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More than 50% of fish species are known to be gregarious at some point in their life-cycle, forming shoals (groups that remain together for social reasons) or schools (synchronized and polarized swimming groups). These groups are thought primarily to serve an anti-predator function by allowing fish to avoid predators. Shoaling may also enhance foraging by allowing fish to locate patchy food faster, reducing time needed for predator vigilance. Another consequence of fish aggregation is a change in strategy for predators relying on these fish.
Understanding of fish aggregation is critical for making accurate stock assessments as well as understanding the stability of these fisheries. Aggregations of fish affect their measurable acoustic properties. Individual fish cannot be detected, making assessment of the target strength, or ‘acoustic size’, difficult. Without knowledge of the target characteristics of the individuals in the school, assessment of numerical density and fish abundance using acoustic techniques is not possible. However, school characteristics can lead to acoustically distinguishable properties not available from individual fish. Experiments designed to look for geological sources of acoustic clutter have instead found that shoals or schools of fish are the dominant source of clutter in the Mid-Atlantic Bight. This has direct applied implications for applied Naval sonars.
Given this, this project is focused on:
- Develop new acoustic techniques to measure aggregations of fish taking advantage of high frequency multibeam sonar from AUVs and compare to mid-frequency multibeam sonar,
- Relate mid-frequency acoustic bistatic and back scattering to high-frequency multibeam and split-beam backscatter and both to fish activity
- Relate fish causing acoustic scattering to physical and biological oceanography
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 120KHz Echosounder transect in the Mid-Atlantic Bight illustrating the physical (sea floor, pycnoclines, frontal boundaries) and biological (fish and human ships) acoustic clutter that will be characterized by this proposed program.
 Illustrations of potential horizontal and vertical orientation of collacated splitbeam and multibeam sonar for water column scattering. The beam patterns and dimensions are not to scale.
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