Volume 9, Number 3 • May/June 2001

Satellite Sensors Find Fish and Food

Scientists using satellite data discovered an unusual, long-lasting, whirlpool-like ocean eddy that generated a dramatic increase in the marine food supply off the Hawaiian coast in 1999.

The eddy, named Loretta, began spinning up in the Alenuihaha Channel between the islands of Hawaii and Maui during mid-May 1999 and maintained a presence in the lee of the Hawaiian Islands until January 2000. Over the eight-month period, the eddy’s churning motion brought up a great amount of nutrients from the ocean depths, enhancing the plankton population on the ocean’s surface, and providing a banquet for marine life.

Several organizations collaborated to track Loretta and other Hawaiian eddies and their ecological benefits. The University of Hawaii, NASA and the National Oceanic and Atmospheric Administration’s (NOAA) National Marine Fisheries Service (NMFS) integrated information from two independent satellite sensors that measure sea surface temperature (SST) and ocean color. The Sea-viewing Wide Field-of-View Sensor (SeaWiFS) satellite tracked ocean chlorophyll, and NOAA’s Geostationary Operational Environmental Satellite-10 (GOES) satellite tracked sea surface temperatures. Data from shipboard measurements taken aboard the NOAA ship Townsend Cromwell were also used to see the efforts of Loretta in subsurface waters, since satellite observations are restricted to the ocean surface.

“Eddies naturally occur in this locale for periods of several weeks to a few months, but Loretta persisted for eight months according to satellite data,” said Robert Bidigare of the University of Hawaii. After January 2000, Loretta started to move slowly westward, and eventually weakened beyond detection, but not before bringing a tremendous amount of deep-sea nutrients to the surface of the ocean.

Eddies are usually 30-125 miles (50-200 km) in diameter, and resemble hurricanes in the water. Like hurricanes, each eddy is given a name to keep better track of it. In 1999, researchers named the eddy “Loretta.” Around the Hawaiian Islands, eddies are caused when northeasterly trade winds interact with the topography of the islands. Eddies occur most frequently in the Alenuihaha Channel off the Kona coast of the Big Island of Hawaii, where they can be observed throughout the year.

Eddies bring an increase in organisms that comprise the marine food web, attracting fish and fishers. The swirling motion of eddies cause nutrients that are normally found in colder, deeper waters to come to the surface. Normally surface waters are nutrient-limited, and when an eddy occurs, the cold water upwelling substantially increases chlorophyll and plankton production, as it did with Loretta.

The SeaWiFS satellite tracked Loretta’s movement by monitoring chlorophyll. The chlorophyll are contained in single-celled ocean plants, smaller than the size of a pinhead, called phytoplankton. Increases in phytoplankton cause higher levels of the green chlorophyll pigment, which in turn change the color of the ocean surface. Although microscopic, phytoplankton can bloom in such large numbers that they can change the color of the ocean so much that they can be measured from satellites.

By looking at the color of an area of the ocean, the concentration of phytoplankton can be estimated. Because phytoplankton changes an ocean’s color, they are ideal candidates for tracking eddies and currents, detecting pollution and observing meteorological events. SeaWiFS generated eight-day composite pictures of the ocean color that showed scientists where the nutrients and the eddy were located.

NOAA’s Coast Watch program also monitored Loretta using imagery from the GOES-10 satellite to generate three-day composites of sea surface temperatures. By watching where the colder water moved, they were able to track Loretta’s movements. According to Michael Seki of the NMFS, “Some of the strongest temperature gradients associated with Loretta occurred during late August-early September 1999. The sea surface temperature in the center of Loretta was 23.5 degrees Celsius (74.3 degrees Fahrenheit), a lot cooler than the waters outside of the eddy.”

Fishers constantly monitor eddies using satellite data because they are such large circulation features that they are difficult to see with the naked eye. When an eddy is spotted, fishers set their hooks and lines around and through these features. Because of the increase in food, eddies are known to increase the concentration of fish and thus, fishers harvest greater catches.

A paper authored by Michael Seki of the U.S. NMFS appears in the April 15, 2001 issue of Geophysical Research Letters, and details the increased plankton observed in response to eddies in the open ocean near Hawaii. The article focuses on the increased ability to measure sea surface temperature in real-time by using the GOES satellite. Co-authors of the paper include Jeffrey Polovina and Russell Brainard of NMFS, Honolulu Laboratory; Robert Bidigare and Carrie Leonard of the University of Hawaii, Department of Oceanography; and David Foley of the Joint Institute of Marine and Atmospheric Research, University of Hawaii and NOAA.

This project is collaborative research effort between the NMFS and NASA-funded projects. The study was also partially supported by the Pelagic Fisheries Research Program administered through the University of Hawaii, School of Ocean and Earth Science and Technology.

Additional information is available on the Internet at: http://www.gsfc.nasa.gov/gsfc/earth/eddy/eddy.htm. For more information, contact Michael Seki, National Marine Fisheries Service, NOAA Honolulu Laboratory, mseki@honlab.nmfs.hawaii.edu, 808/983-5393, 808/983-2902. Please mention you read about it in Innovation.