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

Ajitha, B.S.

Balasubramanian, N.K.

Ritakumari, S.D.

Aquatic Commons

J. Indian Fish. Assoc., 33: 11-17,2006 II 
FOOD OF TWO SIZE-GROUPS OF THE CATFISH MYSTUS GULlO 
(HAMILTON-BUCHANAN) IN VEMBLAI CANAL, VYPEEN ISLAND 
S.D. Ritakumari, B. S. Ajitha and N. K. Balasubramanian 
Department of Aquatic Biology and Fisheries, University of Kerala, 
Thiruvananthapuram- 695 581, 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 
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 (7.8- 10.7 em) and Size-
group 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 size-
groups I and II, and c is the number of 
joint prey items shared by the two size-
groups. 
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% 
FOOD OF TWO SIZE-GROUPS OF THE CATFISH MYSTUS GULlO 
(HAMILTON-BUCHANAN) IN VEMBLAI CANAL, VYPEEN ISLAND 
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 
Freguency of occurrence Percentage com~osition 
Food item Size- Size- Size- Size- Total 
group I group II Total group I group II 
Blue-green algae 29.2 15.0 
Filamentous algae 54.2 55.0 
Diatoms 20.8 10.0 
Plant remains 91.7 70.0 
Rotifers 20.8 0.0 
Crustaceans 20.8 35.0 
Insect parts 16.7 25.0 
Prawn 2arts 4.2 40.0 
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 
22.7 6.7 3.9 5.5 
54.5 18.5 16.3 17.5 
15.9 02.6 1.0 1.9 
81.8 52.9 47.9 50.6 
11.4 7.4 0.0 4.1 
27.3 6.3 9.6 7.8 
20.5 4.8 10.9 7.5 
20.5 0.8 10.4 5.1 
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 ) 1.477 1.52i3 
Diet breadth (B) 3.011 3.454 
Prey evenness (e) 0.710 0.783 
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. 
FOOD OF TWO SIZE-GROUPS OF THE CATFISH MYSTUS GULlO 
(HAMILTON-BUCHANAN) IN VEMBLAI CANAL, VYPEEN ISLAND 
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 size-
groups of M gulio. Thus, the species 
studied can be grouped along with the 
fishes showing an adaptation to 
minimise intra-specific competition for 
food. 
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Food of two size-groups of the catfish Mystus giulio (Hamilton-Buchanan) in Vemblai canal, Vypeen Island

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Food of two size-groups of the catfish Mystus giulio (Hamilton-Buchanan) in Vemblai canal, Vypeen Island

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