Review: Phosphorus in Fish Nutrition

(1968) obtained phytases from culture filtrates of different sources of Aspergillus species, and digested phytate in
soybean meal by this enzyme preparation. The authors compared the effects by the chick bioassay. Cain & Garling
(1995) conducted a similar but simplified experiment with rainbow trout. Numerous researchers have reported some
favorable effects of supplemental microbial phytase on the availability of P, trace minerals and other dietary nutrients.
Rodehutscord & Pfeffer (1995), Riche & Brown (1996), Vielma et al. (1998), Forster et al. (1999), Sugiura et al.
(2000) used rainbow trout, Jackson et al. (1996), Eya & Lovell (1997), Li & Robinson (1997) used channel catfish,
van Weerd et al. (1999) used African cat fish, Schäfer et al. (1995) used carp, and Hughes & Soares (1998),
Papatryphon et al. (1999) used striped bass. The degree of effectiveness of phytase seems to be considerably
different from one study to another. The differences are apparently due to different basal diets used by the above
workers rather than to the different species. Rodehutscord & Pfeffer (1995) reported that P availability for
rainbow trout increased from 25 to 57% when a diet containing 55% soybean meal was supplemented with 1000
units of microbial phytase per kg diet. The authors suggested that practical relevance of the finding applies only
to diets in which protein is supplied almost entirely by plant products. In carp, the apparent availability of P
increas ed from 32% to 49% when a diet containing 53% soybean meal was supplemented with 500units of microbial
phytase per kg diet (Schäfer et al., 1995). Since microbial phytase is almost inactive at pH 7 or higher; and the pH
of the gut of agastric fish is about 7 or higher (Maier & Tullis 1984), the enzyme may not be fully effective in carps.
The effect of phytase may be increased i f it is in a low-Ca diet, which is then weakly acidified. Sato et al. (1997)
reported that carp fed a diet supplemented with microbial phytase had markedly lower P excretion than those fed the
unsupplemented diet. However, phytase-supplemented diet contained much less P (less P supplement), and the fish
fed such diet had signs of P deficiency, including lower ash, Ca and P contents and a higher lipid content in the body,
although their growth was apparently unaffected during 56 days of restricted feeding. In striped bass, Hughes &
Soares (1998) reported an increase of P availability by phytase with a plant-based diet, and suggested the level of
1000units per kg diet was optimal. Storebakken et al. (1998) added commercial phytase to soy protein
concentrate (ca. 15400 units phytase/kg soy protein concentrate), and incubated at room temperature overnight to
reduce phytic acid content in the ingredient. This procedure greatly increas ed the solubility of P in the ingredient
from 7% (untreated material) to 70% (phytase-treated material). The test diets were prepared by an extruder, and
contained fish meal, soy protein concentrate (phytase-treat ed or untreated), wheat, dicalcium phosphate and other
ingredients. Fish (Atlantic salmon) were fed in seawat er every hour, 24 hours a day to true satiation (over fed and
uneaten pellets were collected and counted). When fish are fed in such a manner, leaching of dietary components
is inevitable while feed pellets were floating or suspending in the water or by washing in the mouth of the fish.
What the fish ingested and consumed could be di ferent from what the investigators fed as dry pellets. This
difference may not cause a serious effect in determining digestibility of dietary nutrients since soluble components
in diets are generally digestible by the fish (except phytate-P). The leaching loss, however, will be a direct source
of error when calculating nutrient retention based on balance. Oliva-Teles et al. (1998) fed seabass (initial wt 14 g)
with diets containing fish meal (68.6% of dietary protein) or soybean meal (65.6% of dietary protein).
Supplementing the soybean meal diet with 1000 and 2000 units of microbial phytase per kg diet increased apparent
P absorption 72% and 80%, respectively. Vielma et al. (1998) fed rainbow trout (initial wt 52 g) in excess with
diets containing 0 or 1500 units phytase per kg diet, and 2500, 250,000 or 2,500,000 IU cholecalci ferol per kg diet
for 12 weeks. The basal diet provided 5.8 total P and 3.2 g phytate P/kg dry matter. Soy protein concentrate was
the primary source of the phytate P. Weight gain of fish was increased by phytase but decreased by high dietary
cholecalci ferol. Phytase increased apparent availability of P and bone ash, plasma and body P concentrations.
Dietary cholecalci ferol levels did not influence P utilization. Schroder et al. (1996) indicated that, in pigs, about
70% is the upper limit of digestibility of P in plant materials supplemented with microbial phytase. General
reviews on phytase in human and animal nutrition have been reported previously (Cosgrove 1980, Nayini &
Markakis 1986, Jongbloed et al. 2000).
Optimum Ca/P ratio
Lipschutz (1910) found that dietary Ca to P ratio is important in dogs. A ration very low in P content produced a
moderate increase in weight of the dog, but after 7 weeks, there were protracted muscular clamps and other
physiological derangement. A ration containing the same amount of Ca but more P caused normal development.
Sherman & Pappenheimer (1921) found that the addition of Ca-lact ate (3%) to a low-P diet produced rickets in
rats. McCollum (1923) emphasized that the ratio between Ca and P in a diet is very important in producing rickets
in rats. Diets low in Ca and high in P developed rickets that was complicated with tetany. Diets low in P and high
in Ca also developed rickets but not tetany. The composition of a diet that produced the most extreme degree of
rickets in young rats were as follows; wheat (whole) 33%, maize (whole) 33%, gelatin 15%, wheat gluten 15%,
© 2000, 2005. Shozo H. Sugiura. All rights reserved.
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