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Inadequate seed production has historically been a factor limiting the commercial culture of Clarias gariepinus. The need to determine the environmental requirements of larvae was identified, in order to facilitate their successful mass rearing in hatcheries. The effects of key environmental factors on the growth, survival and aspects of the behaviour of C. gariepinus larvae and juveniles were investigated under controlled conditions. A strong emphasis was placed on the interpretation of the observed responses in terms of the natural history of the animal. Larvae were obtained by artificially inducing and spawning feral adult broodfish. Larval growth rates were highest in the temperature range 26-33ºC, with a peak at 30ºC. Survival of larvae was high between 22 and 33ºC. The final temperature preferendum of juveniles, determined in a thermal gradient was 30ºC, and it was concluded that this temperature is probably optimal for most of the physiological processes of the animal. Larvae and juveniles displayed strong negative phototaxis. When reared under different photoperiod regimes, larval growth increased with longer dark periods, however a similar rate of mortality was recorded in all photoperiod treatments. The provision of cover was found to enhance larval growth under conditions of continual light (24L/OD), however under conditions of continual darkness (OL/24D) similar growth rates were recorded regardless of the presence or absence of cover. All growth rates in continual darkness (OL/24D) were higher than those in continual light (24L/OD), regardless of whether cover was provided or not. It was concluded that a OL/24D photoperiod is optimal for larval rearing. At salinities between 0 and 5% similar growth and mortality rates of larvae were recorded. At 7.5 % larval growth and survival rates were lower, and at 10 % all larvae died within 48 hours. The 96h-LCSO for unionised ammonia was found to be 2.3 mg/l. The cytological effects of unionised ammonia were evident as degenerative changes in the gill and liver tissues. In an investigation of tank hygiene, it was found that larval growth and survival rates were highest in tanks not cleaned at all. As the frequency of tank cleaning increased, the growth and survival rates of larvae decreased. When reared at densities ranging from 45-450/1, larval growth was found to be density dependent. A model of hatchery productivity (number larvae of standard size produced/time/vol.) was developed based on the density dependent nature of larval growth. The model predicts that maximum productivity, will be achieved at a rearing density of 1400/1. The high growth and survival rates obtained over a broad range of each environmental parameter investigated serve to explain how larvae survive and grow in their unstable floodplain habitat in nature. The broad environmental tolerances of C. gariepinus are of benefit with respect to the culture of larvae, as successful larval rearing is possible over a fairly wide range of environmental conditions.

Past and current research on the biology and culture of the sharptooth catfish (Clarias gariepinus) has stimulated the development of its commercial production. At the inception of this project in 1985, it was decided that the development of an optimal dry feed was the most important step in developing a more complete culture technology for the species. The objectives of the project were, therefore, to investigate aspects of the nutritional physiology and dietary requirements of the species to provide the necessary information for the formulation of optimum economical feeds. A review of the natural feeding biology of the species provided valuable information with regard to its food preferences. From this information, and by estimating the nutritional composition of its preferred natural diet, it could be concluded that the animal requires a relatively high dietary protein content. This conclusion was substantiated by studies of the functional morphology of the digestive system (gross and fine structure). The characteristics of the digestive system and external anatomy were found to be typical of an opportunistic, omnivorous predator. The ontogenetic development of the digestive system is relatively fast, and enables the utilization of a variety of diets efficiently from an early stage. Since the ability of an animal to digest a given substance is predominantly dependent on the presence of appropriate enzymes the quantification and characterization of C. gariepinus digestive enzyme activities were investigated on an experimental basis. This work revealed that the animal has relatively high levels of digestive enzyme activities, and that its proteolytic enzyme activities correspond with those of other carnivorous fishes, while its starch digesting abilities correspond with those of specialized herbivores. Also, the high levels of lysozyme and alkaline phosphatase indicate an adaptation to detritivory, making this fish a truly opportunistic omnivore. Evidence was found to suggest that sharptooth catfish are able to re-absorb secreted digestive enzymes in the posterior section of its intestinal tract. It was shown that digestive enzyme activities were induced by food intake, and that no inherent rhythmicity in digestive activities seemed to occur. The development of digestive enzyme systems in the early life history of this animal was found to be also extremely rapid (complete within 10-16 days after hatching). It was also shown that artificial dry feed stimulates higher levels of digestive enzyme activities in larvae and juveniles than natural food. Although increased cellulase activity was found in larvae and juveniles that were fed on live zooplankton, the relative contribution of exogenous prey enzymes to protein digestion could not be ascertained. Specific nutritional requirements were investigated by means of feeding trials. The results of these trials confirmed the above indications regarding the dietary requirements of sharptooth catfish. It was concluded that for optimal growth and production performance, this animal requires a dietary protein content of 40-42%, a dietary lipid content of 10-12% and that diets should contain 14-16 kJ/g digestible energy. The optimum protein-to-energy ratio was found to be 26-29mg protein per kJ of digestible energy. Dietary requirements are evidently the same for juveniles and adults, except that younger fish require higher relative feeding levels. Finally, practical feed formulations were evaluated in terms of their economic feasibility. The results of these trials are presented along with recommendations on the formulation of commercial feeds for C. gariepinus. A computer program with which to implement least cost feed formulation by means of linear programming is provided.

Aspects of the biology of the sharptooth catfish, Clarias gariepinus (Burchell, 1822) were studied in a clear coastal lake (Lake Sibaya, KwaZulu.). Catfish reach maturity towards the end of the first year (total length 200-250mm) and breed in summer. Gonadal maturation is cyclical, and most fishes are ripe between September. and January e ach year. Maturation stages and fecundity are described. Twenty-two catfish spawning runs were witnessed. There were no large aggregations of catfish before the Summer rains, as reported in some impoundments and rivers, but catfish accumulated on the shallow terrace immediately before a spawning run. Spawning usually took place in flooded marginal areas after heavy rain on dark calm nights. Peak activity occurred between 20h00 and 02h30. Pre-nuptial aggression, courtship and mating are described and illustrated in detail for the first time from field observations. Gonadal products are released while in amplexus and fertilized externally. The eggs have an adhesive disc for attachment to plants, and develop rapidly. Early development and feeding are described. The growth rate of catfish was determined using rings on the pectoral spines, and validated using recent techniques. C.gariepinus in Lake Sibaya grow rapidly and remain in good condition to a length of 500mm, but larger catfish have a poor growth rate and condition, and few exceed 650mm. The modal length and weight of the Sibaya population is intermediate between that of other populations but the number and condition of larger catfish in Lake Sibaya is lower. Other studies on C.gariepinus growth are reviewed. Very high and low first year increments obtained previously may be due to methodological errors, but catfish do appear to have variable growth rates. The size reached by C.gariepinus is reviewed. Catfish are equipped morphologically and behaviourally to feed on a wide variety of·prey in different situations, but they usually feed at night on exposed, active benthic organisms. Social hunting facilitates the capture of prey which is too fast or manoevrable for a single predator. Food preferences are scored by three different methods and presented for the total catfish population and for different length groups, seasons, and habitats. Juveniles feed more often and on a greater variety of prey, and their condition is better than that of adults. The proportions of several food items in the diet changed at different lake levels as marginal areas became inundated. In particular, increasing lake levels resulted in a change in relative density of three cichlid prey in the shallow water feeding area of catfish. This situation provided the opportunity to study the effect of changing cichlid density on predation by catfish. The world literature on factors influencing the predator-prey relationship in fish is reviewed and several key factors are identified. The interaction of fish predators and prey in Africa, and particularly Lake Sibaya, is examined in detail, and prey density, modified by accessibility, is found to be an important factor. The effect of cichlid prey density on catfish predation was therefore investigated using field collections and experimental observations. Catfish prey on the cichlid species with greatest relative abundance, with some deviations which can be explained in terms of accessibility. The experimental results also describe feeding periodicity, imitative hunting and the effect of water depth , day-night light intensity changes and predator number on predation efficiency on cichlids. Catfish predation is discussed in terms of recent predation theory, and it is concluded that their ability to switch from one prey to another as prey availability (= density+ accessibility) changes , is an an important aspect of their success as predators. Catfish predate more efficiently on invertebrates, but feed largely on fishes after the first year. This preference for fish prey is probably related to food quality, as fishes contributed over 75% of the energy content of all ingested food, as well as abundant protein and the correct amino acid balance. Finally, I conclude that the role of C.gariepinus as a predator on S.mossambicus is three-fold: (a) to enhance species' fitness by removing enfeebled individuals i.e. to act as a 'cleanser' (b) to restrict the distribution of prey under certain conditions i.e. act as a 'restricter' (c) to dampen fluctuations in cichlid abundance i.e. act as a 'regulator'. The relative importance of these roles may alter at different lake levels in Lake Sibaya in response to changes in the vulnerability of fish prey.

Based on a number of attributes, the African sharptooth catfish, Clarias gariepinus is an ideal aquaculture species. Much work has been undertaken on developing the technology for the commercial culture of the species inclusive of determining the nutritional requirements for cost effective feed formulation. However, digestible energy (DE) values for specific feed ingredients were unknown. As DE values are not interchangeable between species, this study set out to determine the DE values for conventional and unconventional feed-ingredients for the formulation of an optimal feed. The ultimate goal in feed formulation is to produce a diet that supports the maximum production at the least cost. The indirect method was found to be most suitable in obtaining DE values for the catfish. Feed samples were marked by adding 2% chromic oxide as an inert indicator. The fish were then force-fed and faecal samples were obtained from the hindgut by stripping the fish 10 hours after feeding as this was found to be when the hind-gut was at its fullest and the feed maximally digested. A linear based computer programme was used to formulate experimental diets using the established DE values as an additional variable. The formulated feed was tested and compared to a commercial catfish diet and a standard reference diet (H-440). All of the newly formulated feeds were found to be cheaper to manufacture (by between 5 and 10%). Statistically (ANOVA), all of the experimental diets resulted in higher specific growth rates (SGR) than the standard reference diet. The commercial diet also resulted in a lower SGR than those experimental diets with similar protein levels.

The need for developing an artificial dry feed for Clarias gariepinus larvae was identified. Dry larval fishfeed requires certain physical properties. Techniques for manufacturing feeds which comply with these requirements were evolved and are recomended for practical use. The techniques that were used for artificial reproduction of C. gariepinus larvae are briefly discussed. The larvae were fed on various dietary formulations and their growth responses were monitored in order to determine their nutritional requirements. A feed was developed which gave superior results to those obtained with natural food organisms. This feed consisted of Torula yeast(69,8%), fishmeal(23,3%), vitamin supplements(O,9%), methionine supplement (150mg/kg), bactericide and preservatives. The feed particles were coated with a lipid supplement which was added at a ratio of 6% to the weight of the feed. The optimum particle size (diameter) for C. gariepinus larvae is 2,2% of their mean total length. The larvae should be given a feed ration of 25% of body weight per day at a frequency of once every four hours. A favourable feed conversion ratio of 1.1g of feed consumed per gram of larval weight gain was experienced. The established dry feed is suitable for use in commercial hatcheries and can be used as a standard and proven food source for further research on the intensive rearing of C. gariepinus larvae.

The aim of this study was to evaluate the aquaculture potential of a hybrid between the two largest African silurid catfish, Heterobranchus longifilis (HL) and Clarias gariepinus (CG) . A morphometric, meristic and karyological study showed that the hybrid shared some of the physical characteristics of both C. gariepinus and H. longifilis, while its chromosome complement and fundamental number (2N = 54, FN = 95), was intermediate between that of H. longifilis (2N = 52, FN = 92) and C. gariepinus (2N = 56, FN = 97). The HLxCG cross could therefore be regarded as a true hybrid. For the characters tested, no morphological or karyological differences were apparent between the HLxCG hybrids produced in West Africa and those produced in southern Africa. In southern Africa, the HL♂xCG♀ cross had higher fertilization and hatching rates than pure strain H. longifilis, C. gariepinus or the reverse cross. There was no difference in the survival of the HL♂xCG♀ hybrid larvae and C. gariepinus larvae up to the onset of exogenous feeding. Pure strain H. longifilis juveniles had a faster growth rate than the H. longifilis ♂ x C. gariepinus ♀ juveniles, but the hybrid always grew at a faster rate than C. gariepinus or the reverse cross. The HL♂xCG♀ cross was consequently chosen as the hybrid with the greatest potential for siluroid aquaculture in southern Africa. The HL♂xCG♀ hybrid showed evidence of partial gonadic, gametic and post-zygotic sterility in both sexes. The hybrid was not completely sterile, as it was artificially induced to spawn, and a small number of viable F₂ hybrid and F₁ hybrid x C. gariepinus larvae were produced. However, in the light of its probable reproductive strategy, it is highly unlikely that the hybrid would pose an ecological risk to the southern African region. The H. longifilis ♂ x C. gariepinus ♀ hybrid was compared to C. gariepinus for selected water quality preferences and tolerances. The hybrid had a wider temperature preference (28 °C to 34 °C) than C. gariepinus (28 °C to 30 °C), but appeared to be more dependent on aerial respiration than C. gariepinus. The air-breathing frequency of hybrid fish began to increase at dissolved oxygen concentrations below 3 .8 mg.ℓ⁻¹, while C. gariepinus only showed an increase in air-breathing frequency at concentrations below 3.0 mg.ℓ¹. The hybrid was more tolerant of un-ionised ammonia (96-hour LC₅₀ = 9.1 mg.ℓ⁻¹) than C. gariepinus (96-hour LC₅₀ = 6.5 mg.ℓ⁻¹), but their 96-hour LC₅₀ salinity tolerances were similar (10.8-11.0 g.ℓ⁻¹). The H. longifilis ♂ x C. gariepinus ♀ hybrid had a higher fillet yield (43.9 %) than C. gariepinus (38.9 %), but the crude protein content and amino acid profile of the two groups were similar. Catfish are traditionally grown in earthen ponds under semi-intensive conditions in southern Africa, at around 4 kg of fish per cubic meter of water (kg.m⁻³). However, the hybrid could tolerate densities of up to 415 kg of fish per cubic metre of water, if the water was exchanged hourly (kg.m⁻³.hr⁻¹), and the density at which yield was optimised was rounded off to 400 kg.m³.hr⁻¹. The high threshold density and water quality tolerances of the H. longifilis ♂ x C. gariepinus ♀ hybrid indicates that it is ideally suited for highly intensive aquaculture. It was concluded that it would be more productive to farm the HL♂xCG♀ hybrid on an intensive basis in southern Africa, than it would be to farm C. gariepinus in the traditional manner.

Made available in DSpace on 2016-06-02T19:22:53Z (GMT). No. of bitstreams: 1 3162.pdf: 1363154 bytes, checksum: 725b7e9d57697bbcc0ab7a78df25145a (MD5) Previous issue date: 2010-06-25 / Financiadora de Estudos e Projetos / Air-breathing fish are classified as obligatory (when breathing obligatory atmospheric air independently of the water O2 tension) or facultative air-breather (using an air breathing organ ABO -, when theirs gills are not able to extract all O2 necessary to maintain the aerobic mechanisms under hypoxic conditions). The catfish, Clarias gariepinus, is airbreathing fish that shows modifications on the gill lamella, forming a ventilatory fan, and on the 2o e 4o gill arches, forming an arborescent organ. These structures form the ABO of this specie.The objectives of the present study were: 1. To determine if C. gariepinus is an obligatory or a facultative air breather. 2. To analyze the cardio-respiratory responses ( VO2 - metabolic rate; VG - gill ventilation; VT ventilatory volume; fR respiratory frequency; EO2 O2 extraction from the ventilatory current; fH- heart frequency) in response to progressive hypoxia. 3. To verify if the critical O2 tension (PcO2) of this specie is correlated with the O2 uptake from the atmospheric air.To classify the air-breathing mode of C. Gariepinus, the fish (Wt ~ 350 g; n = 7) were maintained in normoxia during 24 hours without access to air. The fR maintained constant during all the experiment and there was no mortality, indicating that C. gariepinus is a facultative air-breather. To analyze the cardio-respiratory responses to progressive hypoxia, VO2 , EO2, fR, VT, VG , EO2 and fH were recorded under the following water O2 tensions(PwO2): 100, 70, 50 e 30 mmHg. Fish maintained a constant VO2 until the PcO2 (~ 55 mmHg), below which VO2 decreased significantly. This decreasing was followed with the significant reduction of EO2 in PinspO2 of 62,7 ± 1,30 mmHg reaching values of 19,6 ± 1,9 % in severe hypoxia. The VG and the VT increased progressively until PinspO2 of 28,0 ± 0,5 mmHg, reaching highest values of, respectively, 1545,7 ± 63,5 mLH2O.kg-1.min-1, 33,9 ± 0,8 mLH2O.Kg-1.resp-1 e 57,2 ± 1,4 resp.min-1. The fH reduced progressively from 43,4 ± 0,4 bpm, in normóxia, arriving significant values just above the PcO2 and reaching minimum values (19,2 ± 3,0 bpm) in severe hypoxia. Under progressive hypoxia (100, 70, 50, 30 e 20 mmHg) and with the access to the atmospheric air, C. gariepinus (Wt ~ 610 g; n = 9) presented a 5-fold increase in the air-breathing frequency (fRA). A bradycardia was observed just before the air breath and a tachycardia just after.Concluding, C. gariepinus is a continuous facultative air-breathing fish that regulate the 2 O V until the PcO2 of ~54 mmHg. Below this tension fish increase the VG mainly due to a larger increase of VT (lower metabolic cost of VG ). The hypoxic pre-air breath bradycardia is characteristic of aquatic breathers while the post-air breath tachycardia is typical of air respirators. The fRA increased proportionally with the progressive hypoxia, mainly just above the PcO2. These results show that C. gariepinus is adapted to survive at hypoxic habitats and that this species show a higher dependence of the atmospheric air than the others facultative air-breathing fishes. / Peixes de respiração aérea são classificados como respiradores aéreos obrigatórios (aqueles que respiram ar atmosférico independente das tensões de O2 da água) ou facultativos (aqueles que usam o órgão de respiração aérea ABO quando as brânquias não conseguem extrair a quantidade de O2 necessário para manter o metabolismo aeróbico em condições hipóxicas). O bagre-africano, Clarias gariepinus, é um peixe de respiração aérea que apresenta modificações na lamela branquial, formando um leque ventilatório, e, nos 2os e 4os arcos branquiais, os órgãos arborescentes. Estas últimas estruturas formam o ABO desta espécie.Os objetivos do presente estudo foram: 1. Determinar se C. gariepinus é um respirador aéreo facultativo ou obrigatório. 2. Analisar as respostas cardiorrespiratórias ( VO2 - taxa metabólica; EO2 - extração de O2 da corrente ventilatória; VG - ventilação branquial; VT - volume ventilatório; fR - frequência respiratória; fH- frequência cardíaca) em resposta a hipóxia gradual. 3. Verificar se a tensão crítica de O2 (PcO2) desta espécie está relacionada com a tomada de O2 do ar atmosférico.Para classificar a modalidade da respiração aérea de C. gariepinus, os peixes (Wt ~ 350 g; n = 7) foram mantidos em normóxia durante 24 h sem acesso ao ar atmosférico. A fR manteve-se constante durante todo o experimento e não houve mortalidade, indicando que C. gariepinus é um respirador aéreo facultativo. Para analisar as respostas cardiorrespiratórias em normóxia (controle) e hipóxia gradual, a VO2 , EO2, fR, VT, V G e fH foram registradas durante as seguintes tensões de O2 (PwO2): 100, 70, 50 e 30 mmHg. Os peixes mantiveram VO2 constante até a PcO2 (~ 55 mmHg), abaixo da qual a VO2 decresceu significativamente. Esta diminuição da VO2 foi acompanhada da diminuição significativa da EO2 em PinspO2 de 62,7 ± 1,30 mmHg atingindo valores de 19,6 ± 1,9 %, em hipóxia severa. A VG , VT e fR aumentaram progressivamente até a PinspO2 de 28,0 ± 0,5 mmHg, chegando a valores máximos de, respectivamente, 1545,7 ± 63,5 mLH2O.kg-1.min-1, 33,9 ± 0,8 mLH2O.Kg-1.resp-1 e 57,2 ± 1,4 resp.min-1. A fH diminui progressivamente de 43,4 ± 0,4 bpm, em normóxia, até alcançar valores significativos próximos a PcO2 e valores mínimos em hipóxia severa (19,2 ± 3,0 bpm). Em experimentos de hipóxia gradual (100, 70, 50, 30, 20 e 10 mmHg) com acesso ao ar atmosférico, C. gariepinus aumentou aproximadamente 5x a frequência de respiração aérea (fRA). Em cada tensão hipóxica ocorreu uma bradicardia pré-RA seguido de taquicardia significativa pós-RA (típico de respiração aérea); a fR manteve-se praticamente constante (~ 32 resp.min-1) até as duas últimas PwO2 (20 e 10 mmHg), nas quais a fR diminuiu para 23,0 ± 1,18 resp.min-1.Finalmente, C. gariepinus é um peixe de respiração aérea facultativa contínua que regula a VO2 até a PcO2 de ~ 55 mmHg. Abaixo desta tensão o animal aumenta a VG devido principalmente a um pronunciado aumento na VT (diminuição do custo metabólico da VG ). A bradicardia hipóxica pré-RA é uma característica de respiradores aquáticos, enquanto a taquicardia hipóxica pós-RA é típica de respiradores aéreos. A fRA aumentou proporcionalmente com a hipóxia gradual, principalmente próximo a PcO2. Tais resultados demonstram que C. gariepinus está adaptado a sobreviver em habitats hipóxicos e demonstra uma maior dependência do ar atmosférico do que outros respiradores bimodais facultativos.

The successful production of quality early fish stages is crucial for artificial rearing fish species. Nevertheless, it is quite a problematic process, because fish as poikilothermic organisms are highly sensitive to various environmental conditions. Moreover, both fish embryos and larvae are more affected by biotic and abiotic factors (such as predation, stocking density, temperature, light, pH, salinity, oxygen saturation, feeding availability, or their interactions) than older juveniles or adults. Most fish processes such as reproduction, metabolism, survival, development and growth can be considerably (positively, negatively or not) influenced by these conditions. Therefore, it is important to understand their impacts on fish stages during embryonic, larval, juvenile, adult, and eventually the senescent period. Then, following the application of new knowledge to the practise, especially species- and age-specific optimal conditions, should result in higher survival, better growth rate, well developed fish without deformities and lower cannibalism. The objectives of the present thesis were to assess the effect of water temperature and light intensity on the early development (embryonic and larval period) of African sharptooth catfish as a perspective fish species for freshwater intensive aquaculture. Due to its great characteristics (for instance fast growth, ability to breathe atmospheric air, rearing at high stocking density) this species can meaningfully contribute to the global aquaculture production which is expected to rise in the coming years. However, there are still some difficulties in the early rearing African sharptooth catfish regarding the cannibalism, feeding, survival and necessity of detailed information about the effects of some abiotic factors and their interactions.