ACOUSTICS UNPACKED

A General Guide for Deriving Abundance Estimates from Hydroacoustic Data

 
 

 

 

 

 

 

 

 

 

 

 

 

Acoustic theory and use

Acoustic technologies are effective and efficient methods for sampling fish and zooplankton biomass in the water column.  Fisheries acoustics is a remote sensing technique with advantages over traditional sampling methods such as:

  1. Nearly the entire water column can be sampled quickly (sound travels approximately 1450-1500 m•s-1 in water);
  2. Areal coverage is continuous along a transect;
  3. Data resolution is high, less than a meter vertically and tens of meters horizontally;
  4. Data can be processed in a variety of ways after the survey. 

Limitations specific to acoustics include difficulty in determining identity of targets, variability in TS, inability to sample close to the transducer (near-field) and close to the bottom (bottom dead zone), and inability to acquire biological samples for aging and diet analysis.  Acoustic technologies also require the use of expensive equipment and some understanding of the physics of underwater sound.  In addition, fisheries acoustics retains some limitations of traditional sampling methods such as vessel avoidance.  Fisheries acoustic surveys typically integrate acoustic technologies with other sampling methods, such as net catch and temperature data, to estimate abundance.

Fisheries acoustics uses specialized terminology. (See Table 1 below).  We follow the standard definitions, symbols and units in MacLennan et al. (2002) in this document.

Table 1. Definitions, symbols and units 

a

Transducer active radius (cm or m)
A Survey area (km2 or nmi2)
BD Water column depth (bottom depth, m)
b(θ) or B(θ) Transducer directivity in the direction defined by angle θ (linear or dB scale)
c Sound speed (m•s-1)
D Cruise track length (Distance, km or nmi)
f Frequency (Hz or kHz)
heq Height of the bottom deadzone (m)
i Ping interval (sec)
k Wave number (k=2π/λ)
L Target scattering size (e.g., body length or swimbladder length) (cm or m)
R

Range between two targets or between a target and the transducer (m)
Rnf Near-field range (m)
sa , ABC

Area backscattering coefficient (m2•m-2)

sA, NASC

Nautical area backscattering coefficient (sA = sa/(1852)2 4 π), unit m2•nm-2)

Sa Area back scattering strength (Sa = 10 log10(sa), dB)
sv Volume backscattering coefficient (m2•m-3)
SV

Volume backscattering strength (Sv = 10log10(sv), dB)
T Temperature (ºC)
TS

TS (dB), TS = 10log10(σbs), dB)
TSu Measured value in the TS echogram (dB).  Also called uncompensated TS
V Sampling volume (m3)
w Fish mass (gm or kg)
W System transmit power (W)
α Absorption coefficient (dB•m-1 or dB•km-1)
Ψ or EBA, Ψ Equivalent beam angle (steradians or dB re: 1 sr)
λ Wavelength (m)
σbs Backscattering cross section (m2)
θ3dB Half-beam angle to the -3 dB point (º)
Λ Degree of coverage (no units)
ρv Volumetric fish density (#•m-3)
ρa Areal fish density (#•m-2)
τ Pulse duration (s)