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Edible periwinkle, Tympanotonus fuscata, was collected from the Cross River estuary, allowed to acclimate to laboratory conditions for a week and then exposed to Nigerian light crude oil at different concentrations of 50ml, 100ml, and 150ml per litre of water for a period of 96 hours. Mud (80g) was added to all the 4-litre aquaria used for the experiment. The oil was thoroughly mixed with the estuarine water by stirring vigorously. During the 4-days experimental period, most of the specimens suffered suffocation and became moribund and mortality recorded. Within the first 24 hours mortality was absent in the 50ml and 100ml concentrations, although most specimens moved out of the medium by creeping up the aquaria. Specimens in the control seemed lively and unaffected by oiling throughout the duration of the experiment. Mortalities were as high as 117 out of 180 individuals during the entire experimental period, with higher concentrations recording higher mortalities. There was steady increase in mortality from 4 individuals in the first 24 hours to 12 individuals after 72 hours in the 150ml/l concentration. The LC 50 values were obtained by graphical interpolation. It is concluded that the soluble fraction of the Nigerian light crude oil could be detrimental to shellfish even at a concentration as low as 1000ml/l of the estuarine water.

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

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