Effects of El Niño 1982-83 on Benthos, Fish and Fisheries off the South American Pacific Coast

Abstract

Biological effects of El Niño (EN) were first detected on the Peruvian coast and have been reported for some of the major events during this century, especially those of 1925-26, 1940-41 and 1972-73. EN 1982-83 exceeded all these earlier events in strength and duration, and was the first to be investigated in more detail, both on a latitudinal gradient and including, for the first time, the benthic subsystem of the Humboldt Current upwelling area off the west coast of South America. Despite the fact that the coverage of the area of major impact (over 4,000 km of coastline) was by no means complete, and that most of the causes and mechanisms behind the observed changes remained unclear due to the lack of experimental (e.g. tolerance) studies, the observational evidence and the fluctuations observed in the landing statistics have contributed a great deal to our understanding of the way in which a major EN event affects the upwelling system as a whole. In the pelagic subsystem, a surface temperature increase of up to 11°C, the effects of which could be traced down to at least 1,000 m depth, led to a tropicalization of the ecosystem, the disruption of the normal food web, and induced changes in species composition and migrations of a large number of fish and invertebrate species populations. This resulted in a general impoverishment of this system. The consequences for some commercially important species, which had already been damaged by former EN events and the impact of overfishing, were severe off Peru and Ecuador although there were certain positive effects due to the inshore migration of various oceanic mackerels and the southward migration of (sub)tropical fish species such as dolphinfish and skipjack tuna. These beneficial effects were, however, only of minor importance for the fisheries, which were not equipped to deal with the new target species. In the benthic subsystem, a factor of major importance, besides the increase in temperature, was the marked increase of dissolved oxygen at the seafloor during and after the event. Despite mass mortalities of many of the cold-water adapted invertebrate species on sandy beaches, shallow soft bottoms and in the rocky intertidal zone, the overall effect on fisheries was rather positive. In the case of commercially exploited shellfish (“mariscos”), this was due to an increased immigration of subtropical species, mostly crustaceans, as well as increases in abundance of some local species more tolerant of warm water such as the Peruvian scallop. This led to an unprecedented boom of some fisheries on the northern and central Peruvian coast. For the demersal fisheries and the artisanal finfish fishery, the immediate effects of EN were detrimental due to a general dispersal and downslope migration of many species, but the long-term effects appeared to have been beneficial: improved feeding conditions at the seafloor and decreased fishing pressure caused a major recruitment success of some species such as Pacific hake, and this may provide increased catches several years after EN. The primary EN effects described in this review apparently induced many secondary effects. The disruption of the pelagic food web caused mortalities and reproductive failures of guano birds and seals; mortalities (including marine algae) and increases in population abundance in shallow water led to temporary changes in species composition and successional processes that remain to be studied in more detail. On the whole, however, recovery of most species occurred rapidly, and relatively few long-term effects have remained, demonstrating the remarkable resilience of the upwelling ecosystem.