Chris Jones, Senior Engineer         Ocean Acoustics Department         Applied Physics Laboratory         University of Washington      Office of Naval Research	THE PROBLEM Fish aggregation is important during spawning, predation, and feeding, yet the quantitative analyses of gregarious movements and behaviors remain relatively rare. Measurements of fish aggregations are often difficult, particularly in pelagic environments. In terms of the forward sonar problem, widespread fish aggregations will appear as interfering reverberation, while schools and shoals may appear as false targets or may screen other targets, degrading sonar system performance. OBJECTIVES Observations of large spatial scales in oceanic ecosystems can be made using low- to mid-frequency sonar with the aid of the ocean waveguide to extend the range of backscatter imagery. Such observations use sonar images in horizontal coordinates, in contrast to the more conventional use of high-frequency down-looking sonars for echo integration. Synoptic two-dimensional horizontal images of the ocean can be created by beamforming a towed or fixed horizontal array, as opposed to piecing together vertical profiles of the water column obtained from a moving platform. While high-frequency side-looking sonars have been used to a limited degree for biomass estimates at short horizontal ranges [Smith:1970, Thomas:1987, Thorne:1998, Pedersen:1999], lower frequencies offer the potential of longer range imaging owing to lower attenuation and reduction of spreading loss in the waveguide sound channel. A recent example of this approach is the use of a 400-Hz towed array to observe the formation processes of large fish shoals at ranges up to 15 km during spawning [Makris:2009, Makris:2006]. We developed and deployed a new type of mid-frequency multi-beam sonar that is capable of imaging fish at long ranges in shallow water environments, exploiting the affects of waveguide propagation, while being physically smaller and easier to deploy than existing towed array systems. Conceptually, the system is designed to observe spatial scales on the order of several meters at ranges up to several kilometers, depending on the water depth. This scale is smaller than images created with large towed line-array systems, but larger than the horizontal scales possible with traditional fisheries echo-sounder systems. The system is also capable of being deployed in a fixed location (unlike moving ship systems), enabling observations of fish aggregation behavior as a function of time from a single location. This is useful for observing a broad range of temporal scales. Novel Acoustic Techniques for Assessing Fish Schooling in the Context of an Operational Ocean Observatory Full Report While images may offer immediate insight into fish behavior, they should also provide quantitative biomass information, given a means of compensating for the effects of propagation. As range increases, waveguide boundary reverberation and propagation effects accumulate and can bias the quantitative analysis of imagery. Inverting waveguide backscatter for estimates of fish density, for example, requires a model of the multipath structure and the resultant transmission loss and pulse spreading. So in addition to developing new sonar hardware and imaging methods, numerical models have been developed to simulate waveguide backscatter. The inherent complexity of waveguide propagation, the complex spatial and temporal behavior of fish, combined with the difficulty of making ground-truth measurements over large areas of the ocean, necessitate the use of models to interpret field data. Notes Makris, N., "Fish population behavior revealed by instantaneous continental shelf-scale imaging," Science, 311, 660-663, 2006. Makris, N., "Critical popluation density triggers rapid formation of vast oceanic fish shoals," Science 323, 1734-1737, 2009. Pedersen, B., and M. Trevorrow, "Continuous monitoring of fish in a shallow channel using a fixed horizontal sonar," J. Acoust. Soc. Am., 105, 3126-3135, 1999. Smith, P., "The horizontal dimension and abundance of sih schools in the upper mixed layer as measured by sonar," Proc., Intl. Symp. Biol. Sound Scatering, 00, 563-591, 1979. Thomas, G., and D. Jackson, "Acoustic measurements of fish schools using array phase information," Can. J. Fish. Aquatic Sci., 44, 1544-1550, 1987. Thorne, R.E., "Review: Experiences with shallow water acoustics," Fish. Res., 35, 137-141, 1998.