Competition and comparative life-history strategies of two sardinella species (Sardinella aurita and Sardinella maderensis) of the West-African coasts

Two sardinella spccies (Sardinella auria and Sardinella maderensis) are present in the three West-African upwelling areas (the Senegalese-Slauritanian, the Ivory Coast-Ghanaian and the Congo-Angolan zones). The competition between these two species, morphologieally alike, with similar diets and occupying the same geographical areas, appears to be real. IIowever, while the spatial distribution and resourcc exploitation of the two species, as well as their growth and reproductive patterns, are intimately related to the dynamics of the upwelling systems, they differ in many ways. A spatio-temporal complementarity appears to exist between the dynamics of these two spccies with different lire history strategies. S. auria exhibits a wide plasticity and adaptability to environmental fluctuations and takes advantage of them. This species is more sensitive to climatic fluctuations than S. maderensis which tolerates large environmental fluctuations. Thus, there is a greater variability of biological and demographic parameters for the first species than for the second. Sardinella aurita has a more flexible adaptive strategy resulting from a greater demographic plasticity. This allows a better utilisation of temporary enrichment, but conversely it surfers more drastic population changes when conditions are unfavourable. Although competition between pelagic fish species is not unusual in marine ecology, it is rarely possible to demonstrate its effects on the abundance of the respective stocks. The competition mechanisms in highly specialized species like pelagic ones have to be specified for certain parameters of their lire cycle. The comparative study of lire-history strategies may allow a better understanding of fish stock dynamics, particularly the organization of resourcc sharing. The necessity of considering life-history strategies in stock management is emphasized.