FOOD OF TWO SIZE-GROUPS OF THE CATFISH MYSTUS GULlO (HAMILTON-BUCHANAN) IN VEMBLAI CANAL, VYPEEN ISLAND

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1 J. Indian Fish. Assoc., 33: 11-17,2006 II FOOD OF TWO SIZE-GROUPS OF THE CATFISH MYSTUS GULlO S.D. Ritakumari, B. S. Ajitha and N. K. Balasubramanian Department of Aquatic Biology and Fisheries, University of Kerala, Thiruvananthapuram , India ABSTRACT The stomach contents of two length-groups of the catfish Mystus gulio collected from Vemblai Canal in Vypeen Island (Kochi) were examined by frequency of occurrence and points methods. Analyses using standard indices proved difference in diet composition between the two size-groups. Keywords: Diet analysis, prey diversity, dietary breadth INTRODUCTION The food habits of several fish species are well documented (Clepper, 1979) and the diets of many fish vary seasonally (Weisberg and Janicki, 1990), often as a result of prey availability (Keast, 1985). However, knowledge is far from complete as regards the feeding ecology of fishes. Aspects such as the cause of differences in species composition between habitats, the extent of food resource partitioning in the assemblage and the importance of competitive interaction between size-classes and/or species of fishes are unknown (Gibson and Ezzi, 1987). Hall et al. (1990) pointed out that analyses of feeding patterns and use of diet statistics to formally describe the variation in diet composition between species or between size-classes can be used to reveal some of these aspects. In this paper, an attempt has been made to extend the knowledge on feeding ecology of Mystus gufzo by describing the diet and by assessing the competitive interaction between two size-classes of fish. As culture of catfishes and air-breathing fishes need greater emphasis in coming years for making use of large extent of our swamps and derelict waters, such studies will be of much relevance. MATERIAL AND METHODS The fish samples were collected from January to April 2002 using cast nets from Vemblai Canal in Kuzhupilly Panchayath located north of Vypeen Island, Kochi. This canal has direct link with the Kochi Backwaters, Kerala. Heavy growth of mangrove vegetation occurs along the canal. M. gulio collected during early hours of the day were anaesthetised and preserved immediately in 1 Oo/o formalin to halt digestion. In the laboratory, specimens (88 no.) ranging in standard

2 12 S. D. Ritakumari. B. S. Ajitha and N. K. Balasubramanian length from 7.8 to 15,1 em were grouped into two length-groups, viz., Size-group I ( em) and Sizegroup II (10.8 em and above). The stomach contents of all the collected specimens were analysed. Empty stomachs encountered in five specimens were not included in the present study. The importance of diet items was assessed using the occurrence method. An estimate of the relative condition of each item was assessed using the points method of Hynes (1950). Prey-diversity was calculated using the Shannon-Wiener infonnation measure (H ) (Shannon and Wiener, 1949): s H= :E (pi) In (pi) i=1 where pi is the proportion of each different food item contributing to the whole diet. The data form the dietary analyses were used to calculate diet breadth, using the niche width index (B) described by Levins ( 1968): B= 1 :E[pi]2 This index was used to compare 'specialist' tendencies between species/size classes, with low values of the index indicating specialists and high values generalists (Gibson and Ezzi, 1987).. Prey evenness (Pielou, 1966) that measures how evenly the prey species are distributed in the diet was calculated as follows: e= H where Hmax = ln S and 1" S is the natural logarithm of the number of food types. Index of similarity calculated using presence or absence and joined occurrence of food items was calculated by the following equation: S= 2c A+B where A and B are the number of different prey items in fishes of sizegroups I and II, and c is the number of joint prey items shared by the two sizegroups. RESULTS Analysis of diet The dietary analysis showed that the fish feeds both on plant and animal matter, and the stomach contents do not differ qualitatively with the size offish. The food items identified in the stomachs included blue-green algae, filamentous algae, diatoms, plant remains, rotifers, calanoids, and parts of insects and prawns. Plant remains formed the most preferred food item in size-groups I and II as they made their presence in 91.7% and 70.0%, respectively, of the stomachs sampled. Filamentous algae formed the next preferred item with more than 50%

3 FOOD OF TWO SIZE-GROUPS OF THE CATFISH MYSTUS GULlO 13 frequency of occurrence. Animal food items such as prawn remains, calanoids and insect parts were encountered more frequently in the large size-group (Table 1 ). Table 1: Food items ingested by Mystus gulio: Frequency of occurrence and percentage composition Food item Freguency of occurrence Blue-green algae Filamentous algae Diatoms Plant remains Rotifers Crustaceans Insect parts Prawn 2arts Percentage com~osition Size- Size- Size- Size- Total group I group II group I group II Total Analysis of percentage composition of the food of the species showed that the main item of the diet was formed of plant remains (50.6). Filamentous algae were also important in the diet (17.5o/o) ofthe species. Other non-animal foods assumed almost equal importance (Table 1 ). Though the percentage compositions of plant remains and filamentous algae were high in fishes ofboth size-groups, their contribution slightly decreased with increase in the size of fish. An increase in consumption of animal remains with increase in size of fish was also noticed in the study. Niche measurement The diversity indices of the two size groups are presented in Table 2. H values vary slightly between the two Table 2: Prey diversity indices of the two size-groups Index Size-group I Size-group II Shannon-Wiener information measure ( H ) i3 Diet breadth (B) Prey evenness (e)

4 14 S. D. Ritakumari, B. S. Ajitha and N. K. Balasubramanian size-groups, suggesting a little higher trophic diversity for the large-sized individuals than the small conspecifics. Dietary breadth (B) indicates a wider dietary niche for larger individuals than for small-sized ones, B values being 3.45 for large group and 3.01 for small group. Prey evenness values were also marginally higher for Size-group II (0.783) than for Size-group I fishes (0.710). The Similarity index's' was calculated as 0.93 in the study. DISCUSSION In the present investigation, M. gulio was found to be an omnivore, subsisting both on animal and plant matter from its surroundings. Pandian ( 1971) studied M. gulio in the Cooum Backwater (Madras) and found the species as an omnivore. Jhingran (1982) reported that M gulio, the euryha1ine catfish occurring mostly in freshwater and also in brackishwater of low salinity, is predominately a carnivore, feeding mainly on crustaceans, molluscs and insect larvae. Food and feeding habits of other Mystus spp. have also been studied. Dietary components of M. aor from Ganga river system was analysed by Saigal (1967). Bhatt (1970, 197la, b) studied the feeding biology of M. seenghala, M vittatus and M cavasius. According to McConnel (1975), mystids in African lakes are carnivores in general. Bordet ( 1979) and Sastrawibawa (1979) observed that M nemurus in the Indonesian Archipelago is an omnivore with wide range of feeding regimes. Mirza and Ahmed (1981) noticed seasonal variation in feeding in M. bleekeri. Biological studies of the freshwater catfish M. keletius was carried out by Santhanakumar and Job (1984.) The food of M. vittatus from the highly polluted Hussain Sagar Lake was studied by Reddy and Rao (1993). Vinci (1986) studied the biology of M. seenghala from the. reservoirs of Andhra Pradesh. M. seenghala, the important commercial catfish of riverine systems has been reported to feed on fishes by earlier workers (Alikunhi and Rao, 1948; Saigal and Motwani, 1961; Sehgal, 1967; Bhatt, 1970; Vinci, 1986; Shrivastava, 1993). However, cannibalistic habit as reported by Bhatt (1970) could not be noticed by later workers. Examination of gut contents of M. bleekeri revealed that it feeds mainly on insects and their i larvae, crustaceans, molluscs and other planktonic organisms (Das and Moitra, 1963; Pandey, 1983; Shrivastava, 1993). Both plant (algae and other hydrophytes) and animal matter form the food of M. vittatus (Bhatt, 1971a; Reddy and Rao, 1993). Studies on food, feeding biology and the morphology of associated organs of tropical catfish M nemurus of Malaysia by Khan et al. ( 1988) indicated that the species is usually euryphagous, feeding extensively on a wide variety of food items in the bottom substrate. Thus, ditierent species of Mystus show diversified feeding regimes.

5 FOOD OF TWO SIZE-GROUPS OF THE CATFISH MYSTUS GULlO 15 Variation in diet as a function of size has been reported in a large number of fishes. According to Werner and Gilliam (1984), many species of fish increase their mean food size or shift their food type during their ontogeny. This change in feeding is more or less evident in the species under consideration. It has been noticed that fish of Size-group I subsist both on plant and animal matter, non-animal food forming major portion of the ingested food. The Size-group II fish were also found to feed on the same food items, but a drop in the percentage composition of non-animal food and a rise in animal matter could be observed on a close scrutiny. Werner (1986) was of the opinion that when ontogenic shifts occur, they almost always involve shifts to larger prey as corroborated by his theoretical and empirical studies. This difference in diet also depicts difference in the manner of obtaining food by the two size-groups. This can also be attributed to the ability of growing fish to handle particular food type, in this case, various aquatic invertebrates. These intra-specific differences in resource utilisation indicate that M. gulio is able to partition and share the resources and escape intra-specific competition. In assessing the trophic diversity measure for the two size-groups of the fish, the slightly higher diversity value obtained for larger size-group in the present study shows that the large-sized individuals are with a wider dietary niche than the smaller ones. Levin's dietary breadth calculated was also wider for large fish as compared to small fish. Prey evenness, also reflecting the breadth of diet, was slightly greater in large-sized fish, the higher number indicating maximum evenness. The study, thus, indicates that there occur variations in quality and quantity of food consumed by the fish of two different size-groups. Further dietary analyses using standard indices also proved the variations in diet composition between the two sizegroups of M gulio. Thus, the species studied can be grouped along with the fishes showing an adaptation to minimise intra-specific competition for food. REFERENCES Alikunhi, K. H. and Rao, N. S., An investigation into the food and feeding habits of some of the common freshwater fishes of Madras. In: Proceedings ofthe l3 1 h Indian Science Congress. Indian Science Congress Association, Calcutta, 2001 pp. Bhatt, V. S., Studies on the biology of some freshwater fishes. Part IV. Mystus seenghala (Sykes). J Bombary Nat. Hist. Soc.,67: Bhatt, V. S., 1971a. Studies on the biology of some freshwater fishes. Part V. Mystus vittatus (Bloch). J. Bombay Nat. Hist. Soc., 68:

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