NOAA Oceanographic Expedition Studying Physical and Biological Characteristics of Fisheries Habitat in the Mariana Archipelago

March 18, 2010

Oceanographers from the NOAA Pacific Islands Fisheries Science Center have embarked on a month-long research expedition to study the physical and biological environment of economically important fish in the Mariana Archipelago. The study is designed to expand knowledge of the local marine ecosystem and the effects of environmental characteristics and processes on commercially important fish stocks. An understanding of such relationships is vital to maintaining sustainable fisheries.

A trawl net will be deployed from the <em>Oscar Elton 
        Sette</em> to collect samples of small organisms such as fish, squid, and shrimp which are the dominant prey 
        species for larger fish targeted by local fisheries.
A trawl net will be deployed from the Oscar Elton Sette to collect samples of small organisms such as fish, squid, and shrimp which are the dominant prey species for larger fish targeted by local fisheries.

Led by Chief Scientist Dr. RĂ©ka Domokos, a team of researchers will conduct an array of studies from the NOAA Ship Oscar Elton Sette focusing on two adjacent areas, one north of 12° N latitude and the other south of this latitude. The regions differ in their oceanographic characteristics.

Scientists will conduct studies along an 840 nautical mile north-south survey transect that cuts through the distinct oceanographic regions (see cruise track below). During the survey, they will continuously monitor the directions and magnitudes of ocean currents and the distribution, biomass, and movement of marine organisms in the water column beneath the ship. They will also pause periodically to measure other physical and biological characteristics of the environment at distinct "stations" located at intervals along the transect line.

To measure the abundance and distribution of marine organisms in the water column, scientists will use a bioacoustic instrument that emits pulses of sound downward from the ship's hull. The instrument also records echoes reflected back to the ship, created as the sound pulses encounter organisms in the water. By studying the echoes, or "backscatter", the researchers can identify the kind of organisms present and their size and biomass density. Analysis of backscatter data reveals patterns of distribution and movement of the organisms. The organisms encountered include small fish, squid, and shrimp which make up the so-called "acoustic scattering layers" and are important food for larger fish that support the local fishery.

To improve the accuracy of organism identification based on the acoustic system, scientists will collect samples of organisms from the scattering layers using a large trawl net towed from the ship. Most organisms occupying the deep scattering layer, between depths of 500-900 meters, migrate toward the surface during the night into the upper 200-250 meters. Accordingly, net samples will be taken from the shallower scattering layer during night operations and from both the shallower and deeper layers during the daytime. Scientists will examine the samples to determine what kinds of organism are present and associate these findings with the acoustic data to improve the accuracy and effectiveness of acoustic monitoring.

Scientists will use this CTD instrument to measure 
        temperature, salinity, and oxygen content of the water column and to collect sea water samples for analysis of 
        chlorophyll and nutrient concentrations.
Scientists will use this CTD instrument to measure temperature, salinity, and oxygen content of the water column and to collect sea water samples for analysis of chlorophyll and nutrient concentrations.

In addition to acoustics surveys and trawl sampling, the team of researchers will collect a variety of oceanographic data to characterize the ocean environment and its variability and structure in the region, including observations of temperature, salinity, dissolved oxygen, and chloropigment concentrations. These properties will be measured from the sea surface down to 1000 meter depth using a "conductivity, temperature, and depth" (CTD) instrument. Sea water samples collected at designated depths by the "rosette" attached to the CTD will be analyzed to determine the concentrations of chlorophyll and nutrients, information that will help measure the biological productivity of the local waters. The speed and direction of the ocean currents will also be monitored down to depths of about 700-800 meters using an "acoustic Doppler current profiler" (ADCP) on the ship. All of the oceanographic information will be analyzed along with the acoustic backscatter data and trawl data to assess the effects of environmental features on the distribution, abundance and composition of organisms in the scattering layers.

Planned cruise track of the <em>Sette</em> oceanographic 
        expedition in the Mariana Archipelago. Thick line indicates main survey transect. Boxes show where CTD and 
        trawling operations will be conducted.  The cruise will begin and end in Apra Harbor, Guam.
Planned cruise track of the Sette oceanographic expedition in the Mariana Archipelago. Thick line indicates main survey transect. Boxes show where CTD and trawling operations will be conducted. The cruise will begin and end in Apra Harbor, Guam.