Changes in the diet of hake associated with El Niño 1997−1998 in the northern Humboldt Current ecosystem

Abstract. Hake (Merluccius gayi peruanus) predation plays an important role in the dynamics of the Humboldt Current ecosystem (HCE). Changes in the hake trophic habits associated with physical variability are expected to impact prey populations and to propagate through the food web. Time series (1995–2002) of (a) stomach contents of hake, (b) biomass estimations of fish prey species of hake, and (c) depth of the 15°C isotherm was analysed with the aim of exploring the impacts of El Niño 1997–1998 on the diet of hake. Biomass estimations of fish prey species were used to indicate resource availability, and depth of the 15°C isotherm to represent variability associated with the ENSO cycle in the physical environment of hake. The richness of prey species increased during the months when 15°C isotherm reached its deepest position, supporting the hypothesis of increased biodiversity (tropicalization) of the HCE during El Niño events. An increased variability in stomach fullness of hake was detected after 1999 which could indicate high heterogeneity in the food supply as a consequence of impacts of the warm event in the biotic community structure of the HCE, a physiological impairment of hake or an effect of the abrupt reduction in the mean total length of hake, postulated as a compensatory response to fishery pressure. Hake can be characterized as an opportunist predator according to the observed changes in its diet during 1995–2002. Overall, the diet of hake in the northern HCE exhibited transitory (e.g. increased richness of prey species in the stomach contents) and medium term (e.g. increased variability in feeding activity) responses associated with El Niño 1997–1998, which should be incorporated both in population dynamics and food web analyses.