Holliday Report

### Acoustical Sensing of Zooplankton on Georges Bank

    INVESTIGATORS:

    D.V. Holliday
    Tracor Applied Sciences
    holliday@tracor.com

    R.E. Pieper
    University of Southern California
    pieper@usc.edu

This project involves the deployment of an instrumented mooring on Georges Bank to measure zooplankton abundance and size spectra. We anticipate that the mooring will be deployed in August 1995 at the long term mooring site. The mooring and first data set will be retrieved in late 1995 or early 1996, depending on cruise schedules. If additional funding becomes available, the mooring will be redeployed for a period depending on the funding level and duration.

Instrumentation on the mooring will consist of two sets of acoustical sensors, one to be deployed at a shallow depth (ca 20 m) and one deeper (ca 50 m). These sensors will measure and record volume backscattering at frequencies of 0.104, 0.165, 0.265, 0.420, 0.700, 1.1, 1.85 and 3.0 MHz. On half hour intervals measurements are made at four frequencies (0.104, 0.265, 0.700 and 3 MHz) in a small (0.01 cubic meter) volume at a distance of 1.25 meters from the mooring. Each hour measurements are made at all eight frequencies in the small volume and additionally are made at 0.104, 0.265 and 0.700 MHz in a larger volume (10 cubic meters) centered at 4.7 meters horizontal displacement from the mooring. Each sensor package also includes a thermistor which is sampled both on the hour and on the half hour. Ratios of the mean squared scattering strengths to the square of the mean scattering strengths are also computed and recorded. This measure provides information on the statistical nature of the scattering (many plankters in the beam, or just a few animals dominating the scattering).

The acoustical data from these sensors are converted to size-abundance estimates via an inverse algorithm (Holliday and Pieper, "Bioacoustical Oceanography at High Frequencies", ICES J. Mar. Sci., 1995 - in press). Plankton biomass (biovolume) and size-abundance distribution estimates are then compared to a variety of physical variables to examine hypotheses regarding the effects of those variables on the plankton biomass. In addition to collecting and storing the acoustical data, our Georges Bank mooring will be sampling weather data (air temperature, irradiance, wind speed and direction) every half hour. Water temperature and conductivity, measured at a depth of one meter, will also be sampled every half hour. The samples of weather, temperature and conductivity data will be transmitted every three hours via the GOES satellite. All data are recorded on the mooring in the event the GOES data link is intermittent or fails. The funding level for this project did not permit implementing quasi-real time telemetry for the acoustical data on Georges Bank.

We anticipate making the data available to the interested community in a format that can be retrieved electronically. Current plans are to provide it on a WWW page. Links to other pages in the GLOBEC community are anticipated.