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ISSN: 2155-9546
Journal of Aquaculture Research & Development
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Nutritional Characteristics of Black Rockfish (Sebastes schlegeli) Fed a Diet of Fish Skin

Sung-Ju Rha1, Jae-Kwon Cho2, Seon-Jae Kim3, Wook-Min Park3, Tai-Sun Shin4 and Jae-Ho Hwang1*

1 College of Fisheries and Ocean Sciences, Chonnam National University, Yeosu 550-749, Korea

2 Southwest Sea Fisheries Research Institute, National Fisheries Research and Development Institute (NFRDI), Yosu 556-823, Korea

3 Department of Marine Bio Food Science, College of Fisheries and Ocean Science, Chonnam National University, Yosu 550-749, Korea

4 Division of Food Nutrition Science, Chonnam National University, Gwangju 500-757, Korea

*Corresponding Author:
Jae-Ho Hwang
College of Fisheries and Ocean Sciences
Chonnam National University
Yeosu 550-749, Korea
Tel: +82-70-7612-0537
Fax: +82-61-724-0538
E-mail: [email protected]

Received Date: April 28, 2014; Accepted Date: June 19, 2014; Published Date: June 29, 2014

Citation: Rha SJ, Cho JK, Kim SJ, Park WM, Shin TS, et al. (2014) Nutritional Characteristics of Black Rockfish (Sebastes schlegeli) Fed a Diet of Fish Skin. JAquac Res Development 5:239 doi:10.4172/2155-9546.1000239

Copyright: © 2014 Rha SJ, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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Abstract

This study investigated the effects of diets substituted with different levels (0, 5, 10, and 20%) of flounder skin meal (FSM) on the nutritional composition of black rockfish Sebastes schlegeli. Fish (10.05 ± 0.44 g) were fed to apparent satiation twice daily for 8 weeks. Adding FSM decreased crude lipid levels and increased crude protein and ash. The abundant fatty acids in the FSM-added group were C16:0, C18:1-cis (n9), and C22:6n-3. The major amino acids in the samples were glutamic acid, aspartic acid, glycine, leucine, alanine, lysine, and arginine. The abundant free amino acids in the FSM-added group were taurine, glutamic acid, alanine, leucine, and arginine. Six free sugars were found in all groups. Glucose was predominant, followed by mannose, rhamnose, fucose, fructose, and ribose. Among the three organic acids in the whole body of black rockfish, lactic acid was predominant, followed by citric acid and oxalic acid. Total organic acid content in the control was significantly higher than those of FSM substitution groups.

Keywords

Fish skin; Black rockfish; Organic acid; Free sugar; Fatty acid; Amino acid

Introduction

Numerous studies have been investigated vegetable and animal proteins that could replace fishmeal in fish feed. In particular, there have been many studies on the use of vegetable protein sources such as soybean meal [1-4], cottonseed meal, and rapeseed meal [5,6], which have a relatively stable supply compared to fishmeal, to replace fishmeal as a source of protein. However, plant resources are constantly in competition with livestock and human consumption, and the recent development of plant extract fuels such as bioethanol will eventually lead to an increase in the price of the plant resources usable as protein resources [7]. The by-products of the processing of terrestrial livestock such as cows, chickens, and pigs could be used as animal protein sources, since they have a relatively high protein content and qualitatively similar amino acid composition to fishmeal, and are inexpensive and stably supplied. Various studies have been conducted on their use as protein sources to replace fishmeal in fish feed [8-13]. However, the rise of safety issues due to serious infectious diseases like mad cow disease, swine fever, and avian influenza has gradually restricted the use of livestock by-products lately. Thus, as there are economic and safety issues with using terrestrial protein sources to replace fishmeal, securing economic and safe protein sources from marine products rather than terrestrial products is necessary. Many researchers have investigated by-products obtained from processing marine animal as potential protein sources, including shrimp by-products [14], tuna muscle by-products [15], shrimp and fish by-products [16], squid liver meal mixing soybean meal with by-products of squid processing [11], fish bone and crab by-products [17-19], and fish by-products [20]. Of the fishery by-products, even though fish skins obtained from the consumption of raw fish are a good protein source because of high collagen content, by-products such as bones and internal organs are only partially used and mostly discarded.

Therefore, this study was conducted to investigate improvement of quality and physiological function on cultured black rockfish fed diets substituted with different levels (0, 5, 10, and 20%) of flounder skin meal (FSM).

Materials and Methods

The skin of Paralichthys olivaceus, which has the highest farming yield and raw fish consumption in Korea, is easy to secure in large quantities due to its low use, thickness, and high collagen content, and was obtained from nearby fish markets. The fish skin was washed with fresh water and was subjected to hot air drying (50-60°C) followed by grinding via a high speed grinder (ZM-1000, Retsch Co., Japan) to prepare flounder skin meal (FSM).

After acclimation for 2 months in a square stock tank (running water system, 6.0 m×6.0 m×1.2 m) at the Fisheries Science Institute, Chonnam National University, Korea., 45 juvenile fish (mean body weight, 10.05 ± 0.44 g) were randomly selected from the stock tank and transferred to separate 300-L rectangular tanks (running water system, 1.0 m×0.8 m×0.8 m). The flow rate of filtered seawater in each tank was adjusted to 5 L/min. Mean water temperature, salinity, and dissolved oxygen were 20.2 ± 2.3°C, 32.0 ± 1.2 psu, and 6.3 ± 0.4 mg/L, respectively, and were measured using a YSI-85 (YSI, Ohio, USA) probe. The rearing trial was conducted in triplicate for each tested diet. The fish were fed twice a day (at 0800 h and 1600 h), until apparent satiation, for 8 weeks. The amount of feed given to each tank was recorded daily to calculate feeding efficiency.

Ingredients and proximate compositions of the experimental diets in response to FSM substitution and the results of vitamin C analyses are shown in Table 1. Proximate analyses were carried out to evaluate the nutritional composition of the prepared diets, and the vitamin C content of the diets were analyzed using the 2,4-dinitrophenyl hydrazine (DNP) colorimetric method [21].

Ingredient g/100g    
Control (0) 5 10 20
White fish meal 41 41 41 41
Casein 20 15 10 -
Flounder skin meal (FSM) - 5 10 20
L-ascorbic acid 0.02 0.02 0.02 0.02
Wheat flour 22.58 23.28 23.98 25.28
Feed oil (squid liver oil) 8.4 7.7 7 5.7
α-potato starch 3 3 3 2
Vitamin premix1) (vitamin C free) 2 2 2 2
Mineral Premix2) 2 2 2 1
Choline Chloride 1 1 1 1
Total 100 100 100 100
Vitamin C in diets 212.41 197.16 204.67 203.12
*Proximate analysis        
Protein 46.71 47.21 47.11 46.11
Lipid 9.21 9.51 10.11 10.81
Ash 8.21 8.61 8.71 8.71

Table 1: Ingredient and proximate composition of experimental diets with various levels of FSM.

FSM is a high protein meal containing more than 80% crude proteins but is lacking in essential amino acids compared to fishmeal. When fishmeal is replaced by FSM, unknown factors present in essential amino acids and fishmeal may affect the experimental fish, making it difficult to evaluate the influences of FSM substitution on experimental fish. Therefore, to maintain proper balances of the essential amino acids and to minimize the effects of the unknown factors in the fishmeal, white fishmeal (FF Skagen LT Supreme, Denmark) was fixed at the same level throughout the experimental diets. Casein, a purified protein, was used to control the protein content of each experimental diet. Squid liver oil (Ihwa, Korea) rich in DHA and EPA, which are essential fatty acids for the black rockfish, was used as a lipid source. Wheat flours (CJ, Korea) and α-potato starch were employed as carbohydrate sources to control energy and bind the diets. To find the relationship between the level of vitamin C and FSM substitution on the fish body, vitamin C (200 mg/ kg) was added based upon the vitamin C requirements of the black rockfish reported by Bai et al. [22]. There were a total of 4 experimental groups, including a control group with fishmeal and casein only and 3 experimental groups with 5, 10, or 20% of the casein replaced by FSM.

Analyzes of proximate composition, fatty acid, total amino acid, free amino acid, organic acid, and free sugar in this study were carried out based on AOAC methods [23] and some modifications of Hwang et al. [24].

All mean values were analyzed via one-way analysis of variance (ANOVA). When differences were found among data, Duncan’s multiple range test was used to compare the mean difference by using the SPSS software package version 17 (Statistical Package for Social Sciences, SPSS Inc., Chicago, IL, USA). Differences were considered significant at p<0.05.

Results

Proximate compositions with various levels of FSM are shown in Table 2. Crude protein was significantly higher in the FSM groups than the control group. FSM 5% was much higher than FSM 10% and FSM 20% (P<0.05). Crude lipid was significantly lower in the FSM groups than the control group, especially in FSM 20% compared to that of FSM 5% and FSM 10% (P<0.05).

Proximate composition (g/100g) FSM substitution level (%)
Control (0) 5 10 20
Moisture 68.14 ± 1.28a 67.93 ± 0.47a 68.73 ± 0.05ab 69.54 ± 0.07a
Crude protein 14.28 ± 0.07a 15.10 ± 0.03d 14.75 ± 0.08a 14.93 ± 0.04c
Crude lipid 10.53 ± 0.22c 9.72 ± 0.02b 9.20 ± 0.25a 8.25 ± 0.53a
Ash 3.77 ± 0.07a 4.11 ± 0.08c 3.92 ± 0.08b 4.04 ± 0.03bc

Table 2: Proximate composition (%) of whole body in black rockfish (S. schlegeli) fed the test diets with various levels of FSM for 8 weeks.

The fatty acid composition of the whole body is shown in Table 3. Saturates were observed to be significantly lower in all FSM groups than the control group (P<0.05), and no significant differences were found in monoenes (P<0.05). Significantly higher polyenes were observed in the FSM groups than the control group, especially in FSM 20% (P<0.05). The control group was also significantly lower in n-3 than FSM 5% and FSM 20% (P<0.05), whereas FSM 10% was not significantly different from the control group. There were no significant differences in n-6 between the experimental groups. The n-3/n-6 ratio was not significantly different between the control group and FSM 5% and FSM 10%, while FSM 20% was significantly higher than the control group (P<0.05).

Fatty acid FSM substitution level (%)
Control (0) 5 10 20
C12:0 0.31 ± 0.01d 0.26 ± 0.01c 0.24 ± 0.01b 0.19 ± 0.00a
C13:0 0.05 ± 0.01ns 0.05 ± 0.00 0.05 ± 0.00 0.05 ± .00
C14:0 7.42 ± 0.05b 6.91 ± 0.23a 7.14 ± 0.17ab 7.33 ± 0.07b
C15:0 0.96 ± 0.07a 1.00 ± 0.02ab 1.00 ± 0.01ab 1.04 ± 0.02b
C16:0 24.07 ± 0.06b 23.23 ± 0.46a 23.69 ± 0.22ab 23.44 ± 0.15a
C17:0 0.79 ± 0.01bc 0.79 ± 0.00c 0.75 ± 0.03ab 0.72 ± 0.02a
C18:0 6.69 ± 0.10b 6.81 ± 0.13b 6.63 ± 0.16b 6.24 ± 0.07a
C20:0 1.64 ± 0.04c 1.03 ± 0.03b 0.96 ± 0.02a 0.92 ± 0.05a
C21:0 0.57 ± 0.02ns 0.56 ± 0.11 0.55 ± 0.01 0.50 ± 0.05
C22:0 0.81 ± 0.01a 0.81 ± 0.01a 0.76 ± 0.01b 0.72 ± 0.01a
C23:0 1.12 ± 0.02a 1.25 ± 0.06b 1.34 ± 0.03c 1.55 ± 0.03d
C24:0 1.43 ± 0.02a 1.50 ± 0.01c 1.45 ± 0.01ab 1.47 ± 0.03bc
Saturates 45.87 ± 0.23b 44.21 ± 0.60a 44.58 ± 0.21a 44.18 ± 0.06a
C14:1 0.32 ± 0.01d 0.27 ± 0.01a 0.30 ± 0.01b 0.31 ± 0.00c
C15:1 0.02 ± 0.00ns 0.02 ± 0.00 0.02 ± 0.00 0.02 ± 0.00
C16:1 6.29 ± 0.03b 5.99 ± 0.10a 6.33 ± 0.02b 6.46 ± 0.07c
C17:1 0.45 ± 0.01a 0.45 ± 0.01a 0.46 ± 0.00a 0.49 ± 0.01b
C18:1n9t 0.53 ± 0.19ns 0.37 ± 0.17 0.46 ± .12 0.39 ± 0.10
C18:1n9c 20.43 ± 0.15b 20.45 ± 0.25b 20.40 ± 0.08b 19.89 ± 0.13a
C20:1 4.65 ± 0.11ab 5.15 ± 0.34b 4.72 ± 0.51ab 4.23 ± 0.20a
C22:1n9 0.96 ± 0.01a 0.97 ± 0.01ab 0.99 ± 0.01bc 0.99 ± 0.01c
C24:1 0.89 ± 0.11ns 1.06 ± 0.17 1.04 ± 0.17 1.14 ± 0.16
Monoenes 34.55 ± 0.28ns 34.73 ± 0.52 34.71 ± 0.61 33.92 ± 0.49
C18:2n6t 0.17 ± 0.01ns 0.17 ± 0.01 0.17 ± 0.01 0.12 ± 0.08
C18:2n6c 0.43 ± 0.01ns 0.42 ± 0.01 0.43 ± 0.01 0.43 ± 0.01
C20:2 0.77 ± 0.00a 0.83 ± 0.02b 0.78 ± 0.02a 0.76 ± 0.02a
C22:2 0.07 ± 0.01a 0.11 ± 0.01b 0.10 ± 0.02ab 0.09 ± 0.02ab
C18:3n6 0.33 ± 0.01a 0.36 ± 0.01b 0.33 ± .01a 0.35 ± 0.01b
C18:3n3 1.31 ± 0.01ns 1.32 ± 0.04 1.30 ± 0.02 1.30 ± 0.01
C20:3n6 0.03 ± 0.01ns 0.02 ± 0.00 0.02 ± 0.00 0.03 ± 0.01
C20:3n3 0.15 ± 0.01ns 0.17 ± 0.04 0.15 ± 0.01 0.16 ± 0.02
C20:4n6 0.26 ± 0.00a 0.28 ± 0.01b 0.29 ± 0.00c 0.31 ± 0.00d
C20:5n3 0.00 ± 0.00a 0.28 ± 0.08c 0.19 ± 0.02b 0.18 ± 0.02b
C22:6n3 16.04 ± 0.47a 17.09 ± 0.87ab 16.95 ± 0.42a 18.17 ± 0.40b
Polyenes 19.57 ± 0.47a 21.06 ± 0.87b 20.71 ± 0.41b 21.91 ± 0.47c
n3 17.51 ± 0.48a 18.87 ± 0.88bc 18.59 ± 0.45ab 19.82 ± 0.43c
n6 1.22 ± 0.01ns 1.25 ± 0.03 1.24 ± 0.02 1.24 ± 0.08
n3/n6 14.31 ± 0.33a 15.07 ± 1.00ab 15.04 ± 0.52ab 16.06 ± 0.73b

Table 3: Fatty acid composition of whole body in black rockfish (S. schlegeli) fed the test diets with various levels of FSM for 8 weeks (g/100 g).

The whole-body amino acid contents are shown in Table 4. The total amino acids and EAA were significantly higher in FSM 20% than the other groups (P<0.05). FSM 20% was significantly higher in all amino acids compared to the control group (P<0.05), whereas FSM 5% and FSM 10% were not significantly different from the control group.

  FSM substitution level (%)  
  Control (0) 5 10 20
Aspatic acid 2.97 ± 0.07a 3.20 ± 0.30a 3.17 ± 0.04a 3.58 ± 0.12b
*Threonine 1.54 ± 0.05a 1.66 ± 0.16ab 1.73 ± 0.21ab 1.82 ± 0.04b
Serine 1.85 ± 0.06a 2.02 ± 0.19ab 2.01 ± 0.10ab 2.23 ± 0.07b
Glutamic acid 3.78 ± 0.09a 4.12 ± 0.39a 4.08 ± 0.15a 4.60 ± 0.16b
Proline 1.59 ± 0.03a 1.73 ± 0.17a 1.67 ± 0.06a 1.97 ± 0.09b
Glycine 4.37 ± 0.11a 4.89 ± 0.45a 4.58 ± 0.11a 5.62 ± 0.33b
Alanine 3.11 ± 0.09a 3.41 ± 0.31a 3.18 ± 0.10a 3.79 ± 0.17b
Cystine 0.11 ± 0.00a 0.12 ± 0.03ab 0.13 ± 0.01ab 0.15 ± 0.01b
*Valine 1.61 ± 0.05a 1.74 ± 0.18ab 1.74 ± 0.03ab 1.93 ± 0.06b
*Methionine 0.81 ± 0.02a 0.86 ± 0.10a 0.86 ± 0.01a 0.97 ± 0.03b
*Isoleucine 1.23 ± 0.02a 1.32 ± 0.13a 1.31 ± 0.01a 1.46 ± 0.04b
*Leucine 2.22 ± 0.05a 2.37 ± 0.23a 2.34 ± 0.01a 2.60 ± 0.07b
*Tyrosine 0.57 ± 0.03a 0.61 ± 0.11a 0.67 ± 0.04a 0.76 ± 0.01b
*Phenylalanine 0.96 ± 0.02a 1.07 ± 0.11a 1.04 ± 0.05a 1.19 ± 0.04b
*Histidine 0.81 ± 0.02a 0.91 ± 0.07b 0.86 ± 0.01ba 0.93 ± 0.02b
*Lysine 1.72 ± 0.04a 1.86 ± 0.21a 1.94 ± 0.03a 2.25 ± 0.08b
Ammonia 2.28 ± 0.05a 2.42 ± 0.17ab 2.32 ± 0.07a 2.58 ± 0.11b
*Arginine 1.42 ± 0.04a 1.48 ± 0.18a 1.52 ± 0.02a 1.76 ± 0.07b
Total 32.96 ± 0.82a 35.80 ± 3.44a 35.17 ± 0.76a 40.18 ± 1.49b
*EAA 12.32 ± 0.31a 13.28 ± 1.35a 13.35 ± 0.34a 14.90 ± 0.45b

Table 4: Total amino acid content of whole body in black rockfish (S. schlegeli) fed the test diets with various levels of FSM for 8 weeks (g/100 g).

The whole-body free amino acid contents are shown in Table 5. The total free amino acids were significantly lower in the FSM groups than the control group, and the FSM groups tended to decreased significantly in a dose-dependent manner (P<0.05).

  FSM substitution level (%)  
Control (0) 5 10 20
Phosphoserine 0.77 ± 0.03b 0.84 ± 0.01c 0.78 ± 0.02b 0.62 ± 0.02a
Taurine 3.31 ± 0.04a 3.27 ± 0.17a 3.41 ± 0.04a 3.57 ± 0.09b
Aspartic acid 3.31 ± 0.02d 2.51 ± 0.02c 1.61 ± 0.04b 1.35 ± 0.02a
Hydroxyproline 1.68 ± 0.05b 1.23 ± 0.27a 1.89 ± 0.13b 1.70 ± 0.18b
Threonine 3.18 ± 0.04a 2.26 ± 0.06b 1.54 ± 0.05c 1.13 ± 0.02d
Serine 2.19 ± 0.02d 1.84 ± 0.09c 1.39 ± 0.04b 0.91 ± 0.02a
Asparagine 0.88 ± 0.06b 0.30 ± 0.02a 0.30 ± 0.01a 0.31 ± 0.00a
Glutamic acid 5.46 ± 0.10d 3.91 ± 0.05c 2.57 ± 0.07b 2.36 ± 0.05a
Proline 1.95 ± 0.04d 1.51 ± 0.10c 1.19 ± 0.03b 0.75 ± 0.03a
Glycine 2.75 ± 0.03c 2.97 ± 0.11d 2.57 ± 0.09b 2.38 ± 0.01a
Alanine 5.95 ± 0.05d 4.92 ± 0.08c 3.79 ± 0.13b 3.02 ± 0.01a
Citrulline 0.34 ± 0.03d 0.22 ± 0.04c 0.11 ± 0.01b 0.05 ± 0.00a
Valine 3.10 ± 0.02d 2.07 ± 0.05c 1.53 ± 0.05b 0.99 ± 0.02a
Methionine 0.86 ± 0.02d 0.48 ± 0.01c 0.41 ± 0.02b 0.32 ± 0.02a
Isoleucine 2.46 ± 0.01d 1.62 ± 0.02c 1.10 ± 0.04b 0.73 ± 0.01a
Leucine 7.33 ± 0.01d 4.33 ± 0.03c 3.34 ± 0.13b 2.54 ± 0.01a
Tyrosine 1.35 ± 0.06b 1.62 ± 0.06c 1.44 ± 0.06b 0.99 ± 0.03a
Phenylalanine 2.84 ± 0.01d 1.61 ± 0.02c 1.33 ± 0.03b 0.95 ± 0.01a
β-aminoisobutyric acid 1.06 ± 0.07b 0.53 ± 0.02a 0.61 ± 0.12a 0.55 ± 0.03a
γ-amino-n-butyric acid 0.03 ± 0.01b 0.01 ± 0.00a 0.01 ± 0.00a 0.02 ± 0.00ab
Histidine 0.95 ± 0.01d 0.56 ± 0.02c 0.45 ± 0.02b 0.20 ± 0.01a
1-methylhistidine 0.06 ± 0.00b 0.03 ± 0.01a 0.02 ± 0.00a 0.01 ± 0.01a
Carnosine 0.21 ± 0.01c 0.13 ± 0.03a 0.14 ± 0.01a 0.07 ± 0.02a
Anserine 0.29 ± 0.06ns 0.29 ± 0.05 0.31 ± 0.07 0.32 ± 0.01
Tryptopan 0.51 ± 0.09b 0.22 ± 0.08a 0.17 ± 0.07a 0.09 ± 0.04a
Hydroxylysine 0.11 ± 0.01b 0.05 ± 0.01a 0.04 ± 0.01a 0.05 ± 0.02a
Ornitine 0.19 ± 0.01c 0.09 ± 0.01b 0.06 ± 0.01a 0.06 ± 0.00a
Lysine 2.99 ± 0.07d 0.77 ± 0.03c 0.49 ± 0.02b 0.33 ± 0.02a
Ammonia 2.20 ± 0.03c 1.86 ± 0.16a 1.42 ± 0.03a 1.55 ± 0.02b
Ethanolamine 0.36 ± 0.08ns 0.40 ± 0.03 0.34 ± 0.03 0.38 ± 0.04
Arginine 8.71 ± 0.12d 5.84 ± 0.18c 5.25 ± 0.16b 3.57 ± 0.05a
Total 67.37 ± 0.26d 48.31 ± 0.99c 39.61 ± 1.29b 31.88 ± 0.22a

Table 5: Free amino acid content of whole body in black rockfish (S. schlegeli) fed the test diets with various levels of FSM for 8 weeks (g/100 g).

Seven kinds of free sugars were analyzed, and fucose, rhamnose, glucose, mannose, fructose, and ribose were detected but not galactose (Table 6). The total free sugars were not significantly different in the control group, FSM 5%, and FSM 10%, while they were significantly lower in FSM 20% (P<0.05).

  FSM substitution level (%)
Control (0) 5 10 20
Fucose 150.20 ± 15.39ns 155.82 ± 10.22 132.53 ± 14.89 145.44 ± 15.64
Rhamnose 354.54 ± 35.57d 284.62 ± 27.03c 136.03 ± 32.66b 82.35 ± 0.55a
Glucose 280.99 ± 5.25a 424.50 ± 31.76b 464.12 ± 27.22b 289.80 ± 19.75a
Mannose 190.91 ± 11.39a 217.63 ± 32.91a 319.56 ± 14.43b 207.09. ± 17.14a
Fructose 76.05 ± 8.64a 87.12 ± 2.18ab 105.61 ± 7.69bc 109.35 ± 15.86c
Ribose 81.90 ± 6.05c 75.70 ± 7.02bc 65.63 ± 4.73b 49.10 ± 7.70a
Total 1,134.59 ± 82.29b 1,245.38 ± 111.11b 1,223.49 ± 101.61b 883.14 ± 76.63a

Table 6: Free sugar content of whole body in black rockfish (S. schlegeli) fed the test diets with various levels of FSM for 8 weeks (mg/L).

Six kinds of organic acids were analyzed, and lactic acid, oxalic acid, and citric acid were found, but not malic acid, tartaric acid, or maleic acid (Table 7). The total organic acid content was significantly lower in the FSM groups than the control group (P<0.05).

  FSM substitution level (%)
Control (0) 5 10 20
Lactic acid 12.12 ± 0.34c 10.11 ± 0.09b 7.79 ± 1.63a 9.09 ± 0.13ab
Oxalic acid 0.19 ± 0.01b 0.15 ± 0.02a 0.13 ± 0.01a 0.15 ± 0.03b
Citric acid 0.72 ± 0.03a 0.68 ± 0.02ab 0.65 ± 0.03a 0.71 ± 0.02b
Total 13.03 ± 0.12c 10.90 ± 0.18b 8.57 ± 1.60a 9.95 ± 0.16ab

Table 7: Organic acid content of whole body in black rockfish (S. schlegeli) fed the test diets with various levels of FSM for 8 weeks (mg/L).

Discussion

In the previous result, it was confirmed that there was a high collagen content (approximately 20% dry weight) in Paralichthys olivaceus skin [25]. Flounder skin meal (FSM) replaced various fractions of the casein substitution in the fishmeal, and this was fed to the fish for 8 weeks to confirm the substitution effect.

Comparing the results of such fishmeal substitution is difficult because of the various protein sources and fish utilized. However, the nutritional characteristics of the alternative protein sources affect lipid metabolism in the body. In particular, as FSM contains relatively higher lipid content (17%) compared to white and brown fishmeal (7- 8%), it would affect lipid accumulation and metabolism. In the FSM substitution group, the protein content in the whole body increased and the lipid content decreased, and the ash content increased significantly compared to the control group. The body composition of fish is affected by various factors such as intraspecific strain differences, water temperature, and increased body weight, and is influenced the most by the amount of feed supplied and the mix proportions of the feed [26,27].

Among whole-body fatty acid responses to the FSM substitution in the feed, saturated fatty acids significantly decreased while polyunsaturated fatty acids increased, especially n-3 HUFA, which is an essential fatty acid in black rockfish [28]. The white fishmeal used in this study contained about 0.7% n-3 HUFA, and fish oil, like the squid liver oil used as a lipid source, has more than 20% n-3 HUFA with an appropriate EPA/DHA ratio [29]. Accumulation of n-3 HUFA in fish bodies increased because of the FSM substitution. This seems to be because of FSM-specific amino acids rather than the differences in n-3 HUFA in the feed.

Amino acids in fish were significantly higher only in FSM 20% than the control group. Although such differences may occur due to the FSM substitution, this is not certain because significant differences in the amino acids were not observed between the control group and the experimental groups. However, the free amino acids in the fish body were significantly decreased in the FSM groups compared to the control group, suggesting that the FSM substitution influenced amino acid metabolism in the fish. In marine animals, free amino acids provide chemical signals for behaviors, communication, and metabolism through sensory organs [30]. Moreover, free amino acids act as substrates for protein biosynthesis or aerobic catabolism and provide osmolality stably during embryonic stages through intrinsic nutrients in marine fish [31,32].

Lactic acids were the most abundant organic acids, especially in the FSM substitution groups. Lactic acids are known to greatly differ based on the amount of activity at the time of harvesting and the storage conditions [33]. However, lactic acids showed significant differences in the FSM substitution group compared to the control group in the present study. Given that the amount of activity at the time of harvesting and storage conditions were similar in this study, lactic acids seem to have an influence on the energy metabolism of glycolysis with respect to the FSM substitution.

The glucose content in the free sugars was significantly higher in FSM 5% and FSM 10% than the control group; but no significant differences between control and FSM 20%, indicating that FSM substitution affect glucose metabolism, resulting in differences in body glucose. Ribose differed in a similar manner, leading to glucose levels decreased in response to the FSM substitution. Free ribose is abundant in the muscles of living fish but is also released from inosine, which is produced by ATP decomposition after death. Consequently, ribose content can depend on the pretreatment conditions of the samples immediately after the instant killing [1]. As ribose, along with fructose, is a quantitatively important factor in glycolysis, it is considered to be somewhat associated with glucose metabolism.

Acknowledgements

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0011204).

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