The Bio-Ecology and Culture Potentials of the Grey Mullet, Mugil Cephalus (Linnaeus) from the Lagos Lagoon, Nigeria

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Soyinka, O.O
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The seasonal abundance, age and growth pattern, food and feeding habits, reproductive biology and culture potentials of the grey mullet, Mugil cephalus (Linnaeus) from the Lagos Lagoon were investigated. The physico-chemical parameters of the lagoon were also monitored. The air temperature ranged from 23.5 to 320C (mean: 27.9 ± 2.100C), while the surface water temperature ranged between 24.5 and 30.80C (mean: 27.9 ± 2.200C). The salinity ranged from 0.0 to 23.5‰ (mean: 9.8 ± 9.13‰), while the hydrogen-ion concentration (pH) ranged between 6.8 and 7.8. The dissolved oxygen ranged from 3.8 to 9.5 mg/l (mean: 5.5 ± 1.65mg/l). The size of the 2661 specimens of M. cephalus used for this study ranged from 1.7 – 29.5cm standard length (total length: 1.9 to 39.0cm) and 0.02 to 450.00g weight. The relative abundance of the species varied, with larger size M. cephalus being more available during the dry season (December – May) and smaller size ones or juveniles predominant in the rainy season (June – November). Growth markings on the scale and otolith of the species did not show discernable marks for determining age. Length-frequency method indicated five age groups representing ages 0 – 1yr (1 - 3cm), 1 – 2yrs (4 - 6cm), 2 – 3yrs (7 - 9cm), 3 – 4yrs (10 - 23cm) and >4yrs (24cm standard length and above). The growth pattern of M cephalus showed a positive allometric growth for both sexes and immature specimens. High correlation (r) values ranging from 0.9003 to 0.9784 obtained was a strong indication of an increase in standard length of the species with a corresponding increase in body weight. The mean monthly condition factor (K) values ranged from 1.39 to 2.30. The species was an omnivore, feeding on algae (blue–green, green, diatoms and desmids), plant materials, annelids, crustaceans, bivalves, fishes and detritus. Sand grains were also found in the stomachs of the fish. The sex ratio for M. cephalus was 1:0.53 and this showed a statistically significant (p<0.05) difference from the expected 1:1 ratio. The fecundity estimates varied from 635,568 – 1,520,185 eggs with a mean fecundity of 832,270 ± 156,016 eggs. The diameter of the eggs varied from 0.36mm to 0.46mm (mean = 0.41 ± 0.04mm). The maturation stages of the testes and ovaries were classified as immature (Stage 1), developing (Stage II), ripening (Stage III), ripe (Stage IV) and spent (Stage V). The immature fish occurred most in the lagoon (75.6%). However, the males were significantly (p<0.05) more abundant than the females. Five major developmental stages of male gonads were observed: spermatogonia, primary spermatocytes, secondary spermatocytes, spermatids and spermatozoa; while six major developmental stages of oocytes were observed: oogonium, primary oocytes, secondary oocytes, vitellogenic oocytes, post-vitellogenic oocytes and hydrated oocytes. In the feeding trial experiment using formulated coppens feed and varied percentage substitutions with low-cost feedstuffs (dried water hyacinth and poultry waste), the highest gain in weight of 125.81% was recorded in the 40% water hyacinth substitution diet. This suggested that water hyacinth incorporation in M. cephalus feed will produce better yield and reduce the cost of fish production. The salinity tolerance experiments showed that the fish tolerated a salinity range of 5‰ to 25‰. The highest survival of 40% was recorded at salinities 10‰ and 15‰. Statistical analysis of the mortality with ANOVA showed no significant (p>0.05) variation. The heavy metals: Cd, Zn, Pb, Ni, Fe and Mn analysed were present in the samples of fish flesh, egg, water and sediment. In the rainy season, the concentrations of heavy metals in the fish were Cd (0.03ppm); Mn (0.074ppm); Pb (0.482ppm); Zn (0.78ppm); Fe (1.332ppm) and Ni (-0.085ppm). The heavy metals concentrations in the fish in the dry season were Cd (-0.059 to -0.027ppm); Mn (- 0.145 to -0.066ppm); Pb (0.234 to 0.298); Zn (1.089 to 2.349ppm); Fe (1.433 to 1.729ppm) and Ni (-0.293 to -0.201ppm). The level of heavy metals in the fish tissues in this study was below the FAO / WHO standard of safety for human consumption. In conclusion, the existence of M. cephalus in large numbers in the Lagos Lagoon, its high fecundity, wide range of food spectra of both plant and animal origin and heavy metals contamination below the FAO/WHO recommended safety standards for human consumption are indicative of a favourable environment of the lagoon to support a viable brackish-water fishery of the species. In addition, desirable baseline information needed for a sustainable brackish-water cum marine culture of the species and a reduction in cost of fish feed by employing unconventional but available and efficient feedstuff in M. cephalus culture are provided in this study.
A Thesis Submitted to the School of Postgraduate Studies, University of Lagos
Fisheries , Lagos Lagoon , Reproductive Biology , Fish , Research Subject Categories::NATURAL SCIENCES::Biology::Terrestrial, freshwater and marine ecology
Soyinka, O.O (2010). The Bio-Ecology and Culture Potentials of the Grey Mullet, Mugil Cephalus (Linnaeus) from the Lagos Lagoon, Nigeria. A Thesis Submitted to University of Lagos School of Postgraduate Studies Phd Thesis and Dissertation, 240pp.