Ž .
Ž .
550 8C AOAC, 1984 , organic matter calculated as 100y percentage ash , lipid by
Ž .
Ž ether extraction Tecator Soxtec System HT2 1045 Extraction Unit following acid 4N
. Ž
. Ž
HCl hydrolysis 1047 Hydrolyzing Unit , total nitrogen by the Dumas method Ebling, .
Ž .
1968 using a Leco nitrogen determinator model FP-228, Leco, St. Joseph, MI with protein calculated as N
6.25, moisture by weight loss after drying for 24 h at 105 8C
Ž .
Ž AOAC, 1984 and gross energy by an adiabatic bomb calorimeter Parr Instrument,
. Moline, IL . Chromium content of diets and faeces was measured spectrophotometri-
Ž .
cally using a micro-method outlined by Suzuki and Early 1991 . 2.4. Statistical procedures
All statistical analysis on growth data were performed according to Steel and Torrie Ž
. 1960 . A 5 level of probability was chosen in advance to sufficiently demonstrate a
statistically significant difference. Statistical analysis was performed using Statistical Ž
. Analysis System 1992 . Treatment means were differentiated using least square means
SEM if ANOVA showed significant differences. Mortality data were compared by Ž
. G-test based on the intrinsic null hypothesis a y 2 degrees of freedom that treatment
Ž .
had no significant effect on mortality Sokal and Rohlf, 1969 . Where the null
hypothesis was rejected, pair-wise comparisons were made by calculating the 95 Ž
. confidence interval by use of the Bonferroni Z statistic Neu et al., 1974 . Within the
Ž .
51 protein treatment, one of the tanks had significantly P - 0.05 higher mortality than the other two replications, therefore, data from this tank were removed from the
analysis of the growth data.
3. Results and discussion
3.1. Growth Chemical analysis of the experimental diets indicated that the formulated protein
Ž .
Ž .
levels were achieved. Desired analyzed crude protein levels percentage as fed in the Ž
. Ž
. Ž
. Ž
. Ž
. diets were 35.0 34.6 , 39.0 39.1 , 43.0 43.1 , 47.0 47.5 and 51.0 50.6 .
The pattern of growth was similar prior to 56 days on test. The mean weight of fish Ž
. fed diets containing 47 and 51 protein were significantly
P - 0.05 higher than those fed the 35, 39 and 43 protein diets. This difference persisted for the duration of
the experiment with final weights of fish fed 47 and 51 protein being similar to each Ž
. other and approximately 17 higher P - 0.05 than those fed lower amounts of protein
Ž .
Ž .
Table 2 . SGR was 19 higher P - 0.05 when dietary protein levels were 47 and Ž
. 51 than those fed the lower protein diets Table 2 . The most efficient FCR was
Ž .
Ž .
observed in groups fed 47 protein Table 2 . This FCR was significantly P - 0.05 better than when eels were fed the lower protein diets by 26. Feed intake was the
highest for fish fed 51 protein at 17.0 grfish, where growth was lower than that of the 47 protein diet. The higher herring meal content of this diet may have led to higher
palatability, but not higher growth and these two factors were reflected in the signifi- cantly poorer FCR.
Table 2 Average weight, growth rate and feed conversion of American eel fed diets containing graded levels of protein
for 84 days
1
Dietary Initial weight
Final weight Specific growth
Feed intake Feed conversion
Mortality Ž .
Ž .
Ž .
Ž .
Ž .
Ž protein
grfish grfish
rate rday grfish
ratio g feedrg .
gain
a a
ab a
a
35 8.00.37
18.10.64 0.960.04
15.20.47 1.530.05
3.71.9
ab ab
a ab
a
39 8.20.46
18.90.64 1.000.04
14.90.47 1.400.05
0.00.0
a ab
ab ab
a
43 8.10.33
18.40.70 0.980.04
15.50.51 1.490.05
3.71.9
c c
b c
b
47 8.10.34
22.30.70 1.200.04
16.80.51 1.170.05
14.13.5
bc bc
b bc
a
51 8.20.17
21.00.82 1.130.05
17.00.59 1.340.06
4.42.1
1
MeansSEM of three replicates and values within the same column with different superscripts are Ž
. significantly different P - 0.05 .
The growth performance of eels measured as final weights, SGRs and FCRs improved as protein level in the diet increased to 47 protein then either leveled off or
declined when 51 protein was fed. Results indicate that by increasing dietary protein above the 47 level, there is no significant benefit for growth improvement of juvenile
American eel.
The growth performance results support past researchers who have concluded that high extremes of protein are not used for normal protein metabolic functions, such as
tissue growth, but are instead used as a comparatively inefficient energy source Ž
. Wattendorf, 1980 and that excesses of protein are, therefore, not likely to be beneficial
Ž .
Van Limburgh, 1975 . Feed conversion not only leveled off, but declined after 47. These findings are consistent with other reports, where reduced growth performance was
Ž .
observed when high protein diets were fed to catfish Dupree and Sneed, 1966 and Ž
. Japanese eel Nose and Arai, 1973 .
By the end of the experiment, mortality of eels fed 47 protein was higher than the Ž
. other groups. All mortalities observed were small eels mean weight, 7.9 0.23 g that
displayed bite marks on their bodies. Based on this, it is felt that the higher growth rate in the 47 group led to higher variation between large and small eels and, ultimately,
more aggressive behaviour within these tanks. It should be noted that this high mortality may slightly overvalue the final mean weight of animals in this group.
Ž .
The results obtained are in close agreement with Nose and Arai 1973 who reported that beyond 45 crude protein, growth and protein accumulation were nearly constant in
Ž .
Japanese eel. The small difference 2
observed in protein requirement may be attributed to the differences in the diet components, growth stage and water temperature.
Ž .
Nose and Arai 1973 used a purified diet with casein as a major source of protein compared to a practical diet containing fish-meal in this study. The protein digestibility
Ž .
of casein is significantly higher than fish-meals National Research Council, 1993 , therefore, the quantitative dietary protein need would tend to be lower than for
fish-meals. The present study was undertaken to obtain information that can be directly applied for the feed formulation of commercial juvenile American eel diets. De La
Ž .
Higuera et al. 1989 also indicated that protein requirement should be determined by Ž
. using protein sources such as fish-meals. Moreover, Nose and Arai 1973 reared
Ž .
smaller fish initial mean weight, 3.0 g at 25 8C, whereas our study was conducted with
larger fish at 22 8C. Although the effect of water temperature on protein requirement is
relatively small, some of the differences may be attributed to increase in nutrient Ž
. retention at high temperatures. Earlier work of Wattendorf 1980 also suggests that
protein requirement of juvenile American eel is close to 48. 3.2. Digestibility
Several faecal sampling methods were attempted including sedimentation and collec- Ž
. tion via a column of still water Guelph system , manually stripping and siphoning. The
Guelph system was unacceptable due to the eels’ ability to escape through the perforations on the tank bottom intended to collect faecal material. Manually stripping
was very laborious and produced poor samples fouled with skin surface mucous and, in some instances, blood. Siphoning of faecal material from the stand pipe area of the tank
yielded intact faecal pellets and the leaching of the nutrients was minimal. Faecal samples obtained in this manner appeared intact, firm, unspoiled and of high quality
Ž with dry matter contents consistent with those from the literature Schmitz et al., 1984;
. De La Higuera et al., 1989 . On this basis, siphoning method was adopted for this study.
Digestibilities of organic matter, protein and energy ranged from low for the 35 Ž
. protein diet to high for diets containing 51 protein Table 3 . The remaining diets had
similar digestibility to the 51 protein diet. Organic matter digestibility of the experimental diets showed a significant increase
Ž .
P - 0.05 from 82 to 90 as protein content of the diet increased. All these values were higher than 82 indicating that all experimental diets were well utilized by the
Ž .
eels. Digestibility of organic matter for the 35 protein diet was significantly P - 0.05 lower than all the other diets. Organic matter digestibilities for the 39–47 protein diets
Ž .
were similar P 0.05
at 87, while the 51 protein diet showed the highest Ž
. digestibility value of 90. These findings further support those of Schmitz et al. 1984
carried out with European eel that digestibility of similar fish-meal-based diets are relatively high at 87.
Ž .
Protein digestibility of the experimental diets showed a significant increase P - 0.05 when more than 35 protein was fed. Digestibility for the 35 protein diet was 85,
Table 3
1
Ž . Apparent digestibility coefficients
for organic matter, protein and gross energy for the diets containing graded levels of protein fed to juvenile American eel
Dietary Organic matter
Protein Gross energy
Ž . protein
a a
a
35 82.40.66
84.90.97 85.20.98
b b
b
39 86.80.66
89.40.97 88.80.98
b b
b
43 87.00.66
90.80.97 89.90.98
b b
b
47 87.00.66
90.60.97 90.60.98
c b
b
51 90.40.66
92.10.97 91.70.98
1
MeansSEM of two replicates and values within the same column with different superscripts are Ž
. significantly different P - 0.05 .
Table 4
1
Ž .
Body composition wet weight basis of juvenile American eel fed the diets containing graded levels of
protein after 84 days Ž .
Ž . Ž .
Ž .
Moisture Protein
Lipid Gross energy kJrg
Initial 69.6
17.1 10.0
7.0 Dietary
Ž . protein
a ab
d a
35 65.00.75
15.90.38 12.10.27
9.60.19
ab a
ac b
39 66.30.75
15.30.38 11.00.27
9.10.19
abc ab
a c
43 67.30.82
16.00.42 10.20.30
8.60.21
c ab
cd d
47 69.40.82
15.50.42 11.40.30
8.20.21
bc b
a cd
51 67.80.75
16.70.38 10.10.27
8.60.19
1
MeansSEM of three replicates and values within the same column with different superscripts are Ž
. significantly different P - 0.05 .
whereas the remaining diets were similar, averaging 91 digestibility. Schmitz et al. Ž
. 1984 reported a similar protein digestibility coefficient of 94 for a fish-meal-based
diet. Digestible energy coefficient of the experimental diets showed a similar pattern as the
protein digestibility. Digestible energy coefficient for the 35 protein diet was 85, whereas the remaining diets were similar, averaging 90 digestible. The lower di-
gestibility of the 35 protein diet may be explained on the basis of diet formulation and ingredient composition. Corn starch was used to balance the energy content of the diets.
In these diets, protein level increased from 35 to 51, whereas carbohydrate level decreased from 33 to 22. It is likely that the eels were not able to utilize the high
amount of starch present in the low protein diet. Similar effects have been reported for
Ž .
rainbow trout fed similar diets Luquet, 1971; Bergot, 1979 . In general, nutrient
digestibility was not significantly affected by the level of protein in the diet when the carbohydrate level was below 30. These finding are in agreement with De La Higuera
Ž .
et al. 1989 who found that nutrient digestibility was independent of dietary protein Ž
. level. Arai 1991 also observed that the optimum dietary level of carbohydrate for
Japanese eel was in the range of 20–30.
Table 5 Carcass protein, lipid and energy gain of juvenile American eel fed the diets containing graded levels of
protein after 84 days
1
Dietary Protein gain
Lipid gain Energy gain
Ž . Ž
. Ž
. Ž
. protein
grfish grfish
kjrfish
a a
abcd
35 1.50.10
1.40.07 120.05.40
a ab
abc
39 1.50.10
1.20.07 114.75.40
a b
ab
43 1.60.11
1.10.07 105.75.87
b c
d
47 2.20.11
1.80.07 135.95.87
b a
cd
51 2.20.10
1.40.07 128.15.40
1
MeansSEM of three replicates and values within the same column with different superscripts are Ž
. significantly different P - 0.05 .
3.3. Nutrient retention Ž
. Ž
. Body composition Table 4
and nutrient retention Table 5
showed a similar Ž
. response to the growth parameters. The highest carcass protein and lipid gains P - 0.05
were observed in fish fed diets containing 47 protein, further supporting our view that an increase in dietary protein beyond 47 produces no significant benefit in terms of
Ž .
Ž growth. Similar results were reported for grass carp Dabrowski, 1977 , snakehead Wee
. Ž
. Ž
and Tacon, 1982 , Japanese eel Nose and Arai, 1973 and common carp Ogino and .
Saito, 1970 . Little difference in carcass energy gain was observed due to the change in dietary protein level. This was not surprising as the five diets were formulated to be
isoenergetic and differences in energy digestibility were negligible.
4. Conclusions
