Research Technical Report

Temporal Variability in the Cetacean Assemblage of a Coastal Upwelling System Spanning an El Nino Event

Benson, S.R., D.A. Croll, B. Marinovic, and J.T .Harvey (1999)

13th Biennial Conference on the Biology of Marine Mammals, Nov 28 - Dec 3, 1999.

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 (area size: 909 km²) during the summer upwelling periods of 1996-98, including impacts of the 1997/98 El Nino. 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. Underway sampling of sea surface temperature and hydroacoustic backscatter complemented the station data. The abundance of several of the California Current's most common cetaceans varied among the years. Dall's porpoise (Phocoenides dalli) and common dolphin (Delphinus sp.) abundance mirrored oceanographic patterns. During 1996, temperatures were low (mean=13.1 degrees C, S.D.=0.43, maximum=13.5 degrees C), the thermocline was well-developed, Dall's porpoise were abundant (Abundance, N=408, C.V.=0.30), and common dolphins were rare (N=6, C.V.=~1.0). As temperatures warmed in 1997/98 (mean=15.5 degrees C, S.D.=1.52, maximum=17.0 degrees C), and the thermocline weakened, Dall's porpoise abundance decreased to N=46 (C.V.=0.51), while common dolphin abundance increased to a peak of N=6,018 (C.V.=0.49). theses patterns are consistent with previous coastwide studies. Rorquals responded strongly to the El Nino conditions. A sharp drop in abundance occurred between 1996 (N=32, C.V.=0.27) and 1997 (N=6, C.V.=0.75) as zooplankton productivity decreased. In 1998, while zooplankton volume slowly increased, rorqual abundance sharply increased to the highest observed abundance (N=83m C.V.=0.26). We hypothesize that a dramatic reduction in productivity offshore concentrated rorquals in the remaining productive coastal upwelling area, 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.