Stomach and gut content of Long Neck Croacker–Pseudotolithus typus (Bleeker,1863) from Lagos Lagoon, Nigera

Feeding ecology is an important aspect of the life-history strategy of a species to understand the functional role of the fi sh within their ecosystems [1]. One of the most economically important and dominant species in the Nigerian coastal waters is Pseudotolithus typus .This species occurs in the estuaries and saline creek systems. Pseudotolithus typus is generally classifi ed as omnivore or predator feeding mainly on aquatic insects, fi sh and plant debris. Pseudotolithus typus utilize various kinds of food resources available in their habitat [2].

their growth within their environment. Therefore, the dietary analysis of Pseudotolithus typus in their natural habitat enhances the understanding of their growth, abundance, productivity and distribution and seasonal fl uxes in the type and magnitude of food available as well as the season it occurs. This study was designed to have the broad knowledge of the different species of prey in order to understand the quantitative connection between fi sh and their food organisms . Furthermore, this study adopts an ecosystem approach by quantifying the food organisms based on seasonal variation. This study presents more information on the gut and stomach content of Pseudotolithus typus in Lagos Lagoon. Previous reports on the Croakers in Nigeria include [3,4] and [5,6], also worked on Food habits of two species of Pseudotolithus (Sciaenidae) off Benin (West Africa) nearshore waters and implications for management and [7] also worked on Biometric characteristics, Food and feeding Habit and Sex Dimorphism of the Long Neck Croacker( Pseudotolithus typus ) from Lagos lagoon.

Study area
Lagos State lies between longitude 3°21"24"E and latitude

Laboratory procedure
The nomenclature of the fi sh samples taken to the laboratory for this study conformed to [9]. The fi sh specimens were treated individually to determine their biometric data. In the Laboratory, after blotting dry with fi lter paper Specimens were dissected and the gut were carefully removed with the aid of a forceps after dissection. The Stomach and the of dissected specimens were slit open and the contents displayed in Petri dishes with a small drops of distilled water added to agitate them and examined under a stereo light microscope(magnifi cation up to 100×) to identify the food items using appropriate taxonomic guide (FAO Marine Fish identifi cation guide). Information on Total Length, Standard length, Weight and identifi ed food items were recorded into a data sheet for data analysis.

Food composition determination
Two methods were used to assess the food composition of the fi sh. These methods included frequency of occurrence and numerical method [10].

Frequency of occurrence
Stomach contents were examined and the individual food organisms sorted and identifi ed. The number of stomachs in which each item occurs were recorded and expressed as a percentage of the total number of stomachs examined.

Frequency of Occurrence = Ji÷P
Where, is number of fi sh containing prey i and P is the number of fi sh with food in their stomach. iJ This method demonstrates what organisms are being fed upon, but it gives no information on quantities or numbers and does not take in to consideration the accumulation of food organisms resistant to digestion [11].

Numerical method
This involved counting the number of each food item present in the stomach of the species and summing up these numbers to obtain the grand number of all food items in its guts [12]. The number of each food was expressed as a percentage of the grand total number of food items. Usually expressed as:

Index of fullness
This is measured as the ratio of food weight to body weight as an index of fullness. This index can be applied to the food in the stomach, or to that in the whole digestive tract. It is usually expressed as parts per 10,000 ( or parts per decimals).
Gut contents were analyzed quantitatively. The various food items are identifi ed. The food content found in the stomach was divided into three groups.

Guts Repletion Index (GRI)
Guts repletion index is number of non-empty guts divided by total number of guts examined, multiply by 100.
Represented as: Number of non empty guts GRI 100 Total number of guts examined   

Statistical analysis
Data obtained from the study were subjected to descriptive statistics (mean and standard deviation) and presented in graphs, pie chart and bar chart. Data were subjected to one way analysis of variance (ANOVA) and signifi cant differences accepted at P < 0.05 using statistical package (SPSS version 14). Relationship between variables (Length and Weight) was analysed using regression.  Table 1 shows the gut repletion indices of Pseudotolithus typus samples from Lagos Lagoon during the wet and dry season. The gut repletion index was lower during the wet season with a mean value of 71.11 and higher during the dry season with a mean of 79.26.The maximum gut repletion index of 80.00 was obtained during the wet and dry season while the minimum of 60.00 was obtained during the wet season. Table 2 shows the gut repletion index Pseudotolithus typus samples for the combined sampling period of six month and has a maximum value of 80.00 with the minimum of 60.00. The gut repletion index of Pseudotolithus typus samples from Lagos lagoon has a mean of 75.18 for both season. Figure 4 shows the gut repletion index of Pseudotolithus typus samples for each month samples from Lagos lagoon (September, 2018 -February, 2019) respectively.

Diet composition
The diet of Pseudotolithus typus samples from Lagos lagoon consisted mainly of crustaceans (shrimps) and Pisces (fi shes). Other food items were unidentifi ed partially digested fi sh, Unidientifi ed partially digested Crabs, Cephalopods      Figure 4 shows the frequency of occurrence dietary composition of Pseudotolithus typus samples from Lagos lagoon, Nigeria (September, 2018 -February, 2019), while Figure 5 shows the numerical dietary composition of Pseudolithus typus samples within the specifi ed sampling period. Figure 6, shows comparison between the frequency of occurrence and numerical dietary composition of Pseudotolithus typus in Makoko Lagos lagoon, Nigeria (September-February) respectively.

Gut repletion index of Pseudotolithus typus
Gut repletion index mean of 75.18% with minimum of 60% and maximum of 80% was observed during the combined sampling period. The observed mean gut repletion index for wet (71.11%) and dry (79.26%) season is an indication of high feeding intensity. The present information is not in agreement with reports of [13] who observed the 100% GRI in C. Auratus in Cross river estuary of Nigeria. The high % GRI exhibited by P.typus might be due to their ability to switch diets due to high abundance of food items.
Fish are capable of changing their gut morphology in response to available food resources and demonstrate high degree of plasticity. However, contrary to the report of high gut      repletion index in this study [14], reported a low GRI in T. teraia (4.35%), E. aeneus (20.00%).Based on their feeding intensity observed in this study, Pseudotolithus typus from Lagos lagoon, Nigeria were considered active feeders.

Dietary composition of Pseudotolithus typus
The diet composition of Pseudotolithus typus shows to be a specialized feeders depending on same food sources with streamlined number of food items. The major food materials reported by the current study were shrimps (Crustaceans).
These results confi rmed those of [15] who observed and reported that four stomachs out of fi ve contained shrimps [16].
also reported the importance of shrimps in the food regime of P. Typus [17]. obtained the similar results while studying the community of Sciaenidae off the Guinean coasts [18]. noted that P. typus feeds mostly (80%) on shrimps.Similar results have been reported in previous studies in Nigeria [19].
The present study revealed that Paeneus notialis was the most important shrimps species fed on by P. typus in Lagoon during the sampling period [18]. reported similar results for these two Sciaenid fi sh species off Cameroon. This importance of shrimps in diet composition may be due to their abundance [20] and nutritional benefi ts. The observed high diet specialization can be attributed to the fact that the dietary sources are largely autochthonous. This could however lead to intra and inter-competition. However, the presence in P. typus attests to the inherent ability of the specie to expand their food items, options should there be any dramatic negative alteration in the availability of the favoured food item.

Seasonal variation in Dietary Composition of Pseudotolithus typus from Lagos lagoon, Nigeria
In this study, it was observed that there was a variation in diets fed on by P.typus during the wet and dry season. There was an obvious absence of Sphyraena barracuda and Bivalves during the dry season while they are present during the wet season.
Bivalve molluscs are largely used as bioindicators to monitor the health of aquatic environments in both fresh water and the marine environments [21]. reported that a defi nite pattern of seasonal variations in pollution could not be established for

Conclusion and recommendation
The stomach and gut content of Pseudotolithus typus from Lagos lagoon show a great autochthonous pattern and have been grouped into predatory or carnivores feeder. An overlap does not exist in the diet which increases competition among the fi sh and ensures a narrow spectrum of dietaries. From the study, it can be concluded that Pseudotolithus typus in Lagos lagoon utilize more than one source of food.
My recommendation is that the research should be intensifi ed in other area of pseudotolithus typus nutrition and microbial gut content in other to be able to determine the nutritional technicality of rearing the fi sh in a cultured environment. Also the pollution level of Lagos lagoon should be determined especially during the dry season(when the water level is low) to be able to determine the remediation methods to use in controlling such.