Density per unit volume (ρ_{v})
If density is expressed as #•m^{3}, s_{v} and σ_{bs} are used to calculate average density (ρ_{v}, m^{3}) by Equation 11:
where:
s_{v} is volume backscattering coefficient (dB re: 1 m^{1}), and;
σ_{bs} is backscattering crosssection (m^{2}).
This method is sometimes referred to as sv/ts scaling. Note that this density estimates is based on echo integration (s_{v} or s_{a}) values and an estimate of σ_{bs}. If fish populations are sufficiently sparse, echo counting or trace counting are possible. This may be the case in Lake Superior. We have not dealt with this issue in this SOP as echo counting cannot be used in all the Great Lakes.
Areal or total density (ρ_{a})
For the calculation of stock size, a density estimate based on area is more useful. ABC (s_{a}) or NASC (s_{A}) may be used to calculate total density ((ρ_{a}, m^{2} or nmi^{2}):


[31] 


[32] 
where:
ABC (s_{a}) is area backscattering coefficient (m^{2});
NASC (s_{A}) is nautical area scattering coefficient (nmi^{2}), and;
σ_{bs} is backscattering crosssection (m^{2}).