Sengupta, M. and Dalwani, R. (Editors). 2008. Proceedings of Taal2007: The 12 th World Lake Conference: 141-148 Food Resource Utilization OF Three Co-Occurring Carnivorous Fish Species in A Shallow Lowland Reservoir in South Eastern Sri Lanka: Approach to Minimize Competition. W.A.H.P. Guruge and N.J. De S. Amarasinghe Department of Zoology, Faculty of Science, University of Ruhuna, Matara, Sri Lanka. Email:guruge@zoo.ruh.ac.lk; jdesilva@zoo.ruh.ac.lk ABSTRACT Food resource exploitation pattern of three carnivorous fish species namely, Mystus spp, Glossogobius giuris and Channa striata, co-occurring in a shallow South-Eastern Sri Lankan reservoir was investigated. The fish community of this reservoir was represented by two introduced exotic tilapias and eight indigenous riverine species including five cyprinids, a clarid, a hemiramphid and a gobid. Fishes were sampled by three types of gear i.e. gill nets, bottom trawl, and cast net, at four sampling stations during day and night time. To determine the ontogenical diet changes, fishes were divided into six length classes. The stomach contents were analyzed based on species, fishing time and size class using the fishes collected on each sampling occasion to establish their feeding ecology. Point method was used to estimate the relative biovolume of different food items. Each fish specimen was assigned a value ranging from 1-5 based on its stomach fullness. The Schoener s formula was employed to calculate the proportional overlap in food consumption between any two species pair and, any two size classes of a particular fish species. The statistical package SPSS (version 10) was used to analyze data. Three carnivorous species fed mainly on fish, insects and zooplankton, but not in same proportion. Gut fullness categories exhibited significantly different variations between day and night time as well as among different size groups in the three fish species. Differences in diet composition and feeding intensities were observed in all the three species according to their respective size. Habitat partitioning of three cooccurring species was found to center upon spatial (surface, column and bottom) and temporal (day and night time) segregation. Role of these three carnivorous fish species in the reservoir is highlighted and they seemed to play an invaluable role, to maintain a healthy stable reservoir, in addition to their food and ornamental value. Keywords: Gut content, diet overlap, niche breadth, Gut fullness, length class, ontogeny, Mystus spp, Glossogobius giuris, Channa striata INTRODUCTION Being a tropical island Sri Lanka is blessed with remarkable bio diversity and still the Sri Lanka fauna is not well known. This is especially true for freshwater fishes of the island. According to the recent studies, freshwater fish fauna of Sri Lanka consists of 78 fish species, of which 39 are considered as threatened and among them 32 are endemic to Sri Lanka (IUCN, 2000). Fish communities in tropical regions are found to be highly complex, and structured with many species co-existing in the same environment. This high diversity considered to be maintained by localized environmental disturbances and preferences for different microhabitat and food items. (Lowe- McConnal, 1975; Costa and Fernando, 1967). Studies of resource partitioning, distribution and feeding ecology among co-evolved species in a community provide considerable insight into the nature of interspecific interaction and critical dimension of niche. Sri Lanka has the largest number of reservoirs for a country of its size and almost the entire inland fish production, which accounts for 20% of the total fish produced in the country, comes from capture and culture-based fisheries of the reservoirs. The available fish yield records from the reservoirs are equally impressive. Predation and competition pose important effects on fish community structure, at least in certain system. To maintain the equilibrium and dynamics of any ecosystem, both predation and competition play vital role and these two events interact in complex ways in fish communities and thereby produce novel community dynamics. These interactions arise mainly in three ways (Werner, 1986). Firstly, the mere presence of predators can greatly affect habitat use by prey species and hence the prey s interactions with other species. Secondly, both growth rates and predation on a species vary with body size and therefore change during ontogeny. Thirdly, the above factors interact to cause ontogenetic niche shifts in most fish species and such shifts can even change the sign of the interaction between two species. Predation can dramatically influence aquatic communities, both directly and indirectly (Carpenter, 1987). By removing prey, predators directly alter community species composition.
Feeding behaviour of four fish species were studied: Mystus gulio (Hamilton - Buchanan) - Longwhiskered catfish, Mystus vittatus (Bloch) - Striped dwarf catfish. Glossogobius girius (Hamilton - Buchanan)- Bar eyed goby and Channa striata (Bloch)- Murrel, indigenous to Sri Lanka. These four species possess economical value since rural poor people utilize them as cheap protein source and some have ornamental value especially Mystus species. MATERIALS AND METHODS For the practical reason the catches of the two Mystus spp., Mystus gulio and Mystus vittatus, were pooled and treated as one Mystus species (MYS). Four different types of macrohabitat were available within the reservoir. The four sampling stations, A, B, C, and D are each representative for one of these four macrohabitat types A - Bottom layer of open water zone; B - Pelagic zone (Surface layer of open water); C - Shallow intermediate zone (without vegetation); and D - Littoral zone with vegetation (Figure 1). Sampling was carried out with four types of gear. The distribution of the sampling gears over the stations is given in Table 1. Table 1. Distribution of sampling gears over four stations Gear type Gill Nets Bottom Trawl Larvae Net Cast Net Sampling station A,B, C and D A and C B and D A, C and D Fish sampled from small mesh nets (mesh sizes ranging from 12.5 to 37 mm stretched mesh) were used for gut content analysis to determine feeding behaviour of fish. Small mesh nets were set at 0600 hours and lifted at 0800 hours for day-time sampling while for the night time sampling setting time was 1800 and lifting time was 2000 hours. To prevent digestion of gut contents small mesh size nets were kept only 2 hrs in the water. Since the feeding habits and behaviour of the fish species depend on the size of the fish, each species was divided into six length classes. The six length classes and their respective lengths are given in Table 2. After fish were removed from the respective nets they were sorted according to species, stations, gear type and their relative length classes and put in jars with 4% formalin. Fish larger than 14.0 cm, their body cavity was injected with 4 % formalin. Then fish were brought into the laboratory for detailed analysis. In the laboratory emphasis was on gut content analysis, gut fullness and ecomorphological characters. Table 2. The six length classes and their respective lengths. Length class L1 L2 L3 L4 L5 L6 Total length (cm) 3.0-4.4 4.5-6.9 7.0-9.9 10.0-13.9 14.0-18.9 > 19.0 Figure 1. Morophometric map of Tissawewa, with water depths measured at a water level of 4.2m and a maximum depths measured at a water level of 4.2m and a maximum depth of 3.3m. position of the sampling station are indicated on a transect from West (left) to East (right). For gut content analysis the percentage of the biovolume per food items were used. This is the modified point method of Hynes (1950). For gut content analysis maximum of ten fish was taken for one species, station, gear type, fishing time and, length class. Gut content analysis was done separately for stations, fishing gear, fishing time, species and their length class to illustrate if there are any relationships with each other. Stomach was used for gut content analysis. Five different fullness conditions were identified and they were rated as 1 to 5: 1. Completely filled and swollen; 2. Just filled over full length, but not swollen; 3. Content divided in different patches; 4. Very few feed particles; 5. Completely empty. 142
Table 3. Number of fish dissected per size classes of three piscivorous fish species for gut content analysis. L1 3.0-4.4, L2-4.5-6.9, L3-7.0-9.9, L4-10 - 13.9 L5-14 - 18.9, L6 >= 19.0 (cm) Fish species Number of fish dissected per length class L1 L2 L3 L4 L5 L6 Total G. girius 21 45 46 55 64 68 299 Mystus species 5 24 89 129 82 14 343 Channa striata 1-2 3 3 11 20 For gut content analysis a maximum of ten stomach contents per size group per species per station were pooled and this sample was subjected to gut content analysis. Following categories of food item were identified. Animal food organisms: fish, shrimp, adult insects (aquatic or terrestrial), insect larvae (mainly chironomids, chaoborus and others), mollusc (gastropods and bivalves), zooplankton (copepods cladocereans and rotifer), microbenthos (harpactocoids, cladocerens and ostracods), and poriferons spicules. Plant materials consisted of: macrophytes, filamentous algae (benthic and planktonic), other phytoplankton and pollen and seeds. Digested matter or/and detritus and an organic materials were also recorded. Diet overlap (S) between each pair of species and any two size classes of particular fish species was determined using Schoener s (1970) formula; S = 1-0.5 (Σ ai=1 n P xi -P yi ). Where S is the dietary overlap coefficient of fish species x and y, P xi is the proportion of food category i in the diet of species x, P yi is the proportion of food category i in the diet of species y, and n is the number of food categories. The values for this similarity index range from 0.00 to 1.00, with 1.00 indicating complete overlap and 0.00 indicating no overlap. As indicated by Moyle and Senanayake (1984), the similarity indices having values less than 0.33 were considered to indicate a low overlap while the values above 0.67 were considered to indicate high overlap. The niche breadth was calculated for each fish species using the niche breadth coefficient, B i (Levins, 1968). RESULTS Number of fish dissected per size classes of three piscivorous fish species for gut content analysis is given in Table 3. Stomach Content of Three Co-Occurring Piscivorous Fish Species Stomach content of three piscivorous fish species is shown in Figure 2. All three species fed mainly on fish, insects and zooplankton, but not in the same proportions. For C. striata and G. girius fish was the most important food item, but Mystus species fed more on insects than on fish. For C. striata and G. girius insects were also important food items. Proportions of insect matter in the gut of three piscivorous species were more or less equal. Only Mystus species fed substantial amounts of zooplankton, this was much less in G. girius and almost lacking in the diet of C. striata. Food consumption by three fish species belonging to same feeding guilds was significantly different (Table 4). Within species, the amount of food consumed of different food categories was often significantly different. B i = -Σ n j=1p j * log Pj. Where P j is the proportion of food category j consumed by species i, B i is the niche breadth coefficient of a fish species i, and n is the number of resource states available. For the data analysis SPSS (ver 10) statistical package were used. Spatial, temporal and size related feeding behaviour relate to gut contents was investigated using ANOVA. Variation of gut fullness according to size of the fish, and sampling station were analysed using non-parametric test on SPSS. Figure 2. Main food categories of the gut of three cooccurring piscivorous fish species in Tissawewa. Values below the species abbreviationi indicate number of fish dissected for gut content analysis. Abbreviations used are Mystus: Mystus species; CS : C. striata; GG: G. girius; Fi: fish, shri: Shrimp; Ins: Insect matter; Mol: Mollusc Zoop : Zooplankton; Mic: Microbenthos; Mac: Macrophytes Det/di: Detritus and digested matter. 143
Table 4. Mean Bio volume of food category present in stomach of piscivorous fish species. Letters next to the each value indicates significantly different groups recorded from Student-Newman-Keuls test. Similar letters indicate no significantly different food categories within the fish species. Food category Fish species G. girius Mystus species C. striata Fish 31.59 a ± 2.60 16.88 c ± 1.89 53.40 a ± 11.17 Shrimp 3.13 c ± 0.82 0.30 d ± 0.16 - Insect matter 24.11 b ± 1.97 25.24 b ± 1.77 30.50 b ± 9.63 Mollusc 0.43 c ± 0.20 0.17 d ± 0.10 - Zooplankton 5.18 c ± 0.68 18.95 c ± 1.26 0.10 c ± 0.01 Microbenthos 1.50 c ± 0.43 1.32 d ± 0.23 - Macrophytes 0.64 c ± 0.17 0.84 d ± 0.27 0.60 c ± 0.50 Detritus/digested matter 33.21 a ± 1.19 35.23 a ± 1.75 14.90 bc ± 6.79 Table 5. Proportion of food categories per length class in G. girius. Letters next to the each value indicates significantly different groups recorded from Student-Newman-Keuls test. Similar letters indicate no significantly different food categories within the fish species. * Indicates significantly different size related food categories at p < 0.05. Food category abbreviation are Fi: fish; Shri: shrimp; Ins : insect matter; Mol: mollusc; Zoop: zooplankton; Mic: Microbenthos; Mac: Macrophytes; Al; Algae; Det: Detritus/digested matter Food Mean Bio volume ± standard error category L1 L2 L3 L4 L5 L6 Fi** 0.00 2.55 c ± 2.14 15.60 bc ± 5.17 29.49 b ± 5.76 47.01 a ± 6.09 58.59 a ±5.88 Shri 0.00 1.55 ± 1.25 3.54 ± 2.10 5.36 ± 2.23 6.37 ± 2.67 0.00 Ins** 37.14 a ± 7.78 40.80 a ± 5.14 45.48 a ± 5.88 22.25 b ± 4.12 12.26 b ± 3.42 7.25 b ± 2.78 Mol 0.00 1.55 ± 1.10 0.43 ± 0.30 0.56 ± 0.36 0.00 0.29 ±0.20 Zoop** 21.19 a ± 4.18 13.22 b ± 2.73 5.19 c ± 1.38 1.89 c ± 0.56 1.80 c ± 0.83 0.70 c ± 0.39 Mic 1.67 ± 1.20 2.75 ± 1.50 3.45 ± 1.59 2.03 ± 1.38 0.70 ± 0.36 0.07 ± 0.05 Mac** 1.90 a ± 1.95 a ± 0.77 0.36 ab ± 0.17 0.03 b ± 0.02 0.12 b ± 0.06 0.52 ab ± 0.20 Det. 38.09 ± 6.11 35.33 ± 4.46 25.91 ± 4.31 38.56 ± 5.24 31.68 ± 5.19 32.34 ± 5.31 Ingestion of food categories such as fish by G. girius and Mystus species was significantly different (p<0.05) from the other food categories. Proportions of insect matter in the guts of G. girius and Mystus species differed significantly (p< 0.05) from the other food categories. Proportion of digested matter in the gut of C. striata was comparatively lower than in the guts of G. girius and Mystus species. Size related feeding behaviour of G. girius Feeding behaviour of different size classes of G. girius is given in Table 5. Food composition of G. girius was significantly different in the different size classes. Consumption of fish, insect matter, zooplankton, and macrophytes by each size classes was significantly different. Remarkable ontogenetic "diet shift" was observed in G. girius. Smaller G. girius individuals (L1 and L2 length classes) mainly depend on zooplankton and insect matter (about 60%) and as they grow switched to the predominately prey fish based diet (> 58%). Proportion of insect matter and macrophytes in the guts of G. girius length classes L1, L2 and L3 were significantly different (p< 0.05) from the other size classes. Significantly higher (p< 0.05) proportions of prey fish were observed in the guts of length classes L5 and L6. Size related feeding behaviour of Mystus species Size related feeding behaviour of Mystus species is given in Table 6. Mystus species exhibit significantly different diet compositions among size classes. Consumption of fish and macrophytes significantly increased with increasing size while consumption of insect matter, zooplankton and microbenthos significantly decreased with increasing length. The proportions of zooplankton in the gut of length classes L1, L2 and L3, of Mystus species were significantly different (p < 0.05). The proportion of prey fish in the guts of Mystus L6 length class was significantly higher than (p < 0.05) in the other length classes. Size related feeding behaviour of C. striata In C. striata guts, among six different food categories only insect matter showed significantly different proportions (p < 0.05) between size classes (Table 7). Proportion of insect matter in the gut of L5 length class fish was significantly different (p< 0.05) and higher than in length classes L4 and L6. 144
Table 6. Proportion of food categories per length class in Mystus species. Letters next to the each value indicates significantly different groups recorded from Student-Newman-Keuls test. Similar letters indicate no significantly different food categories within the fish species. * Indicates significantly different size related food categories at p < 0.05. For the food category abbreviations refer table 5. Food Mean Bio volume ± standard error category L1 L2 L3 L4 L5 L6 Fi** 0.00 0.62 b ± 0.30 4.26 b ± 1.73 17.90 b ± 3.14 24.73 b ± 4.53 75.64 a ± 11.31 Shri 0.00 0.00 0.00 0.00 0.89 ± 0.56 2.14 ± 1.12 Ins** 41.00 a ± 11.33 34.33 ab ± 7.42 30.52 ab ± 3.41 23.43 ab ± 2.80 21.84 ab ± 3.78 6.92 b ± 4.78 Zoop** 23.00 a ± 7.52 29.33 a ± 5.85 25.18 a ± 2.66 18.36 ab ± 1.97 12.76 ab ± 2.21 1.71 ab ± 0.88 Mic** 18.00 a ± 6.63 6.08 b ± 1.59 1.95 c ± 0.44 0.33 c ± 0.13 0.00 0.00 Mac** 0.00 0.20 b ± 0.97 0.47 b ± 0.23 0.53 b ± 0.20 1.00 b ± 0.32 6.64 a ± 4.20 Al 0.00 1.58 b ± 0.97 0.48 b ± 0.20 0.90 b ± 0.47 0.98 b ± 0.50 0.00 Det** 17.00 ab ± 8.30 27.83 ab ± 5.48 36.77 a ± 3.09 38.15 a ± 2.95 37.11 a ± 3.93 6.78 b ± 5.37 Table 7. Proportion of food categories per length classes in C. striata. Letters next to the each value indicates significantly different groups recorded from Student-Newman-Keuls test. Similar letters indicate no significantly different food categories within the fish species. *Indicates significantly different size related food categories at p < 0.05. For food category abbreviations refer table 5. Food category Mean bio volume ± Std error L4 L5 L6 Fi 66.66 ± 12.34 23.33 ± 6.88 72.54 ± 9.83 Ins** 33.33 ab ± 9.27 70.00 a ± 12.22 2.72 b ± 1.68 Zoop 0.00 0.00 0.18 ± 0.26 Mac 0.00 0.00 1.09 ± 0.70 Al 0.00 0.00 0.91 ± 0.51 Det 0.00 6.67 b ± 3.32 22.54 ± 9.73 Gut filling variation of piscivorous fish species Gut fullness of three co-occurring piscivorous fish species is shown in Figure 3. In G. girius and Mystus species a filling category 4 (gut with very few food particles) was the most frequent filling category. Completely filled and swollen gut was the predominant filling category observed in C. striata, but numbers of observations were low. Figure 3. Gut fillness of piscivorous fish species. For the fish species abbreviations refer figure 2. Values below the fish species abbreviation indicate number of fish dissected for gut fullness analysis. Gut filling categories indicated in legends are: 1. completely filled and swollen. 2. Just filled over full length, but not swollen. 3. Content divided in different patches. 4. Very few feel particles. 5. Completely empty. Statistical analysis of gut fullness in relation to length of the three piscivorous fish species is shown in Table 8. Non-Parametric analysis, Kruskal-Wallis Tests (Rank Sums), was performed to examine differences of gut fullness within and between length classes. All filling categories of G. girius exhibited significantly different (p<0.05) gut filling among length classes. In G. girius, the proportion of filling category completely filled and swollen guts was significantly different (p<0.05) in length classes L1, L3, L4 and L6 compared with the other two length classes. In length class L6 the proportion of completely empty guts were significantly different (p<0.05) compared with the other length classes. In Mystus species, gut filling categories 2, 3, and 4 showed significantly different (p<0.05) variation with length classes. Gut filling category 2, was significantly different (p<0.05) in length class L1 compared with the other length classes. Gut filling categories 3, and 4 of Mystus species were significantly different (p<0.05) in length class 6 compared with the other size classes. None of the length class of CS showed significantly different gut filling variation (Table 8). 145
Table 8. Size related gut filling variation in three co-occurring piscivorous fish species. Letters next to the each value indicate significantly different (p < 0.05) groups recorded from Kruskal-Wallis (Rank Sums) test. Similar letters indicate no significant difference within length classes. * Indicates significantly different gut filing categories at p < 0.05. G. girius Filling Length Classes / Mean rank values category L1 L2 L3 L4 L5 L6 1* 118.83 a 113.71 b 144.93 a 158.65 a 170.63 b 163.19 a 2* 155.61 a 162.03 a 158.66 a 161.50 a 147.60 a 129.63 b 3* 164.26 a 164.77 a 169.31 a 167.63 a 147.19 a 113.41 b 4* 212.54 b 175.22 a 152.38 a 147.79 a 126.13 b 138.77 a 5* 187.31 a 163.02 a 130.32 a 160.25 a 158.48 a 128.93 b Mystus species L1 L2 L3 L4 L5 L6 1 215.10 171.63 156.47 168.96 181.99 224.57 2* 284.20 b 192.58 a 171.28 a 183.86 a 150.34 a 121.75 a 3* 264.00 a 145.65 a 185.17 a 186.00 a 150.60 a 96.42 b 4* 84.00 a 123.97 a 171.71 a 203.82 b 158.41 a 71.46 b 5 117.00 129.87 177.82 181.17 172.19 138.14 C. striata L1 L2 L3 L4 L5 L6 1 21 7 10.3333 13.6667 10.3333 2 9 14.25 9 9 11.625 3 8.5 13.75 12 12 10.25 4 8.5 8.5 11.8333 8.5 12.0417 5 10 10 10 10 11.75 Table 9. Size related dietary overlaps within fish species in Tissawewa. Bold and underlined values indicate high dietary overlap and bold and italic values indicates low overlaps between length class pairs. Fish species abbreviations are MYS: Mystus species; GG : G. girius; CS: C. striata GG MYS CS L1 L2 L3 L4 L5 L6 L3 L4 L5 L6 L1 0.80 0.73 0.59 0.52 0.16 L3 0.33 0.57 0.55 L2 0.89 0.87 0.72 0.64 0.16 L4 0.73 0.78 L3 0.70 0.80 0.84 0.77 0.20 L5 0.83 L4 L6 0.64 0.66 0.72 0.91 0.34 L5 0.47 0.51 0.60 0.81 0.42 L6 0.41 0.44 0.50 0.70 0.87 DIETARY OVERLAP The similarity indices of the diets among the pairs of three piscivorous fish species studied are G. girius Mystus species 0.82; G. girius - C. striata 0.71; Mystus species- C. striata 0.82. Similarity index between G. girius - C. striata is low compared to other two species combination. Size related dietary overlaps among three co-occurring fish species was shown in Table 9. It was observed for any species overlaps values between very close length class pair i.e. L1-L2, L1-L3, is higher than overlaps between far length classes i.e. L1-L5-L6. Of the 15 possible length class pairs of G. girius and Mystus species, 7 (47 %) pairs exhibited high overlap values while 3 (20 %) pairs of Mystus species exhibited low overlap values. Among 6 possible length class combination of C. striata, 3 (50 %) displayed high overlap values. NICHE Breadth Niche breadth values of three co-occurring fish species calculated for the whole research period are G. girius: 3.67; Mystus species: 3.96 and C. striata: 2.50. G. girius, and Mystus species showed relatively large niche breadth (B i > 3.6) based on food resources used and thus are more general feeders. In contract C. striata was marginally specialised in food utilization. Change of niche breadth according to size of fish species is shown in Table 10. It is noticed that the niche breadth of G. girius increased with their length except length class 5. Other two species did not exhibit any clear pattern. 146
Table 10. Size related niche breadths in major co-occurring fish species in Tissawewa. Fish species abbreviations are GG: G. girius; MYS: Mystus species; CS: C. striata Fish species L1 L2 L3 L4 L5 L6 CS - - 1.02 3.15 2.07 2.51 GG 3.04 3.21 3.29 3.46 2.93 - MYS 3.54 3.50 3.40 3.75 3.80 1.70 DISCUSSION The low abundance of C. striata compared to Mystus species and G. girius reveals true riverine origin of C. striata. Besides, C. striata is in highly demand as food fish for villagers and due to this a high exploitative pressure acts on C. striata. This high fisheries mortality is probably also responsible for its relatively low abundance. Of the three piscivores in Tissawewa, C. striata has the highest proportion of fish in its diet Mystus spp the lowest and G. girius is intermediate between Mystus spp. and C. striata. All three species have eaten also substantial proportions of insects, and Mystus spp. feeds also substantially on zooplankton. Therefore, none of the three species is purely piscivorous and all three species are facultative piscivorous. The high diversity of food categories in the guts of G. girius and Mystus species revealed that these two species are feeding in the water column and the benthic layer. Same findings were recorded by Weliange et al (2003), research work carried out in three reservoirs of Sri Lanka (Minneriya, Udawalawae, and Victoria). Mystus vittatus sampled from Parakrama Samudra reservoir Sri Lanka, was feeding upon animal food items such as zooplankton, chironomidae, Chaoborus larvae and others insects (Schiemer & Hoffer, 1983). Prominent ontogenetic diet shift were also observed in G. girius and Mystus species of the piscivorous feeding guild. Early ontogenetic stages (small size classes) highly depend on insect matters and zooplankton, while the older ontogenetic stages (larger size classes) switch their diets towards fish. Weliange et al. (2003) also observed size related diet switches in GG and Mystus species. According to his findings, smaller individuals of G. girius and Mystus species feed on adult insects, insect larvae and copepods, while younger individuals feed on fish and worms. For other piscivorous fish species also size related diet switches were reported. Gut contents of the black crappie, largemouth bass, northern pike, smallmouth bass, walleye and yellow perch, in Spirit lake, Iowa, USA, recorded 41 prey taxa including 19 fish species and there were distinct differences in diets among species, among size classes and over time also observed (Liao et al., 2002). One reason for many of these ontogenetic shifts seems clear. Many fish prefer larger prey when they grow to a larger size (Werner, 1986). Individuals transferred from water a body where planktivory was obligatory because young fish was lacking, to a water body where piscivory was possible started to feed on fish immediately and their growth rates increased accordingly Knowledge of the mechanisms behind prey selection in piscivorous fish is important for our understanding of the dynamics of freshwater system. Prey selection can involve active predator choice or be a passive process. Optimal foraging theory, predicting that predators will choose larger prey sizes, giving highest energy return per time spent foraging, is assumed to explain active choice. A change of diet with increasing body size of fish may be an adaptation to reduce intraspecific competition among different size groups. In the present study, it is also revealed that larger fish eat proportionately more voluminous food types such as fish and insect matters. The proportion of completely filled and swollen guts in CS can be explained by the type of food categories present in their guts. Diet of CS is mainly composed of fish which is a highly voluminous food item compared to the other food categories. Clear ontogenetic gut filling variation is observed in the three piscivorous fish species. Completely filled guts increased with increasing size of the fish. This is attributed to the higher proportion of high voluminous food particles such as fish, insect matter etc., present in the guts of the larger size classes of fish. As fish grow they are able to eat larger maximum size prey, and bigger prey become more profitable. The measurement of resource overlap among fish species remains a problem. The incorporation of resource availability into an overlap index would enhance measurements of overlap if technical and theoretical problems could be solved. When resource availability data are absent, the Schoener s index is one of the least objectionable indexes available (Wallace, 1981). Niche overlap between two fish species quantifies the sharing of food and habitat resource categories between two species. Fish species belong to the same feeding guild recorded high niche overlaps. It is well accepted that most species exhibit ontogenetic diet shifts. At the beginning of their life most species depend on zooplankton and as they grow to a larger size they switch to other food categories. During their ontogeny, the morphology of their feeding structures, body and gut is changing too. These changes are adaptation to food collection, processing and digestions. Present study, revealed that, high niche overlap registered between adjacent length classes and low overlap registered between more distance length classes. This difference is attribute to the ontogenetic diet switch of most fish species. 147
Niche breadth quantifies the diversity or breadth of the food resources utilized by a given species. Niche breadth and niche overlap within feeding guilds are often inversely related, the higher the niche breadth the lowers the niche overlap. CONCLUSION For the Tissawewa fish assemblage, predator avoidance and habitat profitability are probably the two most important factors to explain the spatial distribution of species. Because of the size and relative homogeneity of the reservoir, differences in profitability between habitats are expected to be small. Therefore, for the species or size-classes subject to predation, predator avoidance will be the decisive factor in determining their distribution. For the fish assemblages in Tissawewa, ontogenetic diet and niche shifts are common phenomena and juvenile bottlenecks can occur because juveniles or several larger species share zooplankton as an important resource with each other. ACKNOWLEDEMENTS Financial support from the Netherlands Foundation for the advancement of Tropical Research (WOTRO), the centre for Limnology of the Netherlands Institute of Ecology is highly acknowledged. REFERENCES Carpenter, S.R. (ed.). (1987). Complex interactions in lake communities. Springer. pp 283. Costa, H.H. & Fernando, C.H. (1967). The food and feeding relationships of the common meso and macro-fauna of the Maha Oya, a small mountainous stream at Peradeniya, Ceylon. Ceylon J. Sci. 7:74-90. Hynes, H.B.N. (1950). The food of fresh-water sticklebacks (Gasterosteus aculeatus and pygosteus pungitius) with a review of methods used in studies of the food of fishes. J. Anim. Ecol. 19: 36-58. IUCN, 2000. The (1999) List of Threatened Fauna and Flora of Sri Lanka. IUCN Sri Lanka, Colombo, Sri Lanka. Pp 113. Levins, R. (1968). Evolution in changing environments: some theoretical explorations. Princeton University press, Princeton. Liao, H., Pierce, C.L & Larscheid, J.G. (2002). Diet dynamics of the adult piscivorous fish community in Spirit Lake, Lowa, USA 1995-1997. Ecology of Freshwater Fish. 11(3): 178. Lowe-McConnell, R.H. (1975). Fish communities in tropical freshwaters. Longmans, Lond. 337 Moyle, P.B & Senanayake, F.R.(1984). Resource partitioning among fishes of rainforest streams in Sri Lanka. J. Zool. Lond. 202: 195-223.Schiemer, F. & Hofer, R. (1983). A contribution to the ecology of the Parakrama Samudra Reservoir. P. 135-148. In: Schiemer F. (ed.) Limnology of Parakrama Samudra - Sri Lanka. W. Junk Publ., The Hague Schoener, T.W. (1974) Resource partitioning in ecological communities. Science 185: 27-39. Wallace, R.K. (1981). An assessment of diet-overlap indexes. Trans. Am. Fish. Soc. 110:72-76 Weliange. W.S and Amarasinghe. U.S. (2003). Accounting for Diel Feeding Periodicity in Quantifying Food Resource Partitioning in Fish Assemblages in Three Reservoirs of Sri Lanka. Journal of Asian Fisheries Society. 16: 203-213 Werner, E.E. (1986). The mechanisms of species interactions and community organization in fish. In: J Diamond and T.J. Cade (eds.). Community Ecology. Harper and Row. New York. 148