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.
When the environmental conditions are generally cold (La Niña, La Vieja), the upsurge of deep-oceanic cold water, rich in nutrients, is intense. The size of the anchovy habitat increases in these conditions. In parallel, the frontal zone between the colder coastal waters and the warmer oceanic waters, highly suitable for sardine to develop, is pushed back towards the open waters. In the process, conditions become unfavourable for sardine again, especially so in that their larvae are dispersed towards nutrient-poor parts of the ocean.
However, when a warm climatic regime arrives (El Niño, El Viejo), upwelling becomes less effective, primary production diminishes, considerably reducing the habitat favouring anchovy, sometimes as far as making them disappear temporarily. The sardine habitat then extends towards the coast, giving the opportunity for their populations to grow.
The El Niño event of 1997-98 was one of the most powerful episodes of the XXth century, yet it had very little impact on anchovy populations of Peru. The research found that after this episode anchovy was abundant, suggesting the fish were able to adapt and exploit a "loophole" inside the prevailing conditions which were unfavourable to their development.
When this episode began in 1997, upwellings persisted in some areas very close to the coast (about 1 km). The anchovy populations, highly abundant at the time owing to the influence of a cold regime (La Vieja) exerted since the beginning of the 1990s, took refuge and aggregated in these small zones teeming with planktonic production. They are not subjected to any massive predation. The natural predators (sardine, jack mackerel, giant squid, birds, marine mammals) prove to be rare in such zones. Also the latter were protected from industrial-scale anchovy fishing, which was strictly limited by quotas and the obligation to practise beyond 5 nautical miles (about 9 km) from the coast ( 4 ). Adult anchovy have therefore adapted to the changes in the environmental conditions, by extending their period of reproduction and staggering their egg-laying, so that the larvae have more chances to find favourable conditions again. That is what happened for the egg-laying periods of April and June 1998, performed just before the transition, which was exceptionally quick that year, of El Niño to a new cold La Niña period favourable for extending the habitat of anchovy.
The existence of this habitat "loophole" would therefore be the result of a combination of factors, linked in particular to the characteristics of this El Niño episode and to the ability organisms have or have not of taking advantage of such a "loophole". Overall, the El Niño events would not seem systematically to be unfavourable for anchovy and favourable for sardine.
(1) This research work was carried out in a joint effort between the research unit UR ACTIVE, the IRD Centre de recherche halieutique méditerranéenne et tropicale (Sète, France) and the Instituto del Mar del Peru (IMARPE, La Punta Callao).
(2) upwelling: the transfer of cold deep-ocean waters, rich in nutrients, towards the ocean surface.
(3) These are: climatic fluctuations of the El Viejo-La Vieja type, of the intensity and duration of El Niño events, the state of fish populations before the event, of the natural and anthropogenic predation pressure (fishing), reproduction characteristics and the presence of local upwellings.
(4) Consequently, the anchovy have not been captured, nor integrated into fisheries statistics. The fishing effort has therefore been refocused on sardine. The abundance of anchovy has also been underestimated in assessments by acoustic methods, as the standard oceanographic research vessel cannot operate in the "loophole" habitat zones, situated too near the coast.
Fish and Fisheries