Acta agriculturae Slovenica, 118/1, 1–9, Ljubljana 2022 doi:10.14720/aas.2022.118.1.1549 Original research article / izvirni znanstveni članek Use of watermelon seed meal as a replacer of soybean meal in African catfish diets: effect on growth, body composition, haematology, and profit margin Wasiu Adeyemi JIMOH 1, 2, Ayodeji Ahmed AYELOJA 1, Mohammed Olayemi SHITTU 3, Yusuf Olatunji YUSUF 1 Received February 29, 2020; accepted March 04, 2022. Delo je prispelo 29. februarja 2020, sprejeto 04. marca 2022 1 University of Ilorin, Faculty of Agriculture, Department of Aquaculture and Fisheries, Ilorin, Nigeria 2 Corresponding author, e-mail: jimoh.wa@unilorin.edu.ng 3 Federal College of Animal Health and Production Technology, Department of Fisheries Technology, Ibadan, Nigeria Use of watermelon seed meal as a replacer of soybean meal in African catfish diets: effect on growth, body composition, haematology, and profit margin Abstract: The effects of replacing soybean meal with wa- termelon (Citrullus lanatus) seed meal (CLM) on growth, body composition, haematology and profit margin in catfish (Clarias gariepinus) breeding was evaluated. Juvenile catfish (n = 150) were acclimatised for a week, weighed and allotted into five dietary treatments; D1, D2, D3, D4 and D5 containing 0, 15, 30, 45 and 60 % replacement of soybean meal with watermelon seed meal, respectively. The diets were isonitrogenous and iso- lipidic. Each treatment was conducted in triplicate with ten fish per replicate. The results from the study indicate that there was no significant difference (p > 0.05) in growth, carcass composi- tion, and nutrient utilization. However, a significant variation (p < 0.05) existed in the haematological parameters among the fish fed the different dietary treatments. The incidence of cost showed that the production of fish was cheaper when CLM was used as a replacement for soybean meal. The higher carcass yield and profit per kg of fish fed CLM justifies the use of CLM as a substitute for soybean meal in the diet of African catfish. Key words: aquaculture; fish farming; fish; African cat- fish; Clarias gariepinus; animal nutrition; watermelon seed meal; growth; body composition; haematological parameters; economics; profit margin Uporaba moke iz lubeničnih semen kot nadomestka sojinih tropin v prehrani afriških somov: vpliv na rast, sestavo telesa, hematologijo in dobiček Izvleček: V raziskavi smo ocenili učinke zamenjave so- jinih tropin z moko iz lubeničnih (Citrullus lanatus) semen (MLS) na rast, telesno sestavo, hematološke parametre in do- bičkonosnost v prireji afriških somov (Clarias gariepinus). Mlade some (n = 150) smo po tednu dni aklimatizacije stehtali in razdelili v pet skupin, ki so prejemale različne krmne me- šanice; v poskusnih skupinah smo 0 % (D1), 15 % (D2), 30 % (D3), 45 % (D4) in 60 % (D5) sojinih tropin nadomestili z MLS. Vse krmne mešanice so vsebovale enako količino beljakovin, pa tudi maščob. Vsak tretma smo izvedli v treh ponovitvah z desetimi ribami na ponovitev. Rezultati študije kažejo, da med skupinami ni bilo statistično značilnih razlik (p > 0,05) v rasti, sestavi trupa, izkoriščanju krme in hematoloških parametrih. Analiza stroškov je pokazala, da je bila prireja rib cenejša, če smo del sojinih tropin nadomestili z MLS. Boljša klavnost in večji dobiček na kilogram ribjega mesa ob uporabi ribje krme, kjer del sojinih tropin nadomestimo z MLS, upravičuje uporabo MLS kot nadomestka sojinih tropin v prehrani afriških somov. Ključne besede: akvakultura; ribogojstvo; ribe; afriški som; Clarias gariepinus; prehrana živali; lubenična semena; moka iz lubeničnih semen; rast; telesna sestava; hematološki parametri; ekonomika; dobiček Acta agriculturae Slovenica, 118/1 – 20222 W. A. JIMOH et al. 1 INTRODUCTION Soybean meal (SBM) is known for its high protein content, high digestibility, and relatively well-balanced amino acid profile and is widely used as a feed ingre- dient for many aquaculture species (Storebakken et al., 2000). It is currently the most commonly used plant protein source in fish feed (El-Sayed, 1999; Fadel et al., 2017; Fagbenro et al., 2003; Jimoh et al., 2020a). Lim and Akiyama (1992) and Jimoh (2020) reported that soybean products have been used to replace a significant portion of fish meal in fish feed with nutritional, environmental, and economic benefits. However, the wider utilization and availability of this conventional source for fish feed is limited by the increasing demand for human consump- tion and by other animal feed industries (Jimoh et al., 2020b; Siddhuraju & Becker, 2001). The rapid expansion of fish culture in recent years requires the development and improvement of low-cost and nutritious fish feeds, mainly because increasing the feed cost may increase the cost of fish production by 50–80 % (Cavalheiro et al., 2007; Jimoh et al., 2019). Feed contributes between 60 and 70 % to the variable cost of fish production (Gabriel et al., 2007), making it one of the factors that determine the profitability of aquaculture production (Jimoh et al., 2019). Hence, the need to focus on using less expensive and readily available vegetable sources of protein to re- place soybean meals without reducing the nutritional quality of the feed is imperative (Barros et al., 2002). In the past, research was mostly focused on the under- utilised vegetable proteins in the fish diet among which were groundnut cake (Fasakin & Balogun, 1996), lima bean (Adeparusi & Ajayi, 2004), pigeon pea (Adeparasi, 1994), sunflower, sesame (Fagbenro et al., 2010a, 2010b, 2013), and jack bean (Fagbenro et al., 2007; Jimoh et al., 2010). Watermelon (Citrullus lanatus) is a drought-toler- ant crop that belongs to the family Cucurbitaceae. It is cultivated in a wide range of tropical, semi-tropical, and arid regions of the world (Razavi & Milani, 2006). The seeds of the watermelon have a nutritional quality com- parable to that of oilseed proteins including soybean and other conventional legumes (Mustafa & Alamin, 2012). Wani et al. (2011) reported that watermelon seed meal contains an adequate amount of nutritional protein that could be used as an ingredient in feed products. More so, there is a paucity of information on the use of wa- termelon seeds as a dietary protein source in fish feed. Therefore, this work seeks to study the replacement of soybean meal with watermelon seed meal in the diet of African catfish (Clarias gariepinus). 2 MATERIALS AND METHODS 2.1 SOURCES AND PROCESSING OF INGREDI- ENTS The dried watermelon seeds were obtained in Bodija market, Ibadan, Nigeria. The watermelon seeds were rinsed with water and boiled for 15 minutes, after which they were sundried for several days and ground in a hammer mill. The oil therein was removed using the pressure generated from a locally made screw press (cassava-presser type). The cakes and other feedstuffs obtained from commercial sources in Nigeria were sepa- rately milled and screened to fine particles size. Triplicate samples were analysed for their proximate composition (AOAC, 2010). 2.2 ExPERIMENTAL DIETS Based on the nutrient composition of different sources of protein (Table 1), the experimental diets were formulated, containing cooked watermelon seed meal (CLM) substituting soybean meal at the shares of 0, 15, 30, 45, and 60 % instead of soybean meal (designated as D1, D2, D3, D4, and D5), respectively (Table 2). The di- ets were isolipidic and isonitrogenous, containing 40 % crude protein and 12 % crude lipid. The feedstuff was separately ground mixed with hot water, introduced into a Hobart-200T pelleting and mixing machine to obtain a homogenous mass, and then passed through a mincer to produce 2 mm size pellets, which were immediately sun- dried at 30–32 °C. After drying for three days, the diets were kept in a freezer (−4 °C). The diets were analysed for their proximate composition. It was observed that the value of crude fibre increased as the amount of CLM in the diet increased. However, there was no significant dif- ference (p > 0.05) in the proximate composition param- eters of the diets. Parameter Fish meal Soybean Meal CLM** Moisture 9.75 10.70 9.69 Crude Protein 72.4 45.74 27.55 Crude Lipid 10.45 9.68 11.35 Crude Fibre - 5.10 4.97 Ash 8.32 4.48 5.39 NFE* - 30.00 41.05 Table 1: Proximate composition of the different sources of protein in the experimental diets * Nitrogen Free Extract ** Watermelon (Citrullus lanatus) meal Acta agriculturae Slovenica, 118/1 – 2022 3 Use of watermelon seed meal as a replacer of soybean ... African catfish diets: effect on growth, body composition, haematology, and profit margin 2.3 ExPERIMENTAL FISH AND THE AqUACUL- TURE SYSTEM The experiment was conducted at the hatchery unit of the Federal College of Animal Health and Produc- tion Technology, Moor Plantation Ibadan, Nigeria. The fingerlings were obtained from a reputable hatchery, Ibadan, Oyo State, Nigeria, and transported to the ex- perimental site inside an aerated bag. The initial average weight of the fish ranged from 10.80 to 10.97 g. A total of 150 fingerlings were acclimated to laboratory conditions for 14 days before the feeding trial while being fed on a commercial pelleted diet. Experimental diets were as- signed randomly to the tanks with three replicates per di- etary treatment. Each culture tank contained 10 fish that were fed 5 % body weight per day in two equal propor- tions between 9:00–10:00 a.m. and 5:00–6:00 p.m. for 56 days. Fish – from each tank were batch – weighed every other week and the amount of feed was adjusted accord- ingly. The mortality was monitored daily and recorded. The growth performance and feed utilization indices were estimated following the method explained in Jimoh and Aroyehun (2011). The water quality parameters were monitored and recorded throughout the experiment (ox- ygen 6.84 ± 0.55 mg/l, temperature 28.28 ± 0.29 °C and pH 6.88 ± 0.30) using a combined digital YSI dissolved oxygen meter (YSI Model 57, Yellow Spring Ohio) and the pH was monitored weekly using a pH meter (Mettler Toledo – 320, Jenway UK). Eight catfish per treatment were euthanized in clove oil (100 mg/l) at the beginning and end of the feeding trial and analysed for their carcass composition (AOAC, 2010). 2.4 BLOOD SAMPLING AND ASSESSMENT The assessment of the haematological parameters was conducted following the methods explained in Ji- moh et al. (2015a). Briefly, fish (n = 6) from each treat- ment were mildly euthanized with clove oil (100 mg/l) at the end of the feeding trial for blood sampling. The blood (1 ml) was obtained by caudal vein piercing using a 1ml disposable syringe and 25G EDTA treated needle and placed in EDTA treated test tubes for haematological ex- amination. The primary haematological parameters such as packed cell volume (PCV), haemoglobin concentra- tion (Hb) were measured by the microhematocrit meth- od and the cyanmethaemoglobin method (Coles, 1986; Schalm et al., 1975) and total blood cell counts such as red blood cell count (RBC) and white blood cell count (WBC) were determined by the use of hemocytometer, respectively. The secondary haematological parameters such as mean corpuscular volume (MCV), mean corpus- cular haemoglobin (MCH), and mean corpuscular hae- moglobin concentration (MCHC) were calculated using the standard formulae (Coles, 1986; Schalm et al., 1975). Ingredients D1 D2 D3 D4 D5 Fish meal (72%) 27.70 27.70 27.70 27.70 27.70 SBM (45%) 44.40 37.70 31.10 24.40 17.78 CLM (27.55%) - 10.88 21.77 32.66 43.55 Fish oil 5.00 5.00 5.00 5.00 5.00 ϕVitamin premix 5.00 5.00 5.00 5.00 5.00 Starch 17.90 13.72 9.43 5.24 0.97 Proximate Analysis Moisture 9.38 ± 0.08 9.87 ± 0.31 9.95 ± 0.11 9.56 ± 0.92 9.50 ± 0.72 Crude Protein 40.17 ± 0.08 40.19 ± 0.02 40.14 ± 0.01 40.19 ± 0.11 40.18 ± 0.06 Crude Lipid 12.00 ± 0.16 11.74 ± 0.83 12.29 ± 0.45 12.00 ± 0.04 12.16 ± 0.23 Crude Fibre 5.51 ± 0.26 5.53 ± 0.28 5.86 ± 0.59 6.20 ± 1.18 6.51 ± 0.37 Ash 6.20 ± 0.06 6.30 ± 0.04 6.00 ± 0.17 5.95 ± 0.33 5.44 ± 0.33 *NFE 26.76 ± 0.32 26.42 ± 1.40 25.76 ± 0.22 26.11 ± 0.44 26.19 ± 0.22 Table 2: Gross (g/100g) and proximate composition (%) of experimental diets containing watermelon seed meal (CLM) Means without superscript in the same row are not significantly different (p > 0.05) from each other * Nitrogen free extract ϕ Specification: each kg contains: Vitamin A = 4,000,000 IU; Vitamin B = 800,000 IU; Vitamin E = 16,000 mg, Vitamin K3 = 800 mg; Vitamin B1 = 600 mg; Vitamin B2 = 2,000 mg; Vitamin B6 = 1,600 mg, Vitamin B12 = 8 mg; Niacin = 16,000 mg; Caplan = 4,000 mg; Folic Acid = 400 mg; Biotin = 40 mg; Antioxidant = 40,000 mg; Chlorine chloride = 120,000 mg; Manganese = 32,000 mg; Iron = 16,000 mg; Zinc = 24,000 mg; Copper = 32,000 mg; Iodine = 320 mg; Cobalt = 120 mg; Selenium = 800 mg, manufactured by DSM Nutritional products Europe Limited, Basle, Switzerland. Acta agriculturae Slovenica, 118/1 – 20224 W. A. JIMOH et al. 2.5 ECONOMIC ANALYSIS The economic analysis of feeding watermelon seeds was assessed following the procedure explained in Jimoh et al. (2015b). Cost of Feed Incidence of Cost Weight of Fish = Value of FishProfit Index Cost of Feed = Profit / kg Value of 1 kg fish Incidence of cost= − 2.6 STATISTICAL ANALYSIS Data obtained from the experiment were expressed in mean ± SD and subjected to one-way analysis of vari- ance (ANOVA) using SPSS version 16.0. Duncan mul- tiple range tests were used to compare differences among individual treatment means to reveal significant differ- ences (p < 0.05). 2.7 ETHICAL STATEMENT Standard regulations and guidelines of Federal Col- lege of Animal Health and Production Technology, PMB 5029, Ibadan, Nigeria on the care and use of laboratory animals were followed throughout the experiment. 3 RESULTS AND DISCUSSION 3.1 CARCASS COMPOSITION Uys and Hecht (1985) reported that the best growth rate and feed conversion efficiency in juvenile and sub- adult African catfish (Clarias gariepinus) are achieved with diets containing 38–42 % crude protein and lipid content of 10–11 %. The carcass composition of African catfish-fed diets containing CLM is presented in Table 3. Significant differences (p < 0.05) were observed only in carcass protein content between the fish at the beginning and at the end of the experiment. However, no significant difference was recorded in the carcass protein content of fish fed different experimental diets. A similar observa- tion was reported by Tiamiyu et al. (2015), using CLM as a replacer of soybean meal. The groups fed with the inclusion of CLM up to 30 % (D2 and D3) had higher Initial D1 D2 D3 D4 D5 Moisture 77.88 ± 0.18 75.57 ± 0.11 74.89 ± 0.46 74.16 ± 1.09 74.20 ± 2.20 74.58 ± 3.19 Crude Protein 15.11 ± 0.14b 17.32 ± 0.16a 17.45 ± 0.12a 17.37 ± 0.12a 17.10 ± 0.44a 17.14 ± 1.21a Crude Lipid 3.13 ± 0.18 3.84 ± 0.25 3.41 ± 0.21 3.59 ± 0.23 3.56 ± 0.38 3.62 ± 0.33 Ash 3.89 ± 0.18 3.84 ± 0.25 4.26 ± 0.13 4.88 ± 0.74 5.14 ± 1.20 4.67 ± 1.66 Table 3: Carcass composition of African catfish fed diets containing watermelon seed meal (CLM) Row means with the different superscripts are significantly different (p < 0.05) from each other D1 D2 D3 D4 D5 Initial weight (g) 10.88 ± 0.02 10.97 ± 0.04 10.80 ± 0.03 10.87 ± 0.11 10.88 ± 0.03 Final weight (g) 24.31 ± 2.09 32.34 ± 7.81 30.95 ± 5.56 27.66 ± 6.42 21.37 ± 0.45 1Weight gain (g) 13.43 ± 2.11 21.37 ± 7.83 21.65 ± 7.71 16.79 ± 6.53 10.49 ± 0.47 2% weight gain 124.50 ± 21.92 190.93 ± 76.27 200.56 ± 71.89 154.78 ± 61.72 96.37 ± 4.61 3SGR 1.43 ± 0.16 1.90 ± 0.44 1.94 ± 0.43 1.64 ± 0.44 1.20 ± 0.04 4FCR 1.47 ± 0.45 1.16 ± 0.08 1.30 ± 0.03 1.33 ± 0.06 1.36 ± 0.04 5PER 1.64 ± 0.76 2.16 ± 0.16 1.92 ± 0.04 1.88 ± 0.08 1.84 ± 0.05 6% Survival 77.77 ± 15.71 88.89 ± 15.72 88.89 ± 15.72 88.89 ± 15.72 88.88 ± 15 Table 4: Growth and nutrient utilization of African catfish fed diets containing watermelon seed meal Row means without superscript are not significantly different (p > 0.05) from each other. 1 Mean weight gain = final mean weight − initial mean weight; 2 Percentage weight gain = (final weight − initial weight / initial weight) × 100; 3 Specific growth rate = (In final weight − In initial weight) × 100; 4 Feed conversion ratio = dry weight of feed fed / Weight gain (g); 5 Protein efficiency ratio = fish body weight (g) / Protein fed; 6 Percentage survival = ((total number of fish − mortality) / total number of fish) × 100 Acta agriculturae Slovenica, 118/1 – 2022 5 Use of watermelon seed meal as a replacer of soybean ... African catfish diets: effect on growth, body composition, haematology, and profit margin was not significant (p > 0.05). Lower growth at higher inclusion is customary of alternative vegetable protein sources used in fish feed, as they may be deficient in some essential amino acids and may possess antimetabo- lites which may reduce the growth performance of fish (Jobling, 2012). Antimetabolites at higher inclusion can reduce palatability and bioavailability of nutrients in the feed (Jimoh et al., 2014). 3.3 HAEMATOLOGICAL PROFILE Table 5 shows the haematological profile of African catfish fed the experimental diets. Fish fed diets contain- ing CLM had significantly higher (p < 0.05) values of PCV, RBC, WBC, and lymphocytes than animals from the control group. The difference in haemoglobin con- tent was not significant (p > 0.05) between the groups. The values recorded for haemoglobin contents, PCV, RBC of the fish were all within the range of normal carcass crude protein levels than the controls (D1), but the difference was not statistically significant (p > 0.05). 3.2 GROWTH AND NUTRIENT UTILIZATION The growth and nutrient utilization of African cat- fish fed different diets are shown in Table 4. The results of this experiment indicated that the growth and nutrient utilization of C. gariepinus were not significantly affected (p > 0.05) by up to 60 % replacement level of soybean meal with CLM in the diet. This result agrees with the studies conducted by Davies et al. (2000) using sesame and other oil seeds residue as fish meal replacer in diets fed to Nile tilapia (Oreochromis niloticus), Olvera‐Novoa et al. (2002), and Sahar et al. (2003) using sunflower seed meal as a protein source in diets fed to red beast tilapia (Tilapia rendalli) and common carp (Cyprinus carpio), respectively. The D5-fed group had a lower weight gain compared to the control group (D1) but the difference D1 D2 D3 D4 D5 PCV (%) 16 ± 1.41c 17 ± 1.41bc 19 ± 1.41ab 20 ± 1.41a 20 ± 1.41a Hb (g/dL) 5.0 ± 1.41 5.0 ± 1.41 6.4 ± 0.57 6.8 ± 1.41 6.9 ± 0.14 RBC (×1012/L) 1.42 ± 0.03d 1.62 ± 0.03b 1.73 ± 0.00a 1.50 ± 0.00c 1.60 ± 0.00b WBC (×107/L) 158.8 ± 0.28d 205.2 ± 1.41c 342.6 ± 3.39b 340.2 ± 0.28b 400.2 ± 0.28a MCV (fL) 112.7 ± 1.41c 123.0 ± 1.41b 109.8 ± 2.83c 133.0 ± 0.00a 125.0 ± 1.41b MCH (ρg) 35.2 ± 0.83c 40.4 ± 0.34b 37.0 ± 1.41c 40.0 ± 0.00b 43.1 ± 1.41a MCHC (g/dL) 31.3 ± 1.41b 33.3 ± 0.42a 34.0 ± 1.41a 34.0 ± 0.00a 35.0 ± 1.41a WBC Differential Lymphocytes (%) 69.0 ± 1.41c 78.0 ± 2.83b 78.0 ± 2.83b 81.0 ± 1.41b 88.0 ± 1.41a Neutrophil (%) 30.0 ± 2.83a 22.0 ± 2.83b 21.0 ± 1.41c 19.0 ± 0.00c 11.0 ± 0.71d Table 5: Haematological profile of African catfish fed the experimental diets Row means with different superscripts are significantly different (p < 0.05) from each other. Hb = Haemoglobin content; PCV = Packed Cell Volume; WBC = White Blood Cell Count; RBC = Red Blood Cell Count; MCHC = Mean Corpuscu- lar Haemoglobin Concentration; MCV = Mean Corpuscular Volume; MCH = Mean Corpuscular Haemoglobin Price (N)/kg* D1 D2 D3 D4 D5 Fish meal 480 132.96 132.96 132.96 132.96 132.96 SBM 136 60.43 51.36 42.29 33.18 24.18 C.L.M 48 - 5.22 10.44 15.67 20.90 Fish oil 500 100 100 100 100 100 Vit. Premix 262 52.40 52.40 52.40 52.40 52.40 Starch 200 35.80 27.44 18.86 10.48 1.94 Cost (N/kg) 381.59 369.38 356.95 344.69 332.38 Table 6: Cost of producing 1 kg feed containing watermelon seed meal * 1 Euro = N194.85 Acta agriculturae Slovenica, 118/1 – 20226 W. A. JIMOH et al. healthy fish (Clark et al., 1979; Erondu et al., 1993; Fag- benro et al., 1993; Khan & Abidi, 2010, 2011; Omitoyin, 2006; Rastogi, 2007). According to Lenfant and Johansen (1972), an erythrocyte count greater than 1 × 106/mm3 is considered high and is indicative of the high oxygen- carrying capacity of the blood, which is characteristic of fishes capable of aerial respiration and with high activity. Watermelon seeds and flour are known to contain bioac- tive compounds such as tannins, stachyose, phytic acids, raffinose, and verbascose that could have immunomodu- latory and immunostimulatory properties thus could en- hance the innate defence mechanism of fish (El-Adawy & Taha, 2001; Erhirhie & Ekene, 2014; Tarazona‐Díaz et al., 2011). We observed a significant rise (p < 0.05) of WBC in fish fed CLM-enriched diets, demonstrating possible ability to boost innate immunity when compared to the control-fed group (Hoseinifar et al., 2020). Fish fed di- ets D2, D3, D4, and D5 had decreased neutrophil counts compared to the fish fed the control diet. Similar was ob- served in in African catfish fed dietary combinations of onion-pawpaw where Fawole et al. (2020b) discovered an inverse relationship between lymphocyte and neutrophil counts, while Tiamiyu et al. (2019) discovered the same in African catfish fed Talinum triangulare. 3.4 COST OF PRODUCING 1 KG OF DIET Table 6 shows the cost of producing 1 kg of feed con- taining CLM. There was a reduction in the cost of produc- ing 1 kg of diets with an increasing replacement level of soybean meal by CLM. 3.5 ECONOMIC ANALYSIS Table 7 reveals the incidence of a cost analysis of pro- ducing 1 kg of African catfish with diets containing CLM. The cost analysis of producing 1kg of fish, showed that it was cheaper to produce 1 kg of fish with diets contain- ing CLM than with diets containing only soybean meal. A significant difference (p < 0.05) was shown in the in- cidence of cost and profit (N/kg) of fish fed the different dietary treatments. The cost of the feed D5 was signifi- cantly the lowest, while D1 was significantly the highest. A reverse trend was noted for Profit (N/kg) of fish fed the different dietary treatments. However, there was no sig- nificant difference (p > 0.05) in the value of fish produced and their profit margin among the fish fed various dietary treatments. The profit index reveals a trend of increas- ing profitability when feeding CLM based diets to Afri- can catfish (p > 0.05). A profit index above one (Table 7) shows that it is profitable to feed the fish with the diet. There was a general increase in the profit index observed with an increased dietary level of inclusion of CLM. This agrees with our earlier studies (Jimoh et al., 2012; Jimoh, 2004; Jimoh et al., 2019) that reported a general increase in the profit index with an increase in the replacement level of lesser-known vegetable protein Gross margin was reported to be a good measure of profitability (Olagunju et al., 2007). The experiment showed that it is profitable to replace soybean meal with watermelon seed meal. This result agrees with the find- ings of Fagbenro et al. (2001) and Abu et al. (2010), who reported that feeding fish with cheaper and lesser-known feed ingredients left some profit margin. Although the economic implication of using the different dietary treat- ments might not be well appreciated since the margin is small, it will be much clearer when the magnitude of total cost and expected revenue of its large scale operation is critically and objectively considered (Faturoti, 1989; Ji- moh et al., 2019). Adeparusi and Balogun (1999) reported profit margin increasing when the fish meal was replaced by roasted pigeon peal meal in a diet fed to African catfish. Jimoh (2004) also reported an increase in the profit mar- gin in the production of tilapia by replacing up to 30 % of soybean meal with jack bean meal. Jimoh et al. (2019) and Jimoh et al. (2020b) observed similar trends when Ja- tropha curcas was fed African catfish (Clarias gariepinus) and Nile tilapia (Oreochromis niloticus), respectively. D1 D2 D3 D4 D5 Cost of feed fed 3.55 ± 1.01 3.73 ± 3.61 3.91 ± 3.44 3.51 ± 2.17 3.65 ± 0.67 Weight gain of fish 0.013 ± 0.02 0.014 ± 0.02 0.015 ± 0.01 0.017 ± 0.01 0.019 ± 0.01 Value of fish 5.20 ± 1.05 5.60 ± 3.92 6.0 ± 3.85 6.80 ± 3.27 7.60 ± 0.23 Profit index 1.46 ± 0.01 1.50 ± 0.01 1.54 ± 0.01 1.93 ± 0.57 2.08 ± 0.33 Incidence of cost 273.08 ± 1.25a 266.42 ± 2.40ab 260.67 ± 4.07b 206.47 ± 8.31c 192.10 ± 9.86d Profit (N)/kg of fish 126.92 ± 1.05e 133.58 ± 2.43d 139.33 ± 2.19c 193.53 ± 4.24b 207.90 ± 1.21a Table 7: Cost analysis of producing 1kg of African catfish fed diets containing watermelon seed meal Row means without superscript are not significantly different (p > 0,05) from each other. 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