Interest in El Niño began in the 1800s because of its adverse affect on the guano birds (i.e., sea birds such as cormorants, gannets, and pelicans) in Peru. Though demand for guano declined in Europe, these birds were still being "mined" throughout the rest of the century. By the 1900s, these birds were being mined at an alarming rate, and, thus, the Peruvian government established a Guano Administration Company (GAC) to oversee the digging up of guano.

Guano birds along the Peruvian coast live off fish populations that dwell near the ocean surface called pelagic fish. These fish include the anchoveta. The normal conditions in Peru’s coastal waters are optimal for the anchoveta populations. However, some conditions perturbed by El Nino can be devastating to sea bird populations.

Coastal upwelling has proved to be excellent for the local fish populations since it brings many nutrients to the surface of the ocean. When El Nino events occur, coastal upwelling processes are altered to such an extent that fish behavior within and among species becomes modified in major ways. Anchoveta, for example, disperse and dwell deeper in the ocean. Patterns of reproduction and migration change for different fish species, with some reproducing less in the temporary altered marine environment. Though some fish populations prosper in this new environment like the sardines, anchoveta are mostly hurt by this. The stock of anchoveta becomes reduced for a variety of reasons, including higher mortality and lower fecundity. As a result of the decline in the anchoveta populations, the fish become much less accessible to the guano birds, causing starvation and death of hundreds of thousands or even millions of birds.