Improved carp diets based on plant protein sources reduce environmental phosphorus loading

Transcription

1 Blackwell Science, LtdOxford, UK FISFisheries Science Blackwell Science Asia Pty Ltd Less polluting carp diets with plant proteins P Jahan et al /j x Original Article219225BEES SGML FISHERIES SCIENCE 2003; 69: Improved carp diets based on plant protein sources reduce environmental phosphorus loading Parveen JAHAN, Takeshi WATANABE, Viswanath KIRON* AND Shuichi SATOH Department of Aquatic Biosciences, Tokyo University of Fisheries, Minato, Tokyo , Japan ABSTRACT: As part of on-going efforts to reduce environmental phosphorus (P) and nitrogen (N) loading from culture systems, five experimental diets were formulated containing 5 15% of fish meal (FM) and different levels of soy protein concentrate (10 20%), corn gluten meal (3 5%) and defatted soybean meal ( %). These diets had total P ranging between 1.04 and 1.29% and available P between 0.62 and 0.63%. A FM-based (43%) commercial diet was used as the control and this diet had total P of 1.87% and available P of 0.82%. Waste loading was calculated after feeding the diets to 3.4 g carp for 10 weeks. Feed gain ratios were not significantly different for fish fed control and experimental diets having 10 15% FM, but the protein efficiency ratio was lower for the control group. The rates of P absorption and retention in the experimental diet groups were significantly higher than those of the control group. The lowest N retention was obtained for the control group, although absorption was not markedly different among the treatment groups. Total P and N loading (kg/ton production) produced from the experimental diets ranged from 7.1 to 8.9 and from 36.1 to 41.3, respectively, whereas the corresponding values for the control diet were 15.2 and KEY WORDS: discharge. carp feed, fish meal, nitrogen, phosphorus, plant protein source, waste INTRODUCTION Two nutrients that have a great impact on the environment are nitrogen (N) and phosphorus (P) and, in aquaculture operations, these elements originate mainly from fish feeds. 1,2 For this reason, it is important to develop feeds that contribute towards sustainable aquaculture. In this regards, Johnsen and Wandsvik demonstrated a reduction in pollution load by optimizing the nutrient balance of the feeds and by improving feed efficiency. 3 However, not all efforts are equally rewarding because another report has indicated that the high-energy diets introduced recently to improve the growth of salmon and to minimize nutrient discharge did not supply sufficient P for skeletal development. 4 When less P-polluting fish feeds are formulated, the adequacy of available P should be considered, such as to support growth. The rapid growth in world aquaculture production over the past few decades has resulted in a concomitant increase in demand for fish meal *Corresponding author: Tel: Fax: vizi@tokyo-u-fish.ac.jp Received 12 February Accepted 5 August (FM). 5 Fish meal also contains P that is present as tricalcium phosphate that cannot be absorbed by fish such as common carp. 6 Therefore, greater amounts of unavailable P that correspond to dietary FM levels are excreted. This results in hypernutrification or eutrophication of farm areas and a countermeasure would be to use alternative feed ingredients. However, the complete replacement of FM with alternative plant or animal protein ingredients creates an imbalance of amino acids, negatively affecting fish growth. 7,8 Therefore, experiments are needed to identify a combination of alternative protein ingredients that can be an optimal replacement for FM, while maintaining the nutrient loading levels at a bare minimum. Less N-polluting carp feeds are currently being used in Lake Kasumigaura, a main culture site in Japan. 9 In previous studies, we confirmed that by keeping FM levels low and combining the FM with certain animal protein ingredients, carp feedrelated P loading could be reduced However, not all such ingredients can be used freely and it is also necessary to obtain information on plant protein ingredients in the search for low-polluting aqua feed. Therefore, in the present study we combined FM with plant protein sources, such as corn gluten meal (CGM), defatted soybean meal (dsbm) and soy protein concentrate (SPC).

2 220 FISHERIES SCIENCE P Jahan et al. Table 1 Ingredients and nutrient contents of the experimental diets Diets Control* Ingredients (%) Fish meal Soy protein concentrate Corn gluten meal Defatted soybean meal Wheat flour Defatted rice bran Oil mix Phosphorus-free mineral mix NaH 2 PO 4 2H 2 O Chromium oxide mix Others Nutrients Moisture (%) Crude protein (%) Methionine (% protein) Cystine (% protein) Crude lipid (%) Crude ash (%) Gross energy (kcal/g) Digestible energy (kcal/g) Total phosphorus (%) Available phosphorus 7 In diet (%) % Total phosphorus * A commercial diet that is commonly used for rearing juvenile carp in a concrete tank. Feed oil, mineral mix, calcium mono phosphate, vitamin mix. 1 Chilean jack mackerel meal. 2 Aquafeed grade (DANPRO TM ; Central Soya European Proteins, Aarhus Olie, Aarhus, Denmark). 3 Beef tallow : soybean oil = 3 : 2 v/v. 4 Chromium oxide : dextrin = 1 : 1 w/w. 5 Vitamin mix 1.5%; vitamin E (50%) 0.1%; choline chloride 0.5%. 6 Calculated values. 7 Water extractable phosphorus content. MATERIALS AND METHODS Experimental diets As shown in Table 1, five experimental diets were formulated by combining FM (5 15%) with three alternative plant protein ingredients, such as SPC (10 20%), CGM (3 5%) and dsbm ( %). Wheat flour and defatted rice bran were the carbohydrate sources, whereas beef tallow and soybean oil were the lipid sources. Monosodium phosphate was the inorganic P source and a chromium oxide mix served as the digestibility indicator. A commercial diet that had provided good feed performance for carp culture in Lake Kasumigaura was selected as the control diet. However, it was later found that the diet was used for preliminary feeding of juvenile carp in concrete ponds before they were transferred to net cages in the lake. Therefore, the feed contained a high amount of FM (43%) and the crude protein level was well over the official limit set for the lake. In contrast, all experimental diets confirmed to this stipulation of < 35% crude protein. The high FM inclusion in the control diet met the methionine and cystine requirements and the same was the case for the formulated diets based on the calculated values. 13 Crude lipid was higher in the experimental diets, whereas the highest crude ash was noted for the control diet. In the experimental diets, total P ranged from 1.04 to 1.29% and available P ( %) was within the requirement level for carp. 14 These values were highest in the control diet (total and available P of 1.87 and 0.82%, respectively), the available P surpassing the requirement of carp.

3 Less polluting carp diets with plant proteins FISHERIES SCIENCE 221 Feeding and sampling Juvenile carp reared initially on a commercial diet (Nippon Formula Feed, Yokohama, Japan) and weighing 3.4 ± 0.2 g were selected for the study. Duplicate groups of fish were fed each of the test diets three times per day, 6 days a week, to near satiation for a period of 10 weeks in twelve 60 L glass tanks (30 fish per tank). The water temperature ranged between 19.0 and 26.8 C during the experiment. The fish husbandry techniques and sampling procedures, including feces collection methods, have been described in previous reports At the start of the feeding trial, 30 fish were sampled from stock, pooled and stored at 20 C for the initial fish whole-body analysis. Similarly, at the end of the experiment, 10 fish were collected from each tank and preserved for the final fish wholebody analysis. Feces were collected during the last 2 weeks and stored for further analyses. Analytical methods and calculations Proximate composition analyses and the P contents of diets, feces and fish, as well as dietary and fecal chromium oxide analysis, were performed according to methods applied previously, 12 whereas the gross energy contents of diets and feces were determined by a bomb calorimeter (CA- 4P; Shimadzu, Kyoto, Japan). The protein efficiency ratio, net protein utilization rate, absorption and retention of P and N, and energy digestibility were calculated as described in previous studies. 15,16 The total P loading (T-P; values expressed as kg/ton fish production) was calculated as an apparent estimate (considering only fecal excretion) based on P absorption (P consumed P absorbed), followed by an accurate estimate (considering fecal and non-fecal excretion) on P retention (P consumed P retained). The accurate estimate based on N retention (N consumed N retained) was used for expressing the total N loading (T-N; values expressed as kg/ ton fish production). Additional information on T- P and T-N can be found in an earlier publication. 9 Statistical analysis Statistical analysis was conducted by one-way analysis of variance (ANOVA). Post hoc analysis was performed using Scheffés test (P < 0.05) to resolve the differences between means of duplicate groups. Fig. 1 Growth of the carp and changes in water temperature during the 10-week feeding trial. RESULTS Fish growth and feed performances As can be seen in Fig. 1, until 4 weeks of feeding, all groups showed similar growth; however, by the 8th week, differences became evident and fish fed the control diet started to show a higher growth. At the end of the experiment, the final body weights were the maximum for control diet followed by diets 1, 2, 4, 3 and 5, the lowest growth being significantly (P < 0.05) different from the control. Consequently, percent body weight gain was highest for the control diet, but it was not significantly different from diets 1 4 (Table 2). Daily feed consumption rates increased with a decrease in dietary FM levels. Thereby, the feed gain ratios obtained for diets with low FM levels were inferior among the treatments and that of the control diet was the highest. However, due to the dietary crude protein content that exceeded the requirement level, the protein efficiency ratio and net protein utilization rate were markedly lower for the high FM-containing diet (control), even lower than those of the lowest FM diet (diet 5).

4 222 FISHERIES SCIENCE P Jahan et al. Table 2 Growth of carp and feed performance at the end of the trial Diet group Control Initial body weight (g) 3.4 ± ± ± ± ± ± 0.2 Final body weight (g) 52.4 ± 12.8 b 46.3 ± 13.1 ab 46.1 ± 17.3 ab 41.4 ± 15.9 ab 41.6 ± 12.8 b 39.0 ± 14.2 a % Body weight gain 1115 ab 1432 b 1040 a 1253 ab 1246 ab 1108 ab Daily feed consumption rate 2.85 a 3.02 ab 3.04 ab 3.09 b 3.17 b 3.21 b Feed/gain ratio 0.98 a 1.05 a 1.06 a 1.09 a 1.12 ab 1.15 b Protein efficiency ratio 2.52 a 2.84 b 2.82 b 2.73 ab 2.67 ab 2.63 ab Net protein utilization rate 35.2 a 45.0 b 43.7 b 41.9 b 42.8 b 41.3 b Values within the same row not sharing common superscript letters are significantly different at P < Where appropriate, data are given on the mean ± SD (n = 30). Table 3 Proximate composition and phosphorus content in the fish whole body Initial Diet group* Control Proximate composition (%) Moisture Crude protein Crude lipid Crude ash Phosphorus (%) * Values at the end of the 10-week feeding period. No significant differences were observed. Fish whole-body proximate composition Although the proximate composition was not significantly different among the treatments, lower moisture and crude ash contents and higher crude protein and crude lipid contents were obtained for the final fish compared with their initial counterparts (Table 3). The whole-body total P contents were almost identical ( %) among the treatments, but were slightly lower than those of the initial fish. Phosphorus and nitrogen absorption Figure 2 shows the P and N absorption rates determined in the present experiment. The P absorption rate increased gradually from the control diet to diet 5 following the decrease in dietary FM levels. Moreover, the available P content of each experimental diet (data not shown), as calculated from the absorption rate, was approximately similar to the estimated available P (Table 1) and all diets met the level required by carp. Only the control diet showed slightly lower available P (0.75%) than that of the estimated value (0.82%). In the case of N absorption, the control diet showed a slightly lower rate, but there were no significant differences (P > 0.05) among the experimental diet groups. Fig. 2 Phosphorus and nitrogen absorption in carp determined at the end of the trial. Values are expressed as a percentage of the element in the diet that was absorbed. Same patterned bars with identical letters are not significantly different at the level of P < Phosphorus and nitrogen retention The retention rates of P and N determined at the end of the feeding trial are shown in Fig. 3. The P retention rate (as a percentage of total dietary P) was significantly lower for the control group and the values for diets 1 4 were similar; however, the P retention rate of diet 5 (lowest FM) was higher.

5 Less polluting carp diets with plant proteins FISHERIES SCIENCE 223 Table 4 Total phosphorus and nitrogen loading from carp, estimated at the end of the feeding trial Diet group T-P (kg/ton production) based on T-N (kg/ton production) based on P absorption P retention N retention Control 12.4 d 15.2 c 48.1 b c 8.8 b 36.1 a bc 8.9 b 36.3 a ab 8.3 b 38.7 a ab 8.1 b 39.7 ab a 7.1 a 41.3 ab Values within the same column not sharing common superscript letters are significantly different at P < T-P, T-N, total phosphorus and nitrogen loading, respectively; P, phosphorus; N, nitrogen. Fig. 3 Phosphorus and nitrogen retention in carp determined at the end of the trial. Retention values are given either (a) as a percentage of total dietary P and N or (b) as a percentage of absorbed P and N. Same patterned bars with identical letters are not significantly different at the level of P < Similarly, the lowest N retention rate (percentage of total dietary N) was observed for the control diet; however, this was not significantly different from the rest, excluding diet groups 1 and 2, which gave the highest values in this study (Fig. 3a). For P retention as a percentage of absorbed P, the lowest value (65.6%) was that of fish fed the control diet, whereas those of the experimental diets ranged between 68.6 and 79.0%, the highest value being for diet 1 (Fig. 3b). Similarly, N retention as a percentage of the amount absorbed, was highest for diets 1 and 2 (42.6 and 42.7%, respectively) and these values were significantly higher than that of the control diet (36.3%), but the differences among the experimental diet groups were not significant. Estimation of total phosphorus and nitrogen loading The T-P values produced by feeding the test diets are shown in Table 4. Based on P absorption, the T- P ranged from 5.2 to 7.4 kg/ton production among the experimental diet groups and it was significantly higher (12.4 kg/ton production) in the control group. When based on P retention, the T-P was slightly higher: kg/ton production among the experimental diet groups compared with 15.2 kg/ton production for the control diet group. Table 4 also shows the T-N rates, which ranged from 34.6 to 41.3 kg/ton production for the experimental diets and were significantly higher in the control group (48.1 kg/ton production). DISCUSSION The major strategy recommended to reduce P discharge from aquaculture operations is to minimize P levels in feeds through the selection of feed ingredients that have a high P bioavailability. 1 3 Previous studies have indicated that the unavailable P contents in carp feed could be lowered by the part replacement of FM with low-p ingredients The present study focuses on the use of soybean products for the partial replacement of FM because they are cost effective, sustainable and practical for low-p fish feed formulations The ingredients tested in the present study, namely SPC, dsbm and CGM, are highly digestible to carp 23,24 and the total P (mainly the unavailable portion) content of the diets decreased corresponding to a reduction in the FM content. Although an amino acid imbalance related to methionine may be suspected when using soy products, the diets we formulated contained CGM as well as FM as a protein source. Another factor that needs consideration when using soy products is their phytate content, which can inhibit P absorption. 17 The proper selection and combination of adequately processed plant ingredients with suboptimum synthetic P supple-

6 224 FISHERIES SCIENCE P Jahan et al. mentation effectively improves P availability. 21 Thus, the plant proteins were selected based on their water-extractable P content, ensuring its availability to carp, appropriately combined and supplemented with monosodium phosphate to the required level. Significantly higher P absorption rates of the experimental diets confirm the former report and support our formulations. The better absorption efficiency of dietary nutrients produced interesting results in growth performance. A lack of significant differences in feed gain ratios of fish fed either control or the experimental diets 1 4 indicates that reduction of the FM level to 10% and inclusion of SPC up to 20% had no adverse effect on carp growth. In addition, SPC inclusion was also beneficial for the improvement of protein utilization and the overall performance data agree with previous studies, even though those formulated diets incorporated higher SPC than the experimental diets used in the present study The results of whole-body proximate composition confirm the adequacy of protein, energy and P in the diets, as well as the better quality of dietary protein sources. 9,20 It must also be noted that inclusion of up to 5% CGM in carp diets had no negative effect on pigmentation in carp, unlike the yellow orange coloration in the operculum, fin, scale and muscle of gilthead sea bream following administration of a 20% CGM diet. 25 The retention rate of dietary nutrients is an important measure of the quality of feed ingredients and feed. In fact, the retention of any element depends on fish growth and the amount of the respective element in the diet. Despite a superior feed gain ratio, the lower N retention in the control diet in the present study was probably because its protein content was above the requirement of carp. This indicated that excess dietary protein, even though originating from a quality protein source (FM), led to a higher N discharge. In contrast, results obtained with the experimental formulations suggest that maintainance of the protein content at a level required by the fish through proper combination of plant protein sources can efficiently increase protein retention. From the P retention rates, the effect of the nutrient content on retention efficiency was clear. In the experimental diets, there was only a slight variation of dietary P ( %), resulting in retention values that ranged from 36.2 to 43.4%, corresponding with a decrease in dietary total P. Consequently, the control diet, which contained the highest P (1.87%), gave the lowest P retention rate (25.8%). Médale et al. also found that, in rainbow trout, P retention varied from 20.3 to 35.1% for diets containing total P contents of 1.9 and 1.3%, respectively. 21 In fish, a certain amount of N and P is excreted as non-fecal discharge through the urine and gills after absorption. 26 It has been reported that nonfecal P excretion is inevitable and that it increases with an increase of available P intake. 27 In previous studies, we also found that complete retention of absorbed P was never achieved in carp, but the retention efficiency decreased with an increase of intake and a decrease in fish growth. 18,19 Matching results were observed in the present study; a certain proportion of absorbed P and N was not retained and this portion can be considered as non-fecal loss. Similar and adequate amounts of protein and available P in the experimental diets helped to cut excess non-fecal discharge, although the slight variations could be linked to the small differences in feed gain ratios. In contrast, the high protein and available P contents in the control diet, exceeding the requirement levels, resulted in the lowest retention values, despite giving the best fish growth. It should be remembered that the standard of our research is Lake Kasumigaura, where T-P calculation is based on P absorption. To make comparisons with data from the lake, the apparent estimates of T-P were considered in the present study. These values for the experimental diets were much lower than the average value of 13 kg/ ton production reported for the lake. 28 However, the accurate values based on P retention were slightly higher than the apparent ones and this additional portion is probably due to the nonfecal P. 27 Moreover, higher non-fecal P excretion was confirmed from the apparent and accurate T- P values for the control diet and agreed with results from a previous study. 26 Despite this, highest T-N was obtained for the lowest FM diet (diet 5), which had the lowest T-P. Our previous studies indicated the same relationship between T-P and T-N when FM was replaced with alternative proteins Furthermore, all experimental diets in the present study gave T-N values that were lower than the value of 45 kg/ton production reported for Lake Kasumigaura. 28 The higher T-N rate in the control diet was probably due to its excess protein content and, if it had matched the experimental diets, we postulate that the T-N may have been low too. Therefore, carp feed formulation by combining FM with plant protein sources was effective in reducing waste loading without hampering growth. Fish feed should contain adequate amounts of dietary P and N using the strategies listed above, such that waste loading is reduced. Further research is required to develop economical and environmentally friendly aqua feeds that are less dependant on FM.

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