Acoustic Hake Survey Methods on the West Coast
"Seeing" the ocean through sound during the Joint U.S.-Canada Integrated Ecosystem and Pacific Hake Acoustic-Trawl Survey.
Acoustic Sampling Theory
Acoustic fisheries surveys use sound to estimate the abundance of fish in a particular area of the ocean. Hydroacoustics is similar to echolocation used by marine animals like dolphins. Boats that have specialized SONAR equipment can 'see' into the water beneath them.
The Joint U.S.-Canada Integrated Ecosystem and Pacific Hake Acoustic-Trawl Survey, known as the hake survey, uses echo sounders that transmit and receive sound waves. The transmitter emits short pulses of sound, also called pings, into the water. When the sound wave hits an object like a school of fish, the sound bounces off, or scatters, in many directions. Some of that sound gets reflected to the echo sounder, called an echo or backscatter.
The strength or loudness of the echo is affected by how strongly different ocean elements reflect sound. The seafloor usually makes the strongest echo because it is composed of rocks and other hard materials. Fish, especially fish with gas-filled swim bladders, also create strong echoes. Perhaps surprisingly, the swim bladder's air reflects sound more strongly than fish bones because the density of air is so much different from water. This is the same for bubbles in the water, which can also create strong echoes.
Each fish species has a unique target strength or amount of sound reflected to the receiver. The size, shape, and anatomy of the fish influence the target strength. Knowledge of different fish species' target strengths allows scientists to use the echoes from fish to identify and count them.
A 2-D display of returned echoes is called an echogram. This picture shows the strength of the returned sound versus depth. Scientists from the Fisheries Engineering and Acoustic Technologies (FEAT) team mark areas, called regions that are thought to contain organism(s) of interest like fish schools and zooplankton swarms, also called targets. These areas are then verified, or ground-truthed, using a fishing net to collect a sample of the targets from the water. Scientists then use the information from the fishing sample to identify the species of fish or zooplankton and other details about the animals, like size, sex, and age.
Fish are usually too close together to count individually in the echogram, so we use echo integration to estimate the total number of fish. We combine ground-truthed regions. We use the species-specific target strength of fish in the sampled aggregation to partition the backscatter and estimate the total number of fish in the survey.
For our survey, we focus on Pacific hake (hake, Pacific whiting, Merluccius productus). Our survey is considered a fishery-independent survey because we collect data separately from the commercial fishery for hake. Our estimate of hake biomass is then used in combination with fishery-dependent sources for the stock assessment component of the U.S.-Canada International Hake/Whiting Treaty process.
Evolution of Transducer Technology
In the early 1990s, the Simrad EK500 scientific echo sounder was the standard echo sounder for acoustic fisheries surveys throughout the world. We used it until our 2005 hake survey. In the early 2000s, the Simrad EK60 scientific echo sounder started to replace the EK500 as the next-generation echo sounder. We began using this new system in 2005 and continued using it through the 2019 hake survey. The more advanced EK60 system allowed us to maintain a high-quality time series for abundance and biomass estimates of Pacific hake.
Both the EK500 and EK60 were narrow-band and multi-frequency echo sounder systems. Recent technological advances in both hardware and software have allowed for the development of a broadband echo sounder system called the Simrad EK80. The EK80 system can operate in both continuous wave (CW, or narrow-band) mode and frequency-modulated (FM) mode, providing information on acoustic signatures of marine targets over a much broader and continuous frequency band. We will begin using the EK80 system for the 2021 hake survey.
Echo Sounder Calibration
Calibrating echo sounders is very important to ensure that the data we collect are precise and accurate. We typically conduct calibration of the acoustic system at the start and end of the survey. Ideally, this takes place in protected waters that are deeper than 50 meters and where the water column is mostly empty of fish and other marine life so that the data are not contaminated with extra noise.
Calibration measures the accuracy and precision of the echo sounder system using a standard target with a known target strength to develop parameters that adjust for any differences. Currently, we use a tungsten carbide sphere as the standard target. It is suspended below the ship's hull using downriggers and monofilament lines. These downriggers are positioned in three locations along the ship's railings, with one downrigger in line with the echo sounder and the other two on the opposite side of the ship, thereby creating a triangle that suspends the sphere in the center of the echo sounder's sound beam.
- For more information on calibrations, see the ICES Cooperative Research Report, Chapter 2: Calibration of acoustic instruments (PDF, 136 pages).
- The Food and Agriculture Organization of the United Nations (FAO) has a more in-depth manual for using acoustics in fisheries.
- Discovery of Sound in the Sea