Automated monitoring of pelagic fish assemblage during El Ni¹o/Southern Oscillation (ENSO) The El Ni¹o/Southern Oscillation (ENSO) is an interannual pattern in the ocean-atmosphere circulation in the Pacific Ocean. During El Ni¹os the trade winds weaken and vary in direction, decreasing upwelling. Nutrients no longer reach the euphotic zone and phytoplankton production is depressed. This reduces the amount of biomass in the lower trophic levels and results in the migration of the larger predators from the geographic area in search of alternative forage. During La Ni¹as, the antithesis of El Ni¹os, the eastern Pacific waters are cooler than normal, biomass in the lower trophic levels is high, and large predators are abundant. The physical processes during the ENSO and the effect on the distribution and migratory behavior of members of the higher trophic levels, i.e., the large vertebrate predators, are poorly known. Such studies to date have been confined to salmonids off the coast of Oregon, sea turtles in the eastern tropical Pacific, and pinnipeds in the eastern temperate Pacific. The observed sea surface temperature anomaly along the western coast of the Baja Peninsula area reached +5à C during September 1997. The multivariate ENSO index indicates that as of June-July its intensity exceeds all others of the century and should have a profound ecological effect on the oceans. We propose to study the response of an assemblage of pelagic fishes to an El Ni¹o by visual censuses of adults, sampling ichthyoplankton, and tagging members with coded acoustic beacons and detecting their presence with electronic listening devices. These devices will be moored at seamounts and islands within the Gulf of California and off the Pacific coast of the Baja Peninsula. Water temperature will be recorded hourly at these sites with loggers attached to the monitors and related to AVHRR satellite images of sea surface temperature. This study will compliment and geographically extend a long-term collaborative project with Mexican colleagues investigating the effects of mesoscale oceanographic processes on the seasonal movements of marlins, sharks, tunas, etc. Another primary objective of this study is to train doctoral students from Mexico in the use of state-of-the-art fish tracking equipment.
