Nutrition plays a critical role in intensive aquaculture as it influences not only the production cost but also fish growth, health and waste production [1] . Cost effective diets are essential for successful fish farming. The profitability and success of compound feed production depends on the cost, availability and digestibility of the feed ingredients to be used. The conventional feed ingredients for fish are mainly from plant and animal products especially by-products of processing plants. These products are also used as human food [2] . Therefore there is a need to search for lesser known materials suitable for fish feed. Legumes are less expensive sources of protein that have been identified to be capable of reducing the cost of fish feed when combined as complementary ingredients to meet the nutritional requirement of fish. Soybean meal has high protein content and the best protein quality among plant protein feedstuffs used in fish feeds [3] . It has been reported to partially or totally replace fish meal in diets of many aquaculture species [4] [5] . However, wider utilization and availability of this conventional source for fish feed is limited by increasing demand for human consumption and by other animal feed industries [6] . This phenomenon according to [7] has hindered the expansion and profitability of aquaculture enterprise in many developing countries and has to encourage the need to look for cheaper alternative protein source for the development of low-cost feed that can replace this conventional feedstuff without reducing the nutritional quality of the diets. It then becomes a priority to look for cheaper, alternative protein source. In the recent past, researches were mostly focused on the under-utilized plant proteins in fish diets. Prominent among which are soybean meal [8] [9] , groundnut cake [10] , Lima bean [11] , Jackbean [12] [13] , African Yam bean [14] , Pigeon Pea [15] . Relatively, work on the use of other oilseeds residue such as sesame meal in warm water fish nutrition is limited [16] . Sesame seed (Sesamum indicum) is one of the important annual crops of the world grown for oil. They have nutrient density comparable to other oilseed proteins including soybean meal and other conventional legumes [17] - [19] and their potentials as dietary protein sources are well recognized [20] [21] . In fish feeding, sesame seed cake had been tested with species such as Clarias gariepinus [21] - [23] , common carp [17] [18] [24] . The meal leftover after oil production containing 34% - 50% protein can be used as a feed for livestock and poultry [25] . Furthermore, Sesame seed cake is known to be rich in methionine [26] - [28] and also tryptophan [29] [30] , amino acids lacking in most plant protein feedstuffs and its incorporation in fish diets has been little investigated [22] . This study therefore investigates the replacement value of differently timed wet- heat-treated sesame seedmeal with soybean meal in the diet of Clarias gariepinus using growth performance, digestibility and nutrient utilisation as indices.

Fish meal, soybean meal and other feedstuffs obtained from commercial sources in Nigeria were separately milled, screened to fine particle size (<250 µm) and triplicate samples were analysed for proximate composition. Sesame seeds were obtained from a farm in Kebbi State. Three batches of sesame were cooked for 10, 20 and 30 minutes dried, milled in a hammer mill and mechanically defatted using the pressure generated from locally made screw press. The cake thereafter was analysed for its proximate composition [31] . Amino acid analysis of differently-processed sesame seedmeals were determined by using the ion exchange chromatography (IEC). The samples were defatted, hydrolyzed and evaporated in a rotator evaporator and then injected into the Automatic Technicon Sequential Multi-sample Amino Acid Analyser (Model No 0209, Technicon, Ireland). The gross energy content of samples was calculated based on the physiological value of 5.61Kcal/g protein, 9.50 Kcal/g lipid and 4.11 Kcal/g carbohydrate [33] .

Based on the nutrient composition of the protein feedstuff (Table 1), a control diet and nine test diets (40% crude protein, 12% crude lipid and 18.45 Mj/kg gross energy) were formulated. The control diet (CTR) contained soybean meal, providing 50% of total protein. Nine tests diets contained each of these differently pro- cessed seedmeals at three replacement levels, 15, 30 and 45%, for soybean meal (Table 2). The feedstuffs were ground and hot water was added to aid binding after which it was then introduced in a Hobart-200T pelleting and mixing machine to obtain a homogeneous mass and then passed through a mincer to produce 0.8 mm (long) 0.2 mm (diameter) pellet which was immediately be sun-dried (30˚C - 32˚C). After drying for some days, the diets were kept frozen in a refrigerator.

Clarias gariepinus fingerlings were acclimated to experimental condition for 7 days prior to the feeding trial. Groups of 15 catfish fingerlings (3.38 ± 0.015 g) were stocked into aquaria comprising 60 litre-capacity rectangular plastic tanks. Each diet was fed to the catfish in triplicate tanks twice daily (09.00 h, 16.00 h) at 5% body

^*Values obtained from [32] ; CMS10: Sesame seeds cooked for 10 minutes; CMS20: Sesame seeds cooked for 20 minutes; CMS30: Sesame seeds cooked for 30 minutes.

*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, 2000 mg; Vitamin B₆, 1600 mg, Vitamin B₁₂, 8 mg; Niacin,16,000 mg; Caplan, 4000 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.

weight for 56 days. Fish mortality was monitored daily, total fish weight in each tank was determined at two weeks intervals and the amount of diet was adjusted according to the new weight. Growth response and feed utilization indices were estimated. Water temperature and dissolved oxygen were measured using a combined digital YSI dissolved oxygen meter (YSI Model 57, Yellow Spring Ohio); pH was monitored weekly using pH meter (Mettler Toledo-320, Jenway UK). Eight catfish and 6 catfish per treatment were respectively sacrificed at the beginning and end of the feeding trial respectively and analysed for their carcass composition [31] . All data were subjected to one-way analysis of variance (ANOVA) test using SPSS 13.0 version. Where ANOVA revealed significant difference (P < 0.05), Duncan multiple range test [34] was applied to characterize and quantity the differences between treatments.

Comparable performance in growth, nutrient utilization and carcass crude protein deposition in Clarias gariepinus fed diets with processed sesame seedmeal based diets showed that the nutritive value of raw sesame seedmeal could be improved by cooking. This is evidenced in more superior growth performance and nutrient utilization of Clarias gariepinus fed differently cooked sesame seedmeal even at higher replacement level than those fed raw [23] at level to level comparison. [35] reported good growth performance that was not significantly different from control was recorded when Clarias gariepinus was fed diets containing autoclaved nor roasted winged bean (Psophocarpus tetragonolobus) replacing up to 80% of fishmeal. Similarly [13] reported that 60% of soybean meal protein could be replaced be replaced with Rosselle (Hibiscus sabdariffa) seed meal without affecting growth performance and nutrient utilization in Clarias gariepinus. [21] reported that it is possible to replace soybean meal in Clarias gariepinus diet with cooked and mechanically defatted sesame seed meal with optimum growth response at a 25% replacement level though at 50% replacement with sesame seedmeal the growth response was different from that of control however it was similar to that of fish fed diets containing cooked and mechanically defatted sesame seed meal at a 25% replacement level with soybean meal. Similarly [36] reported that growth was positively affected by the replacement of up to 52% fish meal by sesame oil cake without amino acid supplementation in a practical pelleted feed for rainbow trout without reducing growth performance when compared to a fishmeal control diets. [37] reported that 62.5% of fishmeal protein replacement by plant proteins showed similar growth performance and feed utilization. [38] submitted that thermal pro- cessing of raw pea seed meal improved the quality of the ingredients and that dry heat treatment in particular (180˚C:30 min) led to a greatly improved feed utilization and consequent better growth performances. The response of C. gariepinus with respect to SGR was in accordance with those typically obtained for this species by other workers. [9] reported similar values for growth and feed utilization data for soybean meal as a fish meal substitute in practical diets for C. gariepinus. Similarly, [38] reported mean SGR of 2.2 - 2.4%/day. Furthermore [39] reported mean SGR values of 2.7%/day for juveniles C. gariepinus. [40] also reported a superior growth performance and feed utilization for juvenile rainbow trout fed diets in which pea seed meal was subjected to cooking.

The apparent digestibility coefficient for organic matter, protein, energy, lipid, fibre, carbohydrate in Clarias gariepinus fed processed seedmeal based diets in this study were more superior than those fed raw sesame seedmeal by [41] and comparable with the results obtained for fish fed control diets. The values of apparent digestibility coefficient of nutrients recorded in this study were in agreement with the values reported for carp [17] [42] - [44] .

The organic matter digestibility coefficient reported in this study was slightly higher than the value reported by [45] the variation may be attributed to processing methods, and or the variation may be attributed to processing methods, and or experimental methodology; different seed meal and fish species was used as it is known that digestibility of nutrients are species specific however the result closely related to that reported in [46] and [47] for juvenile hybrid tilapia fed cottonseed meal. [20] reported AOMD of the sunflower diets fed to Tilapia rendalli was over 90%.

The values of apparent digestibility of protein and lipid for the control diet and diets containing processed sesame seedmeal obtained during the experiment were almost similar to those obtained by [17] [44] [48] . The apparent protein digestibility coefficients of sunflower and sesame meal based diets were similar to the values reported by [49] . [50] reported that sunflower protein concentrate fed to rainbow trout had protein digestibility coefficients ranging from 79.5 to 90.6%. The results of this study also indicated that irrespective of the replacement levels (15, 30 and 45%), the protein in them were well digested by Clarias gariepinus. The ranges of APD values in various ingredients of plant origin as reported earlier are 76.2% - 94.0% in rohu [51] , 78.9% - 85.8% in common carp [17] , 52.5% - 94.1% in catla [52] [53] and 81.2% - 92.8% in mrigal and grass carp [53] [54] . It could therefore be established that sesame oil cakes, may be used for partial replacement of soybean meal in formulating the diets for Clarias gariepinus, as they are high in protein content with a fairly good digestible protein. [49] concluded that many of the ingredients tested, particularly the oil cakes, may be used for partial or total replacement of scarce and costly fishmeal in formulating the diets for silver barb, as they are high in protein content with a fairly good digestible protein.

The earlier reported ranges of apparent lipid digestibility (ALD) coefficient for plant-derived feedstuffs are 73.4% - 100% in red tilapia [55] 79.6% - 90.2% in Nile tilapia [56] , 69.6% - 86.4% in catla [52] , 90.4% - 94.1% and in rohu [51] . A range of 76% to 97% fat digestibility of various sources of fat has been reported for channel catfish. [57] reported that the ability to digest fat appears to be influenced by temperature and the level of fat in the diet. Sesame oil has excellent nutritional properties. They are practically free of toxic composition. In this study, the ALD did not vary widely among the differently processed diets containing sesame seed meal. This indicates that the C. gariepinus has the capacity to digest oil that are available in the plant feedstuffs as effectively as the animal oil from fishmeal and therefore, may use both the plant and the animal oil efficiently as source of energy. The high lipid digestibility by this species was found to be in line with what was reported in [58] for rainbow trout [51] . Then [49] did not observe much difference in lipid digestibility between plant- and animal-derived feedstuffs for rohu, which agrees with the present finding for C. gariepinus [51] . And [49] observed an ALD value of over 90% in soybean meal for rohu, the results of which were consistent with the ALD of diet containing sesame seed meal for the C. gariepinus in this study [11] . Then [59] reported that nutrient digestibility of Oreochromis niloticus fed lima bean (Phaseolus lunatus) and jackbean meal (Cannavalia ensiformis) diets respectively was improved with toasting and autoclaving.

The low carbohydrate digestibility recorded in this study was similar to that reported by [11] for Oreochromis niloticus fed lima bean. The digestibility of carbohydrate has been known to vary with their complexity of carbohydrate, source treatment and level of inclusion in the diet [60] - [63] compared the digestibility and growth performance of rainbow trout fed diets containing peas processed by different methods and reported that autoclaving increased starch and energy digestibility.

The authors appreciate Alh Ali Jamiu Alaba (Port Harcourt, Nigeria) for his contribution towards the publication of this article.