The warm El Niño episodes are generally accepted to be harmful to the development of cold-water anchovy populations, but favourable for abundant populations of sardine, adapted to warmer waters. IRD researchers and their Peruvian partners (1) have been studying fluctuations in pelagic fish populations in the world’s richest oceanic ecosystems for fish, the Peru-Humboldt Current system, off Peru. They showed that the traditional explanation does not always hold true. During the 1997-98 El Niño event, one of the strongest of the XXth century, anchovy in fact adapted and reproduced by taking advantage of refuge habitat ‘loopholes’ located very close to the coast. The existence of these areas, with specifically different climatic conditions, appear to have favoured the survival of these populations in spite of generally unfavourable climatic conditions.
Near the coasts of Peru and Chile, the Humboldt Current ecosystem is the world’s most productive fishing zone. This cold-current zone, with frequent coastal upwellings ( 2 ), occupies less than 1 % of the world’s ocean surface and provides 15 to 20 % of global maritime catches. Unlike other large regions of upwelling, this ecosystem proves to be more exposed to variations in climate. Its geographical location brings it under the direct influence of disturbances generated by the El Niño-La Niña events which arise every 3 to 7 years. Other climatic cycles, called El Viejo-La Vieja by reference to the first two, also bear influence, but on a longer time-scale with a period of about 50 years. Large-scale alternation of abundance of sardine and anchovy populations corresponds to these warm (El Viejo) and cold (La Vieja) climatic regimes. At smaller scale, the El Niño events would induce massive die-offs in anchovy, adapted to cold, nutrient-rich coastal waters, whereas the populations of sardine (and of other species like jack mackerel or mackerel), which live in the warmer waters, would experience an upsurge in numbers during or just after these episodes.
A recent study conducted by IRD researchers and their Peruvian partners ( 1) in this part of the Pacific, has called this traditional theory into question. Indeed, as there is no single type El Niño event, each one different in intensity, length and environmental consequences, pelagic fish would not have one single adaptive response to these events. In order to analyse these adaptive strategies and explain the fluctuations observed in sardine and anchovy populations, the scientists chose an overall approach. This took into account a whole range of available data: climatic, biological and ecological, at different time-scales (3 ). They put forward a hypothesis, based on the variations in habitat size for each species, to interpret the alternate fluctuations of anchovy and sardine populations at decadal scales, not only on inter-annual periods.
Hélène Deval | EurekAlert!
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