Results from both Shelikof Strait and the slope waters of the eastern Bering Sea indicate that the highest abundances of pollock larvae often reside in eddies. To examine the nature of biophysical processes extant in these features and determine their influence upon survival requires in situ observations. Finding a reliable method to locate an eddy for field studies provides a challenge. Although infrared imagery has proved useful, cloud cover and generally weak sea surface temperature gradients limit this approach. High resolution Synthetic Aperture Radar (SAR) eliminates both of these constraints. Mesoscale features are imaged by SAR through several possible mechanisms that are not well understood, including modulation of the short surface waves by current shear, alteration of the stability of the surface wind across a relatively sharp sea surface temperature gradient, surface film damping of short surface waves, shifts in the Doppler frequency due to variability in the surface currents, and current-induced wave refraction. This latter mechanism has been examined for features in Shelikof Strait [ Liu et al., 1994].
During April and early May 1992, three eddies (20-25 km diameter) were apparent in SAR images of Shelikof Strait. In mid-May a larval survey was conducted in the same region. A satellite-tracked buoy deployed in a region of high larval abundance made a circular trajectory around a mesoscale feature that likely was one of the eddies observed in the SAR imagery [ Liu et al., 1994]. During this and a subsequent cruise, anomalous patterns of backscattering appeared on a 38-kHz acoustic system. A strong scattering layer at the surface and in midwater, with the column in between nearly void of sound scattering organisms, characterized the signal. This signal appeared in several sections of data where SAR had indicated the presence of eddy-like features. Analysis of concomitant water property and shipboard acoustic Doppler Current Profiler (150 kHz) observations confirmed the existence of these features. The density of larval pollock in these features was estimated to be an order of magnitude greater than in surrounding waters. Acoustic backscatter signals can sometimes be used to identify and characterize mesoscale biophysical features in the ocean [ Aoki and Inagaki, 1992], thereby permitting real-time studies of these features.