Review: Phosphorus in Fish Nutrition

egg albumin (71%). However, the diets were supplemented with Ca carbonate in amounts of 0.5% and 1.18%,
respectively. The diets therefore had high Ca/P ratios (especially egg albumin diet), which likely decreas ed P
absorption or availability. The large difference in estimated P availability of soybean meal between the two studies
may be due to different Ca contents of the diets since free phytate (phytic acid) and calcium-phytat e (phytin) are
different in bioavailability. Since Bruce & Callow (1934), many workers reported in various species that Ca
interferes with the utilization of phytate P by forming Ca-phytate precipitate in the intestine. If phytate stays in the
soluble fraction in the intestine, it can be digested by bacterial, dietary, and/or endogenous phytases. Apparently,
the diet used by Lovell (1978) contained little Ca since this researcher used a Ca-P-free mineral mixture, whereas
Wilson et al. (1982) used a diet containing a substantial amount of Ca as Ca carbonate and Ca lactate, which was (in
my calculation) capable to precipitate as much as 0.47% phytate-P in the diet. Taylor (1965) wrote, "Phytate-P is
by no means completely undigested in the rat, dog, pig and hen under appropriate conditions, and it appears that the
extent of phytate breakdown in the gut is reduced as the level of dietary Ca increases." (see section “Precipitation
of calcium phytate”) It should be noted that about 60% of phytate in soybean meal are water-soluble and easily
removed by washing (Han 1988). Any portion of dietary phytate that leaches out from the pellets before being
ingested by the fish therefore will be calculated as "digestible". Also, the leaching loss of phytate from feces will
be accounted for as "digested". The degree of thes e sources of error depends on water stability of the pellets, fish
size (pellet size), palatability, feeding level, feeding method, fecal collection method, and the content of cations in
the diet. Both Lovell (1978) and Wilson et al. (1982) collected fecal samples by dissection; however, the former
apparently fed the diet manually, while the latter force-fed the diet. Li et al. (1996) compared bioavailability of P
in feed-grade dicalcium phosphate and 3 defluorinated rock phosphates of high, midium and low solubilities in
neutral ammonium citrate. Channel catfish (body wt. ca.4 g) were fed in aquaria for 12 weeks with egg
albumin-based puri fied diets containing one of the test P sources. Based on weight gain, feed consumption, feed
conversion ratio, bone ash and P levels, they reported that all the test P sources had comparable bioavailability for
channel cat fish. Eya & Lovell (1997) determined P availability of various P supplements for channel cat fish based
on absorption (digestion). They used a non-purified diet containing soybean meal as the basal diet rather than
using a purified low-P diet. The authors subtracted P absorbed from the basal diet portion of the test diet to
determine the amount of P absorbed from the test P source. This method is acceptable i f interaction is negligible.
The calculated values are the true absorption (availability) although the authors called the values net absorption
(which normally refers to as apparent absorption). The authors added CaCO 3 to maintain 1/1 Ca/P ratio in all diets.
The values could have been higher if the diets were low in Ca.
P availability in Tilapia
Viola et al. (1986b) reported, in tilapia, that P in various fish meals was highly available, soybean P was poorly
available, and sorghum P was superior to wheat P. They noted that fast growing fish need more P than slow
growing fish.
P availability in Other fishes
In red sea bream, Sakamoto & Yone (1979) reported that sodium phosphates (mono-, di-, and tribasic), potassium
phosphate monobasic, and Ca phosphate monobasic were more effective than Ca phosphates (di- and tribasic) to
prevent the development of P deficiency. Ca phytate was poorly utilized by the fish as a P source. Yone &
Toshima (1979) showed that the digestibility of P in fish meal was higher in seabream than in carp. Carp fed fish
meal diet grew poorly with very low feed effi ciency, which was improved by supplementing the diet with available P.
Fernandez et al. (1996) studied the absorption of C, N, P and dry matter along the intestine of gilthead seabream,
and compared di fferent fecal collection methods to estimate apparent digestibility coeffi cients. Fernandez et al.
(1998) determined again the apparent digestibility from several regions of the intestine of gilthead seabream (body
wt 10-25 g). Except for P, the apparent absorption increased along the intestine. High correlations were found
between N, C and DM digestibilities, whereas P digestibility showed low or no correlation. Silva & Oliva-Teles
(1998) determined the apparent digestibility coefficients of dry matter, protein, energy and P in two fish meals, a fish
protein hydrolysate, blood meal, meat meal, soybean meal and yellow dextrin using seabass (body wt 40 g). The
optimal inclusion level of a test feedstuff into the reference diet in estimating the apparent digestibility was also
studied (15 and 30%). Satoh et al. (1998) reported that fish meal-bas ed diet containing total P at the level of 2.1%
did not provide optimum growth of red seabream (body wt., initial 2.4 g, final 23 g vs. 30 g), and concluded that
supplementing the diet with available P was necessary. The fish fed the low-P diet, however, had similar or higher
© 2000, 2005. Shozo H. Sugiura. All rights reserved.
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