Density_number
The species of any one region have two key properties:
TSS = Expected target strength returned from an individual specimen of species S (dB re 1m2)
pS = Expected proportion of the total number of specimens which are species S—in Echoview this is a percentage
We also note that:
NS = Total number of species defined in the region being analyzed
These properties can be seen on the Region Class Properties dialog box and defined using the Species Properties dialog box and Species Allocation dialog box.
Density_number is then calculated as follows:
where:
= Volumetric fish density in the region (fish/m3)
= Areal fish density in the region (fish/m2) = Density_number. See Note.
= Expected backscattering crosssection of a member of the species S (m2)
= Weighted mean backscattering crosssection of all species 0 to NS1 (m2)
= The linear mean Sv value for the region (m2/m3) see Sv mean for method of calculation.
= mean thickness of the region being analyzed (m). See Single beam vs. Multibeam (also ABC and NASC).
Note:
A unit conversion is required for exported Density_number. The units for Density_number (fish/m2) used with the generic set of biomass algorithms differs from the Echoview's export for Density_number which can be in (fish/nmi2) or (fish/hectare). In addition, be aware that biomass deductions should consider paired values for Density_number and Thickness_mean/Height_mean. Density_number on its own is an inadequate indicator of school size. Consider the following example:


 School #1 has a Density_number of 100 fish/nmi2 and a mean height of 10 m
 School #2 has a Density_number of 100 fish/nmi2 and a mean height of 1 m
Although the Density_number values are the same, school #1 is bigger and the fish within it are spaced further apart, i.e. its volumetric density (fish/m3) is much lower.

Single beam vs. Multibeam
Echoview provides access to these results in different ways depending upon the type of analysis as follows:
Single beam data 

Multibeam data 

Density_number calculation warning
In Echoview version 3.10 and 3.00 Density_number was calculated using pS to apportion the mean sv (rather than the specimen count) among the species. Version 3.20 and onwards correctly apportion the specimen count among the species defined. If you have used the Density_number (or Density_weight) variable in Echoview 3.10 or 3.00 your analysis results will differ in 3.20 and later. You must redefine pS (Proportion of fish of this species for region class on the Species Allocation dialog box).
See also
About biomass estimation
Density_weight
About analysis variables