ABSTRACT
Dredging of the Benin River for port creation would boost the economy of the populace by ensuring easy transportation of agricultural and industrial goods. This study covered the ecological assessment of the finfish and shellfish as part of the documentation of the current status of the Benin River prior the Seaport Development project.
Four stations were sampled for 2 years spanning across 25.2km2 of the river from Ajoki by Koko, Delta State to Young Town. Water and sediment samples were collected and analyzed for physicochemical parameters. Finfish and shellfish were sampled every month using various sizes of net and woven cylindrical non-return valve traps respectively. Samples were sorted and identified to species level accordingly using their morphological features. Inter station and seasonal comparisons of significant differences were done using ANOVA and Duncan Multiple Range (DMR) test. Principal Component Analysis (PCA) and Water Quality Index (WQI) were used to establish relationship among water quality parameters and status.
For the water parameters, non significant variation (P > 0.05) spatially except for Salinity, Turbidity, COD, Na and Ca while significant variation (P < 0.05) was observed seasonally. The water quality index for the Benin River stretch was of poor quality, Cd was detected as a major pollutant. First seven components of PCA accounted for 88.72% of the total variance while high sorption capacity showed that there was low mobility of heavy metals (Kd > 6 kg/l) in the water, Cu being the least adsorbed. 620 individual finfish consisting of 27 families and 46 species were collected; Bagridae and Clupeidae being the most dominant. Three Prawn Species were identified from the sampling sites as M. macrobrachion, M vollenhovenii and M. felicinum; recording1,573 individuals. Highest number 58.17% (915) from station 3 and lowest in station 1 (8.39%). Female M. macrobrachion and M. vollenhovenii predominated with sex ratio of 1:1.60 and 1:1.33 respectively, chi-square (X2) analysis showing a highly significant difference (P < 0.01) between the expected 1:1 and the observed ratio. The highest Gonadosomatic Index (GSI) was (mean values 30.00 ± 0.0%) for M. macrobrachion and (25.60 ± 0.00%) for M. vollehovenii. Mean fecundity of M. macrobrachion and M. vollenhovenii ranged from 50.00-4440.0 eggs/g to 145.0-6120.0 eggs/g respectively.
The prawn speciesexhibited positive allometric growth while condition factor (Kf) for all the species across the stations was >1 indicating wellness. CPUE for both the fish and prawn species was highest in May 2019 and lowest in April 2019. Heavy metals concentration on Prawn species showed a ranking profile of Fe>Zn>Cu>Pb>Cd>Cr while the ranking for heavy metals on different parts of the prawn showed head>shell>flesh. 36 specimens of crab consisting of 3 families and 4 species were sampled and identified. These include 55.56% Callinectes amnicola, 25.00% Chiromantes angolense, 13.98% Panopeus africanus and 5.56% Perisesarma alberti. The continuous investigation and monitoring of the Benin River stretch during the seaport development which would include dredging is very necessary and is highly recommended.
