Read Prof. Sebastian Chenyambuga Inaugural Lecture paper

Executive summary

Fish is used in many countries as a primary source of protein. Fish is estimated to account for 17% of the global intake of animal protein and 6.5% of all protein consumed. Fish is also a major source of livelihoods and income, particularly in developing countries. More than 158 million people in the world are estimated to depend directly on fish-related activities. Since the number of fish stocks in natural water bodies has greatly declined, more emphasis on fish production has been directed to aquaculture. Aquaculture has the potential of enhancing food security directly through producing fish for household consumption, improving the supply, and reducing the price of fish in the market. Aquaculture also contributes to farm diversification and creation of new employment opportunities and income streams. Thus, aquaculture is currently promoted as a mechanism for rural development with a focus on poverty alleviation in developing countries.

In Tanzania, aquaculture is an emerging industry, which dominated by pond culture of Nile tilapia (Oreochromis niloticus). The Nile tilapia is a good fish for resource poor farmers because it is easy to raise, it grows fast, and it is tasty, it can eat many types of foods, it is highly tolerant to diseases, it is able to reproduce easily under captivity and can tolerate poor water quality conditions. The demand for tilapia for both domestic consumption and export is  high  and  increasing,  but  the production from natural water bodies has shown a declining pattern due to overfishing. Thus, there is a need of improving fish production from aquaculture to complement the declining capture of fisheries. Early sexual maturity and prolificacy are the problems, which are associated with pond culture of tilapia, leading to uncontrolled reproduction and overcrowding which, in turn, results in large populations of small-sized fish that are of low value. Poor nutrition is another problem facing pond culture of Nile tilapia. Protein concentrate feeds contribute a major cost component of fish feeds. Because of high cost of conventional protein concentrates, fish farmers use locally available feeds (rice and maize brans, kitchen leftovers, and garden remains) to feed the cultured fish. These are of low quality and fish reared on these feeds are unable to meet their maintenance and production requirements, especially for protein. This practice prolongs the time of reaching the market weight and consequently leading to the production of poor quality fish and low profitability of fish farming. Therefore, there is a need of identifying appropriate alternative cheap sources of protein.

To address the above problems a number of studies were done between 2009 and 2018 as described below. The first study was carried out to compare the growth performance and survival of Nile tilapia (O. niloticus), Wami tilapia (O. urolepis) and Ruvuma tilapia (O. ruvumae) under earthen pond and hapa-in-pond culture conditions in order to identify the species that can complement O. niloticus in aquaculture. Two growth experiments were conducted for 90 days. In the first experiment, the fish were raised in earthen ponds while in the second experiment they were grown in hapas installed in earthen ponds. On average, O. niloticus had higher (p ≤ 0.0001) overall mean final body weight (47.17 ± 1.95 g), weight gain (42.75 ± 2.04 g), growth rate (0.48 ± 0.02 g/d) and survival rate (92.82 ± 0.01%) than both O. urolepis and O. ruvumae. On the other hand, O. urolepis had higher overall mean final body weight (33.79 ± 2.24 g), weight gain (28.54 ± 2.34 g) and growth rate than O. ruvumae.

To address the problem of over-reproduction of pond cultured Nile tilapia and enable the production of large market size tilapia, two methods for controlling unwanted reproduction were evaluated; polyculture of tilapia with predatory fish (African catfish) and the culture of all-male population. A growth trial was conducted for 120 days in earthen ponds and tilapia were subjected to three treatments, that is, stocking only male tilapia, stocking tilapia of mixed sex together with African catfish at a ratio of 1:10 for catfish: tilapia and stocking tilapia of mixed sex only. The tilapia raised under all-male culture had higher (P ≤ 0.0001) weight gain (65.95 ± 1.19 g) and growth rate (0.55 ± 0.01 g/d) than those under polyculture of tilapia and catfish. On the other hand, tilapia cultured under polyculture system had higher (P ≤ 0.0001) weight gain (55.43 ± 1.81 g) and growth rate (0.46 ± 0.02 g/d) compared to those raised under  mix  sex  culture  without  catfish  (weight  gain of

37.71 ± 3.03 g and growth rate of (0.31 ± 0.02 g/d). Since the culture of all-male tilapias resulted in higher growth  performance than  the  polyculture of tilapia with catfish and manual sexing is tedious, a study was conducted to determine the best ways of producing all- male population. Two methods were compared; hormonal sex reversal using 17α-methyltestosterone (17α-MT) and hybridization of Wami tilapia (O. urolepis) and Nile tilapia (O. niloticus). Sex reversed Nile tilapia were produced by feeding newly hatched fry a diet containing 60 mg of 17α-MT per kg of the diet for

28 days. Hybrid tilapias were produced by making a reciprocal cross of O. niloticus ♂ x O. urolepis ♀ and O. niloticus ♀ x O. urolepis ♂. The resulting F1 hybrids and hormonal sex reversed males were compared in a growth experiment conducted for 98 days. The results showed that hybridization of O. niloticus x O. urolepis resulted in slightly higher percentage of males (94%) compared to 17α-MT hormone treatment (90%). However, the percentages of males produced from the two methods were not significantly different (P > 0.05). The hybrids showed higher growth performance (weight gain = 31.54± 1.0 g, growth rate = 0.32 ± 0.01 g/d) than the hormonal sex reversed tilapia (weight gain = 25.91 ± 0.95 g, growth rate = 0.26 ± 0.01 g/d), which, in turn, had higher growth performance compared to the mixed sex tilapia (weight gain = 19.30 ± 0.98 g, growth rate = 0.20 ± 0.01 g/d).

To address the problem of unavailability and high price of protein concentrates in fish diets, a study was carried out to evaluate the possibility of replacing soybean meal, which is expensive, with Moringa oleifera leaf meal and sunflower seed cake in tilapia diets. Nine diets were formulated with different combination of soybean meal, Moringa leaf meal and sunflower seed cake. Nile tilapia fingerlings were grown in concrete tanks and fed the formulated diets for 90 days. Results indicated that fish fed the diet containing 50% sunflower seed cake and 50% Moringa leaf meal as sources of protein had higher (P ≤ 0.05) average body weight gain (23.03 ± 1.3 g) and growth rate (0.27 ± 0.01 g/d) than the fish fed other diets. Likewise, higher gross margin was obtained from  the fish fed the diet containing 50% sunflower seed cake and 50% Moringa leaf meal as protein sources. Therefore, the diet containing a mixture of Moringa leaf meal and sunflower leaf cake in equal proportions was found to be better than that with soybean meal.

In order to avoid unnecessary cost of feed, a study was carried out to determine the appropriate amount of feed and feeding frequency in order to minimize feed wastage and increase profitability. Three levels of feeding (i.e. 1%, 2.5% and 5% of fish body weight) and two feeding frequencies (i.e. daily feeding and alternate days feeding) were evaluated. Nile tilapia fingerlings were grown in concrete tanks for 90 days under a 3 x 2 factorial experiment. Results showed that the fish fed at the feeding level of 5% of body weight had higher (P ≤ 0.05) weight gain, growth rate, survival rate and feed conversion ratio while those fed at 1% feeding level had the lowest weight gain, growth rate, survival and feed conversion ratio. The fish under daily feeding regime had higher (P ≤ 0.05) weight gain, average daily gain and average feed conversion ratio than those under alternate days feeding regime. Feeding level of 5% of body weight and skipping a day resulted into better growth performance than daily feeding at 2.5% level.

To solve the problem of feed wastage in ponds and, hence, minimize production cost, an experiment was conducted to assess the effects of pond fertilization and supplementary feeding on the growth performance and profitability of Nile tilapia. Three treatments were compared namely, weekly fertilization alone with urea and Di-Ammonium Phosphate (DAP) at a rate of 3 g/m2 and 2 g/m2, respectively, concentrate feeding alone at 5% of fish body weight and weekly fertilization with urea and DAP plus concentrate feeding at 2.5% of fish body weight. Nile tilapia were grown in nine concrete tanks and subjected to the three treatments for a period of 166 days. Results indicated that fish reared in concrete tanks under fertilization plus concentrate feeding at 2.5% of fish body weight had higher (P ≤ 0.0001) weight gain (257.37 ± 5.71 g), growth rate (1.50 ± 0.03 g/d) and  gross margin (TZS 51,692,352 per ha per year) and  lower feed conversion ratio (1.49 ± 0.08) than the fish under concentrate feeding alone at 5% of fish body weight and fertilization alone.

The following conclusions were made: -

  1. Nile tilapia is superior in growth performance and survival compared to other tilapia
  2. The culture of all-male population of Nile tilapia results in higher growth performance and bigger fish than polyculture of mixed sex Nile tilapia and African
  • Growing Nile tilapia of mixed sex together with African catfish in the same pond produces tilapia of relatively larger size than is the case with only growing mixed male and female tilapias in the same pond.
  1. The hybrids of Nile tilapia and Wami tilapia have higher growth performance compared to hormonal sex-reversed Nile tilapia.
  2. The diet containing a mixture of 50% Moringa leaf meal and 50% sunflower seed cake as sources of protein promotes higher growth rate and results in higher profit than the diet containing soybean
  3. Daily feeding at the level of 5% of fish body weight promotes higher growth rate than feeding at 2.5% daily or at 5% on alternative
  • Feeding at the level of 5% of fish body weight on alternative days (skipping a day) results into better growth performance than daily feeding at 2.5% level.
  • Combination of weekly fertilization of ponds and concentrate feeding at 2.5% of fish body weight promotes higher growth rate and results into higher profit than either weekly fertilization alone or feeding alone at 5% of fish body weight.

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