Full Length Research Paper
ABSTRACT
Fish sustainability in an aquatic environment involves continuous and judicious management towards optimum production of resources, for rational utilization and availability for future generation. Stomach composition of Oreochromis niloticus collected from a tropical brackish water environment was investigated to assess food availability towards optimum fish productivity for rational public utilization. Oreochromis niloticus, a fish of high economic importance and a generalize omnivore endemic to fresh and brackish water of Sub-Saharan Africa, and Asia, was reviewed to know the food availability status of the Lagos Lagoon, Nigeria. The conditions of stomach contents of O. niloticus samples collected were visually determined and subjected to frequency of occurrence, dominance, numerical, volumetric and relative index methods. Analytical and descriptive statistics were used to show occurrence, prevalence and availability of food organisms, within Lagos Lagoon. Results of the investigation revealed the presence of phytoplankton, dinoflagellates, sand grains, insect parts, plants parts, fish parts and unidentified constituents in O. niloticus stomach contents. O. niloticus revealed herbivorous feeding habit and Bacillariophyceae food item with highest occurrence. Food items satisfy requirements for optimum sustainability.
Key words: Stomach contents, tropical, Oreochromis niloticus, Lagos Lagoon, Nigeria.
INTRODUCTION
MATERIALS AND METHODS
Study area
The brackish water Lagos Lagoon (Figure 1) is part of the continuous system of lagoons and creeks that are found along the coast of Nigeria from the border with the Republic of Benin to Niger-Delta. This lagoon bordering the Lagos Island is located between longitude 30° 10' and 30° 4' SE and latitude 60° 5' and 60° 36' N. Lagos lagoon lies along the coast for more than 50 km long and 3 to 13 km wide. It is bounded by tidal wetlands and swamps, and is a major lagoon among 10 other lagoons along the coastline. The Lagoon has a surface area approximately 6,354.7sq km, and it receives influx of freshwater from several rivers, the most important of which are the Ogun, Ona/Ibu, Oshun, Shasha and Oni. It is tidal in nature and it receives water from Atlantic Ocean during high tides which recede during low tides. Lagos Lagoon is fairly shallow with averages depth of 2 to 4 m and it is isolated from sea by beach barrier ridges. The lagoon empties inside the Atlantic Ocean via Lagos Harbour where it has depth increase of 10 m, with 0.5 to 1 km width and 10 km long, (Badejo et al., 2014).
Collection of samples
A total of four hundred and eighty (480) live Oreochromis niloticus were collected from Lagos Lagoon with the assistance of fishermen using cast net from January to December 2015. The fish were carried in a 25 L plastic container for analysis in Fish Biology Laboratory for analysis.
Measurements
The Standard Lengths (SL) of fish samples were determined to the nearest 0.01 cm using a graduated board; and the corresponding weights (W) were also measured with Mettler Toledo electronic weighing balance (Model; PB8001).
Sample preparation
The fish were dissected to remove gut samples which were dissected individually to reveal the condition of the stomach. The guts and intestine contents were categorized based on degree of fullness as: {full (4/4), three quarter full (3/4), half full (2/4), one quarter full (1/4), and empty (0/4)}. The stomach contents were fixed in 4% formalin prior to examination. Stomach samples were mixed with 2.0 mL distilled water in a petri-dish for proper separation and easy identification. A drop of the prepared sample was placed on a glass slide with the aid of a dropper and covered with cover slip.
Microscopy and observations
The various items in the fish stomach were analyzed using frequency of occurrences, dominance, numerical, volumetric and relative index methods to show food item prominence and importance.
Frequency of occurrence (FOC) Method (Hyslop, 1980)
FOC = Total number of specific food item occurrence
Percentage occurrence of a food item (% FOC):-
Dominant method (Hyslop, 1980)
Percentage dominance of a food item = Number of stomachs that particular food item is dominant
Total number of stomachs with food X 100
Volumetric method (Hyslop, 1980)
Numerical Method ……………………… (Hyslop, 1980)
Relative Index (I) (Hyslop, 1980; Costal et al. 1992)
I = (%occurrence X % volume) X 10-2
The percentage numerical index of occurrences was expressed to know the food item in stomach of O. niloticus fish samples. The percentage occurrences of organisms were recorded according to Hyslop, (1980) and Costal et al. (1992).
RESULTS
Standard length ranged from (13.72 to 18.45) (cm) (with a mean of 14.67 ± 1.56) ( cm) ; whilst weight ranged from (83.32 to 140.68) g with a mean weight of( 135.40 ± 31.46) g for the 480 samples of O. niloticus collected from the Lagos Lagoon. Table 1 shows fish length range and frequency of occurrence.
The stomach contents of O. niloticus included phytoplankton, insect parts, plant parts, sand grains and some unidentified food items. Table 2 shows percentage occurrence of stomach contents of O. niloticus from the lagoon. Phytoplankton ranked highest in occurrence of food items in the stomach of the fish with a percentage of 62.32%.
Table 3 shows the degree of stomach fullness of the fish samples. This indicated that 70% of O. niloticus stomach revealed half full and above degree of fullness. Hence, it is an indication of food availability in the brackish environment.
From Table 4, Ppercent frequency of occurrence revealed that plant parts was the highest (93.13%) and also dominated the food items with a value of 58.54%. Cyclotella meniggniana species, which is a Bacillariophyceae ranked second desired food item (51.88 %); while Dinophyta Ceratium sp. ranked third (35.92 %) desired food item from the environment. This indicated all food items on rank list were plant source.
Figure 2 shows the percentage occurrence of food item in the stomach of Oreochromis niloticus from Lagos Lagoon. Bacilleariophyta were the highest (38.22%) . Iindicating that the fish highly desired theis food item. Bacillariophyta species were highest in occurrence of food items identified; hence the result revealed that O. niloticus is a generalized grazer with higher relative index recorded in plant source food items (Figure 2).
DISCUSSION
Stomach contents in Oreochromis niloticus fish samples examined is in line with the work of Agbabiaka (2012) and Rini (2013) which reported Tilapia fish have varying food in its gut and shifting feeding behavior. These food contents seen in the stomach of the fish follow the work of Abari et al. (2015) who reported O. niloticus as obligate herbivore.
High desire of Bacillariophyceae by the fish is supported by Mohsen (2003) as Bacillariophyceae constituted 38.22% of the food items and the most prevailing food items, but plant parts had the highest percentage dominance food item (58.54%); while Bacillariophyceae Gyrosigma balticum had the least (10.20%). Hence, stomach contentsobtained corroborate the reports of Fagade and Olaniyan (1972) in the Lagos Lagoon; Houehanou et al. (2016), who reported that cichlids exploit more than one source of food item; having the ability to utilize different varieties of food makes O. niloticus omnivorous in nature. He et al. (2015) also reported high degree of overlap in diet of fishes from the same community.
The work of Rao (2017) supported the report that cichlids fed mainly on detritus, insects and plant materials; as seen in the result of this study that other unidentified food items, plants and insect materials identified from the gut of O. niloticus are members of its stomach contents. Ali et al. (2015) also supported the result that the species feed on plants, leaves, buds and seeds of water lilies and are thus herbivorous feeders.
The result that O. niloticus had higher frequency of plant material is supported by Oso et al. (2006) who also observed O. niloticus to be herbivores having highest percentage frequency of occurrence of higher plant remains as 49.2 %. O. niloticus preference for diatoms than the green algae, despite the latter being more abundant is in line with the work of Kariman et al. (2009) who reported diatom preference in the nutrition of O. niloticus in Abu-Zabal lakes in Egypt.
Teferi et al. (2000) who studied the food and feeding habit of Nile tilapia in Lake Chamom Ethiopia on the stomach contents of adult and juvenile fish reported that O. niloticus fish is phytoplanktivorous, and that the component of the phytoplankton varied seasonally, and Ali et al. (2015) also reported that Nile tilapia are filter and benthic feeder as they consumed plankton, and detritus. These supported the result that the fish is planktivorous in the food item recovered from the stomach contents examined.
Hence, stomach contents of O. niloticus examined in Lagos Lagoon complemented and corroborated previous management assessments on stomach analysis of O. niloticus from Sub-saharan Africa.
CONCLUSION
Lagos Lagoon environment supported food constituent of O. niloticus which has generalized herbivorous feeding habit with great desire for bacillariophyte species of food items among other food items. Food availability and optimum requirement for fish daily need is supported by Lagos Lagoon environment status. And this will enable production of healthy O. niloticus fish for public fish food of plant nutrient source.
CONFLICT OF INTERESTS
The authors have not declared any conflict of interests.
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