Research Technical Report

Temporal Variability in the Cetacean Assemblage of a Coastal Upwelling Center Spanning an El Niño Event

Benson, S.R., D.A. Croll, B. Marinovic, F.P. Chavez, J.T. Harvey (2000)

In review for a special proceedings report summarizing El Niño workshop, MBARI.

ABSTRACT

The world's most productive fisheries and marine mammal foraging areas are located in coastal upwelling centers. Temporal variability in strength of upwelling can affect primary production, zooplankton productivity, and the distribution and abundance of fish and marine mammals. In this study, we report on ecosystem studies in Monterey Bay, California during the summer upwelling periods of 1996-98, including impacts of the 1997/98 El Niño. Monthly line-transect surveys for marine mammals covered approximately 120 km along random-systematic lines from August to November, 1996-98. Hydrographic stations, including conductivity-temperature-depth profiles and zooplankton net tows, were conducted opportunistically and at 10 predetermined locations. Hydroacoustic backscatter was measured continuously while underway.

The abundance of several of the California Current's most common cetaceans varied among years. As temperatures warmed throughout the period, the temperate Dall's porpoise, Phocoenoids dalli, decreased in abundance and the tropical common dolphins, Delphinus spp., increased in abundance, mirroring oceanographic patterns. Rorquals responded strongly to El Niño conditions. A sharp drop in abundance occurred between 1996 and 1997 as zooplankton productivity decreased. In 1998, whereas zooplankton volume slowly increased, rorqual abundance sharply increased to the highest observed abundance. We hypothesize that a dramatic reduction in productivity offshore concentrated rorquals in the remaining productive coastal upwelling areas, including Monterey Bay. These patterns exemplify short-term responses of cetaceans to large-scale changes in oceanic conditions and demonstrate the importance of coastal upwelling systems to higher trophic level predators.