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.
March - April 2013
On the usefulness of bioenergetics and the need for more rational approaches
Bioenergetics, the study of energy transactions in biological systems, has found wide application in animal nutrition, including that of aquaculture species. A century ago, Ege and Krogh (1914) first applied the principles of bioenergetics to fishes. Today, we formulate feeds to a certain digestible energy (DE) basis and ensure that the feeds have a proper digestible protein (DP) to DE ratio (DP:DE). It is also increasingly common for feed manufacturers to alter the essential nutrient concentrations of the diet, and aquaculturists to adjust the ration to be delivered to the fish, on the basis of the DE of the feed used. Bioenergetics-driven models, such as those proposed by my mentor, Dr C Young Cho, have proven very useful and practical for estimation of feed requirement and waste outputs of fish populations held in captivity. The suitability of comparing feeds on the basis of their DE content has been demonstrated on a number of occasions in the scientific literature.Despite its increasing acceptance and popularity in aquaculture nutrition, it must never be forgotten that bioenergetics is a ‘system’ aimed at simplifying interpretation of highly complex of biochemical processes. Hundreds of widely different compounds contain energy (Gibb's free energy). Animals do not simply metabolize this energy per se, instead, they metabolize specific nutrients, each with their specific roles and metabolic fates. Consequently, the widely held belief that ‘animals eat to meet their energy requirement’ is overly simplistic.
While it is true that animals need to consume nutrients that will be catabolized to harness their chemical energy, which will then be used in life sustaining processes, it must be recognised that a very large proportion (well over 50% under most conditions) of the feed intake of an animal is to acquire nutrients that are precursors for the biosynthesis of molecules that are structural or catalytic components (structural proteins, enzymes, phospholipids), storage forms (triglycerides, glycogen) or biologically active molecules (hormones, cytokines, lipid mediators, etc.). The amount of ‘energy’ that needs to be consumed is, thus, largely driven by 1) what the animal seeks to achieve (its growth potential, desired body composition, etc.), 2) the nutritional composition of the feed, and 3) the specific metabolic rules that govern the utilization of the individual nutrients consumed. In this context, to boil down such complex processes to a single term or factor, i.e. the ‘energy’ content of the diet or requirement of the animal, is not sensible.
Evidence suggests that significant differences exist between different aquaculture species in terms of the efficiency of different energy-yielding nutrients (amino acids, lipids, digestible starch) to support protein deposition and growth. Arguably the most significant limitation of bioenergetics models is that they are based on ‘hierarchy of energy allocation’, a concept according to which ‘growth is the surplus of energy after all other components of the energy budget have been covered or satisfied’ (Kitchell et al., 1977). This concept has proven to be a relatively flawed since young fish fed a maintenance ration (ration supporting zero body energy deposition) can still deposit protein and grow.
To quantitatively look at the requirement and utilization of all dietary components in a detailed and integrative fashion is highly desirable but it is also extremely complex. Consequently, bioenergetics offers today a relatively simple and practical way of looking at the global nutrient needs of the animal and the partitioning of these nutrients between catabolism as fuels and anabolism as storage in tissues. However, we should be unsatisfied with this situation and should strive to develop more rational approaches and models based on more or less explicit representation of biochemical reactions and metabolic roles and fates of nutrients.
A number of this type of models has been developed by various research groups for various fish species. Given the complexity of the task, all these ‘mechanistic’ models have been developed with some degree of simplification of metabolic pathways, included numerous assumptions, and been generally driven by more or less transparent and rational partitioning rules. These highly detailed models can work well within the narrow range of conditions for which they are developed. However, they generally fail to accurately describe nutrient utilization by fish under a wide range of conditions (differences in feed composition, environmental conditions, husbandry practices, life stages, genetic background of animals, etc.) encountered in fish culture.
A major bottleneck has been the lack of critical mass in terms of R&D effort invested on this topic. Efforts in the past have largely been idiosyncratic, piece-meal, and short-term in nature. There is a need for more concerted, long-term systematic R&D efforts. More comprehensive and rational approaches and models allowing more accurate description and prediction of the conversion of dietary inputs into biomass would make possible the elaboration of effective strategies aimed at improving the economical and environmental sustainability of aquaculture operations worldwide.
Want to know more? Disagree? Have feedback and suggestions? Contact me at: dbureau@uoguelph.ca, or leave a comment below.
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