Review: Phosphorus in Fish Nutrition

environmental standpoint, studying the precise dietary P requirement for large (1kg) fish is 32 times more important
than studying the same for small (10g) fish. When fish are grown larger than 1kg in size, the importance of
studying the requirement for those fish will become much higher. When the initial fish size is large, however, it
will be difficult to magnify the size relative to the initial. In order to achieve a satisfactory level of growth
magnifi cation, large fish must be fed for a much longer period since their speci fic growth rat e is much lower than
their young as in other animal species. Additional problem of keeping large fish for such a long period is that the
amount of feed they consume should be much more than the amount required for young fish. This makes the use
of a semi-purifi ed diet economically unaffordable in many cases. Ironically, one of the most important discoveries
that have been made in the history of nutrition or physiology research is the use of small or young animals such as
rats and mice (for example, Lavoisier used the guinea pig in a respiration study). Small or young animals grow
rapidly but consume only a small quantity of food, which thereby made the use of purifi ed (nutritionally defined)
diets possible. Nutrition research for large animals, including fish, therefore, may require unconventional
approaches and techniques that differ from those used for small or young animals.
Feeding duration and the fish size will have a profound effect in estimating dietary P requirements, especially
when the initial fish size is large. However, errors from such sources cannot be eliminated completely. Thus, it
will be important to verify the accuracy of estimated values. One way to do this is to determine the requirement at
intervals (e.g., alternate weeks) until the values stabilize. Body pool size or diet history is an important source of
error when working with large fish and with relatively short feeding duration. Gillis et al. (1953) fed laying hens
with diets of varied P levels for 35 weeks. P-deficiency did not reduce egg production until after 12 weeks,
suggesting that at least 12 wks of feeding is necessary when estimating P-requirement based on egg-production.
Singsen et al. (1962) followed 40 weeks to monitor responses of laying hens to varied dietary P intakes. The
dietary P level did not affect the egg production until after 8 weeks. Laying hens and lactating cows have been
studied for their P requirement based on egg or milk production; however, in fish culture, neither of them is more
important than fish growth. Skonberg et al. (1997) tested only 5 dietary P levels, and did not extend the feeding
beyond 8 weeks. However, their data at wk-4 show that P and Ca contents in fish body and skin increased linearly
with the dietary P intake (i.e., no response plateau), whereas at wk-8 fish on high-P diets reached apparent plateaus
in those measurements. It is uncertain, however, if the plateaus (break points) at wk-8 could remain constant or
decrease further i f the fish had been fed for a longer period. Rodehutscord (1996) determined Ca and P contents
of fish per gain rather than per fish body by correcting the background (Ca and P contents of fish at the beginning of
experiment). This procedure, however, does not eliminate or reduce problems described above since the size of the
initial body pool is still variable from fish to fish, which buffers gain or loss of nutrients during growth. Body
pool size or diet history is an important source of error when working with large fish and/or with relatively short
feeding duration. To reduce this source of error, determining the requirement more than once in the course of a
feeding period will be required. Aternatively, it will be more convenient to use highly responsive indicators to
examine the adequacy of dietary P intake in large fish. Several workers have shown in mammals and fish that
urinary (or non-fecal) P output is a sensitive indicator for P. The urinary response is very fast (1-3 days), clear,
consistent and less vulnerable to diet history, demonstrating the distinct advantage of monitoring urinary P to
determine dietary P requirement for large fish. In addition, since urinary P is the most problematic source of P in
effluent, establishing dietary P level that can minimize urinary P is itself rational from an environmental standpoint.
Using the urinary method, however, is not applicable for other important cases such as diagnosing fish on site
(aquaculture ponds). Thus, it is important to explore alternative methods, such as molecular indicators (described
before).
Balance technique
The balance method, as McCay once stated, is "similar to the trade balance between nations". It is an old method
to estimate dietary nutrient requirements, and is still used today with little or no revision. Boussingault (discussed
below) pioneered in the balance trial, especially of nitrogen and fats, using livestock animals. The results were
reported in his famous book entitled "Rural Economy". He used the balance method to study the possibility of N
fixation (from air) by animals, and also fat de novo synthesis (from CHO) by livestock animals. These are both
landmark nutrition studies in history, but not discussed here any further. Almost 50 years before Boussingault,
however, Vanquelin (1799) conducted a balance experiment using hens. He measured Ca phosphate, Ca carbonate
and silica in the food they consumed, in the eggs they laid, and in the excrement during 10 days. His calculation
showed that the excretion was much larger than the intake in the quantity of Ca phosphate. He concluded that a
transformation of other elements into Ca had occurred in the metabolism of hen. He did not recognize that bones
served as storage centers. This conclusion, however, was accepted by his contemporaries. Boussingault (1845)
© 2000, 2005. Shozo H. Sugiura. All rights reserved.
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