Lake Kainji,Nigeria; A summary of the changes within the fish population since the impoundment of the Niger in 1968

The relative composition of the major fish families in Lake Kainji between 1974 and 1975 is analysed and compared with pre- and post-impoundment data collected by previous workers. Trends in the fishery are assessed, and it is concluded that the cichlids, particularly Sarotherodon (= Tilapia) galilaeus Artedi, are becoming the dominant commercially valuable fish. No evidence was found of a significant recovery in the formerly abundant citharinids.The value of the sampling technique is discussed and criticised in relation to under-assessment of certain species, notably S. galilaeus. The Schilbeidae were found to be one of the most numerous families in the lake but are commercially under-exploited.Department of Zoology, University of Reading     

The distribution,abundance and trends in the establishment of the family schilbeidae (osteichthyes: siluriformes) in Lake Kainji,Nigeria

The distribution and abundance of the five species of Schilbeidae in Lake Kainji were studied. The gill net samples showed that Eutropius niloticus was the most abundant and very common species in the lake. Physailia pellucida although greatly under-estimated by the gill nets was the next abundant and fairly common species. Schilbe mystus was not as abundant as the two previous species but was found to be common. Schilbe uranoscopus and Siluranodon auritus were very rare in the Lake. The spatial distribution showed that E. niloticus and P. pellucida occurred in high numbers in the open lake and the shorelines, while S. mystus had its highest number in the riverine areas of the Lake. An upward trend in the abundance of E. niloticus and P. pellucida was indicated while a decrease in the abundance of S. mystus within the lake was noted.     

Ecological comparison of the pre- and post-impoundment macrophyte flora of the river Niger and Lake Kainji,Nigeria

The floristic composition of the macrophyte vegetation of Lake Kainji was investigated and compared with the vegetation of the river Niger before impoundment.Of 18 hydatophytes, 40 tenagophytes and 57 trichophytes listed for the Niger, 18 tenagophytes, 12 trichophytes and 7 hydatophytes were found growing in the lake 17 years after impoundment. Fourteen new tenagophytes, 3 new hydatophytes and 1 new trichophyte were recorded. The hydatophytes which were expected to colonize the lake are limited in number and range.The failure of the hydatophytes to colonize the lake, as predicted by Cook (1968), is discussed in relation to the inflow-to-volume ratio of the lake.Nomenclature follows Hutchinson & Dalziel (1952–1972). Flora of West Tropical Africa.     

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     

Sex,reproductive cycle and variations in the fecundity of the family schilbeidae (Osteichthyes : Siluriformes) in Lake Kainji,Nigeria.

The sex, reproductive cycle and variations in the fecundity of the family Schilbeidae (Osteichthyes : Siluriformes) in Lake Kainji, were investigated. Sexual differences in size and morphology were found to exist in all species. Sex ratio varied with species and season. There were more females than males in the population. Size at maturity varied from species to species with the males attaining maturity at a slightly lower length than the females. Breeding condition was attained mainly during the rains in Eutropius niloticus, Schilbe mystus and Siluranodon auritus and in the dry and rainy seasons in Physailia pellucida. Spawning occurred once during the breeding season in E. niloticus and S. mystus; and more than once during the prolonged breeding season in P. pellucida. Condition factors did not show any correlation with maturation of gonads. Fecundity was found to vary from species to species.     

Contribution to the dynamics of mean-size statistics in a changing fishery: I. Family citharinidae in kainji lake,Nigeria

A model on The dynamics of mean-size statistics in a changing fishery proposed by Henderson (FAO) was tested using the Kainji Lake experimental gill-net data collected between 1969 and 1977. The results revealed distinct trends in mean weight and population abundance (catch/effort) for members of the family Citharinidae. Simultaneous plots of mean weight against log₁₀ of population abundance allowed the diagnosis of the causes of change. Changes were attributed to the effects of fishing mortality, intensive recruitment into the fishery and change in selectivity coupled with the effect of growth. The pattern of change depicted for Citharinus citharus Geoffroy St. Hilaire revealed a three yearly cycle of intensive recruitment. The regression of mean weight on mean length for C. citharus gave a linear relationship thus facilitating the extrapolation of the mean weight values to the mean lengths of fish caught in the different meshes of gill-nets. The conversions permitted the selection of suitable mesh nets for capturing fish at appropriate levels of mean weights in order to safe-guard the survival of sexually mature adults for an intensive spawning cycle.     

The fish fauna changes and the place of clupeids in Lake Kainji,Nigeria

The fish fauna changes and the place of the clupeids was examined. It was found that the conditions in the lake since impoundment have favoured the growth and development of these little sardines. The studies undertaken prior to impoundment and their continuation right from the time of the closure of Kainji Dam in August 1968 has made it possible for these changes to be followed sequentially.     

The food and feeding habits of Eutropius niloticus (Ruppell), family Schilbeidae (Osteichthyes: Siluriformes) in Lake Kainji,Nigeria

Studies on the food and feeding habits of Eutropius niloticus in Lake Kainji, Nigeria revealed that the fish is omnivorous but fed more on insects than on any other food. The most important insects were Povilla adusta Navas nymphs and imagos and adults of the common grasshoppers. Fish, particularly the clupeids Sierrathrissa leonensis Thys Van den Audernaerde and Pellonula afzeliusi Johnels, also formed an important part of the diet. There was seasonal variation of organisms selected for food and there were variations in the dietary items selected by different age groups. Accumulation of fat was found to correlate with seasonal feeding intensity. E. niloticus serves as food for other fishes.     

Fish distribution in Kainji Lake, Nigeria

A study of the distribution of fish populations in the inshore, surface and bottom water habitats of L. Kainji, and comparison with previous data, showed that there has been a relative increase in the populations of the fish families Cichlidae, Bagridae, Schilbeidae, Centropomidae and Cyprinidae and a relative decrease in the populations of the Characidae, Mochokidae, Citharinidae and Mormyridae. The overall composition of fish families caught in the three habitats varied significantly ( P <0.05). Four of the nine major families showed habitat preference: there was a preponderance of Cichlidae and Centropomidae in the inshore water habitat; Bagridae were evenly distributed in the inshore and bottom waters; and Schilbeidae were the most abundant in the surface water habitat. There were significant differences in the total catch among the various sampling stations. However, there was no significant difference in the catch of various family groups except in Cyprinidae. It was established that overall catches from the three major habitats were inversely proportional to water level in the lake.     

An analysis of the distribution of fish species in a large prairie lake

The distribution of fishes in Dauphin Lake, a large, shallow, turbid, prairie lake in west-central Manitoba, was examined using gillnets. Catch per unit effort was used to assess differences in distribution thai may be attributed lo year, season, lake zone (inshore, offshore), substratum, water depth, water temperature, turbidity and presence of other fish species. Although 13 species were caught, only the distributions of the six most abundant was analysed. Variation in number of species caught was attributed mainly to lake zone, water depth and water temperature. White sucker (Catastomus commersoni) distribution can be explained by zone and temperature. Shorthead redhorse (Moxostoma macrolepidotum) catches were related to depth and temperature. Cisco (Coregonus artedii) catches varied with year, zone and temperature. Variances in northern pike (Esox Indus) distribution reflected year and zone. Walleye (Stizostedkm vitreum) varied with year, depth and temperature and differences in the catches of yellow perch (Pertaflurescens) were attributed to year, depth, temperature and turbidity. They were also positively correlated to the numbers of northern pike caught.     

武汉东湖鱼产量时间序列模型及回归分析

随着城市人口的增长和经济的快速发展,武汉东湖的水体富营养化日益严重。80年代以来,东湖通过实施滤食性鱼类治理水华取得了一定成效。利用统计学软件SPSS,应用时间序列模型和回归分析对东湖44年鱼产量数据进行运算,对东湖鱼产量变化进行了分析和预测:东湖最高鱼产量在1700～1850t之间,比较合理的鱼产量应维持在1150t左右。     

An analysis of fish community responses to coral mining in the Maldives

Synopsis Coral mining takes place on shallow reef flats at a number of localities in the Maldives, but not on the adjacent deeper reef slopes. A semi-quantitative census method for fish species abundance and biomass is described. Fish community structure is compared on mined and non-mined reef flats and their adjacent slopes using a variety of univariate, graphical/distributional and multivariate statistical techniques. In general, univariate and graphical distributional methods do not indicate significant differences between mined and non-mined localities with respect to the relative abundances and biomasses of species. Multivariate methods (both classification and ordination), however, indicate very clear-cut effects of mining on the reef flats, and also significant effects on reef slopes adjacent to mined flats. The effect was equally clear using non-quantitative (presence/absence) data. The fish species mainly responsible for the differences between mined and non-mined localities are identified, and the differences are explained in terms of the feeding biology of these species.     

Abundance and daytime vertical distribution of planktonic fish larvae in an oligotrophic South Island lake

The vertical distribution of common bully (Gobiomorphus cotidianus) and koaro (Galaxias brevipinnis) larvae in the limnetic zone of Lake Coleridge were determined using a high-frequency (200 KHz) echosounder. Planktonic bully larvae first appeared in appreciable numbers in January. By February, they formed a scattering layer between depths of 12 to 24 m during the day, where they achieved a maximum density of 0.59 fish m^-3. Larger (> 18mm) fish migrated to the littoral zone and densities declined to < 0.01 fish m^-3 by July, when remaining larval fish occupied greater daytime depths. Their vertical distribution during the day appeared to be influenced mainly by light levels and water temperatures. Larvae grew more slowly (0.12 mm d^-1) than in more productive North Island lakes, and were also present in lower densities for a more restricted period of time. Koaro larvae first appeared in November and December and were found in low numbers (< 0.01 fish m^-3) in summer at depths of 10 to 26 m. Salmonid production in the limnetic zone is probably limited by the small size and relative scarcity of forage fish present.     

Ecological fish production in a small Precambrian shield lake

Synopsis Ecological fish production of 30.8 kg ha^–1 yr^–1 was estimated for a 6.2 ha Precambrian shield lake by treating it with a piscicide and removing all fish. This is one of the lowest productions recorded for any waterbody.Perca flavescens contributed 71% to this total. The five other indigenous species werePimephales promelas, Semotilus margarita, S. atromaculatus, Notemigonus crysoleucas andSalvelinus fontinalis. Seventy-five percent of the production occurred in the first year of life. A conventional capture-mark-recapture experiment conducted onP. flavescens prior to the piscicide treatment underestimated the density by nine times and the total production by eight times. The gravest error occurred in the estimation of density for the younger age groups. Production studies can quantify the ecological success of a species, and provide a measure of stress in aquatic ecosystems.     

Morphometric differentiation among haplochromine cichlid fish species of a satellite lake of Lake Victoria

Lake Victoria holds a young but species‐rich assemblage of cichlid fishes, which form a monophyletic assemblage with additional species from surrounding water bodies, termed the Lake Victoria superflock. Lake Victoria is surrounded by smaller lakes that are somewhat disconnected from the main lake. Lake Kanyaboli is such a small lake, having markedly reduced species diversity, in part comprised of Lake Victoria species and endemics. Here, we studied the modern haplochromine component of the cichlid fauna, represented by Lipochromis maxillaris, Astatotilapia nubila, Xystichromis phytophagus and Astatotilapia sp. ‘Bigeye’, as well as a number of unidentified modern haplochromine specimens. We used landmark‐based geometric morphometrics to study the degree of morphological divergence among those young entities. Twenty landmarks and 14 interlandmark distances were used for shape analysis. Multivariate analysis revealed significant differences between all four species, but principal component analysis and canonical variate analysis did not clearly discriminate between A. nubila and X. phytophagus, demonstrating great overall morphological similarity despite clear dietary differences. Besides coloration there was sexual dimorphism in body proportions, so that only male individuals were analysed further. In all four species, the observed similarities and differences in body shape conform to the type of ecological specialization of the fish. Most unidentified specimens overlapped the range of A. nubila and X. phytophagus, while the assignment test based on the canonical variate analysis suggested 70% of the three overlapping entities as separate units. To test their reproductive distinctness and to demonstrate potential hybridization, nuclear genetic data are needed.     

The annual flood regime as a regulatory mechanism for phytoplankton production in Kainji lake,Nigeria

Phytoplankton production was measured in situ in Kainji lake from December 1970 to September 1972 using the oxygen light and dark bottle technique. Seasonal variations in solar radiation, transparency, temperature, and composition of subsurface light were also measured. Oxygen production per unit area varied from 220 to 4500 mg O₂ m^–2 day^–1, the maximum production rate from 95 to 400 mg O₂ m^–3 h^–1. Seasonal mixing of lake water and river water of varying turbidity changed the optical properties of the lake water and consequently affected phytoplankton production. The annual flood pattern was found to be an important factor regulating phytoplankton production in the lake.     

Spatial and temporal distribution of fish and zooplankton in a shallow lake

Summary 1. We performed both a large- and a small-scale echo sounding study on the spatial and temporal distribution of fish (mainly roach, Rutilus rutilus and perch, Perca fluviatlis ), as well as a small-scale study of zooplankton distribution in the small, shallow and eutrophic Lake Hanebjerg in Denmark. In the small-scale study, sampling was conducted in open water as well as in the edge zone immediately outside two different types of vegetation.
2. Fish daytime abundances differed between the northern and the southern parts of the lake and, on a small scale, small fish aggregated in the edge zones during day, preferably outside dense emergent vegetation. Copepods avoided emergent vegetation, while cladocerans showed no habitat preference. Both small fish and cladoceran numbers were found to be higher during night than day.
3. The relative abundance (number per sample) of cladocerans in the edge zone immediately outside vegetation was negatively correlated with the relative abundance of fish in that zone. There was no correlation between cladoceran and fish abundance in open water, or between the relative abundance of copepods and fish.
4. The presence of pelagic piscivores in combination with avoidance behaviour of both fish and zooplankton is a likely explanation for the observed distribution of small fish and cladocerans in Lake Hanebjerg. Both small- and large-scale distribution patterns may be dependent on the type and distribution of complex structure in the lake. Even in a small lake, large-scale patterns may affect the interpretation of small-scale data.     

The relationship between fish yield and primary production in large European freshwater lakes

The data published on fish yield (Yf) and primary production (PP) in three large European freshwater lakes (Ladoga, Ilmen and Pskovsko-Chudskoe) were analyzed on a long-term basis. The ratios between Yf and PP were found to vary from 0.02% to 0.46%. It was shown that there was an optimal level of PP, above which the efficiency of energy transfer in the pelagic food chain began to decrease. An individual optimum of PP was characteristic of each of the lakes studied. This level was primarily determined by the original trophic status of a given lake as well as by its morphometry and hydrochemistry. The results warn practical ecologists against erroneously predicting commercial harvests from PP.