"We think that the fish larvae are in the thin layers, because that's where all the food is, and during relaxation events they get mixed out, and that's when recruitment occurs," McManus said.
Two key instruments used in the thin layer studies are an acoustic profiler and an optical profiler, both of which McManus helped develop. The combination of these instruments provides a detailed profile of the water column, from near the ocean bottom to the surface, showing not only physical conditions such as temperature and salinity, but also how much phytoplankton (algae and other plant life) and zooplankton (including fish larvae and other small animals) occur at every depth. The new instruments are accurate to within a few inches, provide continuous monitoring, and use radio transmitters to send data streaming back to researchers' offices onshore.
"I can sit in my office and see what the conditions are out in the bay at any time," McManus said.
To find out what was in the layers, PISCO divers collected samples from a location where the instruments showed a distinct layer. Initially skeptical, the divers reported that as they descended they suddenly encountered a six-foot-thick layer with drastically reduced visibility. The water samples they brought back from the layer showed three- to five-times higher concentrations of a wide range of organisms compared to the water above and below the layer. There were many types of zooplankton, including fish larvae, as well as phytoplankton, bacteria, and viruses.
The fish species in these preliminary samples were not identified, but intensive sampling of the layers, including identification of fish larvae, is planned for later this year, McManus said.
"We really want to know why these organisms are concentrated in the layers and how they are interacting," she said.