Target strength distributions of Pacific sardine schools: Model results at 500 Hz to 10 kHz

Schools of fish can cause interference for long-range active sonars. The degree of interference depends heavily on the target strengths of the schools. However, there are few measurements and limited modeling of school target strengths in the frequency ranges of these sonars. During the summers of 2009 through 2013, a comprehensive set of measurements of the characteristics of Pacific sardine schools was collected off the west coast of the United States. This data set has enabled model estimates of the target strength distributions of these schools to be made at frequencies between 500 Hz and 10 kHz. This paper describes the sardine school data, its use in the modeling, the modeling procedures, and the target strength distributions obtained. The analysis indicates that, given the rates at which high target strengths occurred at all frequencies of interest, Pacific sardine schools could be a significant source of interference for long-range active sonars operating in their vicinity.