Keeping fisheries sustainable with groundbreaking new acoustic technology

With fish and other marine life becoming increasingly important for feeding our growing human population, science-based fisheries management is crucial to keep stocks sustainable. How can acoustic-based scientific instruments contribute in this respect, while also opening up opportunities for interdisciplinary scientific research?

To keep stocks sustainable, fishery scientists and managers need to understand the dynamics and structure of fish stocks, as well as the resources those fish depend on. What scientific instrumentation can be used in this respect? Echosounders can help us quantify the biomass and behavior of fish, as well as the plankton and/or krill that many fish species eat. Acoustic Doppler Current Profilers (ADCPs), on the other hand, provide information about currents, which can shape the availability of plankton.

“It used to be that the biological oceanographers could work in one place, while the physical oceanographers worked separately somewhere else,” says David Velasco, lead author of a paper on a combined ADCP and biological echosounder system called Signature100 presented at the Oceans’18 MTS/IEEE Kobe/Techno-Ocean 2018 conference.

With fisheries management beginning to take on a more holistic ecosystem approach and funding becoming more limited, collaborations between physical and biological scientists are coming to fruition more and more. Scientific instrumentation therefore needs to follow suit to match these changing needs.

Providing the tools for collaborations between physical and biological scientists

The Signature100 lends itself well to collaborations between physical and biological scientists, since it uniquely combines the capabilities of an ADCP and a biological echosounder in one. It is currently the only instrument in the world providing this combined capacity.

“The biggest advantage of combining an ADCP and a single-beam wide-band echosounder as on the Signature100 comes down to costs and logistics,” says David Velasco. “Currently, scientists wanting to study both physical and biological aspects need to purchase two separate pieces of equipment, each deployed separately. With the Signature100, however, instead of having to deal with two moorings and two separate installations, you just have one,” he adds.

Echogram Large Reflector
A denser concentration of plankton and/or fish can be observed as strong reflectors in the center of the image. Internal wave structures can also be observed towards the right of the image, near the bottom.

Since the samples from the ADCP and echosounder are already accurately synchronized with each other in the one instrument, processing the data afterwards also becomes much more efficient.

As an added touch for user flexibility, the Signature100 records the raw “demodulated acoustic return” signal. “This gives users the ability to process the signal using different algorithms, and to pick out different information from the signal that maybe our processing doesn’t reveal,” Velasco says.

Providing information on the movement of marine life

As reported in the IEEE paper, sea trials of the Signature100 in the Mediterranean Sea have demonstrated the high performance of this novel instrument. Focusing on the echosounder performance, a 70 kHz pulse (one of the three frequencies available in the system) was able to provide information on the movement of marine life.

Acoustically, a single fish can look very similar to a school of plankton of the same volume. For users to determine whether they are looking at one fish or a mass of plankton, and assess how big the individuals are, it is important they calibrate their echosounder for absolute backscatter. For the purposes of this initial field test, the Signature100’s echosounder was not calibrated for absolute backscatter, but from the movement patterns seen in the echograms the team is confident they detected the migration of plankton up and down the water column. Nortek is currently developing a way for users to calibrate the echosounders to enhance the instrument’s identification potential.

Echogram Plankton Diving
This echogram from November 11, 2017, hasn’t picked up a diver, but a school of plankton or fish! The diel pattern of plankton migration can be clearly seen in this echogram. Starting in the dawn hours of November 11, 2017, migration starts downward through the water column, eventually settling at about 100 m range above the instrument (about 300 m water depth). The pattern reverses as night approaches, and two distinct populations can be observed: one closer to the bottom and another starting upwards from about mid-depth.

Identifying internal waves in the ocean

For those more interested in physical oceanography, the Signature100’s echosounder beam used in the field validation in the Mediterranean Sea also identified internal waves towards the bottom half of the water column. Meanwhile, setting the ADCP to transmit 60 pings at 0.25 Hz on a 5-minute repeating sequence, and with a profile of sixty 10 m depth cells, the tested instrument was able to profile currents through a maximum usable range of up to 420 m.

Enabling long-term deployments for deeper scientific understanding

Although the field test lasted only five days, thanks to the remarkably low power consumption of the unit, in other deployments the Signature100 can collect data for up to a year. The data retrieved from such a long-term deployment can help scientists understand the seasonal dynamics of an area and play a role in understanding the longer-term impacts of climate change on the ocean and implications for fisheries management.

Echogram Passing Ship 3
Echogram showing the passage of a surface vessel. The transient noise reaches almost the entire profiling range of the instrument, with increasing attenuation with depth, an inverted arch pattern typical of a single strong reflector, followed by increased near-surface noise in the vessel’s wake. The echogram also suggests the vessel had an active acoustic source on board, as impulsive noise features are clearly visible in the leading and trailing edges of the main signal.

Echogram And Current Data
The 70 kHz echogram (top), vertical velocity (middle), and horizontal current speed (bottom) for the first two days of the Signature100’s deployment. The black patches in the vertical velocity and horizontal current speed panels represent areas of low acoustic signal strength during daytime, when the plankton has migrated deep down the water column. Although no trawling was conducted during the deployment to ground-truth the nature of the scatterers, it is reasonable to assume that the features shown represent the migration of either plankton or krill (or both). The diel nature of the movement drives measurable vertical currents of approximately 5 cm/s upward during dusk hours, with the reverse pattern at dawn. (Diel vertical migration is the mass movement of plankton up and down within the ocean, primarily in response to sunlight. As the sun goes down, the plankton migrate upwards towards the surface, and as the sun rises, they migrate downward into the deep.)

Instruments in use

  • Current profiler


    Long-range current profiler designed for combined current profile and biomass measurements
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