Robison, B. H.. Monterey Bay Aquarium Research Institute, robr@mbari.org
Reisenbichler, K. H.. MBARI, reki@mbari.org
Sherlock, R. E.. MBARI, robshrlk@mbari.org

 
A LONG-TERM, TIME-SERIES STUDY OF THE PHYSONECT SIPHONOPHORE NANOMIA BIJUGA
 
In 1993 we initiated a time-series survey and sampling program of the midwater fauna in the upper 1000 m of Monterey Bay. We used the ROV Ventana: to conduct quantitative video transects at 100 m depth intervals; for in situ observations of behavior; and to collect specimens for laboratory studies ashore. The dominant gelatinous predator in the mesopelagic fauna was the siphonophore Nanomia bijuga. The population of N. bijuga showed a clear, seasonal cycle of abundance that was strongly coupled to regional upwelling and primary production. Mean abundance within the principal depth range (200 to 400 m) was 5 colonies per 100 m3 and peak abundance was 1 colony per cubic meter. The typical pattern of seasonal abundance was significantly altered during the 1997-98 El Nino period. While the primary prey of N. bijuga (e.g. Euphausia pacifica) are diel vertical migrators, the siphonophore population maintained a broad vertical range both day and night. Stomach content analyses showed that feeding takes place throughout the diel cycle. Feeding behavior was stereotyped, and appears to be optimized for patchy prey. Laboratory measurements indicated that digestion rates are considerably higher at warmer temperatures, and this factor may be influential in Nanomia's vertical movements.
 
Day: Tuesday, Feb. 2
Time: 02:45 - 03:00pm
Location: Hilton of Santa Fe
 
Code: CS67TU0245H