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This study was conducted from November 2007 to October 2008 into the two main fishing sites in Jebel Aulia Reservoir ; these were Jebel Aulia site (45 km south Khartoum city) and Kosti site about 355 km southward Khartoum city .
The study focuses in family Characidae that has two genera Hydrocynus (H. forskalii, H. lineatus and H. brevis) and Alestes (A. baremose, A.dentix and A. nurse).
Water characteristics investigated including temperature, concentration of dissolved oxygen, conductivity, transparency, pH, compositions and abundance of Phytoplankton. However, there is un-normal decline of phytoplankton and zooplankton production at the vicinity of the dam indicating prospected hazard.
The study covered the biological parameter of characid fishes feeding and maturity. In two sites Jebel Aulia and Kosti Sites fishes showed maximal of incidence of full stomachs at the end of the rainy season exactly in August 2008 and the other peak was recorded during the high abundance of planktonic organism in March 2008.
Keywords: food, feeding characids.
Three factors play important role in plankton distribution. These are light penetration, penetration, temperature and movement of water-masses Prows and Talling (1958). These in turn affect the food chain beside the other biological processes.
Water temperature is an important factor, which affects growth, reproduction, maturity of fish and the food chain. Therefore, every individual fish species shows the best physiological performance at an optimum temperature. Furthermore, the temperature affects on the amount of dissolved oxygen and consequently the rate of respiration of fish Bishai (1962). Reported that the normal temperature to which a fish is adapted in temperate regions is between 0-30ËšC. Value above 30ËšC occurs in shallow water in summer FAO (1989). Minimum temperatures prevail during in autumn and winter. Oxygen concentrations and transparency are the major factors limiting phytoplankton abundance and distribution in water Ahmed (1989).
Many investigators have studied transparency of the White Nile. Of them, Hamaza (1981). reported that the water transparency at Al-Kalakla and Um Shabasha showed maximum records during Dec. and Feb., but it decreased in a horizontal gradient towards the dam due to the distribution of the suspended materials such as planktons and detritus and seasonally during flood season (9-13 cm) due to the sediment and accumulation of the water hyacinth. Recorded that turbidity was higher in Dec. and Sep. and lower in Mar. and June Manteifel, (1961).
The dissolved oxygen concentration for most fresh water fish is above 7.0 p.p.m at any time FAO (1982) (. It is fetal at high temperature. Dissolved Oxygen concentration at Jebel Aulia reservoir was above saturation all the year round Ahmed (1985) .The highest concentration was recorded at December and lowest at August. The mean oxygen concentration in the Jebel Aulia reservoir is beyond the demand of fish and other aquatic organisms Ahmed (1985).
Water with pH value ranging about 6.5 to 9 at day break .This is most suitable for fish production Ahmed (1989), In Jebel Aulia reservoir range of pH was 7.4 to 8.6. The high values were recorded during flood season. The importance of pH it is roles in ionizing chemical compound to produce free ions for the readily use of phytoplankton Ahmed (1985).
Composition and seasonal fluctuation of phytoplankton and zooplankton exhibited maximum level in March during the season of low water discharge Bashier ,(2007). The average zooplankton biomass throughout the whole reservoir was 0.192 g/m2 in April with greater value found near the dam (Russian Research Expedition, (1964) and Adam (1977)). reported two annual peaks of plankton growth and a decline in Jebel Aulia reservoir. One of beak growth occurs just before flood season and the second occurs during winter (November - January).
The food supply is thus governed by the condition for obtaining and utilizing the food Manteifel, (1961).and it is also dependent on a biotic condition during the feeding period such as temperature, illumination, wind, fluctuation in water level, and changes in size of feeding area. The supply is also influenced considerably the protection from predators during the feeding period.
The purpose of conducting this study, were to determine fish food and feeding intensity of Characids fish in Jebel Aulia Reservoir Dam.
2. MATERIALS AND METHODS
Samples of water were collected at monthly from November 2007 to October 2008 using fishing nets and collecting fishing from the landing sites, interval from the open water at the Jebel Aulia site and at season interval at Kosti site. At the same time measurement of dissolved oxygen, ionic conductivity, pH and degree of water temperature were recorded from the field. Dissolved oxygen (ppm) was measured by Winkler's method. Temperature recorded by thermometer, the pH meter and ionic conductivity meter was measured by Hanna instrument. Water transparency was measured with standard Secchi-disc.
Phyto-and zooplankton were collected by standard plankton nets that were hauled for a minute, the volume of water hauled was estimated from the hauling distance and the dimension of the net (net ring diameter and the height of the net cone). The plankton collected was rinsed in sampling bottles and preserved in 4.0% formalin. Planktonic organisms were examined under a light microscope (x 40 or x 100) and the number of organism per ml of the collected sample was counted and the number of organism per liter of water was computed using the volume of the water sieved during the haul of the plankton net.
Family Characidae classification was done by the following the keys: (8,20 and13) , the genus Hydrocynus (Cuv1817) include predator fish belonging to three species H. forskalii. (Cuv1817), H. lineatus (Blkr), H. brevis (Gthr).The genus Alestes (M. and T, 1844) whose member are non pisciviorous comprised three species, A. baremose (Joann), A. dentex (L), A. nurse (Riip) of these species A. baremose. Of these species A. baremose was the most common.
Rate of digestion have been measured by the rate of disappearance of gut content i.e. the transparency of the gut fullness methods.
3. RESULTS AND DISCUSSION
Degrees of water temperatures recorded during the study duration at reservoir of Jebel Aulia dam and Kosti were as follows:
In Jebel Aulia dam, the highest degree temperature was 31ËšC recorded by the end of flooding season in October 2008, while the lowest degree was 21ËšC during winter in February 2008. In Kosti, the highest temperature was 29 ËšC in summer season in May 2008, while the lowest one was 23 ËšC recorded in winter season in March 2008.
In Jebel Aulia dam, high transparency values (67.3) were recorded in March 2008, while the lowest value (11.4cm) was recorded during the flooding season in July. A high transparency value was recorded during the end of winter season in March 2008. Low one was recorded during commence of flooding season in May 2008.
In Jebel Aulia dam, with minimum of electric conductivity value during the flooding season in July (185 Âµmho); whereas, the highest one (308Âµmho) was observed during months of summer in May 2008. In Kosti, with minimum value at commence of March (165Âµmho). The highest was also observed during commence of summer season in March (243Âµmho).
In Jebel Aulia Site, high value was recoded in November 2007 (8.5) but low reading were recorded in September 2008 (6.22) .In Kosti Site, high value ( 8.3 ) was recorded in mid-autumn in October 2008, while in mid-winter in March 2008 reading had the lowest value (6.5) .
pH in Jebel Aulia dam, collected recorded in April 2008 water highly saturated with dissolved oxygen, while in May 2008 had lowest dissolved oxygen concentration. In Kosti, water the highest saturation with dissolved oxygen was recorded in October 2008, where, as March 2008 showed lowest value of dissolved oxygen in water.
In Jebel Aulia dam, the annual maximums abundance of phytoplankton and zooplankton were recoded during the end of the autumn in March 2008, while the lowest levels were during the flooding season in July 2008. In Kosti, the annual maxima were recoded during the start of the summer season in March 2008, while; the lowest level was recoded during the start of flooding season in July 2008.
Table (1): The average of water characteristic in Jebel Aulia Dam during 2007/2008
Phytoplankton abundance (No. /ml))
Zooplankton abundance (No./ ml )
Table (2): The average of water characteristic in Kosti area during 2007 / 2008
Phytoplankton abundance (No.ml))
Zooplankton abundance(No./ ml )
Fish food and feeding intensity
The fish food in stomach of family Characids was determined at both Jebel Aulia dam and Kosti, fishes showed full stomachs at the different times through out the study period. The maximal index of stomach fullness was at the end of the rainy season exactly in August 2008. The other peak of incidence of full stomach was recorded during the high abundance of planktonic organism in May 2008. Results of both sides indicate the seasonality of availability of food. Rainy season coincides with the breeding season of most Nile fishes when there is a plenty of fry and fingerlings, the main food for genus Hydrocynus and the aquatic insect larva and grains of aquatic weed for genus Alestes coincide of full stomach were higher for Alestes species in almost all the seasons specially summer, fish with empty stomach were high for Hydrocynus species. Full stomachs were in summer then winter and least in autumn. . Explanation of high incidence of full stomach in May, it may be explained by the low water level and more possibility of successfully preying for both fish and other aquatic organelles.
Table (3): Fish food and feeding intensity of Characidae fish in Jebel Aulia Dam from (2007/2008)
Table (4): Fish food and feeding intensity of Characidae fish in Kosti during 2007/2008
Fig (1): Fish food and feeding intensity of Characidae fish in Jebel Aulia Reservoir Dam from (2007/2008)
Fig (2): Fish food and feeding intensity of Characidae fish in Kosti from (2007/2008)
Temperature degrees during winter were low and similar at all study stations; where water transparency was relatively not high and ranged between 17.4 cm and 67.3cm (Table 1). Decline of fish in winter were probably correlated with low temperatures and high transparency, presence of fishes into deep waters and low fishing.
The results of water temperatures and transparency during all the year of present study are in corresponding with those of Sinada, (1972), Saeed (1976). and Ahmed (1989) and with those of Muso, (2002). ( whose sampling site was 700 km south of Jebel Aulia dam.
The dissolved oxygen content of the water is second factor to temperature as the biological significant factor in aquatic resources .Absence or two low level of water oxygen limit to distribution of plant and animal. The water of Jebel Aulia dam was normally over saturated with oxygen (6.07 p.p.m to 7.93 p.p.m). The concentration of aquatic oxygen was observed to decrease during the flood season. However no oxygen attention was recorded. These results are in harmony with findings of Hamaza, (1981) and Kawai, (1994).
In Jebel Aulia dam reservoir the pH of water ranged "between 6.22 to 8.7". The high values were probably a result of carbon oxide release that accompanied the annual out break of plankton production (Abu Gideri, (1962) and Prows &Talling, (1958)).
The observed seasonal fluctuations electric conductivity (winter :240Â±12 Âµmho; during flood: 200.5 Â±14.5 Âµmho and summer :269.7 Â±24.7 Âµmho cmË‰1) were most likely due to leaching of salt by flood water that over flow the river banks and wash from rain stream into the river .
Plankton flourished and densely reproduced during the time of slow water flow in period from March to May and November to December .These observation are in agreement with these of (Hamaza, (1981) and Ahmed .,1989). Production was minimal due to flood and high water discharge (Rzoska and Brook, 1954) in addition to the effects of the biotic and a biotic factor.
Although the previous result Adam, (1977), record higher flourish of planktons (primary and secondary) at the vicinity of dam in comparison to down stream, the present recorded the reverse i.e. lowest densities. This indicates change in the habitat at the vicinity the dam is more likely to place either due to change in the regime of the dam operation or due to electro - generating power plants instead to the dam.
Feeding intensity governed by food availability which in related to environmental habitat parameter. Other factors that were effective, cloud formation, decreased water discharge and hence high water levels. Fluctuations in the environmental conditions of this habitat clearly suggested that fish responded to environmental variables in the same general way as the rest of vertebrates and that their maturation and breeding activities are still primarily governed by light cycles.
The percentage of full stomach was decline in the piscivorous fish i.e. Hydrocynus .The role of the vision in feeding has tested experimentally by previous studies Petts, (1988) and FAO (1982) who recorded threshold of light intensity is required for searching power for prey and the daily feeding period vary considerably with season .
So high siltation and low transparency can be considered within the main factor .This can be considered within the main factor. This can be further influenced by the high water current that prevail during this season with accompanied by the flood of white Nile .However , the ability to search for prey depend on the distance depend on the movement of the prey ,its orientation and abundance in the specific area.
Further studies in type of food and feeding of Characidae fish in White Nile.