Acoustic Technology -- Small Zooplankton Acoustics

## Small Zooplankton Acoustics Working Group

Chair: Rick Pieper
Rapporteur: Mark Ohman

Participants: Marvin Blizard, Jack Green, Charles Flagg, Van Holliday (briefly), Mark Huntley (briefly)

### Introduction

The organisms addressed by this group were initially defined as those for which the fluid sphere acoustic scattering model is appropriate. We later extended this definition to encompass somewhat larger, euphausiid-sized organisms.

A high priority was placed on developing multiple-frequency instruments that simultaneously sense several size classes of zooplankton. This resolution of body size is important given that zooplankton growth rates and predator-prey interactions are known to vary with body size. The ability to interpret acoustic backscattering profiles, to understand behavioral differences among species, and to distinguish changes in marine ecosystem structure also require resolution of acoustic backscattering into several size classes. Acoustic validation of population and ecosystem models will also require size class resolution.

### Modes of Deployment

The mode of deployment for zooplankton acoustics instruments may vary with the GLOBEC study site. We discussed the advantages and limitations of both Eulerian and Lagrangian techniques and agreed that a system must be sufficiently flexible to be deployed in several ways: 1) from surface ships on vertically profiling instruments, or on nets; 2) on moorings; or 3) on drifters.

### Priorities for Instrumentation Development

The following subjects pertaining to the development and acquisition of instrumentation and related basic research are considered appropriate for inclusion in future "Calls for Proposals" in support of GLOBEC science objectives.

#### Priority 1

The highest priority was placed on the development of a single, standardized acoustic system to resolve multiple classes of zooplankton. Such a system must be sufficiently versatile to serve a variety of purposes and sufficiently inexpensive to allow deployment of several within one study region. The Working Group agreed that resolving ca 5 size classes of zooplankton in the size range of 1-20 mm in length would be useful for many purposes, while retaining the essential attributes of instrument portability and low cost. The processed output from the instrument should be either number of organisms per m3 or biomass of organisms per m3 in each size class. The chief characteristics of such a system would include the following:

- Capability to resolve ca. 5 size classes of zooplankton;

- Relatively simple to deploy and operate;

- Relatively inexpensive;

- Low power requirements;

- Internal signal pre-processing (in replaceable EPROM's to retain algorithm flexibility); and

- Option for either real-time transmission of data or internal storage.

The approximate size classes and a few representative crustacean organisms are provided in the table below.

Representative Organisms and Size Classes of Interest
Length (mm) | ESD (mm) | ESR (mm) | Representative Organisms
--- | --- | --- | ---
1.2 | 0.5 | 0.25 | Small copepods, Pseudocalanus, Acartia, Paracalanus, Calanus copepodites
2.5 | 1.0 | 0.5 | Adult Calanus, Metridia
5.0 | 2.0 | 1.0 | Adult Eucalanus, Neocalanus, Euchaeta, larval euphausiids
10.0 | 4.0 | 2.0 | Juvenile euphausiids, mysids, amphipods
20.0 | 8.0 | 4.0 | Adult euphausiids, mysids, amphipods

The probable end points for transducer frequencies are ca 3 MHz for the smallest size class and ca. 100 kHz for the largest size class. Final selection of the target size classes and acoustic frequencies should be done by examination of existing zooplankton size frequency distributions from different ocean basins, and in consultation with acousticians (e.g., computer modeling). The instrument should be built in modular fashion so that different transducers may be substituted; the number of frequencies may be greater than the number of desired size classes, depending on the method(s) employed and cost considerations.

Fundamental to the design of the instrument is a low profile, versatile underwater package. The package must be capable of being deployed in the following ways:

- Lagrangian drifters (at a fixed depth or in Cyclosonde mode);

- Moorings;

- CTD systems;

- Surface vessels; and

- MOCNESS/BIONESS towed net systems.

The instrument should have an internal built-in-test capability to verify the stability of the electronics and should also be calibrated periodically (e.g., annually) at a transducer calibration facility.

Flexible post-collection data processing software should be developed in conjunction with the acoustic device. This software should permit data to be aggregated in variable bin sizes; means and standard errors computed by depth, time, or scan; contouring; and plotting of vertical profiles, sections, and time series plots. The software should have "open architecture" to allow other variables (e.g., fluorescence, CTD, or thermistor data) to be processed in a similar manner and should accommodate user customization (e.g., flexible database structure).

Procurement Considerations and Timing

The development and construction of an acoustic instrument with attributes similar to those discussed above were considered to be appropriate for inclusion within future "Calls for Proposals" from the GLOBEC program. In order to have a maximal impact on GLOBEC science, development of this instrument should strive for prototype completion within 18 months, including comparisons with pump and net zooplankton samples. Potential manufacturers for the production instrument should be identified as early as possible. The manufacturer(s) should be encouraged to vigorously pursue construction and marketing. The advantage of such an instrument to a manufacturer is the establishment of a "standard" instrument with a worldwide market. Two of the operational advantages of this instrument to the scientific community are the broad base of user knowledge and the ability to compare results between study sites.

Other considerations

We recognize that gelatinous zooplankton (e.g., salps, larvaceans, medusae, ctenophores) can dominate marine zooplankton assemblages. In some cases, they do so as transients in rapid population bursts. Better methods are needed to acoustically distinguish gelatinous organisms from non gelatinous. Experimental study of target strengths and unique acoustic signatures of these organisms are needed.

We recognize the need to sample eggs, nauplii, and juvenile stages of many species of zooplankton. We recommend that acoustic methods be compared to other methods for achieving this. If sensor costs imposed by the need to offset the extreme acoustical attenuation at high frequencies substantially increase the overall system cost, then sampling these smaller organisms might best be done with optical measurements (e.g., High Definition TV) in combination with the acoustic instrument described here.

We underscore the need for collection of pump and net samples to collect "ground truth" data, (e.g., species identification) for the acoustic instrument. This will necessitate accelerated development of rapid, automated means to enumerate and identify zooplankton samples.

#### Priority 2

Swimming speed of zooplankton is a critical parameter for prey-predator encounter models. Swimming speed is also useful for understanding vertical migration behavior and may help identify the organisms in specific acoustic size classes. We recommend using doppler shift/spread from swimming zooplankton to estimate swimming speeds of organisms in situ. High frequencies (ca. 3 MHz) will likely be most useful for this purpose.

The doppler shift due to swimming zooplankton can be obtained by Fast Fourier Transform methods (among others). Changes in the average doppler shift and the doppler spectrum width of volume reverberation from a volume of water may indicate the mean and extreme swimming speeds of a group of zooplankters. Relatively straightforward modifications to the Priority 1 instrument should permit the doppler parameters to be measured, combining measurements of zooplankton abundance with measurements of zooplankton swimming speeds.

#### Priority 3

A low-cost Expendable Acoustic Profiler (EAP) could be a useful tool to aid in conducting rapid surveys. Deployment of EAP's from ships-of-opportunity, or possibly aircraft, would increase spatial and/or temporal coverage of more isolated study sites. EAP's could also enable a large-scale region to be mapped prior to conducting more detailed sampling. Although there was not unanimous agreement on the utility of this instrument, some working group members supported development of an EAP capability. EAP's might be manufactured with a variety of single frequency transducers.

### Development of Backscattering Models and Bioacoustical Instruments

The working group concentrated its efforts on the specifications and need to develop a simplified, standardized acoustic system for use by the general community of researchers. We also recognize and support the need to refine and improve acoustic backscattering models and instrument design. In conjunction with the simplified system described above, we support the continued development of more sophisticated multi-frequency acoustic systems, along with acoustic scattering model development, for advanced research and development.