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| Figure 1. Map of Ethiopia (A), Lake Tana and study river mouths (fishing grounds) of Reb, Gumara and Gelda rivers (B) |
| Livestock Research for Rural Development 26 (5) 2014 | Guide for preparation of papers | LRRD Newsletter | Citation of this paper |
The infection rate by Ligula intestinalis plerocercoid has been studied in large Barbus fish drawn in the fishing grounds of rivers tributary to Lake Tana, namely Reb, Gumara and Gelda. A total of 384 large Barbus fish were sampled using gill nets with 28-35 mm mesh and Ligula plerocercoid has only been recovered from 33(8.6%).
A prevalence was observed of 4.9% in male and 3.6% in female Barbus fish, and spatially 2.1%, 2.6% and 3.9% in fishing grounds of Gelda, Gumara and Reb, respectively. From the results obtained it was concluded that sex and fish sampling site differences did not have significant effect on Ligula intestinalis plerocercoid infection. However, the slight variations observed between the two sex groups might be attributed to the differences in the number of fish trapped in each sex cluster. The slight difference in the infection rates observed among the fishing grounds could be predominantly related to the distribution pattern of the aquatic piscivorous birds, which are more numerous in fishing grounds where there is high fishing activity and disposal of fish offal is maximized.
Keywords: fishing ground, prevalence rate, sex group
The fish sector makes a vital contribution to the food and nutritional security of 200,000 000 Africans and it provides income for over 10 million people engaged in fish production, processing and trade (FAO 2003). However, fish diseases are very important aspects of modern fish farming given the enormous impact on profitability and also cause of human diseases in many areas of the world (Ruiter 1995).
Although stress appears to play a considerably larger role in causing disease, fish are susceptible to the same type of agents that affect warm blood animals, including virus, bacteria, fungi, parasites and various non-infections problems (Noga 1996). Helminth parasites are frequently occurred within the body cavity and viscera of fish. Some of the largest and most harmful parasites of the body cavity of fish are cestode plerocercoides (Khalil and Polling 1997) and among them the genus Ligula has a global distribution (Lewis 1999).
The plerocercoids of Ligula (L.) intestinalis have been recorded from the body cavity of a wide range of fish hosts, particularly members of the Cyprinidae, from worldwide locations (Hoole et al 2010). This cestode present a complex life cycle with a copepod as a first intermediate host and fish becomes infected consuming the infected copepod. Fish eating birds serve as the final host (Carter et al 2005, Trubiroha et al 2009). Heavy infestations induce host mortality, which is a threat to the commercial fisheries industry. It also causes pathological conditions such as the distension of the host’s body wall, muscle and gonad atrophy, maturation at reduced size, lower absolute fecundity and a number of other physiological and immunological disorders (Dejen et al 2006, Bouzid et al 2008, Trubiroha et al 2009).
The plerocercoids L. intestinalis have been reported from small Babus fish at Lake Tana (Eshete Dejen et al 2003), different fish species at Lake Tana (Zhokhov and Pugacheva 2012) and small and large Barbus fish sampled from different fishing ground (Dirma, Sekela, Gedamat and Warka Ber) of Lake Tana (Eshetu and Mulualem 2003). Despite these studies, no other study has been conducted targeting all morphotypes of large Barbus fish specifically at fishing grounds of Gelda, Gumara and Reb rivers, which are the largest tributaries of Lake Tana. Therefore, the objective of this study was to determine the prevalence of larvae of Ligula intestinalis infection in large Barbus fish inhabited the river mouths of Gelda, Gumara and Reb.
The study was conducted at fishing grounds of tributaries of Lake Tana, namely Reb, Gumara and Gelda rivers (Figure 1). Lake Tana is found in the northwest part of Ethiopia and accounts for half of the total fresh water area of the country. It covers an area of approximately 3150 km2 (Eshetu and Mulualem 2003).
The main commercially important fish species in Lake Tana and its tributaries are Nile tilapia (Oreochromis niloticus), African Catfish ( Clarias gariepinus), Beso (Varicorhinus beso) and large Barbus (Barbus intermedius complex). The latter represent more than 60% of the commercial catch (Wudneh 1998).
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| Figure 1. Map of Ethiopia (A), Lake Tana and study river mouths (fishing grounds) of Reb, Gumara and Gelda rivers (B) |
The study animal consists of large Barbus fish (Barbus intermedius complex) classified under the family Cyprindae. The largeBarbus (Figure 2) are referred to as ‘large’ to distinguish them from the small sized (maximum adult fork length is < 12 cm), diploid small Barbus species (Eshete Dejen 2003). Fifteen distinct species of ‘large’ Barbus have been identified from Lake Tana, and these are believed to form a unique Barbus species flock (Nagelkerke 1997, Nagelkerke and Sibbing 2000).
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| Figure 2. Large Barbus fish of 78.7 cm in length. Source: Abebe Ameha 2004 |
The study was cross-sectional and had conducted from October 2009 to March 2010. During the study period a total of 384 commercial sized fish were sampled using gill nets with 28-35 mm mesh. Fish were sampled from the study river mouths of Lake Tana and which are the natural fishing grounds for the nearby fishermen.
The sample size was determined based on the formula used for simple random sampling technique (Thrusfield 2005) and the total sample obtained was roughly divided to each study sites based on the population density of the fish estimated by the local fisher men. Accordingly, 116, 117 and 151 Barbus fish were sampled and examined for plerocercoid of L. intestinalis from fishing grounds of Gelda, Gumara and Reb tributaries, respectively.
The morphological features described by Pandey (2004) were used to identify the large Barbus fish from other fish species. Following routine post-mortem examination procedures the sex of the fish was identified by viewing the gonads based on Nicolsky (1963). The body cavity of each fish was searched for plerocercoids of L. intestinalis and the keys in Yamaguti (1959) and Schmidt (1986) were used to identify the parasites.
Infection rates were determined using simple mathematical calculations. Chi-square tests were used to determine the significant differences among the categories of each risk factor in respect to infection with Ligula plerocercoid. P<0.05 was considered statistically significant in all cases.
From a total of 384 large Barbus fish sampled, 8.6% (33/384) were found infected with plerocercoid L. intestinalis. On average of 2 Ligula plerocercoides per fish were recovered and all were found in the abdominal cavity.
Prevalence of 4.9% in male and 3.6% in female Barbus fish were recorded but the association between the variables and Ligula plerocercoid infection rate was found statistically none significant (P = 0.82) (Figure 3).
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| Figure 3. Frequency of L. intestinalis plerocercoid in relation to sex group |
Prevalence of 2.1%, 2.6% and 3.9% were recorded in Barbus fish sampled at fishing grounds of Gelda, Gumara and Reb tributaries, respectively. However statistically no significant differences were observed in the occurrence of the Ligula intestinalis plerocercoid in large Barbus fish drawn among the different fish sampling sites (P = 0.68) (Figure 4).
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| Figure 4. Frequency of L. intestinalis plerocercoid infection based on the different fishing grounds |
The overall prevalence of L. intestinalis plerocercoid infection in this study was found 8.6%. Relatively high Ligula plerocercoid infection were reported in small Barbus at Lake tana (10%) and its tributaries (40%) by Eshete Dejen et al (2003) and Eshetu and Mulualem (2003), respectively. As compared to the finding obtained in this study in large the relative high infection rates reported in small Barbus fish could be related to the feeding habit, where the small sized Barbus fish prefer copepods which are the primary intermediate hosts of L. intestinalis (Eshete Dejen et al 2003).
Prevalence of 8.14% in large Barbus fish (B. tsanensis) was also reported in the study conducted by Eshetu and Mulualem (2003) at Lake Tana and was found more closer with the finding of 8.6% reported in this study. However the slight variation observed between the two studies might be due to the differences in the study sites where different concentrations of aquatic birds are found. It could also be linked to the season when the sampling and examination of Ligula plerocercoid were done. The seasonal variation could as well related in host feeding pattern, availability of infected intermediate hosts, hormonal and immunological changes (Woo 2006, Turgut et al 2011). The availability and the numbers of aquatic piscivorous birds (which are most of the time migrate during the rainy season), water quality and temperature as well as other general physical factors of the habitat could also have contribution in the disparity of the prevalence rate of Ligula pelerocercoid (Post 1987). Most likely, birds predate more efficiently on barbs in shallow clear waters than in shallow turbid and deep waters (Dejen et al 2006).
Even though the differences were statistically not significant slight variations have also been observed in Ligula plerocercoid infection between the two sex groups and among the different study fishing grounds. These findings were similarly described in different study results reviewed by Scholz (1999). However the slight difference observed between the sex factions could be attributed to the variation in the number of Barbus fish sampled during the study period (male = 214 and female = 170). The high infection relates, which were recorded at Reb (3.9%) and Gumara (2.6%) as compared to Gelda (2.1%) fishing grounds might also be credited to the presence of large number of aquatic birds (personal observation and communication with the nearby fisher men) and the comparative distance of each study site to the surrounding villages. Thus the amount of fishing and offal discarded could determine the number of fish eating birds attracted to the surrounding river mouth and the number of infected primary intermediate hosts of Ligula intestinalis (copepods).
On average 2 Ligula plerocercoides per fish were recovered and their main location was abdominal cavity. High parasite level up to 30 for Ligula infection have been observed in European and North American cyprinid fish (Szalai et al 1989, Loot et al 2001). African Barbus species are also apparently able to survive infestations of more than ten parasites per fish (Mashego 1982). The infection levels of L intestinalis plerocercoid might depend on the age of the host fish sampled and infection levels may continue to increase during the life of the fish as more plerocercoids are added to the plerocercoids already present in the fish (Post 1987).
The authors acknowledge Bahir Dar Fisheries and Research Centre for the provision of working facilities and technical staffs during the study period.
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Received 14 August 2013; Accepted 1 January 2014; Published 1 May 2014