How do we define growth? Layman definition of growth is an increase in size or perhaps in weight. But this definition seems to direct animal husbandry men to the wrong direction. This is even a more sensitive issue to the pet lovers. Would they want growth for their pets when they are past the linear area of the sigmoid curve for growth? Or put in another way, would they like to feed their pets which have stopped 'getting bigger' since attaining maturity to gain more or 'grow' more? They don't want this to happen since it will only result in the accumulation of unnecessary fat in the body which may lead to sluggish movement or perhaps cardiovascular diseases as dogs are prone to have. When pets are the topic, the idea of feeding at around maintenance level seems very logical but when you talk about food animals such as fish in aquaculture, it is hard to convince the fish farmers. But I agree that in the fish farm, it is hard to be so exacting about feeding the fish just the 'right amount' and so the rule of the thumb is that you feed them proportionately less in amount with time. The fact is that fish may not have reached the plateau yet during the culture period or even towards harvest time but more likely that the growth curve is gradually flattening out. So what's the fuss about growth all about?
Strict definition of growth is what is needed by scientists, the animal nutritionists, to be stringent on their statement of hypothesis and whether their data supports the hypothesis they have put forth. Thus, it is primarily needed by nutritionists to generate basic knowledge so that application of such knowledge (which is technology) is assured. Does this mean that fish farmers do not care and that they do not have anything to do with strict definition of growth? They should care because the technology everybody uses in their daily activities should be based on sound science, otherwise, the technology based on trial and error would soon crumble and we are back to square one.
And what is the strict definition of growth? It is the accumulation of body protein in the body and with an acceptable deposition of fat. This means to say that it is not enough to say that an animal gained so much weight per se but what is the composition of that gain. Is it mostly protein or mostly fat or bones/scales? Of course when it comes to farm animals, there are various objectives in raising the animals - is it for the hair (e.g. wool of sheep), is it for the amount and quality of milk, etc.? In aquaculture, it could be as food fish, broodstock fish which are judged according to the quality of eggs and their hatchability, it could be for aquarium display, etc. Assuming that we are concerned mainly on food fish such as tilapia, milkfish, grouper, salmon, grouper etc., then definitely we are interested in the body protein accumulation as against fat accumulation as the definition of growth.
If you happened to have read journal articles on feeding experiments in animals, you would notice that some experiments try to evaluate some feed ingredients mixed with other ingredients to make a balanced diet. All factors equal except the ingredients, say A, B or C. Diets which incorporated A, B or C are fed separately to each lot of fish and the experimenter will evaluate and rank the efficacy of each diet. Assuming that digestibility of the ingredients are equal, the indexes that will be measured will be growth in its usual sense of weight gain. But in addition to this, the composition of the diet and the composition of the whole body of fish (yes, they will have to be sacrificed at the end of the experiment) will be determined. So that not only growth in terms of weight gain, but what kind of growth in terms of protein accumulation, fat accumulation would be considered in the evaluation (i.e. ranking) of diets A, B or C. The findings will benefit the feed manufacturer directly and the fish farmer indirectly if these feed ingredients are cheaper replacements of some feeds that they are currently using.
That goes the story of the strict definition of growth, impractical in the actual raising of fish but very essential in establishing scientific basis of some fish nutritional findings that indirectly benefits the fish farmers.
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