Scattering properties of Atlantic mackerel over a wide frequency range

Abstract

Atlantic mackerel do not have a swimbladder, which is the dominating scatterer in most fish. Schools of Atlantic mackerel can be identified by the level of backscatter at 38, 70, 120 and 200 kHz, but these are too few points to describe the continuous (broadband) frequency spectrum of the school or to identify which components of the fish are responsible for the different characteristics of the spectrum. In this study, backscatter from different mackerel body parts was investigated in a laboratory tank in the frequency range 68–450 kHz, and tilt angles ±30° (defined as the angle between the sound axis and the normal to the fish axis). Measurements were averaged over tilt angles to simulate the results from a fish school. Our results show that scatter from the backbone is significant around 0° tilt angle, but less important when the fish is tilted. The head, including gills, appears to be an important scatterer when the fish is tilted. Segments of the mackerel body, including both flesh and backbone, give strong backscatter with wide directivity, and show an increase in the backscatter with frequency.