Manipulation of Graded Levels of Jack Mackerel Meal in Diets Replacing Fish Meal with Corn Protein Concentrate in the Diets of Rockfish (Sebastes schlegeli): Effects on Growth Performance, Feed Utilization, and Economic Analysis

Abstract

Incorporating protein feed ingredients that exhibit strong attractiveness to the target fish species is an effective and sustainable feeding strategy to improve feed intake and enhance growth performance. An 8-week feeding experiment was performed to elucidate the manipulation impact of graded levels of jack mackerel meal (JMM) in diets replacing 10% fish meal (FM) with corn protein concentrate (CPC) on the growth, feed utilization, and blood chemistry of rockfish (Sebastes schlegeli), as well as to perform an economic analysis. A total of 450 fish were randomly assigned to 15 plastic tanks (30 juveniles/tank). Five experimental diets were formulated to be isoproteic (50.0% crude protein) and isolipidic (15.5% crude lipid). The control (Con) diet contained 55% FM. In the Con diet, 10% of the FM was substituted with CPC, and graded levels of JMM at 0%, 20%, 40%, and 60% were subsequently incorporated instead of the FM, referred to as CPCJ0, CPCJ20, CPCJ40, and CPCJ60, respectively. Triplicate groups of rockfish were carefully hand-fed the diets to satiation throughout the feeding experiment. The rockfish fed the CPCJ60 diet produced significantly higher weight gain (WG) (p < 0.002) and specific growth rate (SGR) (p < 0.003) than those fed the Con, CPCJ0, and CPCJ20 diets but showed no significant (p > 0.05) differences compared to those fed the CPCJ40 diet. Both the WG (Y = 0.9367X + 17.0500, p < 0.0001, Adjusted R² = 0.8468) and SGR (Y = 0.0005X + 0.0165, p < 0.0001, Adjusted R² = 0.8580) of the rockfish increased linearly with increased dietary JMM inclusion levels when 10% of the FM was replaced by CPC. The rockfish fed the CPCJ60 diet showed a significantly higher feed consumption (FC) (p < 0.03) compared to those fed the CPCJ0 diet. Among the dietary treatments, however, no significant (p > 0.05) differences were found in the feed utilization, proximate composition, amino and fatty acid profiles, and blood chemistry of the rockfish. The CPCJ60 diet resulted in the highest economic profit index (EPI) among the dietary treatments. Conclusively, JMM was found to be effective in improving the FC of rockfish fed the diets replacing 10% of the FM with CPC. Furthermore, the WG and SGR of the rockfish fed the diets replacing 10% of the FM with CPC improved linearly with elevated JMM inclusion. Thus, incorporating 60% of JMM into the diets substituting 10% of the FM with CPC was the most recommended strategy according to the growth performance and FC of the rockfish, providing the highest EPI for fish farmers.

1. Introduction

In aquafeed formulations, fish meal (FM) is traditionally used as the main protein source due to its rich protein content, well-balanced essential amino acids (EAAs) and fatty acids (EFAs), high digestibility, and abundant minerals and certain vitamins [1,2]. However, the rising global demand for FM, combined with its limited supply, has resulted in elevated prices, thereby constraining its use as the sole protein source in aquafeeds [3]. Therefore, feed nutritionists have been actively exploring cost-effective and year-round available alternatives to FM.

Over the last few decades, substantial research efforts have focused on substituting FM with plant-derived protein sources in aquafeeds [4,5,6] because of their wide availability, low cost, and environmental sustainability. Nevertheless, the application of plant-derived proteins in aquafeeds is often limited by nutritional drawbacks, including imbalanced AA profiles, the presence of non-starch polysaccharides, and anti-nutritional factors (ANFs), all of which can impair nutrient availability and digestibility [2,7]. To address these issues, several innovative processing techniques have been introduced in recent years to enhance the nutritional quality of plant-derived ingredients by reducing ANFs and increasing protein concentration, resulting in products commonly referred to as plant protein concentrates [8,9].

Corn protein concentrate (CPC), an innovative protein extract from corn, is produced by enzymatically extracting non-protein elements from the by-product of corn wet milling, which is used to produce corn starch, syrup, and oil [6,10]. Upon refinement, CPC’s dry weight can predominantly comprise 80% crude protein (CP) and a minimal starch content of less than 1% [11]. Unlike soybean-derived feedstuffs, CPC is deficient in lysine content [2]. Specifically, Shekarabi et al. [12] highlighted that substituting 25.2% of FM with CPC resulted in comparable growth performance and feed utilization of rainbow trout (Oncorhynchus mykiss) to fish fed a 45.2% FM-based diet. Our earlier study [13] revealed that 10% of FM could be substituted with CPC without significantly impairing weight gain (WG) and feed intake (FI) of rockfish. However, the substitutability of CPC for FM in some fish diets has been reported to improve with AA supplementation [8,14,15]. For example, Minjarez-Osorio et al. [14] demonstrated that AA supplementation allowed FM to be substituted by up to 50% in diets for red drum (Sciaenops ocellatus) and up to 75% in diets for shortfin corvina (Cynoscion parvipinnis), without adversely affecting WG.

Reduced feed palatability is a major challenge associated with high levels of FM replacement by alternative protein sources, particularly plant-based ingredients, in aquafeeds. This can result in poor FI, which may eventually lead to stunted growth, increased feed waste, and deteriorated water quality [16,17]. In response to this issue, feed attractants and stimulants have been effectively incorporated into fish diets to enhance feed palatability, thereby increasing FI, particularly when plant-based proteins constitute the primary dietary ingredients [17,18]. For instance, Blaufuss and Trushenski [19] demonstrated that inclusion of feed attractants and stimulants could elucidate the palatability of low-FM diets for hybrid striped bass (Morone chrysops × M. saxatilis). Furthermore, betaine, taurine, inosine, nucleotides, and nucleosides are among the synthetic substances typically used to elevate the taste appeal (palatability) of feeds for a variety of fish species [20,21]. Nevertheless, the extent to which fish are attracted to these synthetic chemicals varies considerably depending on the fish species, feeding behavior, and the type and amount of feed attractants or stimulants [20,22]. The effectiveness of the synthetic feed attractants and stimulants in enhancing FI and promoting fish growth in practical feeding contexts remains a contentious issue [18,23]. In contrast, natural marine ingredients such as jack mackerel meal (JMM) not only enhance feed palatability, but also provide functional nutrients, including EAAs and nucleotides [24], which are often limited in plant-origin protein sources such as CPC, thereby conferring both sensory and nutritional benefits in fish diets.

Several marine-based ingredients have been reported to possess feed attractant and/or stimulant properties that enhance diet palatability and, consequently, improve FI and growth performance in various fish species. According to Takakuwa et al. [24], AA and nucleotides, especially inosine monophosphate (IMP), are abundant in jack mackerel (Trachurus japonicus) muscle extracts, which showed the highest efficacy in stimulating feeding behavior in greater amberjack (Seriola dumerili). Chotikachinda et al. [16] demonstrated that the inclusion of 3–4% tuna viscera hydrolysates in a diet containing 54% poultry by-product meal effectively improved feed palatability, resulting in increased FI and enhanced growth performance in Asian sea bass (Lates calcarifer). It has been reported that JMM acts as a potent protein feed ingredient, exhibiting effective feed attractant and stimulant responses in various marine fish species, including rockfish [25], olive flounder (Paralichthys olivaceus) [26], and yellowtail (Seriola quinqueradiata) [27]. Additionally, Choi et al. [28] revealed that grower walleye pollock (Gadus chalcogrammus) fed diets with JMM as a main protein source exhibited markedly enhanced WG and FI compared to those fed diets containing pollock meal or anchovy meal as a main protein source.

Rockfish (Sebastes schlegeli) is commonly considered to be the second-most farmed and economically important marine finfish species in the Republic of Korea (hereafter, Korea) after olive flounder. In 2024, annual production of rockfish reached 14,463 metric tons (MT), accounting for a substantial proportion of the total marine finfish aquaculture production of 81,911 MT in Korea [29]. Rockfish require high dietary protein [30], and FM is therefore commonly used as the primary protein source in rockfish diets [31], which contributes to elevated feed costs. Exploring alternatives to FM in rockfish diets is essential for promoting sustainable and economically viable rockfish production. To date, considerable efforts have been made to improve the nutritional quality and enhance the productivity of rockfish [25,32,33,34,35,36,37].

Manipulating feed ingredients to effectively enhance the attractiveness of low-FM diets for target fish species can be a viable and sustainable strategy for promoting fish growth through improvements in FI [9,35]. Given the attractiveness of JMM and the suitability of CPC as a replacer for FM, this experiment aimed to examine the impacts of manipulating graded levels of JMM in diets that substitute 10% of FM with CPC on growth, feed utilization, and biochemical composition of juvenile rockfish, and perform an economic analysis.

2. Materials and Methods

2.1. Formulation of the Experimental Feeds

Five diets were prepared to be isoproteic (CP; 50.0%) and isolipidic (crude lipid, CL; 15.5%) (

Table 1. Ingredients and chemical composition of the experimental diets (%, dry matter basis).

	Experimental Diets
	Con	CPCJ0	CPCJ20	CPCJ40	CPCJ60
Ingredients (%, DM)
Fish meal (FM) 1	55.0	49.5	38.5	27.5	16.5
Corn protein concentrate (CPC) 2		5.2	5.2	5.2	5.2
Jack mackerel meal (JMM) 3			11.0	22.3	33.6
Fermented soybean meal	12.0	12.0	12.0	12.0	12.0
Wheat flour	21.5	21.4	21.3	20.9	20.5
Fish oil	4.5	4.9	5.0	5.1	5.2
Soybean oil	4.5	4.5	4.5	4.5	4.5
Vitamin premix 4	1.0	1.0	1.0	1.0	1.0
Mineral premix 5	1.0	1.0	1.0	1.0	1.0
Choline	0.5	0.5	0.5	0.5	0.5
Nutrients (%, DM)
Dry matter	94.4	96.8	96.3	96.1	96.5
Crude protein (CP)	50.4	50.4	50.1	50.4	50.0
Crude lipid (CL)	15.5	15.5	15.4	15.1	15.2
Ash	9.1	8.8	9.1	8.9	8.7

¹ Fish meal (FM, anchovy meal) (CP: 73.4%, CL: 10.7%, ash: 14.0%) was imported from Chile. ² Corn protein concentrate (CPC) (CP: 79.5%, CL: 3.3%, ash: 1.0%) was imported from Cargill (Minneapolis, MN, USA). ³ Jack mackerel meal (JMM) (CP: 72.2%, CL: 9.9%, ash: 14.3%) was imported from Chile. ⁴ Mineral premix contained the following ingredients (g/kg mix): MgSO₄·7H₂O, 80.0; NaH₂PO₄·2H₂O, 370.0; KCl, 130.0; ferric citrate, 40.0; ZnSO₄·7H₂O, 20.0; Ca-lactate, 356.5; CuCl, 0.2; AlCl₃·6H₂O, 0.15; KI, 0.15; Na₂Se₂O₃, 0.01; MnSO₄·H₂O, 2.0; CoCl₂·6H₂O, 1.0. ⁵ Vitamin premix contained the following amounts, which were diluted in cellulose (g/kg mix): L-ascorbic acid, 121.2; DL-α-tocopheryl acetate, 18.8; thiamin hydrochloride, 2.7; riboflavin, 9.1; pyridoxine hydrochloride, 1.8; niacin, 36.4; Ca-D-pantothenate, 12.7; myo-inositol, 181.8; D-biotin, 0.27; folic acid, 0.68; p-aminobenzoic acid, 18.2; menadione, 1.8; retinyl acetate, 0.73; cholecalciferol, 0.003; cyanocobalamin, 0.003.

). The control (Con) diet was prepared to contain 55% FM and 12% fermented soybean meal as the main protein sources, complemented by 21.5% wheat flour for the carbohydrate source and 4.5% each of fish and soybean oils as the lipid sources. In the Con diet, 10% of the FM was substituted with CPC based on our earlier study [13], in which FM up to 10% could be replaced with CPC without significantly impairing the WG and FI of rockfish. Subsequently, graded levels of JMM at 0%, 20%, 40%, and 60% were incorporated instead of FM in the Con diet, named as the CPCJ0, CPCJ20, CPCJ40, and CPCJ60 diets, respectively.

The experimental feeds were prepared to satisfy the protein and lipid needs for supporting rockfish growth [32,38]. Using an SMC-32 pellet extruder (SL Company, Incheon, Republic of Korea), the experimental feed ingredients were processed into pellets after being thoroughly mixed with water at a 3:1 ratio. The experimental feed batches underwent drying at 40 °C for 24 h using an SI-2400 electronic drying apparatus (SIN IL Drying Machine Co. Ltd., Daegu, Republic of Korea) and were subsequently preserved in a freezer at −20 °C until further use.

2.2. Rearing Conditions of the Feeding Experiment

Healthy juvenile rockfish were procured from a commercial hatchery and acclimated for 10 days before initiating the feeding trial. A commercially available extruded feed (50% CP and 13% CL) was provided to the rockfish twice daily throughout this period. A total of 450 juvenile rockfish, with an average weight of 11.3 g, were randomly allocated into 15 50 L plastic tanks (30 juveniles/tank). Throughout the experimental period, a 1:1 blend of underground seawater and sand-filtered seawater was continuously provided to all the fish tanks at a flow rate of 4.2 L/min, ensuring adequate aeration. The AZ-8603 digital multi-meter (AZ Instrument, Taichung, Taiwan) was employed to assess water quality on a daily basis. The recorded water temperature ranged between 17.2 and 23.1 °C (20.7 ± 1.52 °C; mean ± SD), dissolved oxygen ranged between 7.3 and 7.8 mg/L (7.5 ± 0.12 mg/L), salinity ranged between 30.8 and 32.5 g/L (30.5 ± 0.41 g/L), and pH ranged between 7.4 and 7.7 (7.5 ± 0.07). Rockfish in triplicate groups were carefully hand-fed the experimental diets to achieve visible satiation twice daily (08:30 and 17:30) throughout the 8-week feeding study. To avoid deterioration in water quality, siphon-cleaning was performed daily, and dead fish were promptly eliminated from the tanks upon detection.

2.3. Sampling Procedures and Evaluation of Growth Performance

The live rockfish underwent a 24 h fasting period following the feeding trial, after which they were anesthetized with 100 mg/L tricaine methanesulfonate (MS-222). The anesthetized fish from each tank were subsequently counted and weighed collectively to ascertain both survival and WG. Ten rockfish were randomly selected, and their individual total lengths and weights were measured to assess the condition factor (K). The viscerosomatic index (VSI) and hepatosomatic index (HSI) were determined by extracting, isolating, and weighing the visceral organs and liver of the ten dissected fish. The parameters, including specific growth rate (SGR), feed efficiency (FE), protein efficiency ratio (PER), protein retention (PR), condition factor (K), VSI, and HSI, were determined according to the same methods and formulas detailed in our earlier study [13].

2.4. Assessment of Biochemical Composition of the Samples

The whole-body biochemical composition of the rockfish was assessed by sampling and homogenizing 10 fish at the onset of the feeding trial and all the remaining (≥10) rockfish from individual tanks upon the completion of the feeding trial. Following the AOAC standard methods [39], analyses were conducted to measure moisture, CP, CL, and ash content in the formulated diets and whole-body rockfish. A Hitachi L-8800 Auto-analyzer (Tokyo, Japan) was used to determine the AA profiles of the feed ingredients (FM, CPC, and JMM) and the experimental diets used in this study, as well as whole-body rockfish. These analyses were performed in our laboratory, and no database values were used. Prior to analysis, the samples underwent hydrolysis in 6N HCl at 110 °C for 24 h, followed by ion-exchange chromatography. The FA profiles of the formulated diets and whole-body rockfish were assessed by extracting the FAs using a 2:1 (v/v) chloroform and methanol mixture, based on the protocol of Folch et al. [40], and then converting them to methyl esters through transesterification with 14% BF₃-MeOH (Sigma, St. Louis, MO, USA). The same techniques and protocols outlined in Lee et al. [37] were employed to determine the AA and FA profiles of the formulated diets and whole-body rockfish.

2.5. Assessment of Blood Chemistry of Rockfish

At the termination of the experimental trial, blood collection from the caudal vein of six rockfish per tank was performed using both heparinized and non-heparinized syringes (three for each) to assess plasma and serum biochemical parameters. Plasma and serum were separated by centrifuging the blood samples at 2716× g for 10 min at 4 °C and subsequently placed in a deep freezer at −70 °C for later analysis. An automatic chemistry system (Fuji Dri-Chem NX500i, Fujifilm, Tokyo, Japan) was employed to measure various blood plasma parameters, including aspartate aminotransferase (AST), alanine aminotransferase (ALT), alkaline phosphatase (ALP), total bilirubin (T-BIL), total cholesterol (T-CHO), triglyceride (TG), total protein and albumin (ALB). To evaluate immune-related parameters, serum was used to measure the activity of lysozyme and superoxide dismutase (SOD). The serum SOD was assessed using a commercially available ELISA kit (Cat. No. MBS705758; MyBioSource, Inc., San Diego, CA, USA) in accordance with the supplier’s recommended procedures. A turbidimetric assay to evaluate lysozyme activity was conducted following the standard procedures and methods of Ellis (1990) [41].

2.6. Analysis of the Economic Parameters

The economic analysis of the study was determined using the following formulas outlined in Martínez-Llorens et al. (2007) [42]’s study. The economic parameters were determined as follows: economic conversion ratio (ECR, USD/kg fish) = feed consumption (kg/fish)/weight gain of fish (kg/fish) × diet price (USD/kg), and economic profit index (EPI, USD/fish) = [final weight (kg/fish) × selling price of fish (USD/kg)] − [feed consumption (kg/fish) × diet price (USD/kg)]. Taking into account the Korean market prices during the fourth quarter (4Q) of 2025 from Daehan Feed Co. (Incheon, Republic of Korea), the costs (USD/kg) of the individual feed ingredients were as follows: FM (USD 2.02), CPC (USD 1.20), JMM (USD 2.37), fermented soybean meal (USD 0.66), wheat flour (USD 0.52), fish oil (USD 2.59), soybean oil (USD 1.68), vitamin mix (USD 7.76), mineral mix (USD 6.24), and choline (USD 1.22). The selling price of rockfish was estimated at USD 9.53/kg. The feed ingredients and fish prices were calculated based on an exchange rate of USD 1 = 1304 KRW.

2.7. Statistical Analysis

Following analyses of normality (Shapiro–Wilk) and homogeneity (Levene’s test), a one-way ANOVA and Tukey’s HSD multiple range test were employed to detect differences in group means at a p < 0.05 significance level. Preceding the statistical analysis, all the percentage values were subjected to arcsine conversion. An evaluation using the orthogonal polynomial contrast analysis was performed to ascertain if the effects on WG, SGR, and feed consumption (FC) of rockfish versus JMM incorporation levels in the diets replacing 10% of the FM with CPC were linear, quadratic, and cubic. Further regression analysis was applied to statistically significant data to fit the most appropriate model. IBM SPSS Statistics version 25.0 (IBM Corp., Armonk, NY, USA) was used to carry out all the statistical analyses. Principal component analysis (PCA) was performed using OriginPro 2025 (OriginLab Corporation, Northampton, MA, USA), and the resulting PCA biplot and variable contribution plot were generated to visualize the relationships among the experimental diets and the contributions of the measured variables.

3. Results

3.1. AA and FA Profiles of the Experimental Diets

Leucine and phenylalanine, among the EAAs, as well as all non-essential AAs (NEAAs) except aspartic acid and glycine, were present at higher levels in CPC than in FM (

Table 2. AA profiles (% of the diet) of the feed ingredients and experimental diets.

	Ingredients				Experimental Diets
	FM	CPC	JMM	Requirement	Con	CPCJ0	CPCJ20	CPCJ40	CPCJ60
Essential amino acids (EAAs, %)
Arginine	3.87	1.92	3.90		2.97	2.68	2.71	2.74	2.77
Histidine	1.89	1.26	2.83		1.41	1.34	1.43	1.51	1.58
Isoleucine	2.89	2.55	2.76		2.19	1.90	1.87	1.95	2.00
Leucine	5.03	10.89	4.83		3.87	4.15	4.16	4.20	4.24
Lysine	5.36	1.00	5.31	2.99 1	3.73	3.29	3.23	3.33	3.44
Phenylalanine	2.64	4.02	2.60		2.14	2.27	2.28	2.30	2.33
Threonine	2.95	2.15	2.88		2.25	2.16	2.13	2.16	2.22
Tryptophan	0.69	0.32	0.57		0.27	0.18	0.20	0.22	0.24
Valine	3.34	2.76	3.22		2.46	2.36	2.35	2.41	2.44
∑EAA 2	28.66	26.87	28.90		21.29	20.33	20.36	20.82	21.26
Non-essential amino acids (NEAAs, %)
Alanine	4.16	5.67	4.29		2.99	3.09	3.11	3.17	3.23
Aspartic acid	6.07	3.74	5.91		4.75	4.38	4.41	4.51	4.52
Glutamic acid	8.34	13.80	8.34		7.28	7.38	7.38	7.41	7.45
Glycine	3.76	1.68	4.29		2.78	2.51	2.64	2.69	2.78
Proline	2.74	6.47	2.98		2.49	2.58	2.56	2.60	2.63
Serine	2.58	3.26	2.59		2.12	2.18	2.18	2.23	2.26
Tyrosine	1.82	2.84	1.71		1.33	1.49	1.44	1.47	1.49
∑NEAA 3	29.47	37.46	30.11		23.74	23.61	23.72	24.08	24.36

¹ Data were obtained from Yan et al. [43]’s study. ² ∑EAAs: Total essential amino acids. ³ ∑NEAAs: Total non-essential amino acids.

). JMM was richer in arginine and histidine among the EAAs than FM, and all NEAAs, except aspartic acid and tyrosine, were present at relatively higher levels than in FM. Among EAAs, arginine, histidine, leucine, phenylalanine, and tryptophan increased with increasing inclusion levels of JMM in the diets, replacing 10% of FM with CPC. Notably, histidine content increased from 1.34% to 1.58% with higher JMM inclusion levels in diets replacing 10% of FM with CPC

Total saturated FA (∑SFA), monounsaturated FA (∑MUFA), and n-3 highly unsaturated FA (∑n-3 HUFA), including eicosapentaenoic acid (EPA, C20:5n-3) and docosahexaenoic acid (DHA, C22:6n-3) in CPC were lower than in FM, whereas oleic acid (C18:1n-9) and linoleic acid (C18:2n-6) were higher (

Table 3. FA profiles (% of total fatty acids) of the feed ingredients and experimental diets.

	Ingredients			Experimental Diets
	FM	CPC	JMM	Con	CPCJ0	CPCJ20	CPCJ40	CPCJ60
C12:0	0.08	0.00	0.06	0.03	0.02	0.03	0.03	0.03
C14:0	4.20	0.00	4.29	1.91	1.60	1.70	1.78	1.84
C16:0	22.22	14.47	19.97	15.03	14.85	14.43	14.34	14.20
C18:0	8.05	2.26	7.58	4.96	4.81	4.78	4.70	4.66
C20:0	0.10	0.14	0.10	0.20	0.25	0.22	0.22	0.21
C22:0	0.30	0.00	0.16	0.36	0.33	0.33	0.31	0.30
C24:0	0.68	0.00	0.50	0.56	0.50	0.49	0.48	0.46
∑SFAs 1	35.63	16.87	32.66	23.16	22.43	22.06	21.94	21.76
C14:1n-5	0.23	0.00	0.15	0.09	0.09	0.08	0.07	0.07
C15:1n-5	0.15	0.00	0.34	0.11	0.03	0.10	0.10	0.12
C16:1n-7	5.47	0.17	6.50	3.04	2.93	3.04	3.10	3.16
C17:1n-7	0.78	0.00	1.12	0.42	0.38	0.40	0.43	0.45
C18:1n-9	23.30	25.79	23.21	31.89	32.20	32.13	32.07	32.01
C20:1n-9	1.01	0.26	1.54	1.15	0.87	0.98	1.04	1.18
C22:1n-9	0.19	0.49	0.16	0.41	0.47	0.46	0.44	0.43
C24:1n-9	2.69	0.00	4.00	1.20	0.89	0.99	1.15	1.25
∑MUFAs 2	33.82	26.71	37.02	38.31	37.86	38.18	38.40	38.67
C18:2n-6	1.89	53.56	1.33	23.05	25.90	25.48	24.99	24.68
C18:3n-3	0.70	2.00	0.51	3.47	3.61	3.57	3.50	3.48
C18:3n-6	0.30	0.52	0.16	0.58	0.63	0.60	0.57	0.55
C20:2n-6	0.07	0.00	0.24	0.11	0.07	0.11	0.12	0.14
C20.3n-3	0.17	0.00	0.09	0.06	0.01	0.02	0.02	0.03
C20.3n-6	0.08	0.00	0.00	0.03	0.01	0.01	0.02	0.02
C20:4n-6	2.44	0.00	1.74	0.98	0.87	0.83	0.80	0.77
C20:5n-3	7.06	0.00	10.98	2.79	2.61	2.91	3.25	3.37
C22:2n-6	0.60	0.18	0.70	0.37	0.32	0.34	0.37	0.39
C22:6n-3	14.71	0.16	12.18	4.68	4.45	4.39	4.31	4.28
∑n-3 HUFAs 3	21.94	0.16	23.25	7.53	7.07	7.32	7.58	7.68
Unknown	2.53	0.00	2.39	2.41	1.23	1.50	1.71	1.86

¹ ∑SFAs: Total saturated fatty acids. ² ∑MUFAs: Total monounsaturated fatty acids. ³ ∑n-3 HUFAs: Total n-3 highly unsaturated fatty acids.

). The ∑MUFA, EPA, and ∑n-3 HUFA in JMM were relatively higher than those in FM, whereas ∑SFA and DHA were lower. The ∑MUFA, EPA, and ∑n-3 HUFA tended to increase with increasing inclusion levels of JMM in the diets, substituting 10% of FM with CPC, while ∑SFA and DHA tended to decrease.

3.2. Performance, Feed Utilization and Biological Measurements of Rockfish

The survival rate of rockfish ranged from 91.1% to 96.7%, with no significant difference (p > 0.5) among the dietary treatments (

Table 4. Survival (%), feed efficiency (FE), protein efficiency ratio (PER), protein retention (PR), condition factor (K), viscerosomatic index (VSI) and hepatosomatic index (HSI) of rockfish fed the experimental diets for 8 weeks.

Experimental Diets	Initial Weight (g/Fish)	Final Weight (g/Fish)	Survival (%)	FE 1	PER 2	PR 3 (%)	K 4 (g/cm3)	VSI 5 (%)	HSI 6 (%)
Con	11.3 ± 0.08	30.3 ± 0.46	91.1 ± 2.94	0.85 ± 0.076	1.79 ± 0.117	30.35 ± 1.819	1.64 ± 0.056	10.36 ± 0.226	3.22 ± 0.078
CPCJ0	11.2 ± 0.02	29.3 ± 0.32	90.0 ± 3.33	0.83 ± 0.024	1.79 ± 0.003	29.30 ± 0.339	1.59 ± 0.078	9.90 ± 0.078	3.20 ± 0.243
CPCJ20	11.3 ± 0.04	30.0 ± 0.34	94.4 ± 1.11	0.83 ± 0.001	1.72 ± 0.013	28.91 ± 0.742	1.58 ± 0.031	10.48 ± 0.421	3.19 ± 0.122
CPCJ40	11.4 ± 0.15	31.4 ± 0.41	94.4 ± 2.94	0.85 ± 0.018	1.76 ± 0.069	29.35 ± 0.919	1.59 ± 0.036	10.30 ± 0.276	3.15 ± 0.049
CPCJ60	11.2 ± 0.02	32.0 ± 0.04	96.7 ± 3.33	0.85 ± 0.011	1.73 ± 0.046	28.88 ± 0.618	1.63 ± 0.041	10.64 ± 0.195	3.05 ± 0.027
p-value			p > 0.5	p > 0.9	p > 0.9	p > 0.8	p > 0.8	p > 0.4	p > 0.8
Orthogonal polynomial contrast
Linear				0.3548	0.5650	0.7985	0.5494	0.1313	0.4185
Quadratic				0.9839	0.5957	0.9542	0.5783	0.6731	0.7441
Cubic				0.5463	0.4215	0.5856	0.9177	0.3245	0.9333

Values (means of triplicate ± SE) in the same column sharing the same superscript letter are not significantly different (p > 0.05). ¹ Feed efficiency (FE) = weight gain of fish/feed consumption. ² Protein efficiency ratio (PER) = weight gain of fish/protein consumption. ³ Protein retention (PR, %) = protein gain of fish × 100/protein consumption. ⁴ Condition factor (K, g/cm³) = fish weight (g) × 100/total length of fish (cm)³. ⁵ Viscerosomatic index (VSI, %) = viscera weight × 100/fish weight. ⁶ Hepatosomatic index (HSI, %) = liver weight × 100/fish weight.

). Rockfish fed the CPCJ60 diet (20.8 ± 0.02 g and 1.87 ± 0.002%/day) exhibited significantly higher WG and SGR (p < 0.002 and p < 0.003, respectively) than those fed the Con (19.0 ± 0.50 g and 1.76 ± 0.035%/day), CPCJ0 (18.1 ± 0.30 g and 1.71 ± 0.016%/day), and CPCJ20 (18.7 ± 0.36 g and 1.75 ± 0.024%/day) diets, but were alike to rockfish fed the CPCJ40 diet (20.0 ± 0.30 g and 1.81 ± 0.012%/day) (Figure 1 and Figure 2). As the level of JMM incorporated into the diets replacing 10% of FM with CPC increased, orthogonal polynomial contrast analysis revealed significant linear relationships for both WG and SGR (p = 0.0016 for both parameters). The regression analysis revealed that linear models best described the relationship between the level of JMM included in the diets replacing 10% of the FM with CPC and both WG (Y = 0.9367X + 17.0500, p < 0.0001, adjusted R² = 0.8468) and SGR (Y = 0.0005X + 0.0165, p < 0.0001, adjusted R² = 0.8580) in rockfish.

The FC of rockfish fed the CPCJ60 diet (24.0 ± 0.63 g) was significantly higher (p < 0.03) than that of rockfish fed the CPCJ0 diet (20.1 ± 0.29 g), but comparable to those fed the Con (21.1 ± 0.78 g), CPCJ20 (21.7 ± 0.55 g), and CPCJ40 (22.7 ± 1.19 g) diets (Figure 3). According to the orthogonal polynomial contrast analysis, a significant linear relationship (p = 0.0045) was observed between the FC of rockfish and increasing levels of JMM incorporation in diets in which 10% of FM was replaced with CPC. A linear regression model that best fits the JMM incorporation levels in diets replacing 10% FM with CPC and FC (Y= 1.2537X + 19.0450, p < 0.002, adjusted R² = 0.5965) was identified. However, FE, PER, and PR of rockfish varied from 0.83 to 0.85, 1.72 to 1.79, and 28.88% to 30.35%, respectively, but did not differ among diets. The biological indices of K, VSI, and HSI did not differ among dietary treatments.

The biological indices of K, VSI, and HSI did not differ between the treatments.

3.3. Biochemical Composition of the Whole Body of Rockfish

The dietary treatments did not influence whole-body proximate composition (

Table 5. Proximate composition (% of wet weight) of rockfish fed the experimental diets for 8 weeks.

Experimental Diets	Moisture	Crude Protein	Crude Lipid	Ash
Con	70.36 ± 0.152	16.72 ± 0.063	8.00 ± 0.106	3.96 ± 0.091
CPCJ0	70.30 ± 0.292	16.35 ± 0.133	8.13 ± 0.069	3.98 ± 0.129
CPCJ20	70.28 ± 0.188	16.62 ± 0.190	8.09 ± 0.181	4.06 ± 0.049
CPCJ40	70.44 ± 0.269	16.57 ± 0.242	8.02 ± 0.269	4.03 ± 0.097
CPCJ60	70.60 ± 0.287	16.54 ± 0.064	8.00 ± 0.160	3.87 ± 0.098
p-value	p > 0.8	p > 0.5	p > 0.9	p > 0.7
Orthogonal polynomial contrast
Linear	0.3908	0.5131	0.5991	0.4571
Quadratic	0.7448	0.4110	0.9649	0.2755
Cubic	0.8752	0.6775	0.9217	0.9355

Values are presented as means of triplicate ± SE.

), amino acid (

Table 6. AA profiles (% of wet weight) of the whole body of rockfish fed the experimental diets for 8 weeks.

	Experimental Diets						Orthogonal Polynomial Contrast
	Con	CPCJ0	CPCJ20	CPCJ40	CPCJ60	p-Value	Linear	Quadratic	Cubic
EAAs (%)
Arginine	1.00 ± 0.025	0.98 ± 0.020	1.00 ± 0.026	1.01 ± 0.023	1.03 ± 0.029	p > 0.6	0.1685	0.8996	0.9101
Histidine	0.35 ± 0.025	0.34 ± 0.015	0.35 ± 0.017	0.35 ± 0.015	0.36 ± 0.020	p > 0.9	0.6227	0.8988	0.8688
Isoleucine	0.69 ± 0.055	0.61 ± 0.040	0.60 ± 0.052	0.59 ± 0.038	0.57 ± 0.049	p > 0.5	0.5689	0.9722	0.9876
Leucine	1.02 ± 0.028	1.11 ± 0.035	1.10 ± 0.046	1.12 ± 0.013	1.14 ± 0.020	p > 0.1	0.4042	0.6447	0.8001
Lysine	0.75 ± 0.050	0.70 ± 0.058	0.68 ± 0.058	0.65 ± 0.050	0.61 ± 0.066	p > 0.5	0.2815	0.8507	0.9904
Phenylalanine	0.58 ± 0.025	0.65 ± 0.020	0.64 ± 0.032	0.63 ± 0.018	0.61 ± 0.020	p > 0.3	0.1774	0.8381	0.9757
Threonine	0.70 ± 0.028	0.69 ± 0.015	0.68 ± 0.018	0.65 ± 0.018	0.68 ± 0.020	p > 0.5	0.4852	0.1839	0.3578
Tryptophan	0.09 ± 0.015	0.08 ± 0.009	0.07 ± 0.021	0.06± 0.013	0.05 ± 0.009	p > 0.3	0.1023	0.9091	0.7988
Valine	0.69 ± 0.020	0.68 ± 0.020	0.66 ± 0.023	0.65 ± 0.020	0.64 ± 0.017	p > 0.4	0.1759	0.7599	0.8910
NEAAs (%)
Alanine	1.07 ± 0.023	1.08 ± 0.035	1.10 ± 0.029	1.12 ± 0.033	1.13 ± 0.029	p > 0.5	0.2903	0.9218	0.9881
Aspartic acid	1.25 ± 0.075	1.23 ± 0.092	1.22 ± 0.101	1.18 ± 0.075	1.15 ± 0.087	p > 0.9	0.5199	0.9300	0.9436
Glutamic acid	2.05 ± 0.025	2.13 ± 0.017	2.12 ± 0.015	2.11 ± 0.018	2.10 ± 0.020	p > 0.1	0.3023	0.9295	0.9684
Glycine	1.25 ± 0.023	1.15 ± 0.058	1.18 ± 0.040	1.21 ± 0.028	1.24 ± 0.032	p > 0.3	0.1395	0.9383	0.9449
Proline	0.75 ± 0.023	0.77 ± 0.012	0.79 ± 0.020	0.79 ± 0.018	0.80 ± 0.017	p > 0.5	0.2889	0.9273	0.7148
Serine	0.69 ± 0.020	0.71 ± 0.017	0.72 ± 0.017	0.74 ± 0.023	0.76 ± 0.026	p > 0.2	0.1372	0.9418	0.8194
Tyrosine	0.42 ± 0.023	0.47 ± 0.023	0.45 ± 0.029	0.44 ± 0.020	0.43 ± 0.020	p > 0.5	0.2201	0.9465	0.9760

Values are presented as means of triplicate ± SE.

) or fatty acid (

Table 7. FA profiles (% of total fatty acids) of the whole body of rockfish fed the experimental diets for 8 weeks.

	Experimental Diets						Orthogonal Polynomial Contrast
	Con	CPCJ0	CPCJ20	CPCJ40	CPCJ60	p-Value	Linear	Quadratic	Cubic
C12:0	0.03 ± 0.006	0.02 ± 0.003	0.03 ± 0.003	0.03 ± 0.003	0.03 ± 0.003	p > 0.1	0.0585	0.0369	0.5328
C14:0	2.27 ± 0.075	2.17 ± 0.058	2.25 ± 0.061	2.25 ± 0.058	2.28 ± 0.058	p > 0.7	0.2469	0.6605	0.65853
C16:0	14.95 ± 0.326	14.53 ± 0.170	14.71 ± 0.323	14.62 ± 0.329	14.58 ± 0.344	p > 0.8	0.9635	0.7114	0.8230
C18:0	4.37 ± 0.044	4.37 ± 0.136	4.34 ± 0.136	4.30 ± 0.133	4.21 ± 0.139	p > 0.8	0.4107	0.8491	0.9321
C20:0	0.16 ± 0.015	0.18 ± 0.015	0.17 ± 0.015	0.17 ± 0.012	0.16 ± 0.009	p > 0.9	0.4634	0.8979	0.9542
C22:0	0.42 ± 0.009	0.40 ± 0.009	0.40 ± 0.015	0.40 ± 0.012	0.38 ± 0.012	p > 0.1	0.2016	0.4208	0.8066
C24:0	0.52 ± 0.015	0.51 ± 0.009	0.49 ± 0.017	0.49 ± 0.009	0.47 ± 0.015	p > 0.1	0.0652	0.7086	0.6174
∑SFAs 1	22.73 ± 0.250	22.18 ± 0.285	22.39 ± 0.268	22.26 ± 0.146	22.11 ± 0.231	p > 0.4	0.7581	0.4643	0.7719
C14:1n-5	0.10 ± 0.006	0.09 ± 0.015	0.08 ± 0.012	0.07 ± 0.009	0.06 ± 0.012	p > 0.7	0.1387	0.7845	0.9025
C15:1n-5	0.07 ± 0.012	0.06 ± 0.017	0.06 ± 0.017	0.05 ± 0.020	0.06 ± 0.017	p > 0.9	0.9364	0.8585	0.8111
C16:1n-7	4.27 ± 0.015	4.25 ± 0.058	4.28 ± 0.024	4.29 ± 0.052	4.29 ± 0.040	p > 0.9	0.5606	0.7497	0.8863
C17:1n-7	0.60 ± 0.009	0.55 ± 0.017	0.56 ± 0.015	0.57 ± 0.012	0.58 ± 0.017	p > 0.1	0.1972	0.9163	0.8879
C18:1n-9	31.87 ± 0.026	32.31 ± 0.173	32.27 ± 0.188	32.22 ± 0.050	32.18 ± 0.098	p > 0.1	0.4932	0.9815	0.9917
C20:1n-9	1.80 ± 0.130	1.63 ± 0.136	1.70 ± 0.139	1.75 ± 0.095	1.82 ± 0.133	p > 0.8	0.3086	0.9898	0.9773
C22:1n-9	0.25 ± 0.006	0.28 ± 0.009	0.26 ± 0.015	0.26 ± 0.015	0.25 ± 0.015	p > 0.5	0.2742	0.9036	0.7058
C24:1n-9	1.41 ± 0.147	1.19 ± 0.127	1.26 ± 0.130	1.35 ± 0.170	1.42 ± 0.141	p > 0.7	0.2636	0.9730	0.9478
∑MUFAs 2	40.38 ± 0.245	40.36 ± 0.026	40.48 ± 0.248	40.58 ± 0.062	40.66 ± 0.069	p > 0.5	0.1331	0.8749	0.9870
C18:2n-6	18.27 ± 0.198	20.39 ± 0.181	20.12 ± 1.207	19.66 ± 1.005	19.61 ± 1.181	p > 0.7	0.6410	0.9946	0.9831
C18:3n-3	2.75 ± 0.023	2.89 ± 0.055	2.84 ± 0.032	2.79 ± 0.032	2.74 ± 0.058	p > 0.1	0.0373	0.9718	0.9874
C18:3n-6	0.51 ± 0.017	0.57 ± 0.032	0.55 ±0.029	0.52 ± 0.029	0.49 ± 0.012	p > 0.2	0.0511	0.8089	0.8709
C20:2n-6	0.20 ± 0.009	0.18 ± 0.017	0.19 ± 0.020	0.20 ± 0.009	0.20 ± 0.017	p > 0.7	0.3292	0.7696	0.6949
C20:3n-3	0.04 ± 0.009	0.01 ± 0.003	0.02 ± 0.003	0.02 ± 0.003	0.03 ± 0.006	p > 0.6	0.0560	0.6938	0.0795
C20:3n-6	0.03 ± 0.012	0.03 ± 0.012	0.03 ± 0.003	0.04 ± 0.003	0.04 ± 0.003	p > 0.9	0.4423	0.8028	0.9705
C20:4n-6	1.45 ± 0.015	1.43 ± 0.020	1.41 ± 0.023	1.40 ± 0.023	1.37 ± 0.017	p > 0.1	0.0926	0.7599	0.7844
C20:5n-3	4.00 ± 0.315	3.77 ± 0.309	4.09 ± 0.315	4.30 ± 0.315	4.46 ± 0.320	p > 0.5	0.1434	0.8097	0.9725
C22:2n-6	0.40 ± 0.015	0.37 ± 0.015	0.41 ± 0.023	0.42 ± 0.032	0.45 ± 0.026	p > 0.2	0.0567	0.8958	0.7263
C22:6n-3	5.01 ± 0.312	4.89 ± 0.320	4.77 ± 0.335	4.72 ± 0.346	4.65 ± 0.329	p > 0.9	0.6159	0.9342	0.9463
∑n-3 HUFAs 3	9.05 ± 0.006	8.68 ± 0.626	8.88 ± 0.025	9.04 ± 0.658	9.14 ± 0.644	p > 0.9	0.5510	0.9329	0.9948
Unknown	4.22 ± 1.130	2.92 ± 0.368	2.69 ± 1.265	3.08 ± 1.465	3.19 ± 0.288	p > 0.8	0.8281	0.8153	0.9353

Values are presented as means of triplicate ± SE. ¹ ∑SFAs: Total saturated fatty acids. ² ∑MUFAs: Total monounsaturated fatty acids. ³ ∑n-3 HUFAs: Total n-3 highly unsaturated fatty acids.

) profiles.

3.4. Blood Chemistry of Rockfish

The blood chemistry of the rockfish remained unaffected by the dietary treatments (

Table 8. Blood chemistry of rockfish fed the experimental diets for 8 weeks.

	Experimental Diets						Orthogonal Polynomial Contrast
	Con	CPCJ0	CPCJ20	CPCJ40	CPCJ60	p-Value	Linear	Quadratic	Cubic
Plasma parameters
AST (IU/L)	80.1 ± 0.93	80.2 ± 1.36	81.4 ± 3.90	80.7 ± 1.96	82.9 ± 2.46	p > 0.9	0.7635	0.9232	0.8361
ALT (IU/L)	26.1 ± 0.65	27.3 ± 1.61	32.7 ± 2.59	32.4 ± 1.25	28.6 ± 1.15	p > 0.5	0.8282	0.2196	0.9094
ALP (IU/L)	139.1 ± 1.78	165.2 ± 10.72	174.6 ± 14.28	149.9 ± 5.63	150.1 ± 7.60	p > 0.6	0.4609	0.8269	0.5322
T-BIL (mg/dL)	0.4 ± 0.02	0.5 ± 0.09	0.4 ± 0.02	0.5 ± 0.05	0.5 ± 0.07	p > 0.8	0.7115	0.5834	0.6231
T-CHO (mg/dL)	246.7 ± 1.55	244.0 ± 5.87	251.7 ± 1.59	251.3 ± 6.96	242.8 ± 7.86	p > 0.9	0.9426	0.5227	0.9967
TG (mg/dL)	442.4 ± 4.42	441.5 ± 2.41	439.0 ± 0.68	442.3 ± 4.23	444.3 ± 4.78	p > 0.9	0.7119	0.7495	0.8203
TP (g/dL)	3.9 ± 0.03	4.2 ± 0.09	4.3 ± 0.04	4.5 ± 0.13	4.1 ± 0.06	p > 0.2	0.2907	0.2206	0.4206
ALB (g/dL)	0.9 ± 0.01	0.9 ± 0.03	0.9 ± 0.03	0.9 ± 0.01	0.9 ± 0.01	p > 0.8	0.6257	0.7153	0.8699
Serum parameters
Lysozyme activity (U/mL)	301.1 ± 36.57	207.8 ± 48.39	230.6 ± 10.52	263.6 ± 25.20	232.1 ± 8.81	p > 0.8	0.6852	0.6425	0.7743
SOD (ng/mL)	4.0 ± 0.22	3.7 ± 0.19	4.1 ± 0.33	3.8 ± 0.20	4.0 ± 0.46	p > 0.9	0.8167	0.8963	0.6407

Values are presented as means of triplicate ± SE.

).

3.5. Economic Analysis of the Study

There were no significant differences in ECR among diets, although the EPI of the CPCJ60 was significantly higher (p < 0.03) than that of the CPCJ0 diet (

Table 9. Economic parameters [diet price, economic conversion ratio (ECR), and economic profit index (EPI)] of the experimental diets for rockfish.

	Experimental Diets						Orthogonal Polynomial Contrast
	Con	CPCJ0	CPCJ20	CPCJ40	CPCJ60	p-Value	Linear	Quadratic	Cubic
Diet price (USD/kg)	1.64	1.60	1.64	1.69	1.73
ECR a (USD/kg)	1.83 ± 0.113	1.79 ± 0.003	1.91 ± 0.016	1.92 ± 0.077	2.01 ± 0.054	p > 0.242	0.0576	0.7474	0.4852
EPI b (USD/kg)	0.254 ± 0.0057 ab	0.247 ± 0.0026 b	0.250 ± 0.0022 ab	0.261 ± 0.0021 ab	0.263 ± 0.0015 a	p < 0.032	0.0001	0.8842	0.2157

Values (mean of triplicate ± SE) in the same row sharing the same superscript letter are not significantly different (p > 0.05). ^a Economic conversion ratio (ECR, USD/kg fish) = feed consumption (kg/fish)/weight gain of fish (kg/fish) × diet price (USD/kg). ^b Economic profit index (EPI, USD/fish) = [final weight (kg/fish) × selling price of fish (USD/kg)] − [feed consumption (kg/fish) × diet price (USD/kg)].

). The orthogonal polynomial contrast analysis revealed that the EPI exhibited a significant linear relationship (p = 0.0001) with JMM inclusion levels in diets replacing 10% FM with CPC. The best-fitting model between JMM incorporation levels in diets replacing 10% of FM with CPC and EPI (Y = 0.005667X + 0.2412, p < 0.0001, adjusted R² = 0.7455) was identified.

3.6. Principal Component Analysis

The PCA biplot of growth performance, feed utilization, biological indices, and economic parameters showed distinct separation among the dietary treatments (Figure 4A). PC1 and PC2 explained 66.7% and 23.7% of the total variation, respectively, accounting for 90.4% cumulatively. The CPCJ60 diet was positioned on the positive side of PC1, closely associated with WG, SGR, FC, and the EPI, whereas the CPCJ0 and CPCJ20 diets were located on the negative side, and the CPCJ40 diet occupied an intermediate position. The variable contribution plot indicated that WG, SGR, FC, and the EPI contributed strongly and positively to Dim1, consistent with the positioning of the CPCJ60 diet (Figure 4B).

4. Discussion

To foster sustainable aquaculture and reduce dependence on increasingly scarce and expensive FM, numerous studies have investigated plant-derived protein alternatives [4], including CPC [6,12,13,14]. However, despite their potential, the use of plant protein as FM replacers at higher inclusion levels in fish diets has often been associated with reduced FI, which in turn leads to impaired growth performance in fish [9,12,36,44] Rockfish fed the Con and CPCJ0 diets showed no remarkable differences in WG, SGR, and FC in this study, confirming the results of our earlier findings [13] in which up to 10% of the FM could be replaced with CPC in a 55% FM-based diet without negatively lowering the WG and FC of rockfish. However, in the present study, linear improvements in WG, SGR, and FC of rockfish fed the series of CPCJ diets indicate that inclusion of JMM in diets replacing 10% of FM with CPC could effectively enhance growth performance. The observed enhanced WG and SGR of rockfish seemed to be a direct reflection of the improved FC. Considering the highest WG, SGR, and FC observed in the rockfish, the CPCJ60 diet emerges as the most desirable dietary treatment in the current study.

Similarly, Li and Cho [45] demonstrated that increasing the inclusion levels of JMM into low-FM diets, where 20% of FM was replaced with chicken by-product meal (CBM), led to a linear improvement in the growth performance of rockfish along with enhanced FC when juvenile rockfish were fed with a 55% FM-based diet or one of diets replacing 20% of FM with CBM with 0, 20, 40, 60, and 80% JMM inclusion. Islam et al. [46] also revealed that the inclusion of JMM in low-FM diets, in which 50% of FM was replaced with corn gluten meal (CGM), resulted in a linear enhancement in growth performance and FI in olive flounder. Furthermore, Kader et al. [47] reported that complete replacement of FM with dehulled soybean meal (DSM) supplemented with a blend of feed attractants, including 10% fish soluble (FS), 5% krill meal (KM), and 5% squid meal (SM), resulted in red sea bream (Pagrus major) exhibiting growth and FI comparable to those of fish fed a 60% FM-based diet.

Addressing reduced FC is one of the major challenges when substituting FM beyond the optimum level with dehulled soybean meal in fish diets [47]. The incorporation of feed stimulants and/or feed enhancers in low-FM diets has proven to be an effective strategy for improving FI and growth performance in fish, particularly when FM is partially replaced with plant-derived protein sources [47,48]. JMM has been reported as one of the most potent attractants and/or stimulants among 16 commonly used protein sources in fish feeds, effectively improving feed palatability and FC in rockfish [25]. The presence of feeding attractants, including AAs, IMP, and nucleotides, in jack mackerel tissue, as reported by Ikeda et al. [49], might have triggered increased FC in rockfish fed the CPCJ60 diets, leading to the greatest growth performance in this experiment. The findings of Ikeda et al. [50] indicated that among the constituents of the synthetic jack mackerel muscle extracts, AA, particularly histidine, was the most potent in stimulating feeding behavior in olive flounder. Similarly, another study performed by Kim and Cho [34] found that the dietary content of AAs, including alanine, glycine, and histidine, exhibited a correlation with the growth performance of rockfish. In this study, incorporating JMM in diets replacing 10% of FM with CPC resulted in elevated levels of AAs, including alanine, glycine, and notably histidine. These changes may have indirectly enhanced feed palatability and acceptance, thereby contributing to the observed increase in FI and growth performance in rockfish.

Beyond its role in improving palatability, JMM also provides balanced nutrition, highly digestible proteins, abundant EAAs, and bioactive compounds [25,35,50], which may have further supported protein synthesis and feed utilization efficiency. These nutritional attributes, together with enhanced FC, likely acted synergistically to contribute to the superior growth observed in fish fed the CPCJ60 diet.

No remarkable differences were observed in feed utilization (FE, PER, and PR) of rockfish in the present study. Similarly, Baek et al. [9] reported that feed utilization of olive flounder was not significantly altered by the inclusion of 12% JMM in diets substituting 25% and 50% of FM with diverse plant protein sources, including CGM, soy protein concentrate (SPC) and CPC. Furthermore, incorporating various levels of JMM in diets substituting 20% and 50% of FM with CBM and CGM had no significant effect on the feed utilization of rockfish [45] and olive flounder [46], respectively. However, in contrast to the present study, replacement of 25% and 50% of FM with various plant protein sources, including CGM, SPC, and CPC, in diets supplemented with 22% JMM affected feed utilization of rockfish [36]. According to Hancz [51], the inconsistency in results could be attributed to the use of various types and dosages of feed attractants and/or stimulants.

The deficiency of EAAs is commonly considered to be a major bottleneck when formulating low-FM feeds [52]. Since deficiencies in some EAAs can often hinder fish growth, it is crucial to carefully assess the AA profiles in low-FM diets for fish [33,53,54]. Lysine is commonly considered as one of the most limiting AAs in plant-derived protein sources [55]. As JMM was found to be rich in arginine and histidine among EAAs, increased JMM inclusion in diets substituting 10% of the FM with CPC resulted in increased arginine and histidine content. The lysine content (3.23–3.73% of the diet) in all the diets in the current study met the lysine requirement (2.99% of the diet) for rockfish [43]. Although sulfur amino acids, such as methionine and cysteine, were not quantified in the present study, this is acknowledged as a limitation. Methionine is generally a limiting EAA in low-FM diets; its evaluation is warranted in future needed in future study.

In the nutritional profiles of many marine fish, ∑n-3 HUFAs, especially EPAs and DHAs, are EFAs that are known to be critical for ensuring proper growth and survival [56,57,58]. EPAs and DHAs play a crucial role in numerous physiological processes in fish, such as preserving cellular membrane stability, modulating immune and inflammatory pathways, and aiding carbohydrate and lipid metabolic processes [59]. In general, marine fish exhibit a limited or non-existent ability to biosynthesize adequate levels of EPA, DHA, and arachidonic acid (ARA, 20:4n-6) to support their physiological functions, requiring them to obtain these essential FAs through dietary sources [59,60]. In this experiment, due to the very low level of ∑n-3 HUFAs in CPC, the CPCJ0 diet was found to have the lowest ∑n-3 HUFAs. This might be one of the reasons why those fish fed the CPC0 diet achieved the poorest growth among the rockfish. However, increasing the inclusion levels of JMM in diets substituting 10% of the FM with CPC appeared to increase the ∑MUFAs and ∑n-3 HUFAs, including EPAs, but decrease the ∑SFAs and DHAs.

The biological indices of fish are commonly used as reliable markers to determine the nutritional and physiological well-being of fish [61]. No marked changes in K, VSI, and HSI of rockfish were noticed in the present experiment. Similarly, the findings of our recent study [45] demonstrated that incorporating various levels of JMM into rockfish diets, replacing 20% of FM with CBM, had no significant effects on the biometric indices. Likewise, several studies have shown that substituting FM with various plant and animal protein sources in diets supplemented with JMM as feed attractants or stimulants did not significantly influence the biological indices of rockfish [36], red sea bream [62], and olive flounder [9,63].

The biochemical composition of the whole-body rockfish, including proximate composition and AA and FA profiles, was unaffected by the dietary treatments in the present study, suggesting that inclusion of JMM in the diets replacing 10% FM with CPC did not detrimentally alter the biochemical composition of the whole-body fish. Similarly, no alterations were observed in the whole-body proximate composition, AA profiles, and FA profiles of olive flounder when FM was replaced with various plant-derived protein sources supplemented with JMM as a feeding stimulant [9]. Likewise, Kim and Cho [36] found that the whole-body proximate composition and AA profiles in rockfish were unaffected when FM was substituted with diverse plant-based protein sources in diets supplemented with JMM. Contrary to these studies, the substitution of FM with various plant-derived protein sources in fish diets has been reported to markedly alter the whole-body proximate composition [6,8,64], and AA profiles [53,65] in several fish species, including red hybrid tilapia, olive flounder, Asian seabass, and pearl gentian grouper (Epinephelus lanceolatus ♂ × E. fuscoguttatus ♀). The discrepancies among these findings may be attributed to various factors, including species-specific differences, developmental stages of the fish, the source and quality of the feed ingredients used, and experimental conditions [33].

Plasma parameters are commonly used as standard biomarkers for assessing fish health and evaluating the physiological effects of dietary nutrients and their interactions on overall well-being [66,67,68]. No marked differences in the plasma parameters of rockfish were found among dietary treatments in this study, indicating that incorporating JMM into the diets, replacing 10% of FM with CPC, appears to have no detrimental effect on these parameters. This finding is consistent with the earlier studies [9,36,45,62,63], in which incorporation of JMM in diets replacing FM with various animal and plant-derived protein sources did not markedly alter the plasma chemistry of rockfish, red sea bream, and olive flounder. Contrary to the findings of this study, however, Kader et al. [69] found that plasma parameters of red sea bream, such as AST, ALT, total cholesterol, and total bilirubin, were markedly affected when 60% of FM was substituted with SPC in diets supplemented with 10% FS, KM, or SM. Further study on plasma parameters is needed to gain a better understanding of the possible changes that may arise when fish are supplied with low-FM diets.

The serum lysozyme activity and SOD are considered innate immune markers that provide reliable insights into the effects of dietary interventions on the immunological state of fish [70]. Lysozyme, a mucolytic enzyme, assists in safeguarding fish from harmful microorganisms [71], while SOD functions as an antioxidant enzyme, defending fish against oxidative substances that could potentially harm or damage their cells [72]. This study found no significant differences in lysozyme activity and SOD level in rockfish among dietary treatments, suggesting that incorporating JMM in diets replacing 10% FM with CPC did not impair the innate immunity of rockfish. Likewise, Jeong and Cho [63] unveiled that inclusion of JMM in low-FM diets, substituting 25% and 50% of FM with diverse animal-based protein sources, did not distinctively alter the plasma and serum parameters in olive flounder. Contrary to these results, Khosravi et al. [73] reported that incorporating shrimp, tilapia, or krill hydrolysate into low-FM diets had a marked effect on SOD and lysozyme activity in red sea bream.

In aquaculture, fish feed represents a major component of operational expenses, typically accounting for 50%–70% of total production costs [61,74]. Accordingly, a key objective of feeding strategies is to minimize feed costs while enhancing overall economic returns for fish farmers. From an economic standpoint, the EPI is widely recognized as an appropriate metric for evaluating feed profitability [42]. The CPCJ60 diet led to the highest EPI among all the experimental diets in the current study, which strongly supports the findings of superior WG and SGR in rockfish fed the CPCJ60 diet. Moreover, the CPCJ60 diet seems to be the most promising dietary approach for enabling farmers to achieve high economic returns in the context of practical aquaculture. It should be noted, however, that the economic outcomes presented here are influenced by fluctuations in the relative market prices of ingredients, such as FM, CPC, and JMM, which may vary by year and region.

The PCA revealed that the CPCJ60 diet is closely linked to improved growth performance, feed utilization, and economic returns in rockfish. The strong contributions of WG, SGR, FC, and the EPI to the principal components highlight the superiority of the CPCJ60 diet over others, and the multivariate findings further confirm that incorporating 60% JMM in diets replacing 10% of the FM with CPC represents the most effective feeding strategy for rockfish farming.

5. Conclusions

The inclusion of JMM was found to be effective in improving the FC of fish fed diets replacing 10% of the FM with CPC. Furthermore, the WG and SGR of the rockfish fed diets substituting 10% of the FM with CPC improved linearly with increased JMM inclusion levels. However, the feed utilization, proximate composition, AA and FA profiles, and blood chemistry of the rockfish were unaffected by the dietary treatments. The EPI tended to increase linearly with dietary inclusion levels of JMM. Conclusively, incorporating 60% of JMM in the rockfish diets replacing 10% of the FM with CPC was the most recommended strategy based on the WG, SGR, and FC of the rockfish, and this provided the highest EPI for rockfish farmers.