Fisheries Acoustics

Hydroacoustic methods for fish detection, biomass estimation, species classification, and ecosystem assessment

Overview

Fisheries acoustics uses sound to detect, quantify, and classify aquatic organisms. Scientific echosounders transmit acoustic pulses that scatter off fish swim bladders and zooplankton bodies. The returned signal (echo) carries information about the target's size (target strength), abundance, and depth distribution. Multi-frequency and broadband systems enable species discrimination, while acoustic-trawl surveys combine echosounder and net catch data to produce absolute biomass estimates. This technology is fundamental to stock assessment for fisheries management worldwide, particularly for species like Alaska pollock, herring, and Antarctic krill.

18–400 kHz
Typical survey frequencies
EK80
State-of-art broadband echosounder
~3.4M t
Alaska pollock annual catch
Sv (dB)
Volume backscattering strength

Key Concepts

Sonar Equation

The active sonar equation relates transmitted power, propagation loss, target strength, and receiver sensitivity. For fisheries acoustics, the relevant quantity is volume backscattering strength (Sv) for aggregations or target strength (TS) for individual targets.

TS = 10 log₁₀(σ_bs) [dB re 1 m²]
Sv = 10 log₁₀(sᵥ) [dB re 1 m⁻¹]

Target Strength

TS is the acoustic measure of an organism's size and shape. For fish, the swim bladder is the dominant scatterer (~90% of echo). TS-length relationships (e.g., TS = 20 log₁₀L − 66 for pollock) convert acoustic data to biological estimates.

Echo Integration

Echo integration sums backscattered energy over depth intervals and along transects to estimate the nautical area scattering coefficient (NASC, sₐ). Combined with TS and catch composition, NASC converts to biomass density.

ρ_fish = sₐ / (4π × σ_bs)

Multi-Frequency Classification

Different organisms scatter sound differently at various frequencies. Frequency response (e.g., Sv at 120 kHz vs 38 kHz) distinguishes fish from krill from gas bubbles. Broadband systems provide continuous frequency response for improved classification.

Survey Design

Acoustic-trawl surveys follow systematic transect patterns (parallel, zigzag) with statistical design for coverage and precision. Geostatistical methods (kriging) interpolate between transects to produce abundance maps.

Data Processing

Software like Echoview, LSSS, and open-source PyEcholab processes raw acoustic data. Steps include noise removal, bottom detection, calibration, thresholding, species allocation, and integration. The echopype Python package enables reproducible workflows.

Interactive Visualizations

Simulated Echogram (Volume Backscattering Strength)

Target Strength vs. Fish Length Relationship

Frequency Response for Species Discrimination

Key References

  1. Simmonds, J. & MacLennan, D.N. (2005). Fisheries Acoustics: Theory and Practice. Blackwell Science.
  2. Foote, K.G. (1987). Fish target strengths for use in echo integrator surveys. JASA, 82(3), 981–987.
  3. De Robertis, A. & Higginbottom, I. (2007). A post-processing technique to estimate the signal-to-noise ratio. ICES J. Marine Science, 64(6), 1282–1291.
  4. Korneliussen, R.J. et al. (2008). Acoustic identification of marine species using a feature library. Methods in Oceanography, 1-2, 36–47.
  5. Stanton, T.K. et al. (1996). Acoustic scattering characteristics of several zooplankton groups. ICES J. Marine Science, 53, 289–295.