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To ensure efficient and sustainable purse-seine fisheries, the catch process must be monitored to better understand the reactions of fish to the gear. In this study, we monitored the behaviours of herring (Clupea harengus) and mackerel (Scomber scombrus) schools during purseseine capture using a multibeam imaging sonar (Simrad MS70, 75-112 kHz) mounted on a research vessel. The fish behaviours differed between species and purse-seine sets. For both species, the acoustic volume backscattering coefficient increased as 0-80% of the seine was hauled aboard, indicating a corresponding increase in fish spatial density. This increase was significantly greater for herring than mackerel. As 0-40% of the seine was hauled aboard the fishing vessel, schools changed their spatial distribution and volume independent of seine hauling, while for some schools, depth and height decreased. The acoustic volume backscattering strength was up to 25 dB higher in the centre of the school than in the edges. The average lateral target strength was estimated for individual fish in the captured herring schools, and the effect of incident angle on the backscattering strength is considered. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM PUBLISHER]

Korneliussen, R. J. 2010. The acoustic identification of Atlantic mackerel. – ICES Journal of Marine Science, 67: 1749–1758. Calibrated, digitized data from multifrequency echosounders working simultaneously with nearly identical and overlapping acoustic beams were used to generate new, synthetic echograms which allow Atlantic mackerel (Scomber scombrus) to be identified acoustically. The raw echosounder data were processed stepwise in a modular sequence of analyses to improve categorization of the acoustic targets. The relative frequency response measured over as many as six operating frequencies, 18, 38, 70, 120, 200, and 364 kHz, was the main acoustic feature used to characterize the backscatter. Mackerel seemed to have a frequency-independent backscatter below ∼100 kHz, but significantly higher levels of backscattered energy at 200 kHz. Synthetic echograms containing targets identified acoustically as mackerel are presented and evaluated against trawl catches. Although catching fast-swimming mackerel is difficult, trawl catches from three Norwegian research vessels confirmed that the targets identified acoustically as mackerel were indeed that species. Separate experiments performed on mackerel in pens support the findings. [ABSTRACT FROM PUBLISHER]

Nesse, T. L., Hobæk, H., and Korneliussen, R. J. 2009. Measurements of acoustic-scattering spectra from the whole and parts of Atlantic mackerel. – ICES Journal of Marine Science, 66: 1169–1175.Atlantic mackerel (Scomber scombrus) are weak sound scatterers compared with fish that have swimbladders. Accurate acoustic estimates of mackerel abundance require estimates of target strength. Different parts of mackerel may dominate the backscattering spectra. Mackerel schools are acoustically recognized mainly by backscatter four times stronger at 200 kHz than at 38 kHz. Simulations have established that backscatter from only the flesh and the backbone could explain this frequency response, although there are uncertainties in the model parameters and simplifications. In this paper, experiments conducted in a laboratory tank to investigate the complexity of mackerel backscatter are discussed. Acoustic backscatter was measured over the frequency range 65–470 kHz from individual dead mackerel, and their backbones, heads, and skulls. Backscatter from the backbones was measured at several angles of incidence. Grating lobes (Bragg scattering) appeared at different angles, depending on the acoustic frequency and the spacing of the vertebrae. These lobes were evident in backbone backscatter after propagating through the flesh and can be used, in principle, to determine mackerel size acoustically. The frequency response of individual, ex situ Atlantic mackerel estimated from these measurements did not match that from the measurements of in situ mackerel schools. Further investigation is warranted. [ABSTRACT FROM PUBLISHER]

Gorska, N., Korneliussen, R. J., and Ona, E. 2007. Acoustic backscatter by schools of adult Atlantic mackerel. – ICES Journal of Marine Science, 64: 1145–1151. The extent of acoustic backscatter by schools of adult Atlantic mackerel (Scomber scombrus) is investigated to improve biomass estimates. Previous studies involving modelled scattering from individual mackerel showed that backscattering at high frequencies is dominated by the contribution from the backbone. Accurate predictions of the scattering spectra require consideration of backscattering from the entire skeleton, including details of the bone shapes and their acoustic properties. Here, the backscattering cross-sections from mackerel flesh and backbone are estimated theoretically from 18 to 364 kHz and averaged over fish size and tilt angle, then compared with in situ measurements of volume backscattering from mackerel schools. Based on the comparisons, some gross features of the observed relative frequency response are explained, and recommendations for further studies suggested. [ABSTRACT FROM PUBLISHER]