The ecology of the catfish Clarias gariepinus and Clarias ngamensis in the Shire Valley, Malawi

The ecology of two commercially important catfish species, Clarias gariepinus and C. ngamensis , was examined in the Elephant Marsh, southern Malawi. Abundance, distribution, and movements within the marsh system were studied. Growth rates were determined from rings on the vertebrae, with corroborative evidence from length frequency analyses and tagging studies. Breeding seasons, length and age at maturity, and fecundity were determined for both species. Stomach contents were examined and the food preferences of the two species were compared.     

Reproductive strategies of smooth-head catfish Clarias liocephalus (Boulenger, 1898), in the Rwizi-Rufuha wetland system,south-western Uganda

The reproduction of the smooth-head catfish (Clarias liocephalus), a heavily exploited wetland fish in Uganda, in a data-deficient fishery, was studied from January to December 2011. Analyses were based on a sample of 854 fish specimens obtained from a chain of wetlands that fringe the Rwizi River, a tributary of the Nile River. Samples were collected monthly from four sites. Fecundity, gonadosomatic index, size at sexual maturity, condition factor and growth patterns were used to describe the reproduction of the species. Mean female fecundity was 2 484.03 ± 1 289.90 (range 266–3 474). Females attained sexual maturity at a smaller size (12.0 cm TL) than males (13.79 cm TL), but were in better condition than males. Gonadosomatic index peaked in the wettest months of the study period, with highest proportion of mature ova occurring during July to November. Although C. liocephalus seems less fecund than other known clariids and is therefore prone to adverse effects from overexploitation, knowledge of its spawning periodicity and its size at sexual maturity could be useful in the management of the fishery in wetlands where it is still abundant.     

A karyological analysis of the artificial hybridization between Clarias gariepinus (Burchell, 1822) and Heterobranchus longifilis Valenciennes, 1840 (Pisces; Clariidae)

A karyological analysis of an artificial hybridization (reciprocal crosses) between two African clariid catfish, Clarias gariepinus (Burchell, 1822) and Heterobranchus longifilis Valenciennes, 1840, was performed. C. gariepinus has a standard karyotype of 2 n = 56, while H. longifilis has 2 n = 52. The hybrids revealed an intermediate karyotype (2 n = 54), and it appears as if they have totalized the haploid chromosome number of both parental species, excluding gynogenesis or androgenesis. The hybrid karyotype is considered as aneuploid, although the hybrids proved to be fertile. No variation was found in the hybrids karyotypes.     

Male-specific DNA markers from African catfish (Clarias gariepinus)

We searched for sex-specific DNA sequences in the male and female genomes of African catfish, Clarias gariepinus (Burchell, 1822) by comparative random amplified polymorphic DNA (RAPD) assays performed on pooled DNA samples. Two sex-linked RAPD markers were identified from the male DNA pool and confirmed on individual samples, showing good agreement with phenotypic sex. Both markers were isolated, cloned and characterized. The first marker (CgaY1) was nearly 2.6 kb long, while the length of second one (CgaY2) was 458 bp. Southern blot analysis with a CgaY1 probe showed strong hybridizing fragments only in males and not in females under stringent conditions, indicating the presence of multiple copies of CgaY1 in the male genome. When tested by zoo blot on the genomes of two closely related species from the Clariidae family, CgaY1 hybridized to the DNA of Heterobranchus longifilis and generated a faint male-specific band at low stringency. CgaY2 produced similar hybridization pattern in both sexes of C. gariepinus, C. macrocephalus and H. longifilis. Specific primers were designed to the sequences and the markers were amplified in multiplex PCR reactions together with a control band common to all individuals. This allowed for rapid, molecular sexing of the species on the basis of a simple three band (male) versus one band (female) pattern. According to our knowledge these are the first sex-specific DNA markers isolated from a siluroid fish species. This revised version was published online in July 2006 with corrections to the Cover Date.     

Two gonadotropin-releasing hormones from African catfish (Clarias gariepinus).

Two forms of gonadotropin-releasing hormone (GnRH) have been purified from brain extracts of the African catfish, Clarias gariepinus, using reverse-phase high performance liquid chromatography (HPLC) and radioimmunoassay (RIA). The amino acid sequences of both forms of African catfish GnRH were determined using Edman degradation after digestion with pyroglutamyl aminopeptidase. In addition, both GnRHs were studied by mass spectrometry. The primary structure of African catfish GnRH I is identical to Thai catfish GnRH I, pGlu-His-Trp-Ser-His-Gly-Leu-Asn-Pro-Gly-NH2, and the primary structure of African catfish GnRH II is identical to the widely distributed and highly conserved chicken GnRH II, pGlu-His-Trp-Ser-His-Gly-Trp-Tyr-Pro-Gly-NH2.     

Quantitative studies of steroid bioconversions in the seminal vesicles of spawning male African catfish, Clarias gariepinus (Burchell), under natural conditions, and of non-spawning catfish under natural and fish farm conditions

1. Steroid bioconversions in the seminal vesicles of Clarias gariepinus were studied quantitatively in vitro by tissue incubations with [3H]pregnenolone, [3H]androstenedione and [14C]11 beta-hydroxyandrostenedione, respectively. 2. Spawning and non-spawning catfish, collected in the Hula nature reserve in northern Israel during the spawning period, and non-spawning animals, collected from a fish pond in the same region during the same period, were studied. 3.Spawning animals showed a significantly higher production of 17 alpha-hydroxyprogesterone, 5 beta-pregnan-17 alpha-ol-3,20-dione and 5 beta-reduced androgens than non-spawning feral and pond catfish, as a result of a significantly increased contribution of the enzymes 5 beta-reductase and 3 alpha-hydroxysteroid dehydrogenase (HSD). 4. In spawning catfish the concentration of gonadotropin in blood plasma were also significantly higher than in the plasma of non-spawning feral and pond catfish. This increase in gonadotropin level might have induced the rise in enzyme activity of 5 beta-reductase and 3 alpha-HSD. 5. It is concluded that the absence of a shift in steroidogenesis towards the production of 5 beta-reduced steroids may be among the factors preventing spontaneous spawning in male African catfish under husbandry conditions.     

Effects of oxytocin on semen release response in African catfish (Clarias gariepinus)

In silurid fishes, semen collection is practically impossible, even after hormonal stimulation. Instead, males are killed and testes macerated to obtain sperm. To understand the endocrine control of semen release in catfishes, we investigated the role of smooth muscle contractors in semen release and semen quality of African catfish (Clarias gariepinus). In in vitro experiments, testis slices were incubated with oxytocin (1 and 10 IU), isotocin (2 and 20 ug), vasopressin (0.2 and 2 ug), epinephrine (1 and 10 ug), PGF2alpha (1 and 10 ug), purified Clarias LH (300 ng) and partly purified Clarias pituitary extract (containing 300 ng LH). Only oxytocin increased sperm concentration of the medium (assessed by optical density measurements) compared to control incubations. Oxytocin was then tested in vivo in two groups of fish: normal males, and males that had been treated with 17alpha-methyltestosterone during larval stages to inhibit seminal vesicle development (MT males). Both groups of fish received two doses of carp pituitary suspension (8 and 10 mg/kg, respectively i.m.) with or without subsequent oxytocin treatment (5 IU/kg i.v.; cPS-OT treatment and cPS treatment, respectively). There was no effect of oxytocin on the number of strippable males. Of cPS and cPS-OT treated fish, 87% MT males and 60% normal males were strippable. The stripped semen volume was low in both groups but MT males produced higher (P < 0.001) hatching rates (63.1%) than did normal males (2.1%).     

Aspects of the fecundity of the feral catfish,Clarias macrocephalus (Gunther), population obtained from the rice fields used for rice-fish farming,in Malaysia

The catfish, Clarias macrocephalus, harvested from the irrigated rice fields of Malaysia, exhibited spawning activities that are influenced by water level management. The multiple spawning potential of the fish is negated by declining water level and the fish spawned only once per growing season. Although oocyte development started in conjunction with water level increase, subsequent oocyte development did not take place due to low water level. Absolute and relative fecundity also decreased with declining water level, however, mean relative fecundity is relatively constant (17.6–53.7 oocytes g^–1 body weight) and not related to body size. Absolute fecundity ranged between 785 to 14066 oocytes female^–1, whereas relative fecundity is between 7.1 to 83.5 oocytes g^–1 body weight, respectively.     

The ontogeny of the chondrocranium in Clarias gariepinus: trends in siluroids

The ontogeny of the chondrocranium of 31 different stages of the African catfish Clarias gariepinus (Siluroidei: Clariidae) was studied, both from cleared and stained, and sectioned material. The fish ranged from 4.1 (1 day post-hatching) to 127.0 mm SL (100 days post-hatching). The chondrocranium of C. gariepinus seemed to correspond to the general adaptive trends in siluroids, especially in relation to the reduction of eye size and the dorso-ventral flattening of the skull. The platybasic neurocranium involved several modifications related to the trabecular bars, the hypophyseal fenestra, the ethmoid region and even the olfactory nerves. Certain reductions were present, which have been observed in all siluroids (e.g. absence of the pila lateralis, the commissura lateralis, the myodomes) or are part of a variable trend within siluroids (e.g. reduction of the taenia marginalis anterior and the tectum synoticum). Compared with some other siluroid species, the neurocranium of C. gariepinus is well developed, for example in the otic region. The same was observed in the splanchnocranium where some general siluroid trends persist (e.g. isolation of palatine from pterygoquadrate, presence of 'hyo-symplectic-pterygoquadrate' plate). Some trends, as observed in other siluroids, were present also (e.g. interhyal continuous with suspensorium and ceratohyal, Meckel's cartilage initially continuous with the suspensorium). The branchial basket is well developed as all expected elements are present (basibranchials I-IV, hypobranchials I-IV, ceratobranchials I-V, epibranchials I-IV). Based on the observed ontogeny of C. gariepinus and data from the literature, a hypothesis was formulated which indicated the presence of a general reductional trend within siluroids. In C. gariepinus , all four (I-IV) infrapharyngobranchials develop, although the anterior two are much reduced and fused with each other.     

Dietary and seasonal effects on the dorsal meat lipid composition of Japanese (Silurus asotus) and Thai catfish (Clarias macrocephalus and hybrid Clarias macrocephalus and Clarias galipinus)

The effects of dietary lipids and seasonal variation on the lipids of wild and cultured catfish (Japanese catfish, Silurus asotus; Thai catfish, Clarias macrocephalus and hybrid Clarias macrocephalus x Clarias galipinus) were determined by analysis of the lipid content and fatty acid composition of their dorsal meat. The predominant fatty acids of dorsal meat were 16:0, 18:1n-9, 18:2n-6, 20:4n-6 (arachidonic acid, AA), and 22:6n-3 (docosahexaenoic acid, DHA). The DHA content in the diet of Japanese catfish was higher than that in the diet of Thai catfish, and this was reflected in the dorsal meat of the Japanese catfish, which had a remarkably high percentage of DHA compared with the meat of the Thai catfish. Cultured Japanese catfish had a higher percentage of 18:2n-6 than Thai fish and a lower percentage of AA in winter than in summer season. There were also seasonal variations in the percentage of n-6 fatty acids in Japanese catfish. In summer, the fatty acid composition of the cultured Japanese catfish was similar to that of the wild catfish. These fatty acid changes in the lipid classes, triacylglycerol, phosphatidylcholine and phosphatidylethanolamine were similar to those observed for total lipids. These results indicate that the percentage of DHA in the dorsal meat of catfish is influenced by dietary fatty acid, and it may be that it can be increased in cultivated fish by administering a diet containing a large amount of DHA.     

Temperature-induced changes of early development and yolk utilization in the African catfish Clarias gariepinus

Artificially fertilized eggs and yolk-sac larvae of a freshwater tropical/subtropical fish Clarias gariepinus receiving no external food were incubated at 22, 25 and 28° C until full yolk resorption. Developmental time, size and matter composition (CHNS-O Analyzer and ashing) were assessed at egg fertilization, hatching and yolk resorption; respiration was measured every 4–5 h. The course of acceleration of C. gariepinus embryonic developmental rate with temperature ( Q_{10 dev}) was compared over the temperature range to those of Cyprinus carpio and Oncorhynchus mykiss ; they differed greatly, but were similar when compared on the basis of effective temperatures specific to each fish. Specific growth rates for energy (88, 150 and 183% per day at 22,25 and 28° C, respectively) as well as the conversion efficiencies of egg energy (64, 71 and 68%, respectively) and protein (71, 78 and 76%, respectively) in C. gariepinus larval tissues were higher than those known for the endogenous feeding period of coldwater and temperate fish species. In C. gariepinus at the end of yolk resorption, the carbon percentage and caloric values of dry weight, size (in terms of dry matter, minerals, protein and energy per larva) and transformation efficiencies were lowest at 22° C, highest at 25° C and had slightly decreased at 28° C. A tentative mechanism which leads to the positive or negative response of body size to temperature over the viable temperature range is defined.     

The effect of emersion and handling on the nitrogen excretion rates of Clarias gariepinus

African catfish Clarias gariepinus (=119.9&30.6 g) were exposed to periods of emersion (5, 30, 60, 90 and 180 min) and the ammonia and nitrogen excretion rates measured following re-immersion. Immediately following re-immersion (0–30 min), the ammonia excretion and relative ammonia excretion was greatest for the 5-min emersion gro up. Exposure to extended periods of emersion resulted in a decrease in total nitrogen excretion, notably ammonia excretion, although no significant changes were observed in the non-ammonia component of C. gariepinus .     

Gonadal steroids and the maturation of the species-specific gonadotropin-releasing hormone system in brain and pituitary of the male African catfish (Clarias gariepinus)

The effect of testosterone (T), 11-ketotestosterone (KT) and estradiol (E(2)) on the development of the catfish gonadotropin-releasing hormone system (cfGnRH) of male African catfish (Clarias gariepinus), at the onset of puberty [between 10 and 12 weeks post hatching (ph)] was investigated. The cfGnRH neurons, located in the ventral forebrain, were visualized by immunofluorescence and their numbers were determined and the amounts of cfGnRH-associated peptide (cfGAP) in the pituitary were measured by RIA. Steroid treatments did not significantly alter the numbers of immunoreactive GnRH neurons. However, T and E(2) caused an increase in the amount of GnRH, demonstrated by the intensity of the immunostaining of GnRH neurons and fibers in the brain and the amount of cfGAP in the pituitary. Treatment with KT, the main circulating androgen in adult male catfish, neither changed the number of cfGnRH neurons, nor elevated the cfGnRH content in the pituitary. In previous experiments with younger, prepubertal fish (2-6 weeks ph), T caused an elevation of the number of cfGnRH neurons to the same level as present in pubertal fish of 12-14 weeks. We conclude that the onset of puberty in the male African catfish coincides with the completion of the steroid-dependent structural maturation of the cfGnRH system in the brain. T and/or E(2), however, are still able to exert a positive influence on the amounts of cfGnRH during the later stages of pubertal development, thus still playing a role in the control of the cfGnRH system.     

Growth response of African catfish, Clarias gariepinus (Burchell) to varied protein and lipid levels

To study the growth response of African catfish (Clarias gariepinus) to the diets containing two protein (30 and 35%) and two lipid levels (12 and 18%) in a closed recirculatory system for 60 days, fish were fed to satiation twice daily. The experiment was carried out in triplicate groups. Fortnightly sampling of fish and water samples was carried out. The best result in terms of growth was obtained with the diet containing 34.62% protein and 12.12% lipid. The protein to lipid ratio was 18.57 in the diet that gave best percentage weight gain. However best survival, food conversion ratio and protein efficiency ratio were recorded in the diet containing 29.86% protein and 18.08% lipid.     

Photoperiod affects growth, behaviour and stress variables in Clarias gariepinus

Short periods of light or no light (18D : 06L and 24D : 00L) resulted in an increased growth compared to extended periods of light (06D : 18L and 12D : 12L) in African catfish Clarias gariepinus . Fish under longer periods of light (12D : 12L and 18D : 06L) showed higher swimming activity, more aggression (injuries on the body) and higher lactate, free fatty acids and cortisol levels compared to those who were reared at shorter periods of light (24D : 00L and 18D : 06L). Feeding activity during light and dark periods in this experiment showed that C. gariepinus had both night and day feeding activities, with a preference to diurnal feeding in the 12D : 12L photoperiod. The results showed that light plays an important role in the African catfish behaviour and its well‐being. As the hours of light increased during the 24 h cycle, data suggests that the fish were more stressed and aggressive, compared to those under a reduced number of light hours.     

The minimum effective spermatozoa : egg ratio for artificial insemination and the effects of mercury on sperm motility and fertilization ability in Clarias gariepinus

The optimal ratio of spermatozoa : egg (15 000 : 1) for artificial insemination of African catfish Clarias gariepinus gave fertilization and hatching rates of 80 and 67%, respectively. Below a sperm : ova of 3000 : 1 fertilization success decreased significantly. Excessive sperm (>15 000 : 1) partly inhibited fertilization success. Sperm motility was decreased significantly by 0·001 mg 1⁻¹ Hg²⁺ as HgCl₂, but its effect on fertilization was dependent on the sperm : ova ratio, since excess sperm masked the effect of the pollutant. The most sensitive sperm : ova ratio for monitoring pollutant effects on fertilization success was 1500 : 1, which corresponds to half the minimal amount that yields a high fertilization rate in artificial insemination. There was a good correlation between fertilization and hatching rates ( r =0·83; P<0·05). Although both fertilization and hatching rates provide equally good indicators of fertilization success, the more rapid fertilization rate test is recommended since it requires only 12 h.     

Sublethal effects of aquatic pollution in Lake Maryût on the African sharptooth catfish, Clarias gariepinus (Burchell, 1822)

Some aspects of the physiology and biochemistry of the African sharptooth catfish, Clarias gariepinus (Burchell, 1822), were studied along with an array of physicochemical characteristics of the water in Lake Maryût, Egypt. Data were compared to those of a reference fish hatchery. At least 11 of the conventional water pollutants (Cd, Cu, Fe, Pb, Mn, Hg, Ni, Zn, turbidity, chemical oxygen demand and ammonia nitrogen) were elevated in the most polluted main basin of the lake. In turn, serologic analysis of the indigenous catfish C. gariepinus pointed at functional impairment of the liver, heart and kidney as reflected by the elevated activities of alanine aminotransferase, aspartate aminotransferase, lactate dehydrogenase, creatine kinase, cholinesterase, glucose and creatinine. Reduced nucleic acid measurements [the ratio RNA/DNA and the relative RNA content (r)] indicated diminished protein synthesis and impaired growth in polluted fish. In some instances, glucose and nucleic acid measures were elevated in favour of fish from moderately polluted basins of the lake rather than in reference fish. Fish seemed to profit from the typical geographical habitat in the lake, regardless of the virtual contamination there, versus reference fish that seemed to suffer congestion stress and food competition in the confined and overcrowded commercial pond.     

Reproduction in the greenback grey mullet, Liza subviridis (Valenciennes, 1836)

This paper presents data on the reproductive biology of Liza subviridis , a little studied mullet species. The fish is heterosexual, exhibiting external fertilization. Six maturity stages can be macroscopically identified in the testes and seven in the ovaries. The macroscopic changes in the gonads are manifestations of histological changes occurring in the development of sperm cells and oocytes. First sexual maturity is attained in the length ranges of 9.5–11.5 cm and 10.5–11.5 cm in male and female fish respectively. The fecundity for fish measuring 10.3–13.9 cm in standard length ranged from 40 000–145 000 eggs. The relationship between fecundity ( F ) and length ( L ) can be represented as: F = 1.9044 L ^4.2998. The spawning duration in L. subviridis is restricted to a short and definite period, with all ripe ova being released within a single spawning act. A pronounced spawning season can be detected to extend from June to November. However, during off-seasons, some spawning also occurs. The correlations between spawning, rainfall and air temperature is discussed.