Acoustic Technology -- Fisheries Acoustics

## Fisheries Acoustics

Chair: Jim Traynor
Rapporteur: Gordie Swartzman

Participants: Lee Culver, Steve Brandt, Len Zedel, John Simmonds, Ken Foote, Steve Clark, Pat Twohig, Charles Thompson, Bill Michaels, Janusz Burczynski

### Scientific Context

The Fisheries Acoustics Working Group addressed how fisheries acoustics techniques can be applied to answer questions important to accomplishing GLOBEC objectives. The list below includes the questions posed and potential acoustic techniques for addressing these questions.

Does a change in ocean environment result in a change in fish distribution and migration?

Echo integration was considered the most important technique for studying fish distribution. For fish distribution studies near physical phenomena such as ocean fronts, vessel surveys must extend over sufficient distances to describe the distributional (vertical and horizontal) characteristics at an appropriate scale to characterize each environment. To facilitate scaling of echo integration information, target strength data should be collected when possible. The group felt that, in most marine situations, it is dangerous to scale echo integration results by simultaneously collected target strength data because of limitations in target strength measurements related to the stringent conditions required for target strength measurement procedures. Because the interaction between fish species and other trophic levels is going to be important to many GLOBEC questions, multiple frequency instruments are recommended. Frequencies commonly used for echo integration systems include 38, 120, 200, and 420 kHz. We recommend their use, if possible, because information about target strength for many species is available at these frequencies. Some applications which can tolerate lower accuracy in the estimation of density can benefit from the use of sonar, providing better horizontal detail of the spatial distribution of fish schools. In addition, sonars may be used to locate and track the movement of individual fish schools.

Does a change in the ocean environment affect early life history processes (e.g., larval drift, larvae distribution)?

Investigations of early life history (larvae/post larvae) require measurements of size and spatial distribution using multiple frequency systems. Ship-mounted systems can be used for studying short-term variations in distributions over larger areas. Moored systems are appropriate for studies of temporal changes in density at a few key locations over longer time periods. We recommend that multiple frequency systems be developed to assess larval and post-larval fish scatterers.

Does a change in the ocean environment result in changes in species overlap and resultant interactions (e.g., predation processes)?

When a fish predator and its prey are in the same area, the acoustic system must be able to assess a wide range of target sizes. For example, if the larval stage has a mature fish predator, the acoustic system must be capable of obtaining absolute density estimates for both groups. A multiple frequency system (either echo integration/target strength measurement system, or a multiple frequency system using an inversion technique) should provide more appropriate information. As discussed above, fish schools may be tracked through prey density fields to study local interactions (e.g., predation rates).

How is fish behavior and physiology affected by changes in environmental conditions?

Throughout the studies of fish and their relationship with their environment, information about the behavior and physiology of individual fish will be important, particularly since GLOBEC is interested in making process-oriented measurements at the level of individual organisms. Acoustic tags can be used for the study of fish depth, orientation, swimming speeds, and physiology over time. In addition, doppler sonar systems can be used to study swimming speed of both individuals and schools of fish.

How accurate are the various sampling devices (including acoustics)?

Any sampling technique may be affected by the presence of the vessel or sampling equipment. Nets are often used to obtain biological samples. Acoustic apparatus can be used to investigate the possibilities of bias when using nets. For example, sonars mounted on top of the trawl have been used to observe avoidance near the trawl. Body-mounted or vessel-mounted systems have been used to observe avoidance that occurs in front of the trawl or in reaction to the passage of the vessel. Remote, free-floating systems have been used to demonstrate avoidance of the vessel by fish, affecting both echo integration and trawl survey results.

### Status of Instrumentation for Fisheries

The working group proceeded to discuss: 1) equipment that was available off the shelf; 2) equipment that was available by modifying existing equipment; 3) new technology needs; and 4) new experimental studies necessary to accomplish GLOBEC objectives relative to fisheries acoustics.

Available Instrumentation
Item | Cost
--- | ---
| Instrument | Processing
Single frequency Echo Sounder (Integrator/Target Strength) | $20-100K | $60K
Multiple frequency Echo Sounder (Integrator/Target Strength) | $40-100K | $60K
Fisheries Sonar | $100-400K | not available
Net-mounted Sonar | $70K | not available
Acoustic Tags | $100-1000/tag $2000 (receiver) | not applicable

#### Modifications to Existing Equipment

The working group identified commercially available sonar systems as candidates for modification to make them suitable for measuring fish distribution. The signals in most systems are modified using an automatic gain control. Although this procedure facilitates and enhances the display capability of such systems, it removes the ability to reference to absolute echo intensity. It should be relatively easy to modify these sonars to allow measurement of absolute echo levels, for determining fish density estimates, providing the appropriate scaling data can be obtained. (Approximate cost: $25-100K, 1 year development time.)

#### New Technology Needs

The working group identified a critical need for algorithms and computer software to aid in processing, analyzing, and displaying acoustic data from multifrequency echo integration and sonar systems. Several multifrequency systems are currently in use in the community and more are under development. However, their ability to produce data is currently far greater than the community is able to digest. Methods and software have not yet been developed to quickly extract desired data from a group of pings or compare echo characteristics across frequency. Data storage schemes have not been standardized. The need for sophisticated and standardized data analysis, display, and archiving schemes is urgent. Without them we are in danger of being swamped by more data than we can comfortably digest and interpret. (Approximate cost: $1,500 K over 1 to 5 years.)

For some studies, the use of a number of single frequency, autonomous echo ranging devices, mounted on the bottom or looking horizontally from vertical moorings, may be appropriate to answer particular research needs. (Approximate cost: $200 K over one year for a ten transducer system.)

As mentioned above, doppler sonar may be appropriate to answer some behavioral questions. (Approximate cost: $200 K over one year.)

#### Experimental Study Needs

Fish target strength measurements as a function of tilt angle, depth, size, and behavior will be needed for fish species of interest to GLOBEC. This is an ongoing need for any program involved with the estimation of fish density using echo integration techniques. It is also imperative if investigators plan to attempt size estimation using target strength data. (Approximate funding required, $800 K over 3 to 5 years).

There is an urgent need to develop acoustic classification techniques (species and size identification) using echo signatures at single frequencies, multiple frequency echo intensity and target strength information, and other acoustic information from individual fish or fish schools. (Approximate cost: $500 K over 3 to 5 years.)

Since many of the processes that GLOBEC is interested in may occur near the bottom, attention should be paid to developing improved techniques for assessing fish near this boundary. (Approximate cost: $500 K over 3 to 5 years.)

Another need discussed was for studies of the relationship between fish larvae feeding and water turbulence. (Approximate cost: $200 K over one year.)