know. Email me at dbureau@uoguelph.ca, or leave a comment below.
