Evaluation of Apparent Nutrient Digestibility of Novel and Conventional Feed Ingredients in Sobaity Seabream (Sparidentex hasta) for Sustainable Aquaculture

Abstract

This study aimed to evaluate the apparent digestibility coefficients (ADCs) for nutrients and energy of seven conventional and alternative feed ingredients (poultry feather meal, fermented feather meal, mealworm meal, defatted black soldier fly, Chlorella, poultry by-product meal, and corn meal) when fed to Sobaity seabream (Sparidentex hasta), with the goal of identifying sustainable, digestible, and nutritionally viable ingredients for aquaculture feed formulations. A reference diet (RF) was formulated to meet the nutrient requirements of Sobaity seabream while test diets were prepared to contain 70% RF and 30% of the test ingredients. Sobaity seabream (200 ± 8.0 g) were fed the diets for seven days before fecal matter was collected by stripping. The whole length of the digestibility trial was 21 days. The ingredient apparent digestibility of dry matter (34.8–70.4%), crude protein (52.8–107.8%), crude lipid (67.7–112.9%), and energy (52.2–86.1%) were affected by test ingredients (p < 0.01). The dry matter digestibility of mealworm meal was the highest (70.4%) compared to other ingredients. Feather meal, Chlorella, and black soldier fly meal had significantly lower values of dry matter digestibility. Dry matter and crude protein were significantly more digestible in fermented feather meal than the feather meal without fermentation. The crude protein digestibility was significantly higher (107.8%) for mealworm meal. However, feather meal has shown a significantly lower value (52.8%) for crude protein digestibility compared to other ingredients. Energy digestibility showed a significant positive correlation with dry matter digestibility (r = 0.870). The energy digestibility of mealworm meal was significantly higher (86.1%, p < 0.05) than other ingredients. Feather meal had the lowest energy digestibility (52.2%) with no statistically significant difference from Chlorella, corn meal, and black soldier fly meal. This study indicates that mealworm meal is the most easily digestible protein source for Sobaity seabream and should be prioritized in their diets. Fermentation enhances the digestibility of feather meal and is recommended when using it. Ingredients with a lower digestibility, like feather meal, chlorella, and black soldier fly meal, should be used in moderation or undergo further processing to improve nutrient availability.

1. Introduction

Sparidentex hasta, commonly known as the Sobaity seabream, is a high-value marine species in aquaculture due to its fast growth rate, market demand, and adaptability to intensive farming systems [1]. The Sobaity seabream S. hasta, also known as silvery-black porgy or blue-finned seabream, is a native species in the Arabian gulf, the western Indian Ocean, and the coasts of India [2]. Its cultivation holds the potential to contribute significantly to sustainable aquaculture practices, a growing concern in the industry. Traditionally, fish meal has been a primary source of protein in aquafeeds, due to its high digestibility, balanced amino acid profile, and nutrient density [3]. However, the use of fish meal in aquaculture is becoming more challenging due to its increasing price and limited availability. To ensure the long-term viability of aquaculture, it is essential that we explore alternative protein sources that can significantly reduce fish meal [4] in the diets of species like S. hasta without compromising their growth performance, health, or product quality.

Several alternative protein sources show promise in aquafeeds. For example, mealworm meal is a promising protein source due to its balanced amino acid profile and digestibility, making it a valuable ingredient in aquafeeds. Chlorella, a microalgae, is high in protein and essential fatty acids, which support fish health and nutrient absorption [5]. Corn meal is widely used as a carbohydrate source, although its digestibility varies among fish species [6]. Fermented feather meal and feather meal provide a sustainable protein option due to their high keratin content, though their use requires processing to enhance digestibility [7]. Black soldier fly larvae, rich in protein and fats, are becoming popular in fish diets due to their favorable nutrient profile and digestibility [8]. Poultry by-product meal, another animal-derived protein source, has been reported to have high digestibility values across species [9].

Selecting alternative ingredients with a high protein digestibility is essential for ensuring optimal nutrient absorption and utilization in aquaculture species while minimizing waste. These alternative protein sources must not only meet the nutritional needs of cultured species but also exhibit favorable palatability and digestibility characteristics, ensuring they are well-accepted and efficiently utilized in aquafeeds. The high digestibility of alternative ingredients enables optimal nutrient absorption, which directly supports growth performance and helps maintain good health in aquaculture species.

The palatability of feed ingredients in aquaculture is a critical aspect that influences the feed intake, growth performance, and overall health of cultured species. The palatability of insect meals for fish remains a concern at higher dietary inclusion levels, with several studies noting a reduction in feed intake when black soldier fly meal is used to replace fish meal protein in species like gilthead seabream Sparus aurata [10], Nile tilapia Oreochromis niloticus [11], silver perch Perca maxima [12], Atlantic salmon Salmo salar [13], and rainbow trout Oncorhynchus mykiss [14]. Several factors influence the palatability of feed ingredients in aquaculture, including the type of species being cultured, the composition of the feed, feed ingredients, and the presence of attractants or feeding stimulants [15,16]. It has been shown that, in addition to palatability, the nutrient digestibility of an ingredient is among the most influential factors affecting the utility of an ingredient [17]. The digestibility coefficients of feed ingredients provide insight concerning the nutrient qualities of those ingredients and should enable more accurate formulation, allowing better ingredient substitutions in diets designed for target fish species. The nutrient digestibility can vary significantly based on the composition of the ingredients used in the diet. The digestibility of an ingredient reflects the capability of a certain species to utilize the nutrients of that ingredient, predicting its potential as feedstuff [18].

Several studies have reported the potential to advance the field of aquaculture nutrition by providing empirical data on nutrient digestibility, which is critical for optimizing feed formulations, improving feed conversion ratios, and enhancing the overall growth and health of cultured species [19]. Enhanced feed efficiency not only promotes better growth performance and health in farmed species but also reduces nutrient waste, thereby minimizing the environmental impact of aquaculture operations. Furthermore, the development of more sustainable and cost-effective feed formulations will contribute to the economic viability of aquaculture systems. A wide range of ingredients are now used in aquaculture feeds, which are being formulated based on a digestible nutrient and digestible energy basis [5,20,21,22], and the determination of nutrient digestibility is considered one of the initial steps for evaluating potential ingredients in diets for target fish species [23].

To advance the development of low fish meal diets with high levels of alternative proteins, it is essential that we evaluate their ability to supply essential nutrients. Hence, this study was conducted to evaluate the digestibility of various feed ingredients utilized in aquaculture, with the goal of improving feed efficiency and enhancing the nutritional quality of diets for this fish species. By assessing the digestibility of key nutrients, this study aims to provide a comprehensive understanding of nutrient utilization across different ingredients, leading to the sustainable culture of this fish.

2. Materials and Methods

2.1. Ingredient Sourcing and Diet Development

Fish meal, fish oil, wheat meal, wheat gluten meal, soybean meal, corn meal, vitamin and mineral premixes were obtained from commercial feed mills (ARASCO, Riyadh, Kingdom of Saudi Arabia, KSA) and retained for analysis and feed production for the reference diet. Poultry meal and steam-hydrolyzed feather meal were sourced locally from the Almarai Group in the Riyadh, Kingdom of Saudi Arabia, KSA. Defatted black soldier fly larvae meal and mealworm meal were obtained from Qingdao, Shandong, China. Broken-cell wall Chlorella was procured from Xi’an, China.

A diet-substitution formulation strategy was used as the basis for the design of the experimental diets. For this, a basal mash was formulated based on industrial diet specifications for Sobaity seabream and prepared to include approximately 55% protein, 7% fat, and 0.1% yttrium oxide. The dietary protein was set at 55% according to the previously published requirements for the different stages of this fish species [24]. The basal mash was prepared and thoroughly mixed, forming the basis for all experimental diets in this study. The test ingredients (feather meal, fermented feather meal, mealworm meal, broken cell wall Chlorella, poultry meal, corn meal, and black soldier fly meal) of study for each test diet were added at 30% inclusion, with a concomitant (70%) subsample of the basal mash used to make up 100% of the diet mix, respectively (

Table 1. Nutrient composition of different feed ingredients used to formulate the basal and test diets (all values are % as is basis as used unless otherwise indicated).

	Fish Meal	Wheat Meal	Chlorella	Wheat Gluten Meal	Soybean Meal	Feather Meal	Poultry By-Products Meal	Corn Meal	Fermented Feather Meal	Black Soldier Fly Meal	Mealworm Meal
Dry matter	93.6	90.1	93.9	93.9	91.1	96.5	96.9	87.3	92.1	92.6	98.0
Protein	64.0	14.4	58.4	77.5	45.6	87.0	68.4	8.6	84.5	51.7	58.8
Lipid	9.2	2.6	9.2	7.5	3.3	7.3	16.0	3.5	5.8	14.1	18.9
CHO *	2.0	71.4	25.6	8.1	37.0	0.7	0.1	73.9	0.9	15.3	16.1
Ash	18.4	1.7	0.7	0.8	5.2	1.5	12.4	1.3	0.8	11.4	4.2
Energy	19.0	16.7	21.8	22.6	18.4	23.5	22.3	16.2	22.4	20.3	23.9

* Carbohydrate (CHO) content determined based on dry matter minus protein, ash, and lipids, (CHO% = Dry matter − (crude protein + crude ash + crude lipid)).

). A reference diet was prepared without any further addition of any ingredient. Each of the diets was prepared by using Twin-screw Extruder Feed Mill (Coperion ZSK 27 MvPLUS, Stuttgart, Germany) at King Abdullah University of Science and Technology (KAUST). All twelve diets were processed through a 6 mm diameter die. The diets were stored at −20 °C until used to ensure stability and prevent spoilage. The proximate compositions of the different ingredients used in this study is given in Table 1. The reference diet and test diet formulation and their compositions are presented in

Table 2. Diet formulations of basal and test diets (all values are % dry matter as used unless otherwise indicated) to estimate apparent digestibility of different feed ingredients for Sobaity seabream Sparidentex hasta over 21 days trial.

	Reference Diet	Feather Meal	Fermented Feather Meal	Mealworm Meal	Chlorella Meal	Poultry By-Products Meal	Corn Meal	Black Soldier Fly Meal
Ingredients (g)	Diet 1	Diet 2	Diet 3	Diet 4	Diet 5	Diet 6	Diet 7	Diet 8
Fish meal	20	14	14	14	14	14	14	14
Fish oil	7.5	5.3	5.3	5.3	5.3	5.3	5.3	5.3
Wheat meal	16.4	11.4	11.4	11.4	11.4	11.4	11.4	11.4
Wheat gluten meal	30	21	21	21	21	21	21	21
Soybean meal	24.9	17.4	17.4	17.4	17.4	17.4	17.4	17.4
Feather meal	0	30	0	0	0	0	0	0
Fermented feather meal	0	0	30	0	0	0	0	0
Mealworm meal	0	0	0	30	0	0	0	0
Chlorella meal	0	0	0	0	30	0	0	0
Poultry meal	0	0	0	0	0	30	0	0
Corn meal	0	0	0	0	0	0	30	0
Black soldier fly meal	0	0	0	0	0	0	0	30
Vit and Min premix	1	0.7	0.7	0.7	0.7	0.7	0.7	0.7
Choline	0.1	0.07	0.07	0.07	0.07	0.07	0.07	0.07
Yttrium oxide	0.1	0.07	0.07	0.07	0.07	0.07	0.07	0.07
Total (g)	100	100	100	100	100	100	100	100

and

Table 3. Proximate compositions of basal and test diets (all values are in % as is basis unless otherwise indicated) to estimate apparent digestibility of different feed ingredients for Sobaity seabream Sparidentex hasta over 21 days trial.

	Reference Diet	Feather Meal	Fermented Feather Meal	Mealworm Meal	Chlorella Meal	Poultry By-Products Meal	Corn Meal	Black Soldier Fly Meal
%dry basis	Diet 1	Diet 2	Diet 3	Diet 4	Diet 5	Diet 6	Diet7	Diet 8
Dry Matter	97	96	94.3	98	97	95	98	95
Crude Protein	54.6	66	67.7	59.9	58.5	60.9	40.3	56.1
Lipids	7.4	10.2	9.9	9.5	9.2	11.4	6.2	8
Ash	6.1	6.2	4.7	5.3	5.7	8.4	4.9	8.7
Carbohydrates *	28.8	13.6	12	23.3	23.6	14.3	46.6	22.3
Gross Energy (kJ/g)	20.7	21.9	21.9	21.8	21.4	21.2	20	20.2

* Carbohydrate (CHO) content determined based on dry matter minus protein, ash, and lipids, (CHO% = Dry matter − (crude protein + ash + lipids)).

, respectively.

2.2. Fish Handling and Fecal Collection

Fish were obtained from the National Mariculture Center, Kingdom of Bahrain, and raised under commercial settings and held in holding tanks (2000 L), being fed a commercial diet (MarineFish; ARASCO, Al Kharj, Riyadh, Kingdom of Saudi Arabia, KSA), for several weeks. The flow-through tank system supplied filtered marine water (salinity ~42 PSU) with dissolved oxygen typically around 7.4 ± 0.3 mg/L (mean ± S.D.) at a flow rate of about 5 L/min to each of the tanks. To ensure that the fish were disease-free, the health status of the fish was determined by submitting fish samples to the Fish Health and Safety Laboratory of the Jeddah Fisheries Research Center, Jeddah, Kingdom of Saudi Arabia for pathogen and parasite analyses before starting the trial. Fish were tested using qPCR for viral nervous necrosis (VNN), red sea bream iridovirus (RSIV), Streptococcus iniae (SI), and Streptococcus agalactiae (SA), and no infections were recorded. The fish were maintained in indoor holding tanks until they reached the size needed for the digestibility trial. Each of the tanks (850 L) were stocked with 35 individuals of Sobaity bream of 200 ± 8.0 g and fish were fed the reference diet for 1 week to acclimatize under the same conditions.

Following acclimation (1 week) to the system, the fish were then allocated their dietary treatments. Treatments were randomly assigned amongst the tanks for the first replicate. Fish were re-randomized to start second and third replicates of all the treatments by repeating the experimental setup over time. The digestibility experiment was run over a 21-day period. All experimental work was undertaken at the Centre for Marine Oceanographic Research (CMOR) Laboratory (King Abdullah University of Science and Technology, Thuwal, KSA). Water quality parameters were measured daily during the feeding trials [25] and the following data observed: temperature varied from 26.2–28.0 °C, dissolved oxygen 4.8–6.2 mg L⁻¹. Each of the water quality parameters were analyzed using a ProDSS Multiparameter Digital Water Quality Meter (Yellow Springs, OH, USA).

Each tank of fish was hand-fed to satiation twice daily, at 08:00 and 11:00 h. After feeding, the uneaten feed was collected daily and weighed by sieving outflow water from the tank standpipe. Uneaten feed was air-dried and weighed to calculate the actual feed intake of fish to determine the palatability of the diet. The fish were allowed to acclimatize to their allocated diet for seven days before fecal collection commenced, consistent with earlier studies on similar marine species [22,26]. Feces was collected using a stripping technique based on Austreng [27] as adapted earlier [26]. Fish were netted from their respective tank, and placed in a smaller aerated tank containing AQUI-S (20 ppm) until they lost consciousness. The feces was then removed from the distal intestine by applying gentle abdominal pressure. The hands of the person stripping the fish were rinsed with water between each fish to ensure that the feces was not contaminated by urine or mucus. After removal of the feces from the fish, the fecal sample was inspected to ensure it was free from blood and mucus before being placed in a small plastic vial and stored in a freezer at −20 °C. Stripped feces was collected during 20:00 to 22:30 h over a single day for each block, with each fish only being stripped once before being returned to a pooled stock from which the fish were re-randomized for blocks 2 and 3. Fecal samples were kept frozen at −20 °C before being freeze-dried in preparation for analysis.

2.3. Chemical and Digestibility Analysis

All analytical work was undertaken using Corelab facilities of King Abdullah University of Science and Technology (Thuwal, Jeddah, KSA). Six subsamples of the feed ingredients, experimental diets, and three subsamples of fecal matter of each replicate to determine the proximate composition were analyzed using AOAC methods [28]. Diet, ingredients, and fecal samples were analyzed for dry matter, nitrogen, ash, lipid, and energy content. Experimental diets and fecal matter were also analyzed for yttrium. Dry matter was calculated by gravimetric analysis following oven drying at 105 °C for 24 h. Total yttrium concentrations were determined [29] after acid digestion using inductively coupled plasma atomic emission spectrophotometry (G8015 A, Agilent Technologies, Santa Clara, CA, USA). Protein levels were calculated from the determination of total nitrogen by an elemental-analyzer (Flash 2000, Thermo Fisher Scientific, Waltham, MA, USA), based on N × 6.25. Total lipids were determined gravimetrically following extraction of the lipids using the chloroform: methanol solubilization method [30]. Gross ash content was determined gravimetrically following loss of mass after combustion of a sample in a muffle furnace at 550 °C for 12 h (FNC-BX1200-6, Bioevopaek, Jinan, China). Gross energy was determined by adiabatic bomb calorimetry (ACM-6, Bioevopaek, Jinan, China). Total carbohydrates were calculated based on the dry matter content of a sample minus the protein, lipid, and ash. Diet (DADC) and ingredient (IADC) apparent digestibility coefficients were measured and calculated, respectively, according to the following formulae:

DADC_Nutr = 1 − (Y_diet × Nutr_feces)/(Y_feces × Nutr_diet)

where Y_diet and Y_feces represent the yttrium content of the diet and feces, respectively, and Nutr_diet and Nutr_feces represent the nutritional parameter of concern (dry matter, protein, lipid, or energy) content of the diet and feces, respectively. The digestibility values for each of the test ingredients in the test diets examined in this study were calculated according to the formulae:

$$IAD{C}_{ingredient}=\frac{\left(A{D}_{test}\times Nut{r}_{test}-\left(A{D}_{basal}\times Nut{r}_{basal }\times 0.7\right)\right)}{\left(0.3\times Nut{r}_{ingredient}\right)}$$

where Nutr × AD_ingredient is the digestibility of a given nutrient from the test ingredient included in the test diet at 30%. AD_test is the apparent digestibility of the test diet. AD_basal is the apparent digestibility of the basal diet, which makes up 70% of the test diet. Nutr_Ingredient, Nutr_test, and Nutr_basal are the level of the nutrient of interest in the ingredient, test diet, and basal diet, respectively. All raw material inclusion levels were adjusted to account for their dry matter content, and the potential impact of these adjustments on the actual ratio between the reference diet and the test ingredient was considered.

Ingredient digestibility greater than 100% were not corrected because we consider they are potentially indicative of interactive effects between the diet and test ingredient and should be stipulated as determined.

2.4. Statistical Analysis

Before the statistical analysis, all data were tested for normality and equality of variances by means of the Shapiro–Wilk and Levene’s test, respectively. Normally distributed data were analyzed using one-way ANOVA, followed by Fisher’s Least Significant Difference [31] test to compare significant differences between treatments. Data that failed the normality and equal variance tests were analyzed using the Kruskal–Wallis–H test and subsequently analyzed using Student–Newman–Keuls (NSK) test to compare significant differences (p < 0.05) between treatments. The software OriginPro 2020 (San Clemente, CA, USA) was used to employ all the statistical analyses.

3. Results

3.1. Diet Feed Intake Effects

No substantial variability in the feed intake (g/fish/day) among all the test diets was observed across the duration of the trial (

Table 4. Feed intake (palatability) assessment of basal (D1) and test diets (D2–D8) for Sobaity seabream Sparidentex hasta over 21 days trial.

Diets	Test Ingredients	Feed Intake (g/Fish/Day)
D1	Basal diet	3.2 ± 0.81 a
D2	Feather meal	3.3 ± 0.62 a
D3	Fermented feather meal	3.6 ± 0.69 a
D4	Mealworm meal	3.3 ± 0.72 a
D5	Chlorella mela	3.5 ± 0.61 a
D6	Poultry by-products meal	3.6 ± 0.24 a
D7	Corn meal	3.8 ± 0.93 a
D8	Black soldier fly meal	2.6 ± 0.58 a
	p value	p = 0.557

Values (means ± SEM, N = 3) within a column with a common superscript letter are not significantly different from the other dietary groups (p > 0.05).

). Although a lower (p > 0.05) feed intake was observed in fish fed diet D8, which included black soldier fly meal as the test ingredient. However, the other diets (D1–D7) containing different test ingredients did not show any significant differences compared to the D8 diet.

3.2. Ingredient Digestibility

The IADC of the dry matter, crude protein, lipid, and energy of the test ingredients is provided in

Table 5. The ingredient apparent digestibility coefficients (IADC) values for the tested ingredients (D2–D8) fed to Sobaity seabream Sparidentex hasta over 21 days trial.

Test Diets	Test Ingredients	Dry Matter (IADC%)	Protein (IADC%)	Lipid (IADC%)	Energy (IADC%)
D2	Feather meal	34.8 ± 0.9 c	52.8 ± 0.7 d	91.0 ± 0.13 b	52.2 ± 0.4 c
D3	Fermented feather meal	50.1 ± 0.4 b	85.6 ± 0.6 bc	112.9 ± 0.17 a	77.1 ± 0.3 ab
D4	Mealworm meal	70.4 ± 0.9 a	107.8 ± 0.5 a	67.7 ± 0.7 c	86.1 ± 0.5 a
D5	Chlorella mela	36.8 ± 0.9 c	83.8 ± 0.6 bc	108.0 ± 0.17 a	66.6 ± 0.4 bc
D6	Poultry by-products meal	51.1 ± 0.8 b	88.9 ± 0.2 b	106.6 ± 0.5 a	79.4 ± 0.5 ab
D7	Corn meal	51.6 ± 0.7 b	79.7 ± 0.2 c	104.8 ± 0.3 a	68.1 ± 0.9 bc
D8	Black soldier fly meal	40.4 ± 0.06 c	72.1 ± 0.1 c	106.3 ± 0.1 a	59.9 ± 0.5 c
	p value	p = 0.048	p = 0.023	p = 0.00008	p = 0.007

Values (means ± SEM, N = 3) within a column with a common superscript letter are not significantly different from the other dietary groups (p > 0.05).

. The apparent digestibilities of dry matter (34.8–70.4%), crude protein (52.8–107.8%), crude lipid (67.7–112.9%), and energy (52.2–86.1%) were significantly affected among the different test ingredients (p < 0.05). The dry matter digestibility of mealworm meal was significantly higher (70.4%) compared to other ingredients. Feather meal (34.8%) and black soldier fly meal (40.4%) had significantly lower dry matter digestibility values. However, the dry matter digestibility of feather meal was significantly improved (50.1%) after the fermentation of this ingredient. Mealworm meal had the highest (p < 0.05) crude protein digestibility at 107.8%. In contrast, feather meal showed the lowest (p < 0.05) crude protein digestibility (52.8%) for this species when compared to the other ingredients. This value (52.8%) of non-fermented feather meal was significantly lower than that of fermented feather meal (85.6%). The dry matter and protein digestibility were not correlated (r < 0.5) with the ash, carbohydrate, and protein content of test ingredients (p < 0.05). Crude lipid was higher for the fermented feather meal than for the unfermented feather meal. The differences in energy digestibility reflected the differences in dry matter digestibility. The energy digestibility showed a significant positive correlation (r = 0.870) with the dry matter digestibility (p < 0.05). The energy digestibility of mealworm meal was significantly higher (86.1%, p < 0.05) than feather meal, Chlorella, and black soldier fly meal, whereas there was no significant difference when compared to fermented feather meal and poultry meal. Feather meal had the lowest energy digestibility (p < 0.05, 52.2%) than that of other ingredients.

4. Discussion

4.1. Palatability and Digestibility

The palatability of feed ingredients can significantly influence feed intake, which is critical for evaluating the acceptability of alternative protein sources in aquafeeds. In this study, a lower relative feed intake (p > 0.05) in fish fed a diet containing insect meal indicates a lower palatability compared to other groups. This finding aligns with previous studies that have also noted a preference among some fish species for traditional ingredients over insect-based feeds [32]. The reduced palatability of insect meals has been attributed to factors such as fat oxidation susceptibility, high chitin content [12], unpleasant odors from essential oils, flavonoids, and terpenoids [33], and the presence of pupal hormones like ecdysone [34]. However, Moutinho et al. [35] found no adverse effects on the feed intake in S. aurata when fed diets containing defatted insect meal. Other researchers observed an improved voluntary feed intake in Cynoglossus semilaevis with a high inclusion of defatted insect meal [36]. It has been suggested that the inclusion of insect meal can influence diet acceptability due to differences in the nutrient content, which may act as feed stimulants [37,38].

The feather meal, fermented feather meal, mealworm meal, Chlorella, and poultry meal exhibited no significant differences in feed intake, indicating a similar palatability among these ingredients for this fish species. These results aid in refining ingredient selection and processing techniques to enhance feed acceptance in aquaculture diets, particularly when using less palatable ingredients. In this study, the apparent digestibility of the dry matter, crude protein, lipid, and energy of seven different feed ingredients when fed to Sobaity seabream was estimated and the results revealed that the apparent dry matter and crude protein digestibility of the tested ingredients varied from 34.8% to 70.4% and 52.8% to 107.8%, respectively, indicating the different utilization of protein ingredients by this Sobaity seabream.

4.2. Animal-Based Ingredients

4.2.1. Poultry By-Products Meal and Feather Meal

Poultry by-products meal and feather meal are two by-products of the poultry industry and cost-effective ingredients with the potential for use in fish feeds as a source of protein [39]. The results of this study showed that the dry matter digestibility of poultry by-products meal, feather meal, and fermented feather meal was 51.1, 34.8, and 50.1%, respectively. The dry matter ADC was moderate to high in all animal ingredients in earlier research conducted on European seabass (Dicentrarchus labrax) [18]. In that work, the authors reported a 69.5% dry matter digestibility for poultry by-product meal. Crude protein in the poultry by-products meal was found to be 88.9% digestible for this fish, which was similar to that reported for largemouth bass (Micropterus salmoides)—94% [40], sunshine bass (Morone chrysops × M. saxatilis)—75.16% [41], and cobia (Rachycentron canadum)—90.9% [42]. The apparent digestibility coefficients of gross energy for poultry by-products meal (79.4%) in this study were lower than those reported in cobia (91.8%) fed poultry by-products meal [43]. The apparent digestibility coefficients of lipids from poultry by-products meal was recorded to be 106.6%. However, the lipid ADC for cobia was reported at 82.9% for poultry by-products meals [43].

The earlier work reported a 73% dry matter digestibility for hydrolyzed feather meal [18]. In juvenile rockfish (Sebastes schlegeli), Lee [44] reported higher dry matter ADC values for steam-hydrolyzed feather meal (87%) compared to the Sobaity seabream in this study (35%). However, clear advantages were observed with the fermented feather meal when fed to the fish in this study. The fermentation treatment has been identified as an effective method to improve feather meal quality through biodegrading the keratin protein [45,46] and this may account for the elevated ADC of fermented feather meal. In the current research, the crude protein ADC values for fermented feather meal (85.6%) were found to be significantly higher compared to the steam-hydrolyzed feather meal (52.8%) for Sobaity seabream. This may be because of the higher (apparent) carbohydrate contents of the feather meal than the fermented feather meal, which may explain the different ADCs observed, with that carbohydrate content potentially acting like fiber and interfering with protein digestion as is often seen with other sources of non-starch polysaccharides [46,47]. The apparent digestibility coefficients of gross energy for feather meal and fermented feather meal in this study showed a significant difference from each other and were determined to be 52.2 and 77.1%, respectively. The lower values for feather meal and fermented feather meal were recorded in this study when compared to fermented feather meal (79%) and feather meal (77.5%) for cobia [43]. Another study reported a 51.3% gross energy digestibility of feather meal for speckled catfish Pseudoplatystoma coruscans [48]. The apparent digestibility coefficients of lipids from feather meal, and fermented feather meal in this study ranged between 91 and 112.9%. However, the lipid ADC for cobia was reported at 82.2% for fermented feather meal, and 71.1% for feather meal lipid digestibility [43]. The observed differences may be due to interspecies physiological differences in the digestive tract or to the ingredient composition, and, although Sobaity is similar to some species in its digestibility, how it compares to cobia is unknown.

4.2.2. Insects Meals

This study marks the first attempt to assess insect meal digestibility in Sobaity seabream. Insect meal (black soldier fly and mealworms) has shown promise as a potentially sustainable source of nutrients for aquafeeds, offering an alternative to expensive and ecologically undesirable ingredients, in the context of population growth and climate change. The dry matter digestibility of black soldier fly meal was found to be significantly low at 40.4%, which is lower (p < 0.05) than the mealworm meal digestibility (70.4%) when fed to this fish species. Similarly, the crude protein and energy digestibility values were significantly higher for mealworm meal than black soldier fly meal, indicating that mealworm meal may be a better ingredient in terms of nutrient digestibility over black soldier fly meal for this fish species. The traditional nitrogen-to-protein conversion factor of 6.25 may overestimate the insect protein content due to the presence of indigestible chitin in their exoskeletons. Recent studies suggest using species-specific factors to improve the accuracy in protein digestibility assessments [49,50]. Therefore, it is essential that we account for this when assessing the insect protein content in aquafeeds, as it could influence protein digestibility values and comparisons across different studies. The protein ADC (72.1%) observed for black soldier fly meal in this study was lower than an earlier study on barramundi Lates calcarifer (93.2%) [51], European seabream Sparus aurata 84.4% [35], and rainbow trout Oncorhynchus mykiss 85% [14], but higher than the values reported for turbot Scophthalmus maximus 63.1% [12].

4.3. Plant-Based Ingredients (Corn Meal and Chlorella)

Corn meal has a high carbohydrate content of 73.9%, and the ADC of energy for this ingredient was 68.1%, thus demonstrating some capability of Sobaity seabream to digest carbohydrates. The comparatively low digestion of energy of this ingredient was also reported (64.9%) for speckled catfish [48]. However, other researchers (Abimorad and Carneiro [52] and Abimorad, et al. [53]) have reported higher ADC values of energy in corn (75.6–86.7%) for pacu (Piaractus brachypomus) as compared with the results obtained in the present study. The crude protein digestibility was found to be 79.7% for the Sobaity seabream, which is lower than the earlier study conducted (85.8%) for the omnivorous pacu [53]. This result may be related to the feeding habits of fish. The digestibility of crude protein in the ingredients can vary depending on various factors, such as the source of the protein (animal-based or plant-based), the processing techniques used, and the specific amino acid profile of the ingredient, and other compositional features of the ingredient such as non-starch polysaccharide levels and types [54]. Furthermore, the quality of the protein source plays a significant role in determining its digestibility.

The Chlorella meal used in this study had a protein, lipid, carbohydrate, and ash content of 58.4, 9.2, 25.6, and 0.7%, respectively. The significantly lower energy IADC values of 66.6% than the mealworm meal obtained in this study may be due to the high fiber/non-starch polysaccharide content that might have inhibited the proteolytic enzymatic activity. In the present study, the Chlorella meal had an 83.8% crude protein ADC, which compares with the protein digestibility of Chlorella fed to European seabass reported at 85.5% [55]. Although the crude protein ADC of Chlorella for tilapia has been reported somewhat higher at 91% [56]. However, values recorded for the digestibility of Chlorella meal by other species have ranged from 68 to 80% [57]. In one study with Nile tilapia, Sarker et al. [58] have reported that the protein ADC was 80% for Chlorella sp., which is lower than the value reported in the current study for the processed Chlorella (83.8%). The differences in the nutrient digestibility of the ingredients are likely to be significantly impacted due to the technological processing of this algae, and further work needs to examine those effects.

In this study, we observed that ingredient digestibility values (IADCs) for certain test ingredients occasionally exceeded 100%. Several potential sources of error exist when calculating ingredient digestibility. These include inaccuracies in determining the composition of the test ingredient, errors in calculating the diet digestibility, and challenges in accurately assessing the inclusion or dilution of the test ingredient within the diet. The digestibility of an ingredient is calculated by comparing the digestibility of two diets (one containing the reference ingredient and one containing the test ingredient), which introduces additional complexity [59]. These calculations often assume that the inclusion of a test ingredient does not impact the digestibility of other diet components, such as those in the reference diet. However, this assumption is not always valid, as some ingredients can either enhance or reduce the digestibility of other components [17]. For instance, certain ingredients may contain compounds that enhance enzymatic activity, thereby improving the digestibility of the entire diet, leading to apparent digestibility values that exceed 100%. Conversely, some ingredients may contain anti-nutritional factors or indigestible components that can hinder the digestibility of other nutrients in the diet. This potential for interaction between ingredients and diet components makes the calculation of ingredient digestibility non-linear and prone to variability [59].

5. Conclusions

Since the apparent digestibility of feed ingredients is essential in formulating extruded mixed feed for aquaculture species as it helps determine how efficiently the species can utilize nutrients, by assessing the digestibility of nutrients, formulators can select ingredients that provide optimal nutrition while reducing waste. This ensures the feed is both cost-effective and energy-efficient, promoting better growth and health for the species while minimizing the environmental impact through reduced nutrient excretion. In this study, most of the tested ingredients were generally well-digested, with the exception of feather meal, which showed lower digestibility values. Mealworm meal demonstrated superior protein and energy digestibility and is, therefore, highly recommended as a key ingredient. Poultry meal and fermented feather meal also showed good protein digestibility and can be included in practical diets; however, feather meal should be used cautiously due to its comparatively poor digestibility in this species. These findings contribute to identifying nutritionally efficient and environmentally sustainable ingredients for formulating cost-effective diets for S. hasta, a species of growing importance in marine aquaculture.