Increasing the biomass estimation accuracy of a single fish school using a cylindrical multi-beam fishery sonar
Not peer reviewed
MetadataShow full item record
Purse-seining is regarded as one of the most effective methods for capturing migrating pelagic schools (Ben-Yami, 1994; Watson et al., 2006), where the school is encircled and entrapped by the net, pumped aboard into the cargo hold, then delivered to the coast for processing. For avoiding increased mortality during slipping of unwanted catch (Lockwood et al., 1983; Huse & Vold, 2010; Tenningen et al., 2012), the fishermen need reliable information on the school’s total biomass, density and species before shooting the purse-seine. Migrating pelagic fish schools, such as the Atlantic mackerel (Scomber scombrus) and the Norwegian spring spawning herring (Clupea harengus L.), often swim near the sea surface. Therefore, the vertical echosounder may perform poorly if much of the population is within this near-surface blind zone, or if the fish move to avoid the vessel (Misund, 1993b; Ona et al., 2007b; Hjellvik et al., 2008; Totland et al., 2009). Then, calibrated, horizontally-observing tools are needed if this portion of the population is to be quantified.
Schooling fish can be detected and observed remotely using multi-beam sonars, such as the Kongsberg Maritime Simrad SX90 (Simrad, 2007) or the Furuno FSV-30 used in Nishimori et al. (2009). In standard operation, the sonar transmits a conical acoustic beam through the entire water volume around the fishing vessel (Brehmer et al., 2006). During reception, 64 acoustical beams are formed through array processing techniques applied to the transducer element outputs (Blomberg et al., 2012). Calibration of the accessible beams is a necessity if an accurate quantitative measures are required (Aglen, 1994). A precise calibration rig was designed to move the target with adequate control to map a single beam in detail (Paper 1). The initial rig was unsatisfactory as its size and weight limited its capacity to calibrate multiple sonar beams. A second rig was therefore designed for swifter movement of the target through multiple beams from each rig-mounting location. Within-beam target-tracking algorithms were then a prerequisite. The theory behind realizations of both a Split-Beam algorithm and an Interpolated Neighbouring Beam algorithm was described for the cylindrical transducer array with 256 circular elements (Paper 2) where the Split-Beam algorithm facilitated target positioning with precision between ±0.2 and ±0.25º. In a practical field calibration, the reference target was steered to the centre of each accessible beam, or to cross the beam horizontally and vertically, guided by the Split-Beam positioning (Paper 3). Multiple calibration trials have shown accuracy around ±0.5 dB is to be expected in what is regarded as a typical field calibration environment. Still, this accuracy was found to be susceptible to rapid but small variations of both the salinity and temperature of the stratified water, where a 0.8 dB drop of the measured sphere target strength has been seen. A further improvement of the calibration accuracy does not, however, contribute any significance reduction of the total uncertainty when finally computing the school’s biomass.
The volume of a single school is estimated by evaluating its three measured extensions, the length, width and height. The two across-beams extensions, width and height, were seen to be overestimated due to the border effects created by the finite beam width, an effect also reported for echosounders (Diner, 2001). In paper 4, this effect was investigated on several simulated schools of known sizes, where two across-beam smearing effects were identified as the Long Range Smearing and Short Range Smearing effects. Correction of both effects increased the accuracy, giving precisions for the volume estimate between 6.6-8.7 % for the width and 8.5-10.5% for the height. The mean estimated volume of a real school of herring was reduced by 55% by correcting for the smearing effects.
When converting the received acoustic energy into a quantitative biomass measure, the backscattered is divided by a mean backscattering cross-section representative of the species and individual-fish size. For horizontal acoustic transmissions, finding a representative backscattering cross-section is complicated since the cross-section is not only dependent on the distribution of the pitch and roll angles (Nakken & Olsen, 1977), the depth (Ona, 2003) or length of fish (Foote, 1980b), but also the yaw angle (Cutter & Demer, 2007). Circumnavigating the school is proposed as a means of increasing the accuracy of the volume density, where only school data from favourable incidence angles are used, for example close to the broadside of the school. If both low-frequency and a high-frequency fishery sonar are available, a comparison of the frequency response may give an indication of the actual angle of incidence. Such Dual-Frequency analysis may contribute to a more accurate volume density in situations where a full circumnavigation is not possible.
This synthesis represents only part of the total work conducted in one of the working package in the CRISP project, intended to provide the skipper accurate and reliable information on the school biomass during the last stages of an inspection.