A General Guide for Deriving Abundance Estimates from Hydroacoustic Data




























Beam width

Choice of beam width depends on several considerations that can affect data collection or quality.  

Fish density (ρv and ρa)

Narrow beams (i.e., smaller half intensity beam width) increase horizontal resolution and improve the ability to separate echoes from individual fish (Fig. 6).  

Fig. 6: Transducer resolution.

Fig. 6.  Transducer resolution and beam width.  Fish within a pulse volume (delineated with dashed lines) cannot be resolved separately.  More fish are within a pulse volume when the pulse duration is longer and when the beam is wider.  Reproduced from Brandt (1996) with permission from the American Fisheries Society.

Bottom dead zone (heq)

Narrower beam angles have smaller bottom dead zones.  In deadzone height calculation example (120 kHz, θ3 dB=7º, depth=100 m) we had a bottom dead zone of 0.5 m.  A narrower beam (120 kHz, θ3 dB=5º, depth=100 m) would have a dead zone value of 0.4 m. 

Near-field zone (Rnf)

A narrow beam requires a greater active area of transducer elements than does a wide beam at the same frequency.  In the near-field distance calculation example, a 7o, 120 kHz transducer has an active radius of 5.0 cm and a "safe" near-field value of 1.7 m.  A 4o 120 kHz transducer would have an active radius of 8.8 cm and therefore a near-field value of 2.6 m. 

Outside noise

Wider beams allow for a greater sampling volume, an advantage when fish abundance is low, but are more sensitive to omni-directional background noise than narrow beams (Simmonds and MacLennan 2005), making a narrow beam a better choice in noisy environments (Table 3).

Table 3.  Comparison between wide and narrow beam attributes.


Wide beam

Narrow beam

Depth penetration



Horizontal extent



Horizontal resolution at depth









Ambient noise levels



Vessel motion

Data from wider beam transducers are less affected by vessel motion as the greater ping-to-ping overlap in the insonified volume will result during pitch/roll.

Transducer size

Transducers with wider beam widths are smaller than narrow beam transducers.  This is a consideration for portable acoustic systems.

Great Lakes beam widths

Typical beam widths used in the Great Lakes are between 6º to 12º.