Using the School Detection module

Prior to detecting schools

Platform: Fixed

For a fixed platform, select Ping number, Ping time, or Water current as the school detection configuration’s Distance mode. The Water current mode requires a Water current speed source on the Position page of the Platform Properties dialog box.

Platform: Position determined by GPS

Ensure that the cruisetrack has been processed and, if appropriate, smoothed. Cruisetrack data affect distance calculations and can help you determine whether the survey vessel doubled back along its track, potentially resulting in repeated observations of the same schools.

To specify the Distance mode, open the Region Detection page of the EV File Properties dialog box. Under Single beam schools, select a detection configuration and click Properties.... Select the required mode on the Single Beam Schools Detection page.

Detect school settings

On the Data page of the Variable Properties dialog box, apply an appropriate Minimum threshold before detecting schools. A Maximum threshold can also be applied but is generally not required for school detection. The minimum threshold is particularly important because increasing it excludes surrounding subthreshold data points, potentially producing substantial changes in estimates of school shape and energetic properties.

Numerous approaches have been taken to the question of determining what processing threshold to use for school detection (see Richards et al., 1991; MacLennan and Simmonds, 1992; Barange, 1994; Swartzman et al., 1994; Scalabrin et al., 1996). The corrections for beam width effects used by Echoview (Diner, 1998) generally perform best at processing thresholds between -60 and -65 dB (that is, when dST = (Sv mean - Th) is between 10 and 30 dB)+. The ICES study group on echo trace classification has suggested a threshold for school detection of -60 dB for pelagic schooling fishes (Anon. 1998).

Notes:

Lawson et al. (2001) discuss the usefulness of analyzing schools at two different processing thresholds, in order to use the ratio of measurements of school features at the two thresholds for the purpose of species identification.
+ Where Sv mean is the school density and Th is the processing threshold.

On the Analysis page of the Variable Properties dialog box, Exclude above line can prevent detected schools from being linked to surface noise. Similarly, Exclude below line can prevent bottom echoes from being attached to detected schools. The Apply minimum threshold setting on the Data page should generally be selected before school detection. A maximum threshold is generally not required. For appropriate use of corrected school variables, apply the same minimum threshold during echo integration as was used during school detection.

Determine appropriate school detection configuration settings through preliminary analysis. Repeated detection using representative data can help you assess the performance of different settings. Horizontal sizes are expressed in meters, pings, or seconds according to the selected Distance mode, while vertical sizes are expressed in meters.

Consider the biology of the species of interest, e.g., whether schools smaller than the selected minimum sizes contribute meaningfully to stock biomass, or whether an aggregation less than 1 m long should be considered a school.

Postprocessing

Not all 'schools' detected by the module will be true biological schools. Noise, aggregations of organisms other than the species of interest, and so on, may be detected and treated by the module as 'schools'. Some scrutiny may be necessary before or after exporting school variables. In particular regions that have uncorrected lengths smaller or mean energies lower than those appropriate for calculating some corrected schools variables will still be processed and corrected variables exported without filtering. You should filter the exported schools where required.

Volumetric density (individuals/m3), area density (individuals/m2), and school abundance can be calculated using information about the species composition, target strength, and estimated volume of the school.

To further understand the calculation of schools variables please read Notes about School Detection.

Warning: School regions should not extend beyond the available data. This can occur if schools are detected and data files are subsequently removed or replaced, leaving school regions that extend beyond the new data. Inconsistent Uncorrected_length and Uncorrected_thickness calculations may then have undesirable effects on other school analysis variables. For more information, see School Detection module algorithms.

Live viewing

School detection can be performed in near real time while live viewing data. For more information, see About live viewing and School detection while live viewing algorithm.