3.2 Materials and methods
5.2.1 Multibeam equipment and data description
In 1997 Simrad introduced the SM2000 MBE, that enabled the acquisition of digital data for the entire acoustic swath observation range. Its primary purpose was the detection of divers for harbour defence. Up until this time many other multi-beam systems treated water column observations as noise and essentially ignored them, or were only capable of recording acoustic observations as video images. The digital recording of acoustic water column observations is vital for the quantitative assessment of pelagic organisms. The Simrad SM20 MBE is an upgraded SM2000 with a total swath width of 120o, comprised of 128 receive beams, spaced at 0.96o, from an 80-element transmit array, each with a 1.5o across track and 20oalong track beam width. Assuming a flat seabed, the maximum swath width is approximately 3.5 times the water depth. An orthogonally-mounted external transmit, or profiling, transducer was used to reduce the along-track beam width from 20o to 1.5o, which improved the precision of locating targets in the water column and reduced between-ping along track volume overlap. The ping rate was between 1 ping every 1.5 to 3 s, TVG correction was set to 20logr and the transmission power was ‘medium’.
The MBE was housed in a blister fairing, mounted on a rotating frame, which when deployed positioned the SM20 head along the centre line of R/V Roald, with the centre beam of the MBE positioned vertically downwards, giving a 60o swath either side of the boat, perpendicular to the transect (Figure 5.5(b)). This MBE orientation was selected to fulfil the dual requirements of simultaneously observing seabed bathymetry and water column targets. Also, the blister fairing mounting angle was constrained by the design of the rotating frame.
The MBE observations were logged continuously to the SM20 control computer. Two formats of MBE data were recorded: detected bathymetry profiles of the seabed were recorded using Triton ISIS v7.0 (Triton Imaging, Inc.), and acoustic returns throughout the observation range (200 m) were recorded using the Simrad SM20 control software. Water column data were converted to the SM2000 data format using a Simrad utility (MsToSm v1.0) and processed using Echoview v3.50 (SonarData, Hobart, Tasmania). Krill swarms were identified using the proprietary SonarData 3D school detection algo- rithm, and krill swarm descriptive metrics extracted. The sensitivity of the extracted swarm metrics to the selected 3D school detection parameters, minimum longest, middle and shortest dimensions and minimum data threshold, was investigated. The minimum data threshold is the minimum Sv that is included in the analysis.
(a) R/V Ernest (b) SM20 blister fairing
Figure 5.5: Near shore survey equipment. Figure 5.5(a) shows R/V Ernest entering the protected anchorage at Cape Shirreff, Livingston Island. Note the protective dodge to shelter personnel and equipment from the elements. Figure 5.5(b) shows the SM20 multi- beam echosounder, mounted inside a white blister fairing. The blister fairing was attached to a rotating frame that is mounted to the transom of R/V Roald.
5.2.2
Automated krill swarm detection
This investigation sought to examine individual krill swarms. A krill swarm boundary is defined as the interface between a densely packed aggregation of krill and empty water. Krill swarms were identified using the Sonardata 3D schools cruise scanning detection al- gorithm, implemented in Echoview v3.50. The algorithm identified contiguous groups of acoustic returns in each beam and places prisms to bound the extremities of each group. These prisms were triangulated, reducing each prism to two triangles. The perimeter of the 3D school was generated by retaining the visible vertices of the triangles, which were used to create a 3D bounding surface around the contiguous acoustic return. At this point the user-defined size parameters, minimum longest dimension, minimum mid- dle dimension and minimum shortest dimension, were used to eliminate detected swarms with dimensions smaller than these minimum parameters (see following subsection). In addition, the minimum Sv threshold (dB) defined the minimum density of acoustic re- turns that were transferred to the 3D detection algorithm, hence defining the krill swarm boundary.
Acoustic observations of ranges less than 5 m were ignored due to sea-surface noise and near-field effects (Melvin et al., 2003). For the purposes of 3D target detection, the search volume for the 3D algorithm was constrained to water column targets by referencing a sounder detected seabed using the Sonardata MBE sounder detected bottom identification algorithm, offset by 0.5 m shallower. Based on MBE work by Gerlotto et al. (1999) it is assumed that the swarm speed was negligible compared to boat speed of 2.5 to 3.5 m/s .
Sensitivity analysis
In order to select a minimum threshold and minimum dimension parameters for swarm identification, sensitivity analysis of these parameters was conducted. Four transects, numbers 2, 17, 22 and 33, were selected at random from the 41 transects run. The value of each 3D schools detection parameter was varied sequentially across a range of biologically plausible values (given in the following paragraph) and the effect of these variations on the total number of detected swarms and swarm descriptive metrics was assessed.
The sensitivity of the 3D detection parameters was investigated in two stages. Firstly, the effect of varying the three minimum dimension parameters was investigated. Since there was noa priori available information about the 3D shape of krill swarms in the near shore study region, the same length was used for each of the 3D parameters. For example, during the first run of the 3D detection algorithm during sensitivity analysis, the minimum longest, middle and shortest dimension were all set to 2 m. This essentially assumed that krill swarms would be spherical. Subsequent 3D detections using the Sonardata algorithm were performed with all minimum dimensions set to 2, 3, 4, 5, 6, 10, 15, 30, 30 and 35 m, giving 10 sets of detections for the four randomly selected transects. During this section of the sensitivity analysis the minimum detection threshold was fixed at 24 dB.
The second part of the sensitivity analysis investigated the effect of varying mini- mum threshold. Based on results obtained from the analysis described in the previous paragraph, all the minimum school dimensions were set to 5 m, with a school detection performed at minimum threshold settings that ranged from 19 to 29 dB, in increments of 1 dB.
Assessment of swarm avoidance
Following Gerlotto and Paramo (2003), Soria et al. (2003) and Gerlotto et al. (2004) lateral avoidance of the research vessel by krill was assessed by testing the null hypothesis of a uniform distribution of schools with respect to horizontal distance from the transect to the geometric centre of a detected school (dr,s), i.e. H0: no lateral avoidance. Furthermore in
Chapter 6 swarm detectability (see Buckland et al. 2001) was assessed across the survey area.