The population dynamics of the bony tongue, Heterotis niloticus (Cuvier, 1829) in the Cross River, Nigeria

Abstract

The population dynamics of Heterotis niloticus of the Cross River, Nigeria was investigated for 15 months (October 2010 – December 2011) with the aim to determine the population parameters including growth and mortality rates. Time series data on the length - frequency distribution was collected from the artisanal landings using cast nets and gill nets at Ayadehe and Oku Ibuko beaches in Itu L.G.A. of Akwa Ibom State. The length - frequency data were analysed using FiSAT (FAO - ICLARM Stock Assessment tools). Maximum length of H. niloticus obtained from the field was 92.0 cm with a corresponding weight of 6.0 kg. Population parameters from length – frequency analysis were estimated as follows: Asymptotic length (L∞) = 103.87cm total length, growth coefficient (K) = 0.32 per year, amplitude of oscillation (C = 0.5) and winter point (WP) = 0.6. The K value of 0.32 showed that the fish is a slow growing species. The WP of 0.6 suggests that H. niloticus experiences slowest growth rate in the months of June – July possibly associated with spawning activity and low ambient temperature recorded during this period. The longevity was estimated at 9 years 4 months and growth performance index (o’) was 3.54. The fish instantaneous total mortality (Z) was estimated at 1.51 per year; natural mortality (M) was 0.608; fishing mortality (F) was 0.902 and exploitation rate (E) of 0.60. This exploitation rate indicates that fishing mortality is becoming excessive and is therefore being overfished. Possible interventions and recommendations are that the effort be reduced or stabilized. The close of fishery in July - August, the peak of reproduction is recommended and should be implemented. In addition, mesh size be increased to reduce growth overfishing on juveniles and sub adults.
 Key words: Heterotis niloticus, population dynamics, Cross River, Nigeria