This is a joint project between the Centre for Marine Science and Technology (CMST) at Curtin University of Technology and CSIRO Marine Research.

Background

CSIRO Marine Research and Curtin CMST are combining to develop innovative acoustics-based techniques applied to research on zooplankton and micronekton. As part of the Western Australian Strategic Research Fund for the Marine Environment program, a major project in biophysical oceanography is being undertaken. This includes a program of work involving a suite of sophisticated ultrasonic instrumentation. CMST has extensive experience, in Antarctic and temperate waters, in the use of marine acoustics in biomass and seabed assessment. The SRFME Scholarship project is directed to developing a theoretical and interpretative understanding to support the use of the new acoustic techniques to be employed off the Western Australian coast.

Area of research endeavour

Acoustic techniques provide a means of detecting, classifying and measuring the spatial distribution of zooplankton, micronekton, and other organisms that can offer excellent spatial coverage and resolution and are independent of water turbidity. These techniques rely on transmitting short bursts of ultrasound in a narrow beam, either from a vessel, a towed body, or a transducer lowered from the vessel, and measuring the acoustic signal backscattered from the zooplankton or other targets within the beam. If ultrasound at a number of different frequencies is transmitted then the dependence of backscatter on frequency can be used to obtain additional information about the targets. To convert this acoustic data to quantitative measures such as size distributions or biomass there are two ancillary pieces of information required:

1. It is necessary to know the species that are present and their relative abundance, and

2. an accurate model of the acoustic backscatter from each of these species is required.

The first requirement is usually addressed by direct sampling, either via targeted trawls or a plankton pump, although this can necessarily only be done with relatively low spatial resolution.

The primary focus of this research is on meeting the second requirement: ie to produce accurate acoustic backscattering models for the species of interest and to use these to extract as much quantitative information as possible from the acoustic data. Note that such models would also go some way to addressing the species identification issue as different species will have different dependencies of backscatter on frequency.

There is a rich literature on the modelling and measurement of acoustic backscatter from zooplankton and other organisms for the student to build on. In particular Tim Stanton and his co-workers at Woods Hole Oceanographic Institution have published a number of papers on the subject and have developed scattering models appropriate to several different zooplankton groups including euphausids, gastropods, and siphonophores (Stanton, Chu et al. 1998) One species which has attracted particular attention because of its critical importance to the Antarctic food chain is is Euphausia Superba (Antarctic Krill). One of the proposed supervisors of this project (Alec Duncan) has carried out numerical modelling of acoustic backscatter from E. superba as a function of animal size, shape and orientation using a distorted fluid cylinder model and obtained excellent agreement with experimental results (Duncan and Pauly, in preparation). A useful discussion of the practical aspects of applying acoustic models to acoustic field data is given in Holliday and Pieper (1995)

Aims

1. To develop acoustic backscatter models for zooplankton and micronekton species present in Western Australian coastal and oceanic waters.
2. To develop techniques for extracting quantitative information about zooplankton and micronekton abundance and size distribution from acoustic data recorded using the Tracor Acoustic Profiling System (TAPS) and/or the Simrad Underway Macrozooplankton/Micronekton Acoustic Survey System (MMASS) and ground-truth against plankton pump and/or target trawl data.
3. Develop techniques that allow acoustic data to be used to distinguish between species and/or estimate relative abundance.
4. Extend the techniques to epi-benthic species of interest such as sea grass and kelp.

1. Acoustic backscatter models for a range of zooplankton species of interest.
2. An improved understanding of the frequency dependent backscatter of zooplankton and micronekton and its dependence on parameters such as size, shape and orientation.
3. A significantly improved ability to provide quantitative information about zooplankton and micronekton biomass and size distribution from acoustic data.
4. Techniques for the analysis of acoustic data applicable to the quantitative assessment of other species of interest.

The use of acoustic techniques for micronekton and zooplankton abundance assessment and characterisation allows significant savings in logistical effort and improvements in spatial coverage and accuracy. By providing an improved understanding of the physics of acoustic backscatter this project will allow quantitative information to be obtained from acoustic surveys that would otherwise not be available and will reduce the dependence on ground truthing by conventional direct sampling methods, leading to reduced survey time and cost. This will in turn address the following strategic requirements:

• large scale sampling of biotic assemblages,
• cost effective methods of detecting changes in biological communities,
• long term monitoring of biological communities, and
• the provision of environmental input data to integrated bio-oceanographic models.

The most significant scientific problems/questions to be addressed by this research are:

• What are the dominant mechanisms of acoustic scattering for each species of interest?
• What are the values of the required acoustic parameters?
• How can the acoustic backscatter measurements made in the field be related back to parameters of biological interest?

• Review of existing literature.
• Identification of species of interest and determination/estimation of their dominant acoustic characteristics and parameters. The latter would be achieved by a combination of experiment and the use of information already available in the literature.
• Development of appropriate numerical backscatter models.
• Validation of models against field and/or laboratory experimental data and/or data reported in the literature.
• Development of techniques for determining quantitative information and classifying species based on acoustic backscatter data.
• Validation of techniques against field data.
• Extension of models/techniques to epibenthic species.

Professor John Penrose, CMST, Curtin University of Technology,
Mr. Alec Duncan, CMST, Curtin University of Technology, and
Dr. Tony Koslow, CSIRO Marine Research.