From small-scale habitat loopholes to decadal cycles:: a habitat-based hypothesis explaining fluctuation in pelagic fish populations off Peru

The Peru-Humboldt Current system (HCS) supports the world's largest pelagic fisheries. Among the world's eastern boundary current systems, it is the most exposed to high climatic stress and is directly affected by El Nino and La Nina events. In this volatile ecosystem, fish have been led to develop adaptive strategies in space and time. In this paper, we attempt to understand the mechanisms underlying such strategies, focusing on the El Nino 1997-98 in Peru from which an extensive set of hydrographic, capture and acoustic survey data are available. An integrated analysis of the data is crucial, as each has substantial shortcomings individually; for example, both catch data and acoustic surveys may easily lead to wrong conclusions. Existing hypotheses on anchovy and sardine alternations lead us to a 'habitat-based' synthetic hypothesis. Using our data, an integrated approach evaluated how fish responded to habitat variation, and determined the consequences in terms of fish-population variability. Various factors occurring at a range of different spatio-temporal scales were considered: interdecadal regime (warm 'El Viejo'/cool 'La Vieja' decadal scale); strength and the duration of the El Nino Southern Oscillation event (interannual scale); population condition before the event (interannual scale); fishing pressure and other predation (annual scale); changes in reproductive behaviour (intra-annual scale); presence of local upwelling (local scale). During El Nino 1997-98, anchovy was able to exploit a small-scale temporal and spatial 'loophole' inside the general unfavourable conditions. Moreover, sardine did not do better than anchovy during this El Nino and was not able to take advantage of the 'loophole' opened by this short-term event. Our results question the traditional view that El Nino is bad for anchovy and good for sardine.