A Study of Clinostomum (Trematode) and Contracaecum (Nematode) Parasites Affecting Oreochromis Niloticus in Small Abaya Lake, Silite Zone, Ethiopia

Ethiopia has large water resources, with an estimated surface area of 733k km2 of major lakes and reservoirs, 275 km2 of small water bodies and 7285 km long rivers within the country [1]. As a result of these ecological variations, Ethiopia has been the home of highly diversified flora and fauna. More than 200 species of fish are known to occur in lakes, rivers and reservoirs in Ethiopia [2]. The country depends on its inland water bodies for fish supply to its population.


Introduction
Ethiopia has large water resources, with an estimated surface area of 733k km 2 of major lakes and reservoirs, 275 km 2 of small water bodies and 7285 km long rivers within the country [1]. As a result of these ecological variations, Ethiopia has been the home of highly diversified flora and fauna. More than 200 species of fish are known to occur in lakes, rivers and reservoirs in Ethiopia [2]. The country depends on its inland water bodies for fish supply to its population.
Mostly, Nile tilapia (Oreochromis niloticus) has been introduced throughout the country because of its adaptive abilities and its suitability to match Ethiopian consumers' preferences. As a consequence of its natural occurrence plus its introduction into different water bodies, it is contributing about 40.9 % of the 13,253 tons of commercial fish catch in 2007/2008 [3].
One of the problems of the fishery sector in the wild populations are parasites and disease conditions of fish. Parasitic diseases reduce fish production by affecting the normal physiology and if left uncontrolled, it can result in mass mortalities or in some cases, can be served as a source of infection for human and other vertebrates that consumed fish [4]. Presence of a massive number of parasites on each fish might constitute a real threat to the fish population and require immediate action [5].
The digenea parasites are the main endo-parasites of fishes; the greater majority of fish are susceptible to infection with different stages of these parasites [6]. Clinostomum species, digenetic termatode, are common fish parasites throughout the world and the final hosts of this fluke are generally piscivorous birds, including herons and egrets [7]. The metacercariae embedded in the tissues of fish are freed in the host stomach and migrates up to the esophagus, and then attach to the throat or mouth cavity. Human infections are known to be resulting from eating raw freshwater fish. Metacercarial infection in fish is the main source of disease with subsequent economic loss. Metacercariae may affect growth and survival, or disfigure fish so that they loss their market value as a food or ornamental product [8].
Most adult nematodes are found in the intestine of fish but their larval stage, which is infective to human, has the greatest impact on the consumer acceptance of fish as a source of protein [9]. Contracaecum is an anisasakid nematode that infects fish-eating bird and marine mammals. Larval stage of Contracaecum usually occurs in the body cavity and mesenteries of fish while the adults occur in the gut of piscivorous birds, notably pelicans, cormorant's herons and darters [10].
So far, very few diseases have been described from fish of Ethiopia waters. Moreover there is no report of disease and prevalence of parasitic infection in fish in small Abaya Lake.
Therefore the objectives of the current study were: to determine the prevalence of Clinostomum and Contracaecum species in Oreochromis niloticus in Small Abaya Lake.

Materials and Study Methodology
The study area The study was conducted at Lake Small Abaya (7°29 03 ' 65 ' N latitude and 38° 03 ' 17.79 '' E longitude), which is located at altitude of 1835 meter above the sea level. The Lake covers a total area of 1253ha and it is shallow lake with the maximum depth of 9 m. The mean monthly minimum and maximum temperatures varies between 10.8°C significantly varied from 2.95 ± 0.21% with treatment T 6 (banana leaf) at 5 th month (August, 2010) to 13.72 ± 0.36% with treatment T 3 (mustard oilcake) at 4 th month (July, 2010). Carbohydrate significantly varied from 32.85 ± 0.14% with treatment T 3 (mustard oilcake) at 4 th month (July, 2010) to 66.35 ± 0.32% with T 2 (wheat bran) at 3 rd month (June, 2010). In the same feed item no significant difference in the nutrient content was found during the study period (Tables 1-4).

Mean variations
The variations in the mean values of nutrient contents (protein, lipid and carbohydrate) with different treatments of feed items are presented in Table 3 and Figures 1-3. Protein content significantly varied from 6.18 ± 0.13% with treatment T 6 (banana leaf) to 30.53 ± 0.40% with treatment T 3 (mustard oilcake). Lipid content significantly varied from 3.06 ± 0.09% with treatment T 6 (banana leaf) to 13.33 ± 0.10% with treatment T 3 (mustard oilcake). Carbohydrate significantly varied from 32.95 ± 0.29% with treatment T 3 (mustard oilcake) to 66.12 ± 0.47% with treatment T 2 (wheat bran).

Monthly variations of the nutrient contents
Protein content varied from 6.05 ± 0.45% with (T 6 at 6 th month) to 31.20 ± 0.32% (T 3 at 2 nd month). Lipid content ranged from 2.95 ± 0.21% (T 6 at 5 th month) to 13.72 ± 0.36% (T 3 at 4 th month). Carbohydrate to 14.1°C and 22.5°C to 28.7°C, respectively throughout sampling period. Before stocking, there were no commercially important fish species in the lake except for the naturally occurring Barbus species. In the 2005 National Fisheries and other Aquatic Life Research Center stocked Oreochromis niloticus or Nile tilapia and Tilapia zilli fry in to Lake small Abaya.

Study design
A cross sectional study was conducted from November 2013 to April 2014 at Small Abaya Lake to determine the prevalence of parasitic infestation of O. niloticus /Nile tilapia/. The desired sample size was calculated using the formula given by Thrusfield [11]. By considering 95% confidence interval, 5% desired absolute precision and 50% expected prevalence and the total number of sample found to be 384.

Study methodology
Sample collection: A total of 384 O. niloticus species of fish were sampled and examined. All the fish were caught using gill net with mesh size ranging from 6 to 12 cm. Harvested fishes were transported in ice to Ethiopian Institute of Agricultural Research, National Fisheries and other Aquatic Life Research Center, Sebeta, for analysis. The length (L) of fish was taken from the tip of the snout to the posterior tip of the caudal fin and was measured to the nearest ± 0.1 cm. The weight of the fish was measured to the nearest gram using an electric balance.
Laboratory examinations: Each fish was opened and its internal organs were fully examined for parasites. The entire digestive system was removed and placed in a Petri-dish with physiological saline, and the gut was divided into sections. The muscles, gonads, liver, and heart were examined with the aid of a dissection microscope and a phase contrast light microscope at 10 and 40 magnifications. Parasites were counted, their location recorded, and preserved in 70% ethanol. Identification of most parasites was made immediately following standard keys in literature [12][13][14].

Data analysis
The data obtained from the laboratory finding were summarized and then analyzed using SPSS version 16 analyzing software. Chisquare was applied to test association between sex, weight, and standard length with occurrence of the disease. The effect of the parasites on the health of their host was determined by calculating Fulton's condition factor (K), a measure of an individual fish's health that uses standard weight. Proposed by Fulton in 1904, it assumes that the standard weight of a fish is proportional to the cube of its length.

100( / 3) K W L =
Where W is the whole body wet weight in grams and L is the length in centimeters; the factor 100 is used to bring K close to a value of one. Pearson correlation was done to find the correlation between the body conditions of fish with the number of parasites.
Mean intensity was also calculated using the formula given below

Monthly variations
Protein content significantly varied from 6.05 ± 0.45% with T 6 (banana leaf) at 6 th month (September, 2010) to 31.20 ± 0.32% with treatment T 3 (mustard oilcake) at 2 nd month (May, 2010). Lipid content   content ranged from 32.85 ± 0.14% (T 3 at 4 th month) to 66.35 ± 0.32% (T 2 at 3 rd month). Suresh and Mandal worked on the determination of nutritive value of rice bran, mustard oil cake and Azolla for a period of 4 months from July to October. In rice bran they found crude protein and crude fibre as 12.6% and 21.9%, respectively. In mustard oilcake, crude protein and crude fibre was 38.6% and 6.8%, respectively and in Azolla, crude protein and crude fibred was 26.5% and 20.4%, respectively. Sithara and Kamalaveni worked on the formulation of low cost fish feed using Azolla as a protein supplement during September to March and reported 20-25.5% protein in Azolla. Ebrahim used Azolla as tilapia diet for a period of 90 days in summer season and reported 20% protein in Azolla. Fasakin and Balogan worked on the nutritional aspects of Azolla in August, 1997 and reported 20.9% protein in Azolla.
Present findings also indicated that in case of same feed item, no significant difference was found in the nutrient content at different months (Tables 1-4). This might be due to no major change in the temperature was found to affect the growth and composition of Azolla during the study period. This statement was almost agreed with Lumpkin and Plucknett who reported that change in Azolla composition was subjected to change in environment. Statement also agreed with Van-Hove and Ebrahim who reported that change in Azolla composition was subjected to change in species.
The chemical composition of Azolla species varies with ecotypes and with the ecological conditions and the phase of growth. The crude protein content is about 19-30 percent dry matter basis during the optimum conditions for growth. The protein contents of Azolla species are comparable to or higher than that of most other aquatic macrophytes. Aquatic weeds' are highly nutritious with protein content of 20-30%, when cultivated in nutrient rich waters. Importantly, they P > 0.05; df =1 Overall findings indicated that inspite of having variations in nutrient contents, monthly supply of nutrients was almost same respective feed item under non-conventional feeds as with conventional feeds. Mean values of the nutrient contents under nonconventional feed items are found potentials for the development of low cost aquaculture.
Fish feed generally constitutes 60-70% of the operational cost in intensive and semi-intensive aquaculture system. The fish feed used in aquaculture is quite expensive, irregular and short in supply in many third world countries. These feeds are sometimes adulterated, contaminated with pathogen as well as containing harmful chemicals for human health. Naturally there is a need for the development of healthy, hygienic fish feed which influences the production as well as determines the quality of cultured fish. Considering the importance of nutritionally balanced and cost-effective alternative diets for fish, almost similar expression to evaluate the nutritive value of different non-conventional feed resources, including terrestrial and aquatic macrophytes was found with Wee and Wang and Mondal and Ray. However potentials roles of aquatic and terrestrial macrophytes as supplementary feeds in fish farming were also found to be expressed with Bardach and Edwards.

Recommendation
Present findings explored the nutritive aspects of both conventional and non-conventional feed items and question raised about the response of utilizing the feed especially of aquatic weeds to fish growth and economy. Therefore, it is recommended to conduct further study on the evaluation of fish production and economy under different feed and weed based systems in polyculture ponds.