Trophic status and diversity of fish species in selected tributaries of Bentota River.

: Present study was carried out to investigate the, abundance and trophic status of fishes inhabiting tributaries of the Bentota River. Fish were sampled using gill nets, seine nets, cast nets and some were also collected from fishermen’s catches harvested at the tributaries of the Benthota River in Katapola, Ganegoda, Avitthawa, Yagirala and Galatara. To investigate the trophic status, stomach contents were analyzed and quantified using the Point method based on the percentage of bio volume per food category. Ten species of fish were recorded from all five sites, including two endemic species, Clarias brachysoma, Channa orientalis and one invasive alien fish Chitala ornata . Some indigenous species such as Puntius vittatus (n=400), Rasbora daniconius (n=134), Puntius dorsalis (n = 42) and Trichogaster pectoralis (n=71) were also caught in reasonable number. Parts of fish (scales, fins and flesh), mollusks, adult insects, insects larvae, macrophytes and digested/detritus matters were identified as main food categories in the stomach. Based on the gut content analysis, three feeding guilds i.e. piscivorous, herbivorous and zooplanktivorous, were identified. High diet overlap values were recorded in species pairs belonged to the same feeding guild i.e. Chitala.ornata- Channa orientalis while low diet overlap values were recorded in species belonged to different feeding guilds. Remains of fins of Channa orientalis , and Rasbora daniconius and some invertebrates observed in stomach contents of C. ornata, indicated its negative impacts on biodiversity in study sites. Therefore urgent attention should be paid to population control of C.ornata and to prevent its further invasion into new habitats.


Introduction
Food and feeding habits of fish are important biological factors to understand the community structure and selecting fish for aquaculture (Dewan & Saha, 1979). Biological studies of fish species with respect to their diet composition is an important aspect in fish biology. Digestive system of the fish varies with their feeding habits such as carnivorous, omnivorous and herbivorous. There is a temporal variation in gut contents of fish in parallel to seasonal changes in the environment including temperature, and available food organisms Cong et al., (2017). Stomach content analysis is a better tool to predict possible predation, competition and feeding habit of animals. Accurate quantification of fish diets is an important aspects of fisheries management (Manon et al., 2011).
The species-specific differences that allow coexistence can be considered as species' niche. According to the definition of species' niche it consist of four major axes: resources, predators, space and time (Chesson, 2000). Coexistence thus requires niche partitioning between species that increases the strength of intraspecific competition relative to that interspecific competition along at least one of those four niche axes (Amarasekara, 2003). Niche overlap measures the degree to which a niche resource is shared between individuals or population within the community. The greater the overlap between co-occurring species, more similar the  resource use of the individuals of these two populations, and the greater the potential for competition between these two populations.
Diversity of organisms and measurement of diversity have concerned ecologist and conservationist. Biological communities vary according to species richness and evenness (relative abundance of species). A diversity index employed in ecological researches is a measure that reflects the number of different species and how evenly the individuals are distributed among those species (Negi, 2013).
Present study was carried out in Benthota River, which is one of the main rivers in the Southern region of Sri Lanka and its catchment spread in the small mountain in Elpitiya, Pitigala and Pelawatta area. This river play an important role in ecological, social and economic aspects. Fishery of this river is mainly carried out by Small-scale fishermen using non modernize fishing gears i.e. pole and line and gillnets etc..
Many studies have been carried out on the food and feeding habits, distribution pattern of many fish species inhabiting the lotic and lentic ecosystem of Sri Lanka, but detail study on the above aspects of the tributaries of Bentota River is yet to be studied. Therefore present study was carried out mainly to investigate fish diversity, food and feeding habits & size related ontogenetic diet shift in fish species found in the tributaries of Bentota River.

Materials and Methods
Five sampling sites i.e. Katapola, Ganegoda, Avitthawa. Galatara and Yagirala located in the tributaries of Bentota River were selected in the suburbs of Elpitiya area in Southern Sri Lanka ( Figure 1). Bentota River is originated from mountains in Elpitiya area and emptied into the sea near Bentota, a place which has much touristic activities and attractions.
Gill nets, seine nets, and cast nets were used as fishing gears to catch fish. In addition, data on fish diversity and abundance were collected from fishermen. Maximum number of 10 individual of fish caught at each sampling were preserved in Formaldehyde solution in order to carry out gut content analysis. Fishes larger than 10 cm were preserved by injecting 10% formalin into the body cavity immediately after capturing in order to preserve gut contents. All collected samples were kept in formaldehyde bath until analysis. Journal of the University of Ruhuna, Sri Lanka 8(2), 2020 Prior to fixing the fish for gut content analysis, total length and body weight of each individual of each species were measured using meter ruler and top loading balance respectively as soon as possible after sampling them. For gut content analysis, the point method based on the percentage of bio volume per food category was used (Hynes, 1950). Diet overlaps were calculated on the basis of the gut contents of species concerned. Diet overlap (S) between each pair of species was determined using the formula of Schoener's index (1970) ; = 1 − 0.5 (∑ ⌈ − ⌉), where S= the dietary overlap coefficient of fish species x and y: Pxi = proportion of food category "i" in the diet of species "x": Pyi = proportion of food category "i" in the diet of species "y": n = number of food categories.

Shannon-Wiener Index of diversity, Evenness and
Species richness values were calculated for each site. Shannon-Wiener Index of diversity was calculated by using following equation where pi= proportion of total sample represented by species, S = number of species, H= Maximum diversity. Evenness of the respective sites was calculated by dividing Shannon's diversity index (H) by natural logarithm of species richness. Species richness was det ermined by counting the number of fish species present in each site.
The values for this similarity index ranged from 0.00 to 1.00, with 1.00 indicating complete overlap and 0.00 indicating no overlap. To calculate the Jaccard similarity index, following equation was used; = where Sj is the Jaccard similarity index, For the data that exhibited normal distribution, Oneway Analysis of variance (ANOVA) was carried out to statistically compare food items among and between the different fish species using SPSS ver 17 statistical package.

Results and Discussion
Relatively high fish species richness was recorded in all the five selected sites along the tributaries of Bentota River (Table 1). Ten fish species were recorded irrespective to different sites including some endemic species such as Clarias brachysoma, Channa orientalis. Some indigenous species such as Puntius vittatus (n=400), Rasbora daniconius (n=134), Puntius dorsalis (n=42) and Trichogaster pectoralis (n=71) were also caught in reasonable numbers. Chitala ornata an invasive alien fish species was also recorded in reasonably high number (     Percentage similarity index (Gauch, 1982) is another index that can be used to evaluate the similarity and dissimilarity between sites taking in to account, not only the numbers of similar and dissimilar species present in the two respective sites, but also incorporate their abundance.

Galatara -
The highest Percentage similarity index value was exhibited by site combination of Yagirala and Galatara (Table 4). Since the Percentage similarity index also consider the abundance of species found in two sites, less deviation in abundance values of species recorded in the two sites, Yagirala and Galatara (Table 1) could also be attribute for the highest percentage similarity index value between these two sites. Contrary, the lowest percentage similarity index value between Kotapala and Yagirala can be attributed to the higher difference in abundance of species between the two sites Kotapala and Yagirala (Table 1). Journal of the University of Ruhuna, Sri Lanka 8(2), 2020 Results of the gut content analysis of fish species recorded irrespective of the sampling sites are shown in table 5.  (Perera & Weerakoon, 2016), recorded that C. ornata feeds on significant number of native fish species inhabiting in Attanagalu Oya. Results of the one way ANOVA reveals significant difference (P< 0.05) of food items such as parts of the fish, adult insects, and zooplankton among different fish species.
Diet overlap values calculated using Schoenner's index are given in Table  6. Journal of the University of Ruhuna, Sri Lanka 8(2), 2020

Conclusion
Tributaries of Bentota River exhibit comparatively high fish diversity with ten fish species including endemic, indigenous and one invasive alien species. Small minor cyprinids like Puntius vittatus and Rasbora daniconius exist with high abundance in all sampling sites. High diversity of food items were observed in gut contents of the studied fish, ranging from plant materials to animal materials. Feeding guilds of fish species in study sites are mainly piscivorous, insectivorous and herbivorous. Remains of body parts of C. orientalis, which is an endemic fish species found in guts of C. ornata indicates threats to the endemic fauna of Sri Lanka. Presence of flesh and parts of fins of R. daniconius and some invertebrates in the gut contents of C. ornata, indicates the negative impacts on native fish as well as on other aquatic fauna.