Jaffe, J. S. Scripps Institution of Oceanography, jules@mpl.ucsd.edu
De Robertis, A. S. Scripps Institution of Oceanography, aderober@ucsd.edu
Ohman, M. D. Scripps Institution of Oceanography, mohman@ucsd.edu

 
EVIDENCE FOR THE PREDATOR AVOIDANCE HYPOTHESIS IN THE FINE STRUCTURE OF DIEL VERTICAL MIGRATION OF EUPHAUSIA PACIFICA.
 
The risk of attack by visual predators increases with prey body size and with irradiance level. If diel vertical migration (DVM) of zooplankton is principally a predator avoidance behavior, it follows that the timing of ascent/descent into/from the surface layer during twilight changes in illumination should not be identical for all size classes within a population. Rather, less conspicuous prey should enter surface waters earlier and leave later than larger, more conspicuous organisms. The hypothesis of size-class dependent timing of DVM of Euphausia pacifica was tested in Saanich Inlet, British Columbia, in July-August of 2 years by deploying the FishTV acoustic imaging system at a fixed depth (40 or 50 m) during times spanning the vertical migration of euphausiids. Identities of the sonar targets were determined with the OASIS digital camera. Precise locations of the animals' positions (cms) at high frame rates (seconds) were derived from the sonar data and used to determine the distribution of animal sizes passing through the field of view of the sonar as a function of time of day and irradiance level. Results from replicated observations over many day/night cycles indicate that the timing of the vertical migration is a function of body size.
 
Day: Thursday, Feb. 4
Time: 08:30 - 08:45am
Location: Hilton of Santa Fe
 
Code: SS22TH0830H