Spawning periodicity and annual breeding seasons of Tilapia nilotica (L.) in the White Nile

Two annual breeding seasons are indicated for populations of T. nilotica in the White Nile: a major autumnal season (mid-July–September) coinciding with the annual rainfall and a smaller back-up season in February–March. During these seasons reproducing females constituted 80% and 40% of the total population of mature females compared to 10–12% in the intervening summer period. Enhanced gonadal development, fecundity and GSI values characterized the breeding seasons. Gonadal maturation proceeded uninterruptedly even during the summer and repeated fractional spawning is suggested for this species within and beyond the breeding seasons. Most of the spring breeders were either very young or relatively old fish and most medium-sized females bred in the main autumnal season. Older fish were less fecund (by 70–75%) compared to younger fish. In the latter, fecundity increased progressively with body growth (r² = 0.732; p<0.001); the proportionality was lost in medium-sized fish and in older females (0.9–1.5 kg) fecundity correlated negatively with increased body size.     

Studies on the biology of reproduction in the cichlid Tilapia nilotica (L.): gonadal maturation and fecundity

Tilapia nilotica reaches maturity in the first year of life; females mature shortly before males (11·4 and 14·3 cm in length). The distinctive morphological features of the different stages of development of the gonads are characterized. The sex ratio varied in populations of different age (size)-groups: a ratio of 2·81:1 and 0·47:1 (females:males) was obtained in populations of young and old fish. In medium-sized populations the sexes were equally abundant. There is evidence that females breed more than once a season. Fecundity varied more with body length ( r =0·860) and weight ( r =0·806) than with age ( r =0·604). These variations were of the order of 2·02; 0·83 and 0·83 exponentials of length, weight and age, respectively.     

Seasonal abundance,breeding and sex-structure of populations of Tilapia nilotica (L.) and Labeo niloticus (F.) in the Jebel Aulia Dam area of the White Nile

Seasonal abundance, maturation and sex-structure were studied in populations of T. nilotica and L. niloticus in the Jebel Aulia reservoir and in the White Nile below the dam. L. niloticus showed a single annual breeding cycle from mid-July to December and was most abundant in October in the lake, and in early November in the river. T. nilotica showed two distinct annual seasons of abundance: summer (April to June) and autumn (November to October). Spawning activity was highest in summer (April to June) and autumn (late July to September). The sex-ratio varied inversely with age (size); males were predominant in populations of older fish, perhaps because of their greater endurance to adverse conditions as evidenced by their better survival in hypoxic water.     

Bottom-up effects of bream (Abramis brama L.) in Lake Balaton

Enclosures (17 m³) were used in the mesotrophic area of Lake Balaton to determine the impact of benthivorous bream (Abramis brama L.) on the lower trophic levels during summers of 1984–86. In enclosures with a fish biomass similar to the biomass in the eutrophic area of the lake, the number of phytoplankton species was highest. In enclosures with a low fish biomass the phytoplankton was dominated by the greens. A high biomass of bream in the mesotrophic basin caused bacterial production corresponding to that of the eutrophic part of the lake. Crustaceans were dominated by copepods and were unable to control phytoplankton peaks. Bottom-up effects of bream were more obvious than top-down effects and seem to be more important in the possible control of water quality.     

Studies on the biology of reproduction in the cichlid Tilapia nilotica (L.): effects of steroid and trophic hormones on ovulation and ovarian hydration

The effects of intraperitoneal injections of cortical and ovarian steroids and trophic (mammalian) hormones on ovulation, ovarian hydration and the distribution of sodium ions in muscles and ovary was studied in Tilapia nilotica . Cortisol, corticosterone and oestradiol 17p (50–250 mg kg⁻¹ B.W.) induced ovulation in a dose-dependent manner and increased the degree of hydration of muscle and ovarian tissues. The sodium ion content of muscles was reduced and that of ovaries increased following treatment with these hormones. Oestrone, oestriol and progesterone (60–245 mg kg⁻¹ B.W.) failed to induce ovulation but produced the same types of effects on the water and sodium ion content of muscles and ovaries as the other steroids. Human chorionic Gonadotrophin, HCG, (1200–12 000 I.U. kg⁻¹ B.W.) also induced ovulation and increased the degree of hydration of muscles and ovaries and increased the sodium ion concentration in ovarian and reduced that of muscle tissues. Pregnant Mare Gonadotrophin, PMG, in doses similar to those of HCG, did not induce ovulation in any of the treated fish and did not significantly alter the water or sodium ion content of muscles and ovaries.     

Scanning electron microscope observations of the organic matrix in the otolith of the teleost fish Fundulus heteroclitus (Linnaeus) and Tilapia nilotica (Linnaeus)

Scanning electron microscope observations of sagitta otoliths of Fundulus heteroclitus (Linnaeus) and Tilapia nilotica (Linnaeus) have revealed that the “discontinuous zone” is a narrow band of organic matrix consisting of fibers ≈900 Å thick, that in turn are composed of thin fibers ≈200 Å thick. The “incremental zone” is the crystalline layer with crystals elongated perpendicular to the otolith periphery that are usually terminated at the discontinuous zones. The crystals are embedded in organic matrix fibers that appear similar to and continuous with the fibers of the discontinuous zones. Frequently, these fibers aggregate into matrix sheets. Based on these findings, a possible process of otolith formation is proposed.     

Growth,condition and gonad development of bream (Abramis brama L.) in relation to its feeding conditions in Tjeukemeer

The bream (Abramis brama L.) population of Tjeukemeer was studied for three successive years. The growth, condition, gonad development, diet and feeding conditions of the fish are described. Food competition for two years with the then abundant young planktivorous smelt (Osmerus eperlanus) and perch (Perca fluviatilis) resulted in a decreased growth rate, condition and gonad development of the larger bream. The biomass of chironomids was too small in relation to that of the zooplankton to be important for the bream population, although they were eaten more efficiently.     

Response of Faidherbia albida (Del.) A. Chev., Acacia nigrescens Oliver. and Acacia nilotica (L.) Willd ex Del. seedlings to simulated cotyledon and shoot herbivory in a semi‐arid savanna in Zimbabwe

Woody plant seedling establishment is constrained by herbivory in many semi‐arid savannas. We clipped shoots and cotyledons of three woody species 5‐day (=‘early’) or 28‐day (= ‘late’) post‐emergence to simulate herbivory. Seedlings had shoot apex, one or two cotyledon(s) removed, or were retained intact. Survival rates were ≥80%, ≥40% and ≥20% for Acacia nilotica, Acacia nigrescens and Faidherbia albida respectively. F. albida mobilized stored cotyledon reserves faster and consequently shed the cotyledons earlier than the two Acacia species. Cotyledons were shed off as late as 70 days post‐emergence with 5‐day shedding earlier than 28‐day and cotyledon life‐span decreasing with intensity of defoliation. Shoot apex removal 28‐day resulted in higher compensatory growth than 5‐day in all three species. Cotyledon removal had no effect on shoot length, while shoot apex removal reduced shoot length. In F. albida root growth was stimulated by shoot apex removal. We conclude that potential tolerance to herbivory in terms of seedling survival was of the order A. nilotica > A. nigrescens > F. albida, timing of shoot apex and cotyledon removal influenced seedling growth in terms of biomass and that shoot apex removal stimulated compensatory growth which is critical to seedling survival.     

Water temperature affects osmoregulatory responses in gilthead sea bream (Sparus aurata L.)

Sea bream (Sparus aurata Linneaus) was acclimated to three salinity concentrations, viz. 5 (LSW), 38 (SW) and 55psμ (HSW) and three water temperatures regimes (12, 19 and 26 °C) for five weeks. Osmoregulatory capacity parameters (plasma osmolality, sodium, chloride, cortisol, and branchial and renal Na⁺,K⁺-ATPase activities) were also assessed. Salinity and temperature affected all of the parameters tested. Our results indicate that environmental temperature modulates capacity in sea bream, independent of environmental salinity, and set points of plasma osmolality and ion concentrations depend on both ambient salinity and temperature. Acclimation to extreme salinity resulted in stress, indicated by elevated basal plasma cortisol levels. Response to salinity was affected by ambient temperature. A comparison between branchial and renal Na⁺,K⁺-ATPase activities appears instrumental in explaining salinity and temperature responses. Sea bream regulate branchial enzyme copy numbers (V_max) in hyperosmotic media (SW and HSW) to deal with ambient temperature effects on activity; combinations of high temperatures and salinity may exceed the adaptive capacity of sea bream. Salinity compromises the branchial enzyme capacity (compared to basal activity at a set salinity) when temperature is elevated and the scope for temperature adaptation becomes smaller at increasing salinity. Renal Na⁺,K⁺-ATPase capacity appears fixed and activity appears to be determined by temperature.     

Induction of fever in a teleost fish (Lepomis gibbosus, L.)

Although fishes are ectotherms they are nevertheless able to thermoregulate behaviorally by selecting appropriate water temperatures (1). In a temperature gradient fish will congregate to a species-specific range of preferred temperature (“final thermal referendum”) which is unaffected by previous thermal history of the individual (2,3). Several aquatic (and terrestrial) ectothermic vertebrates have been found to exhibit “behavioral fever” which is manifested as an increase in preferred temperature above the final thermal preferendum (4). Fever can be elicited by pyrogens: whole bacteria (alive or killed), components of bacterial cellwall (endotoxins), endogenous pyrogens, prostaglandins or from several other sources (5). Since the results with fever induction in fish using whole bacteria or endotoxins are very scarce the aim of the present work was to compare possible thermoregulatory effects of endotoxins and prostaglandins in the same species (Lepomis gibbosus, L.) by means of identical methods.     

The effects of the anaesthetic quinaldine on oxygen consumption in an intertidal teleost Blennius pholis (L.)

The effects of the anaesthetic quinaldine on oxygen consumption in an intertidal teleost Blennius pholis (L.) were investigated. Oxygen consumption generally decreases with increasing quinaldine concentration in environmental water. However, at low concentrations, oxygen consumption of smaller specimens is slightly increased. Generally, smaller specimens have their oxygen consumption affected less by a given quinaldine concentration than do larger specimens, as the former appear to be at less depressed stages of anaesthesia. This is thought to be due to smaller B. pholis becoming saturated with quinaldine more rapidly but simultaneously being afforded more rapid removal, because of their greater relative gill area per unit body volume. The reduction of oxygen consumption at high concentrations of quinaldine is great.     

Respiratory potential of maize (Zea mays L.) roots exposed to hypoxia

When hypoxia is not too severe, root aerobic metabolism can be partly supported by oxygen delivery via aerenchymateous tissues. In terms of supplying energy, this adaptation is of special importance in plants with a high metabolic demand, such as maize (Zea mays L.). The ability of maize to respond to hypoxia by morphological changes is well documented; however, little is known on the potential for oxidative metabolism in different types of maize roots. In our study, we assessed the root respiratory potential in seminal and adventious nodal roots of maize exposed to mild short-term hypoxia. Plants responded to the treatment with an increased portion of nodal roots per total root length, while there were no changes in the biomass of shoots and roots. Thick nodal roots had much higher respiratory potential (Electron Transport System Activity – ETS) than nodal roots of smaller diameter or seminal roots, irrespective of the aeration rate. The only change in ETS under oxygen deficiency was found for seminal roots where oxygen consumption increased by 25%. Increased root porosity was observed in all roots, the increase was higher in nodal roots. On the basis of ETS data and taking into account changes of root morphology, it can be concluded that large changes of root respiratory potential are not involved in the response of maize to hypoxia. Obviously, maize can cover the respiratory needs with shifts in the growth of different root types which inherently differ in their potential aerobic respiration.     

Cadmium tolerance and phytoremediation potential of acacia (Acacia nilotica L.) under salinity stress

In this study, we explored the effect of salinity on cadmium (Cd) tolerance and phytoremediation potential of Acacia nilotica. Two-month-old uniform plants of A. nilotica were grown in pots contaminated with various levels of Cd (0, 5, 10, and 15 mg kg^?1), NaCl (0%, 0.5%, 1.0% (hereafter referred as salinity), and all possible combinations of Cd + salinity for a period of six months. Results showed that shoot and root growth, biomass, tissue water content and chlorophyll (chl a, chl b, and total chl a+b) contents decreased more in response to salinity and combination of Cd + salinity compared to Cd alone. Shoot and root K concentrations significantly decreased with increasing soil Cd levels, whereas Na and Cl concentrations were not affected significantly. Shoot and root Cd concentrations, bioconcentration factor (BCF) and translocation factor (TF) increased with increasing soil Cd and Cd + salinity levels. At low level of salinity (0.5%), shoot and root Cd uptake enhanced, while it decreased at high level of salinity (1.0%). Due to Cd tolerance, high shoot biomass and shoot Cd uptake, this tree species has some potential for phytoremediation of Cd from the metal contaminated saline and nonsaline soils.     

The food of bream (Abramis brama L.) in two basins of Lake Balaton of different trophic status

Contrary to earlier observations the food composition of bream (Abramis brama L.) in Lake Balaton has changed in parallel with eutrophication. These changes were probably caused by increased population density and connected with density dependent growth as well as sharpened inter- and intraspecific competition in the cyprinid community. According to the frequency of occurrence, zooplankton followed by benthic invertebrates comprised the majority of the bream's food. By weight, however, benthic food dominated in both basins. Size-related dietary changes were pronounced, but statistically not always significant, suggesting the overall importance of the most abundant and available prey types. Bream did not strongly select any of the zooplankton groups, however, seasonal changes in food composition alternated in parallel with the trophic gradient along the longitudinal axis of the lake.     

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.     

Secondary succession in Acacia nilotica (L.) savanna in the Hluhluwe Game Reserve,South Africa

Analysis of aerial photographs indicates that woody plant biomass has rapidly increased in Hluhluwe Game Reserve over the last 40 years. Open Acacia nilotica savanna is being replaced by broadleaf species, especially Euclea spp. We were interested in whether this secondary successional shift was due to high numbers of seedlings establishing and growing to maturity under acacias (facilitation) or due to the release of already established, but suppressed individuals (gullivers) of the resprouting broadleaf species. We examined the recruitment patterns and size-class distributions (height, basal diameter) of important species in this savanna.Densities of euclea seedlings (<0.6 cm basal diameter) under Acacia nilotica were low (median of 0 and mean of 0.06 m²) below adult canopies and effectively zero in adjacent interspaces. No differences in numbers of other broadleaf species were found between open sites and under Acacia nilotica sites. Few large eucleas or other broadleaf species occurred under Acacia nilotica. Few Acacia nilotica recruits were found either under adults or in the open whereas Acacia karroo recruits were more common.The overall size class distribution for eucleas was dominated by individuals in the intermediate size class, suggesting that recruitment is not the dominant demographic process. We propose that the escape of intermediate sized eucleas from the fire trap has caused the increase in woody plants.Analysis of a time sequence of aerial photographs shows that invasion occurred rapidly between 1954 and 1975. A common feature for sites where woody plant invasion has taken place, was the presence of barriers to fire (especially roads). We suggest that the recent and rapid increase in woody vegetation is due to a decrease in the frequency of intense fires, rather than the recent absence of megaherbivores which allowed Acacia nilotica establishment. Few intense fires allow suppressed tree or shrub individuals, ('gullivers'), to escape the grass/fire layer and thereby become tall and fire-resistant. This release may explain the rapid rate of invasion by inherently slow growing broad-leaf species.     

Seasonal Variation of Boron and Fluoride in Tilapia nilotica from an Egyptian Fish Farm in Relation to Human Health Hazard Assessment

This study evaluated the seasonal accumulation of boron and fluoride in different tissues of a common edible fish, Tilapia nilotica, from an Egyptian fish farm as well as their hazard to human health. Among all the determined fish tissues, brain, liver, and bone were the ones that accumulated the highest boron and fluoride contents. Interestingly, flesh tissue showed the lowest boron and fluoride concentrations (0.30 ± 0.06 and 2.50 ± 1.57 μg/g, respectively). Statistical analyses using Pearson correlation matrix and multiple regression procedures indicated that boron accumulation was highest amongst flesh, liver, brain, and bone tissues. Additionally, fluoride content in water affects its abundance in the different fish tissues: flesh, gills, bone, and skin. Human hazard assessment of fluoride and boron from water, sediment, and fish flesh tissue was studied using calculations that considered chronic daily intake from water, sediment, and fish; chronic exposure from dermal contact and ingestion of water; and fish as well as a single acute human dose. These calculations indicated that the consumption of Tilapia nilotica is still safe as regards boron intake, but for fluoride, it may pose a hazard to human health in the long term.     

Further investigation on nuclear transplantation in different orders of teleost: the combination of the nucleus of Tilapia (Oreochromis nilotica) and the cytoplasm of Loach (Paramisgurnus dabryanus).

The nucleus of a blastula cell from Tilapia (Oreochromis nilotica, family Cichlidae, order Perciformes) was transplanted into an enucleated egg of Loach (Paramisgurnus dabryanus, family Cobitidae, order Cypriniformes). From among 3747 nucleo-cytoplasmic hybrid (NCH) eggs two NCH larval fish (0.05%) were obtained; one died on the 6th day and the other died on the 12th day after the operation. Morphological examinations showed that both NCH larval fish had developed normally with an opened mouth except they could not take food after complete utilization of their egg yolk on the 5th day of development. The possible mechanisms for obtaining such inter-order NCH larval fish are discussed. This is the first report indicating that inter-order NCH larval fish can be obtained in spite of their evolutionary divergence.