ACOUSTICS UNPACKED A General Guide for Deriving Abundance Estimates from Hydroacoustic Data

 Main Menu Suggested Standard Operating Procedures History Acoustic Background Equipment Deployment Survey Design System Calibration Data Collection Survey Protocols Data Processing Survey Calculations Uncertainty Example Applications References Equations Figures Tables Contact Us Acknowledgements Useful Links

### Density

Density calculations may be the survey goal or may be needed for abundance calculations.  Either way, the calculation of density assumes that:

• target species or groups have been separated from non-target backscatter, and;
• an appropriate σbs has been identified for each species, group, or depth layer.
• Two calculations are common:

• average density within a vertical bin, and;
• areal or total density within a vertical bin.
•

#### Density per unit volume (ρv)

If density is expressed as #•m-3, sv and σbs are used to calculate average density (ρv, m-3) by Equation 11: where:
sv is volume backscattering coefficient (dB re: 1 m-1), and;
σbs is backscattering cross-section (m2).

This method is sometimes referred to as sv/ts scaling.  Note that this density estimates is based on echo integration (sv or sa) values and an estimate of σbs.  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 (ρa)

For the calculation of stock size, a density estimate based on area is more useful.  ABC (sa) or NASC (sA) may be used to calculate total density ((ρa, m-2 or nmi-2):  where:
ABC (sa) is area backscattering coefficient (m-2);
NASC (sA) is nautical area scattering coefficient (nmi-2), and;
σbs is backscattering cross-section (m2).