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Diurnal variation in food resource partitioning among some co-occurring fishes in the Negombo estuary of Sri Lanka
| Content Provider | Semantic Scholar |
|---|---|
| Author | Shirantha, R. R. A. R. Wijeyaratne, M. J. S. |
| Copyright Year | 2002 |
| Abstract | The diurnal variation in the % overlap of the diet and coefficient of electivity for different food items in 10 species of food fishes namely Acanthopagrus berda, kmbassis commersoni, Chanos chanos, Etroplus maculatus, Etroplus suratensis, Epinephelus tauvina, Lates calcarifer, Leiognathus splendens, Tricanthus brevirostris and Tachysurus caelatus, cooccurring in the northeastern region of the Negombo estuary, a highly productive brackish water body in the western coast of Sri Lanka was studied to get an insight on their ecological segregation. High dietary overlap was evident among A. berda, A. commersoni and T. caelatus and between C. chanos and E. suratensis. Most of the co-occurring fishes feed on different food items resulting in ecological segregation. If they feed on the same food items, feeding is at different intensities. These intensities as well as the preference for a particular food item vary with the time of the day. Some of the food items such as insect larvae, fish and detritus were selected by fish whenever these are the components of the diet. Diurnal variation in the intensity of feeding on different food items and preference for different food items appear to be among the major factors that contribute to ecological segregation of cooccurring fish species in the northeastern region of the Negombo estuary. INTRODUCTION Fish communities in the tropics are highly complex and found to consist of specialized co-evolved populations (Fryer and Hies, 1972; Lowe-McConnel, 1975; Welcome, 1976; Goulding, 1980; Moyle and Senanayake, 1984). High diversity in these fish communities is maintained by localized environmental conditions and preference for different microhabitats and food items (Costa and Fernando, 1967; Lowe-McConnel, 1975; Connel, 1978). Closely related and morphologically similar species in a fish community utilize the same food and habitat resources for their co-existence. Earlier studies have shown that there is a high ecological overlap among the co-occurring fish species with respect to space and food (Costa and Fernando, 1967; Bishop, 1973). Resource partitioning among the co-occurring fish species in some water bodies in Sri Lanka has been studied by several workers (Costa and Fernando, 1967; Moyle and Senanayake, 1984; Edirisinghe and Wijeyaratne, 1986; Wijeyaratne and Costa, 1992; Wijeyaratne and Perera, 2001). Although there appears to be a high overlap in the dietary composition of co-occurring fish species, it is hypothesized that ecological segregation exists due to differences in their feeding chronology. However, not much information is available on the diurnal variation in the food resource partitioning among the co-occurring fish species in highly productive habitats such as lagoons and estuaries in Sri Lanka. The present study was therefore carried out in one of the highly productive estuaries in Sri Lanka, namely the Negombo estuary which extends for about 3200 ha and sustains a commercial fishery yielding around 294,000 kg year" (Samarakoon and Van Zon, 1991) to have an insight on the ecological segregation of co-occurring fish species by analyzing the diurnal variation pattern in the food resource partitioning. * Author for correspondence. Email: jayantha_kln@hotmail.com 26 R.R.A.R. Shirantha and M.J.S. Wijcyaratne MATERIALS AND METHODS The fish samples were collected from the northern region of the Negombo estuary (Figure 1) at every 3 hour interval for a period of 24 hours in November 1996. The samples were collected using a cast net of 2.0 cm stretched mesh operated from a non-mechanized log raft. At each sampling occasion, the net was hauled 10 times and the fishes caught were immediately preserved in 10% formalin for detailed analysis in the laboratory. At each sampling occasion, plankton samples were also collected using a plankton net of 50 u mesh and were immediately preserved in 10% formalin. In the laboratory, the fishes were identified to the species level using De Bruin et al. (1994) and Pethiyagoda (1991). The fishes were then Figure 1. M a p of Negombo estuary showing sampling site eviscerated and the stomach contents were scooped out. The food items in the stomach contents were identified under the optical microscope as much as possible using John (1987) and Bellinger (1992). The stomach contents were quantitatively analyzed by determining the relative volume of food items using a cell of Spirogyra as an arbitrary unit. The relative importance of major taxonomic groups of food items in the diet of each species collected at different times of the day was thus determined. The similarities among the diets of different fish species were determined by calculating the similarity index using the following equation from Schoener (1970). S = 1 0 0 V $ 2 : | P x i P y , | where S = Similarity index PXJ = Relative importance (%) of a particular food item in species x Pyi = Relative importance (%) of the same food item in species y Food resource partitioning among brackish water fishes 27 The values for S range from 0 to 100 and give the % overlap in the diet between the two species considered. The overlap in occurrence during the 24 hour period among the different species was also determined using the same equation taking PXJ as the relative abundance of (% of) species x at time i and Pyi as the relative abundance (%) of species y at the same time. In the present study, values of < 33.3 were considered to indicate a low overlap, values between 33.3 and 66.7 to indicate a moderate overlap and the values > 66.7 to indicate a high overlap as suggested by Moyle and Senanayake (1984). The plankton species in the samples were identified as much as possible using John (1987) and Bellinger (1992). The plankton samples were quantitatively analyzed using a cell of Spirogyra as an arbitrary unit. The relative volumes of major taxa of plankton were thus estimated and the relative importance of each major taxon' in the plankton samples was determined. The preference of each fish species for different food items at different times of the day was determined by calculating the coefficient of electivity using the following equation from Ivlev (1961). Coefficient of electivity = (r-p) / (r+p) where r = Relative importance of a particular food item in the diet p = Relative importance of a particular food item in the environment Values for the coefficient of electivity range from 1 to +1 . Value of +1 indicates complete selection by the fishes and -1 indicates complete avoidance. RESULTS A total of 10 species of fishes were encountered in the northeastern region of the Negombo estuary during the present study. They were Acanthopagrus berda, Ambassis commersoni, Chanos chanos, Etroplus maculatus, E. suratensis, Epinephelus lamina, hates calcarifer, Leiognathus splendens, Tricanthus brevirostris and Tachysurus caelatus. All these species are commercially important as food fish. In addition, 2 species, namely, E. maculatus and E. suratensis being popular aquarium fish, have an ornamental value too. The relative importance of these species at each time of sampling is given in Table 1. The highest number of species was recorded at 1000 h while at 0700 h only one species namely, E. maculatus was recorded. The relative abundance of each species at different times of sampling expressed as a % of the total number of individuals of that species caught during the entire period of sampling is given in Table 2. A. berda and E. suratensis were recorded at 6 sampling occasions while six species, namely E. maculatus, E. tauvina, L. calcarifer, L. splendens, T. brevirostris and T. caelatus were recorded only once during the 24 hour sampling period. The food items recorded in the stomach contents of these fish species are listed in Table 3. Of the 10 species of fish recorded, 4 species, namely E. maculatus, E. suratensis, C. chanos and T. brevirostris were found to be strictly herbivorous while the others were omnivorous. 28 R.R.A.R. Shirantha and M.J.S. Wijeyaratne Table 1 The relative abundance of different species of fish at each time of sampling in the northeastern region of the Negombo estuary during the 24 hour period of the day. (Ab = Acanthopagrus berda; Ac = Ambassis commersoni; Cc = Chanos chanos; Em = Etroplus maculatus; Es = Etroplus suratensis; Et = Epinephelus tauvina; Lc = hates calcarifer; Ls = Leiognathus splendens; Tb = Tricanthus brevirostris; Tc = Tachysurus caelatus). Species OlOOh 0400h 0700h Relative abundance (%) lOOOh 1300h 1600h 1900h 2200h Ab 2.9 5.3 38.5 80.0 88.9 88.8 Ac 88.3 94.7 94.6 Cc 23.0 13.3 Em 100.0 Es 2.9 15.4 6.7 11.1 2.7 5.6 Et 2.7 Lc 5.6 Ls 7.7 Tb 15.4 Tc 5.9 Table 2 The relative abundance of each species of fish at different times of sampling expressed as a % of the total number of individuals of that species caught during the entire period of sampling. (Ab = Acanthopagrus berda; Ac = Ambassis commersoni; Cc = Chanos chanos; Em = Etroplus maculatus; Es = Etroplus suratensis; Et = Epinephelus tauvina; Lc = bates calcarifer; Ls = Leiognathus splendens; Tb = Tricanthus brevirostris; Tc = Tachysurus caelatus). Relative abundance (%) Species OlOOh 0400h 0700h lOOOh 1300H 1600h 1900h 2200h Ab 2.2 6.6 11.1 26.7 17.8 35.6 Ac 25.2 45.4 29.4 Cc 60.0 40.0 Em 100.0 Es 14.3 28.5 14.3 14.3 14.3 14.3 Et 100.0 Lc 100.0 Ls 100.0 Tb 100.0 Tc 100.0 The relative abundance of major taxa of food items in the diet of each species estimated using all samples collected throughout the day is shown in Figure 2. Shrimps were the major food item of A. berda, A. commersoni, E. tauvina, L. calcarifer, L. splendens and T. caelatus. Filamentous green algae were the major food item of E. maculatus, E. suratensis and C. chanos. In T. brevirostris, the major food item was diatoms. The relative importance of major taxa of food items recorded in the stomach contents of the fishes that co-occur at each sampling occasion is given in Figure 3. Food resource partitioning among brackish water fishes 29 T |
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| Language | English |
| Access Restriction | Open |
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