Review: Phosphorus in Fish Nutrition

level in the water but did not exceed it. The fish absorbed 45 Ca readily and concentrated it 100 times the level in
the water. When the fish were fed algae containing these radioactive minerals, the fish absorbed 32 P more than 45 Ca
from the algae. Phillips et al. (1958) observed that brook trout absorbed about 400-1000 times more Ca than P
from water. Increasing P concentration in water proportionately increas ed the quantity of P absorbed, while
increasing Ca concentration in water did not increase the amount of Ca absorbed by the fish. Phillips et al. (1954,
1958, 1959) noted that the labeled P that had been absorbed from the water was included in tissues of the GI tract
(highest in pyloric caeca) and gills of the fish. Phillips et al. (1959) reported that labeled P absorbed from food
was distributed mainly to muscular and bony tissues of the fish. Increasing dissolved P in water decreased the
utilization of P from the food, whereas dissolved Ca increased the utilization of dietary P. Phillips et al. (1961)
showed that the absorption of waterborne P by brook trout was only trace in quantity compared with the absorption
from food. Ogino & Yasuda (1962) reported that larval rainbow trout markedly increased body Na, K and Ca
contents after hatching (but before the first feeding), while P, Mg, Fe and Cu remained relatively unchanged during
the same period. Lall & Bishop (1979) reported that rainbow trout reared in freshwater for 12 weeks had higher
Ca and P and lower Mg, Na and K contents in the body than those reared in seawater with the same diet for the same
period. Shearer et al. (1994) reported similar observations in Atlantic salmon. Mol et al. (1999) also reported
that catfish reared in water low in Ca and/or Mg had higher body P and Ca content than those reared in water high in
Ca/Mg. This suggests possibility that water-borne Ca/ Mg may depress intestinal P absorption of the fish.
Expressing P requirement
Estimated values of dietary P requirement, however, can vary greatly depending on (1) feed efficiency of the diet
(thus, energy density, digestibility of major nutrients, nutrient balance, etc.), (2) availability of P in the diet (thus,
dietary Ca level and acidity), (3) growth velocity of fish (thus, fish size and physiological state), (4) feeding duration
(growth magni fication), (5) diet history (initial body P reserve), (6) response criteria, and (7) statistical methods used.
Reporting or interpreting dietary requirement values out of this context is misleading. Probably there is no such
value like “ dietary requirement”. The requirement is only for growth, reproduction, disease resistance, and other
physiological needs. The diet is merely a vehicle of the nutrient. The amount of P required in a diet changes
every time the composition of the basal diet changes. In this review, therefore, the values of “dietary P
requirem ent” were not listed. (see Shearer 1995, Davis & Gatlin 1996 for tabulated requi rement values for P and
other minerals). Since Adolph's experiment in 1947, it has been shown in many monogastric animals that when
diets are diluted with inert materials to produce diets of varied energy concentrations, the animals are able to adjust
their food intake so that the amount of calorie (and other nutrients) eaten remains constant (Forbes 1986).
Although this compensation is not perfect, the dietary requirement is more accurately expressed as per digestible
energy basis rather than per weight of feed (dry matter basis). However, expressing dietary requirements of
nutrients based on energy content of the diet also involves many problems. The source of energy, whether it is fat,
protein or carbohydrate, has different effects. Utilization of energy, especially carbohydrat e, differs depending on
the dietary level and the state of the ingredient (raw, cooked, retrograded). Amino acid composition and their
bioavailability divert the utilization of protein from growth (deposition) to consumption. Amounts of exercise and
stresses also divert energy flow from growth to consumption. Heinsbroek (1987) says ME decreas es at increasing
feeding levels. Tabulating DE or ME values for many feed ingredients is indeed a daunting task, yet the values
cannot be very accurat e and universal. Expressing nutrient requirements based on per unit body wt/d may be
acceptable for homeotherms for their normal growth rate/d is known for various age groups. For poikilotherms,
however, daily growth rate depends so much on rearing conditions (e.g., temperature, stress, feeding rate), which
makes the use of "per day" expression inappropriate. Dietary requirement of nutrients, particularly for those
constitute the body, may be most accurately expressed as per growth, which is weight gain, but more precisely lean
gain or N retention. The protein/ash ratio of the body of animals of the same species is quite constant over the
different li festages and nutriture except under starvation and malnutrition (Bondi 1987). This indicates that
expressing dietary requirements of the ash components including P based on the retention of protein (N) should be
quite accurate, the concept of which may be supported by Rudman et al. (1975), as mentioned above, in that N, P, K,
Na, and Cl are retained in the body at a fixed ratio at all levels of N intake. Sherman (1920) wrote based on his
P-requirement studies, "We are probably justified in concluding that about one-fortieth to one-fi ftieth as much P as
of protein is required in the maintenance metabolism of man." This account, however, may be better suited during
growth than maintenance. Expressing P requirement relative to N-retention, however, has a practical problem since
N retention or lean-growth can not be accurately predi cted from feed composition. However, when reporting the
result of a requirement study, basing the requirement value per N-gain provides a rational basis of comparison with
the data from other experiments that might be conducted with different diets, fish, and under different conditions.
© 2000, 2005. Shozo H. Sugiura. All rights reserved.
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