INTRODUCTION
Studies on
the dietary analysis of a fish species is fundamental to a successful fisheries
management aimed at fish capture and culture [1]. However, published information on the food and feeding habits of
the clupeid, Ethmalosa fimbriata in Forcados
river estuary which is a dominant and
important economic fish species in the region is scare. Previous works on this
clupeid in Nigeria include those of [2,3,4,5,6,7,8].This study would therefore provide vital information
that could be used by other researchers for future studies.
E. fimbriata
has a superior mouth, a large
head and a deep and compressed body [9]. According to 9, this species is black
bluish green in colour with silver sides and a yellow anal and caudal fin. The
upper portion of the head is golden and possesses a botch behind its operculum.
It also has a deeply forked operculum and is usually abundant from October to
November in this water body. The fish is highly eaten by many people in Nigeria
especially the coastal dwellers, providing basic animal protein to the
populace.
E. fimbriata feeds by filtering planktons [7] and sometimes fish larva [9]. The food and feeding habits of fish in their natural habitats
enhances the understanding of the abundance, distribution, growth and
productivity of an organisms [2] Data on the food and feeding habits of a fish can
provide information on the status of an aquatic ecosystem.
MATERIALS AND METHODS
The study area
The study was carried
out in Forcados River estuary, between Burutu and Forcados in the Niger Delta
Area of Nigeria (latitude 5º 21ꞌ - 50 35ꞌ N and
longitude 5º 31ꞌ - 50 51ꞌ E). The area is known to
have both rainy and dry seasons. The dry season starts from November to April
while the rainy season strerches from May to October [10]. The vegetation
covers include Eichhornia crassippes,
Pistia, Pennisetum purpureum,
Nymphaea spp, Trapa spp, Ceratophyllum spp. Human activities here include
offloading and selling of goods and petroleum products, jetty operations, boat
movement, laundering and washing of boats, log movement, dumping of organic and
inorganic waste, bathing and swimming.
Collection and preservation of fish samples
Samples were
collected monthly between April 2012 and March 2014 from artisanal fisher folks
and taken to the laboratory for analysis. 1133 stomachs of E. fimbriata were analysed. The fish were
sorted and identified to the species level using the keys of [11,12,13,9].
Stomach content analysis
The abdomen of some economic fish was slit open and
the content placed in a petri dish. The aggregate of food items was dispersed
in water. Aliquot samples were taken with a dropping pipette from a stack
and placed in a small petri dish, spread out evenly and subjected to
stereomicroscopic examination (10 - 100x). Large food organisms such as
prawns, bivalve mollusc and fish were counted directly whenever
encountered. The remaining microscopic organisms were counted using a
binocular zoom microscope, and the number of food items present was counted
directly. Stomach content was analysed using the occurrence frequency (Fi)
method and volumetric analysis index [14,15,16,17,18].
Occurrence Frequency (Fi) Method
The
number of stomachs in which each food items occurred was noted and expressed as
a percentage of the entire stomachs examined. This method gives the
proportion of the population of fish that feeds on a particular food item.
Fi
= 100ni/n
Description;
=
occurrence frequency of food i
= number of food
i
= total number
of a digestive tract containing food
Volumetric Analysis Index uses the formula:
Where:
: Volumetric Analysis Index of the i food item in the samples;
: multiplication constant to obtain the percentage;
: mean of the points ascribed for the i food items.
Using the
Index of Preponderance the main food that was eaten was determined, an
integration of occurrence frequency method and volumetric analysis index. Preponderance
index was determined following [19]:
Description:
×100
Vi
= percent volume of food 
= percent
occurrence frequency of food i
= Total of Vi x Oi of all food types
IP = Index of Preponderance (%).
For
selectivity estimations the [20] index was used:
Where Ei is the
percentage by number of taxon i in the stomach contents, and Bi is the percentage by number
of taxon i in the benthos. Positive values indicate that the
fish selected the relevant prey.
The
Gastrosomatic Index (GSI) was calculated to show the trends in the feeding
activity of the fish according to [21] using the formula:
GSI = 100Wf/Wt
Whereas, Wf = Weight (g) of food in the gut;
Wt = Total weight (g) of the fish
The Gut repletion index (GRI) was calculated
according to [22] using the formula:
×100
RESULTS
Food and feeding habits of dominant fish in Forcados
river estuary
Clupeidae
A total of
1133 stomachs of E. fimbriata were analysed. Of these 1095 (96.73%) had food items while 38 (3.27%)
had empty stomachs. Table 1 shows that the species fed predominantly on Microcystis and accounted for 24.99% (the highest percentage) of the
food items by percent occurrence frequency while Macrobrachium sp had
the highest by percent volume of food (42.3%).
The least of 2.12% and 1.07% by percent occurrence frequency and percent volume of food were
recorded for Desmids and Diatoms in Forcados river estuary. Callinectes sp recorded the highest Index of
Preponderance (26.09%) followed by Macrobrachium sp (23.6%)., Microcystis (20.39%),
Closterium (12.89%), Oscillatoria sp
(11.27%), Nitzshia (3.03 %), Rivularia (0.87 %), Desmids (0.74 %),
Cosmarium (0.67 %) and the least was Diatoms (0.58).
Monthly variations in
feeding habits of Ethmalosa fimbriata
Fig. 1
presents the monthly variations in the feeding habits of E. fimbriata. Algae and crustaceans were fed upon throughout the
study period. The food items were fed upon regularly during both seasons. Peak
consumption of algae was in the month of August, 2013 while crustacean had its
peak in the month of October, 2012. The consumption of these food items did not
follow any seasonal pattern.
Monthly variations in
feeding Intensity of E. fimbriata
Variations
in the number of empty stomach (Fig. 2) were used to study the variation in the
feeding intensity of the species. There were no empty stomachs in May, 2012,
October, 2012 and July, 2013. The lowest number of empty stomachs (2) was in
the month of October, 2013, while the highest was in May, 2013. Again, the
number of empty stomach was also generally higher in the rainy season months
than the dry season.
DISCUSSION
Of these 1095
(96.73%) had food items while 38 (3.27%) had empty stomachs indicating that
this species were active feeders. The high GSI and GRI
values recorded is also a confirmation of this finding. The species fed
predominantly on Microcystis and accounted for 24.99% (the highest
percentage) of the food items by percent occurrence frequency which is a
reflection of its availability and abundance while Macrobrachium sp had
the highest by percent volume of food (42.3%).
This could be as a result of its relatively large size and also abundance.
The food items varied from microscopic items
such as planktons to macroscopic ones such as Crustacean implying that the fish
took great advantage of the resources available in the water body. The success of mullets according
to [23,
24] also
lies in their feeding habits and the abundance of their food.
According to [25], seasonal changes in the dietary
composition of a fish species are a reflection of the availability and
abundance of each item. The consumption of food items recorded in
this study did not follow any seasonal pattern. Hence, there were no
fluctuations in the food items. The number of empty stomach was generally
higher in the rainy season months than the dry season, implying that this
species fed heavier in the dry season than in the rainy season. For food
selectivity estimations the macroinvertebrate positively selected by E. fimbriata was Callinectes sp. indicating that the fish made the right choice of
food since crabs are abundant in the water body.
CONCLUSION
The
study indicated
that E. fimbriata fed on
phytoplankton as a major diet. Research on the food and feeding habits of
economic fish species in an aquatic ecosystem should be continuous.
Author’s Contribution
E.O.
conceived and designed the analysis, collected and analyzed the data and
drafted the manuscript. A.J. and B.O.B. constructed aspects of the body of the
manuscript and reviewed the article for spelling, grammar and intellectual
content.
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