A ‘Sound’ Approach to Assessing Fish Stocks

Humans have survived on the ocean’s bounty for thousands of years, and modern fisheries managers frequently assess the size and health of commercially valuable fish stocks in order to set quotas and regulate fisheries. But stocks that are not commercially targeted are typically left unassessed, leaving questions about the location and abundance of fish species that humans do not eat, but other animals do.

A recent paper by scientists studying the decline of Alaska’s Steller sea lions describes the use of acoustic surveying to assess fish stocks critical to the Steller sea lion diet. Acoustic surveying estimates the size, location and distribution of fish populations within an ecosystem, based on the way different species reflect sound to create a ‘signature’ echo.

Drs. Stéphane Gauthier and John K. Horne, of the University of Washington’s School of Aquatic and Fishery Sciences, examined the acoustic signatures of five key forage fish species in the Bering Sea and Gulf of Alaska: capelin (Mallotus villosus), Pacific herring (Clupea pallasii), walleye pollock (Theragra chalcogramma), Atka mackerel (Pleurogrammus monopterygius), and eulachon (Thaleichthys pacificus).

“The intensity of an echo is generally expressed on a logarithmic scale as the target strength (TS),” the authors write. Target strength varies between species and depends on a number of factors, including fish size, shape, anatomical features (such as the swim bladder), and physiological state (such as a full gut or enlarged gonads).

When conducting acoustic surveys in the ocean, it can be difficult to accurately measure target strength in dense schools of fish or where several species appear together. For this reason, Gauthier and Horne used a mathematical model based on individual fish to compare acoustic properties between species and identify ways to distinguish one species from another.

With the help of the Kirchhoff-ray mode (KRM) model, the researchers developed a series of target-strength-to-length equations for each species that will contribute to further research in fisheries acoustics. By understanding the acoustic signatures of key fish species, scientists hope to better predict how fluctuations in these stocks can affect populations of apex predators such as Steller sea lions.

4 April 2005

Publications:
Acoustic characteristics of forage fish species in the Gulf of Alaska and Bering Sea based on Kirchhoff-approximation models. Gauthier, S. and J.K. Horne. 2004. Canadian Journal of Fisheries and Aquatic Sciences 61:1839-1850. abstract Acoustic surveys are routinely used to assess fish abundance. To ensure accurate population estimates, the characteristics of echoes from constituent species must be quantified. Kirchhoff-ray mode (KRM) backscatter models were used to quantify acoustic characteristics of Bering Sea and Gulf of Alaska pelagic fish species: capelin (Mallotus villosus), Pacific herring (Clupea pallasii), walleye pollock (Theragra chalcogramma), Atka mackerel (Pleurogrammus monopterygius), and eulachon (Thaleichthys pacificus). Atka mackerel and eulachon do not have swimbladders. Acoustic backscatter was estimated as a function of insonifying frequency, fish length, and body orientation relative to the incident wave front. Backscatter intensity and variance estimates were compared to examine the potential to discriminate among species. Based on relative intensity differences, species could be separated in two major groups: fish with gas-filled swimbladders and fish without swimbladders! The effects of length and tilt angle on echo intensity depended on frequency. Variability in target strength (TS) resulting from morphometric differences was high for species without swimbladders. Based on our model predictions, a series of TS to length equations were developed for each species at the common frequencies used by fisheries acousticians.
Potential acoustic discrimination within a boreal fish assemblage. Gauthier, S. and J.K. Horne. 2004. ICES Journal of Marine Science 61:836-845. abstract Differences in the acoustic characteristics of forage fish species in the Gulf of Alaska and the Bering Sea were examined using Kirchhoff ray-mode (KRM) backscatter models. Our goal was to identify species-specific characteristics and metrics that facilitate the discrimination of species using acoustic techniques. Five fish species were analyzed: capelin (Mallotus villosus), Pacific herring (Clupea pallasii), walleye pollock (Theragra chalcogramma), Atka mackerel (Pleurogrammus monopterygius), and eulachon (Thaleichthys pacificus). Backscatter amplitude differences exist among these species, especially between swimbladdered and non-swimbladdered fish. Echo intensities were variable within and among species. The effect of morphological variability was indexed using the ratio of the Reducedscattering length (RSL) standard deviation over its mean. Morphological variability was low only at fish length to acoustic wavelength ratios less than eight. Target strength differences between pairs of carrier frequencies (ranging from 12 kHz to 200 kHz) differed among species, and were dependent on fish size and body orientation. Frequency differencing successfully discriminated between fish species but the choice of frequency to maximize target strength differences was not consistent among species pairs. Frequencydependent, backscatter model predictions facilitate comparison of target strength differences prior to acoustic data collection.