Wiebe#1 Report

### U.S. GLOBEC: Broad-scale and time-series acoustic measurements of zooplankton and nekton in the Georges Bank region.

    INVESTIGATORS:

    Peter H. Wiebe
    Biology Department
    Woods Hole Oceanographic Institution
    Woods Hole, MA 02543

    Timothy K. Stanton
    Applied Physics and Engineering
    Department Woods Hole Oceanographic Institution
    Woods Hole, MA 02543

    Charles S. Greene
    Division of Biological Sciences
    Section of Ecology & Systematics
    Corson Hall
    Cornell University
    Ithaca, NY 14853-2701

    GRANT PERIOD:  July 1, 1993 - June 30, 1995

#### STATEMENT OF OBJECTIVES:

As part of the U.S. GLOBEC NW Atlantic/Georges Bank study, we have been determining the broad-scale acoustical distribution of zooplankton and nekton populations on Georges Bank using towed acoustical systems. Our primary goal is to make high resolution volume backscattering and target strength measurements of plankton and nekton throughout the Georges Bank region in order to provide an essential base of acoustical estimates of the spatial distribution of biomass and size frequency of acoustical targets which span the size range of the target species (cod, haddock, Calanus, and Pseudocalanus) and their predators, and which can be directly related to the data generated by the net collections of plankton and nekton. The acoustical spatial maps are also intended to provide an essential linkage between the physical oceanographic conditions on the bank and the biological distributions of the species as determined from the net collections at the stations distributed throughout the Georges Bank region. Specific questions and hypotheses which we are addressing are the following:

- What are the broad-scale distributions of the volume backscattering, numbers of targets, and the frequency distribution of target strengths (as estimators of plankton biomass, numbers of individuals and individual size) as a function of depth in relation to physical variables, and how do these change from winter to summer conditions throughout the Georges Bank region?

- How are predator populations (as indicated by large acoustic scatterers) spatially and temporally distributed with respect to their prey, which includes the target species, (as indicated by the smaller acoustic scatterers) and environmental conditions?

- What is the temporal variability of biological properties as a function of depth at selected sites throughout the Georges Bank region?

Hypotheses:

- Changes in the temperatures on the bank, especially in the winter/spring period will alter the onset of reproduction (warming will bring onset on earlier; cooling will delay the onset) and affect the magnitude of the total production of the target copepod species. This will alter the development of total acoustic backscattering and lead to alterations in the size frequency distribution of the plankton through time.

- Acoustical estimates of the biomass of plankton on the Bank can be used to assess the relationship between the timing of the spring development of the target species and the physical driving forces which control the Bank's horizontal and vertical circulation and vertical stratification.

- The patterns of bank-wide estimates of acoustical biomass and size structure will provide basic evidence for evaluating the exchange of organisms on and off the Bank, especially the role of winter/spring storms, and entrainment features associated with warm-core rings.

#### STATEMENT OF WORK:

Two echo sounding systems have been providing high spatial resolution estimates of volume backscattering. One towed system was deployed during a broad-scale cruise in June 1994 and another during the first broad-scale cruise in February 1995. A hybrid system will also be deployed in July on the last of the six 1995 broad-scale cruises. These surveys of the Georges Bank region are designed to bridge the duration of the pelagic stages of the gadid ichthyoplankton. (Dec-July).

We have been making acoustic measurements along each of the tracklines between stations on two of the ship-board broad-scale survey cruises. The high frequency acoustics packages have been mounted in a down-looking mode in streamlined tow bodies capable of making high quality measurements at speeds of 6+ knots. As currently configured, the systems can only be towed at the sea surface with the transducers downlooking. The more desirable modes of deployment must await acquisition of fiber optic cable mounted on a portable deck winch along with the appropriate slip rings. For the work in this initial phase of the project, we have been using 120 kHz, and 420 kHz.

Acoustic data from the broad-scale survey cruises are processed in real time and plots of volume backscattering as a function of depth and target strength distributions are available during the cruise. Detailed analyses of the data are now occurring at both WHOI and Cornell.

Two different echosounders and two different towed bodies have been used in this work. A dual-beam 420-kHz echo sounder of special design by BioSonics, Inc. and an environmental sensor package were deployed from the R/V Albatross IV in June 1994 for continuous underway data acquisition between stations. The acoustic and environmental sensor packages were mounted in an ENDECO towed 5-foot V-fin fish. It was deployed from a J-frame to starboard off the aft quarter and towed approximately 2.5 m below the surface. The acoustic transducer was operated in a down-looking mode. The system was only recovered from the water for maintenance (battery replacement) or repair (which occurred only a couple of times).

The system used to make the acoustics measurements in February 1995 (R/V ENDEAVOR Cruise 261) was a proto-type instrument package dubbed the Bio-Optical Multifrequency Acoustical and Physical Environmental Recorder (BIOMAPER). The towbody assembly is a simple aluminum weldment consisting of the framework necessary to mount all of the required sensors, with additional space for future expansion. Mounting brackets are included for installation of the acoustic sensors in any direction: up-looking, down-looking, or side-looking. Removable, flat plastic side panels reduce drag and flow noise at high speeds. The framework of the system has dimensions of 3 m length x 1.8 m height x 0.6 m width, and weighs approximately 730 kg in air. A tail section adds another 1.5 m to the length and 0.5 m to the height. On this cruise, as part of a new digital acoustic system, there were two acoustic sensors (120 kHz and 420 kHz). In addition, there were an environmental sensing system with temperature, conductivity, fluorescence, beam transmittance, and downwelling irradiance sensors, and a video camera and lighting system. The transducers were operated in a down-looking mode. The tow-body was deployed from the port stern quarter of ENDEAVOR.

The architecture of the new digital acoustic system includes a transmit module to which transducers are connected in a "daisy chain" fashion for sequential firing. The frequencies have dual-beam target-strength-estimation and target-tracking capabilities. These capabilities will be used to derive more information for acoustically characterizing larger targets resolved in the animal assemblage, including macrozooplankton, micronekton, and fish. Unfortunately, this new system was lost midway through the February cruise at the onset of a gale. For the July cruise, we are re-configuring the v-fin towed body to accommodate the new digital echosounder for temporary use until another BIOMAPER can be constructed.

Acoustical data acquired in May 1992 as part of a two ship coordinated GLOBEC pilot study on Georges Bank, are available on line and are accessible via the U.S GLOBEC Georges Bank home page. The Georges Bank Information System (GBIS) for the NW Atlantic Study is based on the Joint Global Ocean Flux Study (JGOFS) data management software system. This software is a distributed, object-based data system, with the primary data sets residing with the responsible scientists when possible. It is built upon a standard protocol (National Center for Supercomputing Application's HyperText Transmission Protocol) and uses NCSA's HTTP Daemon (HTTPD). Electronic access and viewing of both data and information are done via World Wide Web browser software such as Mosaic and Netscape. Anyone with a computer connected to the network and the browser software can access the Program's information, documentation and data with the uniform resource locator (URL): http://globec.whoi.edu.

Data collected on the June 1994 and the February 1995 broad-scale cruises are ready for installation on the database. The size of these files coupled with the lack of disk storage space or an alternate server has prevented their being put on line; once the data storage capability of the system has been enhanced, they will be placed on line.

#### SUMMARY OF KEY FINDINGS:

Of 48 transect legs between stations during the June 1994 survey of Georges Bank, 45 were successfully completed. The combined length of trackline along which acoustical measurements were made was 1441 km (778 nautical miles).

A striking feature was the great difference in the vertical distribution of the scatterers between the shallow (well mixed) regions of the Bank and the deeper portions (stratified regions). There was generally high scattering throughout the water column in the well-mixed areas and little horizontal layering. Fine-scale vertical lineations from the surface to the bottom were often evident. Lower volume backscattering was observed in the middle portions of the southern flank especially below the surface layer which had the highest volume scattering levels. In the vicinity of the shelf/Slope Water break, volume backscattering increased and heaviest concentrations of the scatterers were at the surface. This was most extreme in the northeast peak region where highest concentrations of C. finmarchicus occurred. Much lower volume backscattering was observed in the offshore Slope Water stations south of the Bank. To the north of the Bank, the levels and distribution of volume backscattering were highly variable. In Georges Basin, moderately high levels were present even at depths of 30 to 50 m, but horizontal variation was pronounced. The dense scattering features which penetrated to depth gave the appearance of downwelling zones or convergences. In contrast, within Franklin and Wilkinson Basins, high volume scattering levels were generally restricted to the upper 15 to 20 m, and there was a very sharp transition to lower levels in the deeper depths.

Of 36 transect legs between stations on the February 1995 survey of the Bank, acoustical data were successfully obtained on 10 of them. High winds and seas prevented deployment of BIOMAPER on a number of the early transects (Stations 5-15). At Station #22, in the early morning hours of 16 February (about 0030), BIOMAPER was lost at sea and now sits on the bottom of Georges Bank (41 31.93N; 65 59.33W) in 100 to 200 m of water. We were in the process of bringing the towed body on board as part of the securing of all gear because of a gale that was just beginning to hit us. With deck hands making the lines ready, a pair of large swells passed by dipping the stern under, raising it and then dipping a second time and raising again. The fish was apparently still going down when the stern came up and the stress was too much for the cable. It parted and the system was lost. Attempts are ongoing to recover it.

A first look at the sections obtained across the Bank in February 1995 provide an interesting contrast between the Bank-wide acoustical structure observed in June 1994 during the previous broad-scale survey, and winter-time acoustical structure. On the February cruise, those portions of the Bank surveyed displayed the "well mixed" acoustical structure; there was essentially no "stratified" acoustical structure. On transects that crossed between Bank and Slope Water, there was a dramatic reduction in volume backscattering that appeared to coincide with the crossing of the Shelf/Slope Water front, and in the Slope Water, there was substantial horizontal and vertical patchiness structure. On the trackline that crossed over the head of a submarine canyon (no name), very intense patches of scattering were associated with the canyon which transitioned abruptly into the "well-mixed" structure.

In spite of the well-mixed appearance over the portions of the Bank surveyed, there were variations. In several sections, there was a drop-off of volume 420 kHz backscattering within 10 to 15 m of the bottom. In some cases, this was mirrored by the 120 kHz backscattering. This pattern did not appear to be related to diel light variations or any hydrographic anomalies.

In Great South Channel, there were large acoustical patch structures interspersed with regions that looked well mixed. In some sections, volume backscattering was substantially reduced at mid-depths (30-50 m) giving the appearance of crossing into a different water mass. Moving up onto the Bank from the southern portion of Great South Channel, a spectacular near-bottom patch was crossed which extended all the way to the surface. A highly speculative guess is that we observed a large, perhaps spawning, school of cod fish.