Condition factor and diet of Chrysichthys nigrodigitatus and Chrysichthys auratus (Siluriformes: Bagridae) from Aiba Reservoir, Iwo, Nigeria

Chrysichthys nigrodigitatus and C. auratus are important, highly valued and threatened freshwater species. To contribute with their ecological knowledge, the condition factor and diet of these two congeneric species were studied between April 2005 and April 2006. Food items of fish were evaluated by occurrence and numerical abundance methods, and the possible changes among sexes, seasons and sizes were considered. Results showed that generally C. nigrodigitatus were better conditioned than C. auratus. The males of C. nigrodigitatus and females of C. auratus were in better condition than their respective opposite sex throughout the year, and during the wet season compared to the dry. The food items of C. nigrodigitatus and C. auratus were similar and comprised twelve species belonging to five groups: Insecta (three species), Crustacea (five species), Arachnida (one species), Rotifera (one species) and Mollusca (two species). Other food items included fish scales, unidentified green eggs, plant parts, detritus and sand. Insecta and Crustacea dominated the food items in both species. For C. nigrodigitatus, insect consumption increased with fish size, while Crustacea items decreased (from 89.59% for 8.1cm - 12.0cm size class to 1.58% for 20.1cm - 26.0cm size class). However, while C. auratus smaller sized fish preferred Crustacea (98.72% for 8.1cm -12.0 cm size class), larger sized fish had both groups in relatively similar amounts. Schoener Overlap Index for both species and between seasons is 1.00. Diet breadth ranged from 0.00-1.47 for C. nigrodigitatus and 0.00-1.32 for C. auratus. Food Richness ranged from 0.00-1.44 for both species. Gut Repletion Index for C. nigrodigitatus and C. auratus are 76.39% and 76.27% respectively. Although, there is considerable similarity and overlap in the utilization of food resource for both coexisting species, condition factor and feeding behavior suggest strategies to reduce intra- and interspecific competition.     

Distribution, reproduction and growth of the catfish, Chrysichthys walkeri (Gunther) in the Lekki Lagoon, Nigeria

An investigation was made on the biology of the catfish, Chrysichthys walkeri (Gunther) occurring in the Lekki Lagoon. The catfish was evenly distributed over the six sampled stations of the lagoon with the sites of the shallow water stations probably serving both as breeding and nursery grounds.
Fecundity estimates for this species ranged from 896 to 4168 eggs. The average number of eggs per female was 2084. Laboratory incubated eggs hatched after 3 days with the yolk sac being completely absorbed on the seventh day. The fry averaged 8.6 mm on their first day of life.
The catfish attained a maximum total length of 25.7 cm within a period of 2 years. There were no significant differences in the growth pattern of this species during the dry and rainy seasons.     

Phylogeography of the African catfish,Chrysichthys maurus (Valenciennes, 1839)

Polymorphism at 19 allozyme loci was studied in 227 individuals of the catfish, Chrysichthys maurus (Valenciennes, 1839), collected from ten populations representing nine different river basins in west Africa. Eleven of the loci studied were polymorphic. Analysis showed that populations clustered according to their geographical origin: eastern populations close to the root represented by a population of a closely related species (C. auratus); then populations from the centre of the species' range, followed by the more western populations. Networks observed, regression of genetic distances versus geographical distances, results of the Mantel test as well as the apomorphic status of two alleles suggested that the genetic differentiation corresponds to an isolation by distance model. These results strongly suggest that the populations initially occurred in the lagoons and rivers of Ivory Coast and from here spread to the western part of the species' range by progressively colonising basins from east to west.     

Diseases of the African estuarine catfish Chrysichthys nigrodigitatus (Lacépède) from the Cross River estuary, Nigeria

A survey based on monthly sampling of commercial catches was carried out in the Cross River estuary in 1984 and 1985 to determine the extent of parasitization and occurrence of externally visible lesions on the bagrid catfish Chrysichthys nigrodigitatus. Infections with the following parasites were encountered: Henneguya chrysichthyi (32%), Protancylodiscoides chrysichlhes (77%). Siphodera ghanensis (4.2%), Aspidogaster africanus (0.1%), Hysterothylacium larvae (64%), Nerolica orbignyi (1.5%), Ergasilus lalus (27%), and Chonopeltis brevis (0.02%). A new kind of epidermal papilloma was found and is described. On a yearly average, less than 0.5% of the catfish population was affected by either skeletal deformities or skin ulceration. Fin rot was observed from December to May, reaching a prevalence of 79% in February. Gill myxozoosis due to Hemwguya chrysichthyi is considered the most important parasitic disease of the fish, occurring throughout the year with a maximum prevalence of 76% in October. Results of the study are discussed in the light of a projected intensive cultivation of the fish in the extensive estuaries and lagoons bordering Nigeria's coastline.     

The food and feeding habits of the catfishes, Chrysichthys walkeri (Gunther), Chrysichthys filamentosus (Boulenger) and Chrysichthys nigrodigitatus (Lacépède) in theLekki Lagoon, Nigeria

A total of 1436 specimens were examined in the study of the food habits of the three Chrysichthys species in Lekki Lagoon. Certain differences were found in their food habits. Chrysichthys walkeri fed mostly on insects, Chrysichthys filamentosus on crustaceans and Chrysichthys nigrodigitatus on molluscs. It is suggested the observed diversity in feeding habits might be due to an active response to interspecific competition.     

Sediment oxygen demand in man-made Lake Ton-Ton (Uruguay)

A study on sediment metabolism was carried out during 1986 in Lake Ton-Ton, Uruguay. Sediment oxygen demand (SOD) from chemical and biological origin was measured in undisturbed sediment cores taken from the deepest part of the lake. Mean SOD rate for the study period (51.56 mgO₂ m^–2 h^–1) corresponded well with the eutrophic state of the lake. During stratification, SOD from chemical origin accounted for 69–87% of total SOD, while SOD from biological origin was dominating for the rest of the year, except in July. Biological respiration was principally of microbial origin. Hypolimnetic temperature was the main factor controlling SOD rates (r = 0.771,p < 0.001). Nevertheless, freshly sedimented phytodetritus from anAnabaena bloom, together with a renewed input of oxygen to bottom water were responsible for the maximum SOD values, recorded at the beginning of a mixing period in April (72.51 mgO₂ m^–2 H^–1).     

Observations on the biology of nymphs of Povilla Adusta Navas (Ephemeroptera: Polymitarchidae) in Lake Kainji,Nigeria

Observations onPovilla adusta Navas nymphs in Lake Kainji revealed that seasonal fluctuations in abundance were due to a shortage of food during the period of rising lake level rather than a depletion of oxygen in the water column; this shortage was caused by the increased turbidity of the flood waters restricting the growth of both planktonic and periphytic algae. Planktonic algae apparently formed the diet of nymphs <4–6 mm and also of larger nymphs if their preferred diet of periphytic algae was in short supply. Both depth distribution and individual size of the nymphs were closely related to the distribution of periphytic algae. Depth distribution patterns of the nymphs revealed a peak between 2–4 m below the surface.The dimensions of the nymphs and the duration of the aquatic stages (3–4 months) in Lake Kainji correspond closely with results obtained elsewhere as does the mean percentage of males in the population (28%±10%). Seasonal fluctuations in the sex ratio were apparently caused by variations in the intensity of predation.Department of Zoology, University of Reading     

On a new species of electric catfish from Kainji, Nigeria, with some observations on its biology

A new species of electric catfish, Malapterurus minjiriya , from Kainji, Nigeria, is described and a key for separating it from the two other known species is given. Notes on some aspects of the biology of the electric catfishes in Lake Kainji, Nigeria are included.     

Age and growth determinations in the catfish, Chrysichthys nigrodigitatus (Lacépède) by use of the dorsal spine

Annual rings on the first dorsal spine were used to determine the age and growth of the catfish, Chrysichthys nigrodigitatus (Lacépède), in the Lagos Lagoon, Nigeria. From back-calculations, it was found that the mean total length ofthe species when the first annulus was formed was 18.5 cm, 32.6 cm for second annulus, 43.5 cm for the third annulus and 53.9 cm for the fourth annulus. It was shown that this species in the Lagos Lagoon attained four years of age and that the fastest growth in length was achieved during the first year of life. Thereafter, increase in length gradually decreased with age. The relationship between the body length and dorsal spine radius was shown to be linear and highly correlated.     

On the biology of an estuarine population of the clupeidPellonula afzeliusi (Johnels) in Lagos Lagoon,Nigeria

The length-weight relationship, condition factor, fecundity and food and feeding habits ofPellonula afzeliusi (Johnels) in Lagos Lagoon were investigated. The standard length of 1832 specimens examined ranged between 3.5 and 9.4 cm (total length 3.9 cm–11.4 cm) and was larger than reported elsewhere in West African man-made lakes. Specimens exhibited positive allometric growth. Their condition factor increased with individual size. Females had a slightly higher condition factor than males. The number of mature ova per female ranged between 330 and 33,649. There was a higher correlation between fecundity and weight than between fecundity and length. The major food items were crustaceans, insects and fishes. There were distinct differences in the feeding habits in relation to size. Variations were observed in the monthly analysis of food items.     

Notes on the reproductive biology ofPortulaca L. (Portulacaceae) in Nigeria

Nigerian species ofPortulaca: P. grandiflora Hooker,P. oleracea L. andP. quadrifida L. combine inbreeding, outbreeding and vegetative propagation in the wild and in the oultivation. Observations have shown that the anthers ofPortulaca species dehisces before the flower bud opens, and are usually ripe at the time the stigmas are receptive. This enhances self-fertilization.     

Distribution and breeding biology of offshore pelagic cyprinids and catfish in Lake Malawi/Niassa

Synopsis Lake Malawi/Niassa is a permanently stratified lake, consisting of an oxygenated layer of 230 m depth that overlies an anoxic zone that extends to the lake bottom at a maximum depth of over 700 m. Some fish are found throughout the oxygenated zone of the offshore waters, and lead a pelagic existence, although many of the species form part of the demersal community in shallower waters where the lake-bed is oxygenated. This paper reports on the distribution of cyprinids and catfish at six offshore locations, sampled with a mid-water trawl and gillnets, along the north-south axis of the lake. These two groups formed, respectively, 5% and 13% of the offshore fish biomass in the trawl catches. The offshore cyprinids comprised the small lake-spawningEngraulicypris sardella, the only species to have pelagic larvae, and the larger river-spawningOpsaridium microcephalum andO. microlepis. These cyprinids occurred mainly in the upper 100 m of the water column. The biomass of both larvae and adults ofE. sardella showed considerable fluctuations over the two years of study. The spawning site ofE. sardella remains a mystery, but is no longer believed to be in the offshore pelagic zone. The most abundant catfish was the mochokidSynodontis njassae, that exhibited diurnal movements from just above the oxic-anoxic boundary layer during the day to the surface layers at night. The clariid catfish of the genusBathyclarias were caught infrequently offshore, but owing to their large size, they could form a significant part of the biomass. Their abundance was difficult to quantify, as it was believed that the majority are active in the near-surface layers and were under-sampled by the trawl and gillnets. Information on some aspects of the breeding biology is included in the paper, although our knowledge in this area is very incomplete.     

On the origin of the Synodontis catfish species flock from Lake Tanganyika

Species flocks within Great Lakes provide unique insights into the factors affecting diversification. Lake Tanganyika (LT) is of particular interest because it contains many endemic groups for which general factors affecting diversification can be discerned. Here, we present the first phylogenetic study of the LT Synodontis (Siluriformes, Mochokidae) species flock using mtDNA sequence data. Our data reveal some previously unrecognized species diversity and indicate that the LT species flock is not monophyletic, and that two closely related clades of endemics may have independently colonized LT. Other comparable small species flocks are characterized by a single colonization event. Molecular date estimates of the timing of the initial within-lake diversification of the LT endemics, based on a fossil calibration, are comparable to those reported for other groups, suggesting that extrinsic factors maybe important common causes of clade diversification. The basal divergence in the sampled Synodontis reveals an East-West African faunal split seen in many terrestrial, but few aquatic groups, the timing of which coincides with East African rifting events.     

Recent changes in the fishery of a new tropical man-made lake, Lake Kamburu (Kenya)

Five years after inundation, two varieties of Cyprinus carpio —the common carp and the mirror carp, have appeared in the commercial fishery in Lake Kamburu. Species of Tilapia continue to remain the most abundant fish in the lake and their numbers are on the increase. The second most numerous species is Barbus while Labeo sp. seems to be declining. The numbers of Mormyrus have declined drastically. Species of the genera Tilapia, Barbus and Cyprinus are widely distributed within the lake; Labeo prefers the confluence of the rivers entering into the lake and Mormyrus has moved upriver.     

On the biology of Amphibia in the ecosystem of Lake Glubokoe

In Lake Glubokoe, the Amphibia are represented by five species. Two numerous species, Rana temporaria L. and Bufo bufo L., spawn in the lake and appear to be a significant component of its ecosystem. Thus, in autumn R. temporaria is a common food for predatory fish (pike, perch). During their wintering R. temporaria participate in the consumption of oxygen from the water. They may be a source of dangerous infections. Their tadpoles and those of B. bufo feed in the lake and annually remove substances from it. Adult R. temporaria in autumn, and the young of the year of R. temporaria and of B. bufo influence the population numbers of small littoral invertebrates. Several dozen pairs of Rana terrestris Andrzejw are observed to spawn regularly. Rana esculenta L. and Rana ridibunda Pallas are scarce. The influence of introducing a nature preserve on the numbers of Amphibia in Lake Glubokoe is discussed.     

On the biology of bream, Abramis brama (L.) in Lake Peipsi in 1994

The population of bream in L. Peipsi was studied with respect to age, growth rate, condition factor (according to Fulton) and length-weight relationship in 1994. That autumn the bream population in L. Peipsi consisted of fishes aged from 0+ to 15+. During the first year bream reached an average body length of 7.9 cm (the commercial legal size (30 cm) was usually attained by the end of the 5th–6th year. The condition of bream in this lake was above the average of Estonian lakes. The relatively good growth rate and condition of bream in the lake indicates that the waterbody is appropriate for this fish.     

Population dynamics of two small cichlid fish species in a tropical man-made lake (Lake Kariba)

The population dynamics of two small cichlid fishes (Pharyngochromis darlingi andPseudocrenilabrus philander) were studied in Lake Kariba, a very large African man-made lake. They are of no economic importance but make up about 14% and 7% respectively of the inshore fish population and are the major components of the diet of fish-eating birds on the lake.P. darlingi isthe larger species (L = 156.5 mm) and is found on both shelving and steep, eroding shores. Its mortality rate differs in each habitat (Z = 0.44 and 0.72 month^–1 respectively), only 0.79% survive for 12 months and its % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamiuaiaac+% cadaqdaaqaaiaadkeaaaaaaa!384D!\[P/\overline B \] ratio is 5.45 (on shelving shores).Ps. philander is smaller (L = 83.9 mm) and is restricted to shelving areas with abundant vegetation. Its monthly mortality rate was high (Z = 7.69), only 0.05% survive to 12 months whilst its % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamiuaiaac+% cadaqdaaqaaiaadkeaaaaaaa!384D!\[P/\overline B \] ratio was very high (7.69). The estimates of growth obtained forP. darlingi differ considerably from those given in an earlier study in Lake Kariba and some possible reasons for this are discussed. In suitable habitats, the combined production of both species could be 40 kg ha^–1 yr^–1 which indicates their potential importance to the ecology of the lake.