Visualization of Marine Habitats Affected by Wind and Tidal Mixing near the Pribilof Islands

FOCI researchers Dr. Ric Brodeur and Dr. Jeff Napp of the National Marine Fisheries Service's Alaska Fisheries Science Center in Seattle, Washington, collaborating with bio-acousticians Dr. Dave Demer and Dr. Roger Hewitt of the Southwest Fisheries Science Center, La Jolla, California, have adapted a dual frequency hydroacoustic analytical procedure. Developed with support from NOAA and its Coastal Ocean Program, the procedure pioneers visualization of distributions of upper-trophic-level predators and their prey in marine ecosystems. The technique lends itself to examination of spatial and temporal patterns of distribution of juvenile fish, principally walleye pollock, and their prey in and around tidally generated fronts near the Pribilof Islands in the Bering Sea. In the echograms below, fish that are distributed throughout the water column by day are seen to migrate to the upper layer at night.

Hydroacoustics is the science of sound in water. One application of hydroacoustics is to estimate the amount of material in a given volume of water by sending out a "ping" of noise and measuring the amount of acoustic energy that is bounced back from sound scatters in the ocean. The frequency of sound used in part determines the size of scatterers that can be detected. Higher frequencies (shorter wavelengths) detect smaller objects. The two echograms above show the difference in volume backscattering measured using two acoustic frequencies, 200 and 120 kHz, along a 50-km transect during September 1994 near the Pribilof Islands. Low differences (contoured red to yellow) indicate that scattering is dominated by fish; high values (blue to purple) imply that the scattering organisms are primarily invertebrate zooplankton such as copepods and euphausiids ­ the main prey of juvenile fish. Interpretation of features contained in the echograms is aided by results of net tows and water property profiles taken concurrently.

In each panel the surface waters and nearshore areas are dominated by small zooplankton (blue and purple). The ocean bottom shows as a thick purple band located at a depth of about 32 m near shore (0 km), deepening to about 70 m offshore. Information below the bottom is to be disregarded. Within this 50-km transect there are three distinct habitats created by wind and tidal mixing. Nearshore in the inner shelf habitat (0 to 12 km), the water is shallow enough that winds and tides together mix the water column to a homogeneous state. Adult fish (pollock and cod) are seen near the bottom. Offshore over the middle shelf (about 22 km and beyond), tidal mixing only affects the lower part of the water column. The character of the upper portion is largely determined by atmospheric processes. This creates a two-layer system with a distinct thermocline at about 40 m depth between the layers. At the nearshore end of this region, adult fish are again found near the bottom. Farther offshore, prey are found near the bottom. The region between the inner and middle shelves (12 to 22 km) is a transition zone sometimes called the inner front. Adult fish are found near the bottom toward the offshore end of the transition zone.

The upper echogram shows daytime conditions. The surface to about 20 m is dominated by zooplankton. Juvenile fish are distributed at mid depth and show a preference for the thermocline area. This could be a strategy for avoiding predators during daylight. At night, in the lower panel, there is a strong upward migration by fish over the inner front and middle shelf, presumably to feed on the zooplankton in the upper layer. Juvenile pollock in the Gulf of Alaska have been shown to be night feeders.

This visualization of the diel pattern in three varying habitats suggests how physics affects the distribution and behavior of fish and their prey through the tidal mixing process. When coupled with net samples and physical measurements, dual-frequency hydroacoustics reveals important information about the ecosystem.