Oceanographic and Ecosystem Sampling During the Pacific Hake Survey
We use sensor technology to see into the ocean.
What We Do
Understanding local ocean conditions is important because environmental conditions at sea influence fisheries health, regional weather, and global climate. Some of the tools used during the Joint U.S.-Canada Integrated Ecosystem and Pacific Hake Acoustic-Trawl Survey, known as the hake survey, include instruments to measure many different conditions contributing to the structure of the ecosystem. While some conditions are continuously measured by sensors onboard the ship, some require a more focused sampling effort. Oceanographic data collected by the Fisheries Engineering and Acoustic Technologies (FEAT) team during the hake survey are used by scientists in the Northwest Fisheries Science Center and a wide range of partners.
During the hake survey, fisheries acoustics and trawling operations take place between sunrise and sunset. After sunset, oceanographic sampling at predetermined stations continues until dawn.
On-Board Sensors
Sea surface salinity and temperature
A thermosalinograph (TSG) is a sampling system mounted on a water intake on the ship's hull. The intake brings a constant flow of seawater from approximately five meters below the surface. The TSG instruments take regular sea surface temperature measurements, salinity (translated from the conductivity cell), and fluorescence (related to the chlorophyll of phytoplankton) from the water. These data are useful in describing habitat for different fish species.
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Scientific Computer System
Commonly known as SCS, the Scientific Computer System combines a suite of oceanographic, atmospheric, and other sensors to provide information in real-time to the crew and scientists. This system is deployed on most of the ships in the NOAA fleet, including the hake survey ship. It provides regularly updated information via graphical displays and text. Simultaneously, the Scientific Computer System is performing basic quality checks on the data, flagging errors, and saving the files for later study.
Atmospheric sensors
- Air temperature
- Barometric pressure
- Wind speed and direction - for regional weather and sea state reporting
- Light levels - for comparison with light levels at depth
Oceanographic sensors
- Sea surface temperature (SST)
- Sea surface salinity (SSS) - for weather and heat budget reporting, as well as aiding in identifying ocean features like upwelling, fronts, and eddies
- Surface water fluorescence (chlorophyll) - to measure primary production
Ocean currents
Information on ocean currents gives scientists insights into the ecology of the California Current Large Marine Ecosystem. Life in the ocean is patchy, and ocean currents influence how and where animals are distributed.
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The Acoustic Doppler Current Profiler (ADCP) is an echo sounder system with four transducers, each angled at 30 degrees down into the water. Each transducer aims in one of the ship's primary directions: fore (forward), aft (back), starboard (right), and port (left). Called a Janus configuration, it is named after the ancient Roman god with two faces, each looking in opposite directions.
The idea behind the ADCP is simple. Have you ever heard the whistle on a passing train? As the train and its whistle approach, we hear the whistle as a high-pitched note. Once it has passed and the whistle moves away, we hear the whistle as a low-pitched note. The whistle has not changed, but the pitch has changed relative to our ear. This change, or Doppler shift, represents the difference in the sound frequency, the number of waves hitting your ear in a given time, that you hear as the whistle is moving towards you or away from you.
Now imagine the ship sailing along with the four ADCP transducers pinging into the water. Many particles are suspended in the water, in all directions, that are moving at the same speed as the water. Some of the energy from the ADCP's pings bounces back from these particles, and the ADCP receivers hear the echoes. Special software, called the University of Hawaii Data Acquisition System (UHDAS), was developed for the ADCP. UHDAS uses data from the ship's navigation system to subtract the ship's motion and determine the Doppler shift for those particles. We use an interface in UHDAS to process the collected information and remove any anomalies, resulting in a description of the speed and direction of the area's ocean currents.
Sampling Stations
Our survey area is between approximately Point Conception, California, and Dixon Entrance, Alaska. It is broken up into parallel lines, called transects, typically spaced ten nautical miles apart. Transects cross the continental shelf, shelf break, and upper continental slope.
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Each odd-numbered transect is designated as a primary line where oceanographic sampling stations are placed. The 20-nautical mile spacing between primary lines is a good general distance for identifying ocean features like boundaries between distinct water masses, or fronts, and circular water currents called eddies. Stations are located at the inshore (50-meter depth) and offshore ends (1500+ meters) of each primary line. Moving offshore, sampling occurs at 150-, 300-, and 500-meter depths. Beyond that, deep sampling stations are placed approximately every five nautical miles.
The ship's crew and scientists can generally conduct 5 to 7 oceanographic stations along a primary line in a night. The profiles collected show the dynamic biological and physical oceanography along the shelf break and upper continental slope. They are usable in many different ways and are critical information for understanding the factors that influence Pacific hake's biology and distribution, and other important species of fish and invertebrates, within the California Current Large Marine Ecosystem.
Measuring Conductivity, Temperature, and Depth
CTD rosette
During the hake survey, stationary water column sampling is done vertically using a winch, hydro-wire, and an instrument system called a CTD (short for Conductivity-Temperature-Depth). The CTD consists of a central core made up of various pumps and instruments. Sampling bottles, called Niskin bottles, often surround the core. These bottles can be closed remotely at specific depths by a survey technician in the ship's lab watching the data stream. Everything is contained inside a white metal cage for protection, whose color increases visibility for the winch operator. The whole system is often called a rosette or carousel.
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The CTD rosette collects a variety of environmental information. As the CTD descends from the surface down to a depth of 500 meters or 10 meters off bottom (whichever is shallower), it captures the water temperature relative to depth. It also measures how well the seawater conducts electricity or conductivity, which relates directly to the ocean's salinity or saltiness. Temperature and salinity together determine seawater density, which is a main driving force behind ocean currents.
Three additional sensors on the rosette are the dissolved oxygen sensor, fluorometer, and turbidity sensor. Dissolved oxygen is an essential parameter in evaluating water quality because oxygen is vital for the survival of fish and invertebrates like crabs and krill. The fluorometer measures fluorescence from the chloroplasts in phytoplankton and is a measure of their productivity. Ocean turbidity, or how clear or cloudy the water is, is a measure of how well light penetrates the water, an important factor in phytoplankton photosynthesis.
UCTD instrument
The Underway Conductivity Temperature Depth instrument is a "fast and furious" sampling device similar to the larger CTD detailed earlier. The instrument is deployed along the transect as the ship continues to move. This enables scientists to capture details of the water column's temperature, salinity, and density along the ship's cruise track without having to slow down surveying or navigation by stopping to get the data using a stationary CTD.
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The sensor probe is a torpedo-like device, about one meter long. A thin, tough spectra line is spooled onto the probe's tail equal to the target depth scientists want to reach. Once at the sampling site, the ship slows to a speed between 6 to 10 knots. The stern-mounted winch is set to free mode, and the probe is dropped nose-first off the ship's stern. The line paying out from the winch prevents the ship from dragging the probe while the line spooling off from the probe's tail allows it to free-fall down through the water. When enough time has passed to allow for the probe's descent, the winch rewinds the line, retrieving the probe for data download.