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Marine Acoustic Surveys

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Jennifer Emery

on 5 October 2015

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Transcript of Marine Acoustic Surveys

Marine Acoustic Surveys
Jennifer Emery
BAT Technology Profile Report

What is an Acoustic Survey?
Krill populations form large swarm-like aggregates, making population size assessments difficult
In general, marine species are difficult to quantify as they are not easily accessible.
Acoustic Surveys for Assessing Krill
Delineating Krill
Using Acoustic Data to Create Abundance and Distribution Maps
After processing the acoustic signatures found within the survey area, data can be extrapolated to create distribution and abundance maps along the survey grid, identifying areas of high or low density.
Acoustics Surveys of Antarctic Krill for Ecosystem Monitoring
Krill is an indicator species
Krill is a vital component of the Antarctic Food Web. Without it, the health of the entire Antarctic Ecosystem is at risk
As shown on the image to the right, many species rely on krill as a staple in their diet (and this is a very simplified food web)
Obstacles of Acoustic Surveys
Calibration Issues
A Tungsten Carbide sphere with a known target strength is lowered below the hull-mounted transducer in order to calibrate the echosounder.
The data for the sphere must be recorded and analyzed to make necessary adjustments to the echosounder settings (as seen on the left with my colleague and I)
Calibrating is imperative to an acoustic survey.
When sea conditions are unfavorable, calibrations cannot be conducted as this procedure must take place under the calmest sea conditions.
Rough conditions cause the ship to move and the sphere does not remain positioned within the acoustic beam.
NOAA Discusses Latest Acoustic Ship Technology
References
Acoustic surveys employ the use of sound to identify the biomass of marine species in a given area.
Echosounders transmit noise and receive the echo that reflects off the marine organisms or seafloor
The returned echos are called backscatter
The target strength, of each "ping" received from the echo helps determine which species has been acoustically sampled.
The amount of time it takes to receive the echo determines the depth of the received backscatter.
Acoustical data analysis software records an acoustic signature for all organisms within the beam width.
Processing data for a specific species incorporates algorithms that eliminate the noise and non-target species.
Images below depict data before and after the application of the algorithm
The map above indicates the abundance of krill around the South Shetland Islands and King George Island, Antarctica.
Threats to the Antarctic Krill Population
Many countries, including the U.S., have established krill fishing fleets
The fishing zones are located within the foraging grounds of many bird and mammal species that depend on krill for sustainability.
The latest fishing technology employs a pump that can run for days, feeding millions of metric tons of krill directly to the fishing vessel without deploying the trawl nets (as seen in the image to the left). It is extremely efficient at hauling the most krill in the least amount of time.
Electrical noise on board ships can create "cross talk" that interferes with the acoustic signal from the echsounder (as seen on the right). The cross talk creates backscattering that will cause over-estimation of biomass if not eliminated from the data.
Additionally, in order to apply the appropriate algorithm to acoustically sample a specific species, it's range of target strength values must be known. If not, it becomes guesswork. Often, net sampling along an acoustic survey transect is required to confirm species composition. Then it could be assumed that the target strengths acquired were consistent with the species from the trawl.
Roger Hewitt, the director of fisheries resource division of NOAA/SWFSC explains how the new ship, the R/V Oscar Dyson was designed as a "quiet ship" for acoustic survey.
Importance of Krill Acoustic Survey Data
The Commision for Conservation of Antarctic Marine Living Resources (CCAMLR) is a multi-nation organization whose main goal is preserving Antarctic Species
Each CCAMLR subsidiary conducts an acoustic krill survey each year. The biomass estimates for krill resulting from these surveys are used to set catch limits for the krill fisheries.
CCAMLR has recently attempted to create an Antarctic Nature Reserve to no avail. The lack of one nation's vote has impeded the progress and the proposal has been tabled. BBC reports on the lates CCAMLR proposal at http://www.bbc.co.uk/news/science-environment-24551855.

Why are Acoustic Surveys Important to me?
My adoration for oceanographic studies goes way back. In 2000, I was hired by NOAA to be part of the Antarctic Ecosystem Research Division. At the time, I was very much a "virtual biologist" because I used technology to study zooplankton. I used sonardata to identify location and abundance, without any consequence to the zooplankton itself. Each year I spent 2 to 3 months on the ship to the left, the Russian R/V Yuhzmorgeologiya, running acoustic transects around the South Shetland Islands, an archipelago just off the Antarctica Peninsula.
On the right, you can see a much younger me and my former boss, Roger Hewitt (now a director at NOAA) deep in thought while contemplating data analyses. The equipment has come a long way since my days in the field and so has the analysis software packages. In a perfect world, I will get my feet wet in the acoustic world again. Until then, let me tell you about the very useful tool of acoustic surveying, especially for Antarctic krill.
(courtesy of NOAA/SWFSC/AERD)
(Hewitt et al, 2003; Lawson et al. 2012); Watkins & Brierly, 2002)
(courtesy of NOAA/SWFSC)
(Watkins and Brierly, 2002)
(Hewitt et al, 2003; Warren & Denery 2010)
(Kawaguch et al., 2006; Nicol et al., 2012)
(Kasatkina et al., 2004; Watkins & Brierly, 2002)
(Croll & Tershey, 1998)
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