Pressions sélectives et nouveautés évolutives : une hypothèse pour comprendre certains aspects des fluctuations à long terme des poissons pélagiques côtiers

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Abstract
The extinction during periods much longer than the duration of a species life cycle followed by a quick resurgence to high abundance levels is sometimes observed in marine coastal pelagic stocks. Japanese sardine (Sardinops melanosticta), Pacific sardine (Sardinops sagax), and Peruvian and Chilian anchovy (Engraulis ringens) are presented as examples. Interpretations of these stocks' fluctuations are reviewed. Climatic events and the influence of commercial harvesting do not always appear sufficient to explain such recoveries. Approaches derived from marine and terrestrial ecology as well as from evolutionary biology may lead to a better understanding of the biological reasons for the maintenance and recovery of depleted biomasses. The importance of interspecific competition during periods of low biomass levels is discussed. The possible consequences of strong selective pressures on population dynamics and the adaptive potential of pelagic species are emphasized. Lastly, evolutionary novelties that appear when a species is becoming depleted are considered. From these considerations, we hypothesize that the long attractive periods of low abundance can result from strong selective pressures and from the reduced influence of climatic events. The recovery of depleted biomasses would be the result of an adaptive process (microevolution). Natural selection could therefore act as another regulation mechanism for marine pelagic populations. A model using catastrophe theory is proposed to ilustrate the long-term regulation of these pelagic systems.

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