Influence of rainfall on the dynamics of two prawn populations in the Cross River Estuary, Nigeria

Abstract

The influence of rainfall on the population dynamics of the prawns, Macrobrachium macrobrachion Herklots 1851 and Nematopalaemon hastatus Aurivillius 1898, in the Cross River Estuary, Nigeria, was investigated. Rainfall accounted for a significant portion of the variations in catch rate, spawning and recruitment indices of M. macrobrachion. This is because the processes of migration to the estuary, and the spawning of the prawns there, are presumably not only triggered off by rainfall but also progressively influenced by it. With significant relationships also among the population variables (catch rate, spawning and recruitment indices) at appropriate lags, it was possible to chart the time steps from rainfall through catch rate, to spawning and recruitment. A certain amount of rainfall in a given month leads to a given abundance of adult population (catch rate) in the estuary two months later, which is followed in another one month by spawning. Recruitment takes place 4 to 5 months after spawning. Thus the time span from rainfall to recruitment of the young of a given cohort is 7 to 8 months. For N. hastatus, the catch rate in the estuary does not seem to proceed linearly with rainfall because after the early rains, the prawns migrate offshore for spawning purposes. However, from the analyses the time span from rainfall through spawning to recruitment of the young was estimated at 6 months. There is still room for studies aimed at clarifying the recruitment processes of these prawn populations in order to enhance their effective management.


KEY WORDS: Rainfall, Catch rate, Spawning, Recruitment, Macrobrachium, Nematopalaemon, Cross River Estuary, Nigeria


Global Jnl Pure and Applied Sciences Vol.10(4) 2004: 525-533