2005. The influence of tilt angle on the acoustic target strength of the Japanese common squid (Todarodes pacificus). e ICES Journal of Marine Science, 62: 779e789.To measure the influence of changes in tilt angle on the acoustic target strength (TS) of the Japanese common squid (Todarodes pacificus), we conducted a series of experiments to estimate TS in relation to tilt angle and swimming angle. Swimming angle was measured in a seawater tank using two infrared, underwater cameras under dark conditions. Ex situ measurements of TS in relation to tilt angle on live specimens using a fishhook and cage method were then conducted at 38 and 120 kHz; mantle length (ML) ranged from 21 to 27 cm (mean 24.75 cm). For the more precise TS measurement with tilt angle, another set of ex situ TS measurements relative to tilt angle was made at 38 and 120 kHz on tethered, anesthetized specimens in seawater. The mean swimming angle was ÿ17.7( (G12.7( s.d.). The mean TS varied from ÿ48.6 to ÿ44.6 dB and was relatively higher at 120 kHz than at 38 kHz, in the order of 0.7 and 2.5 dB. The empirical relationship between TS (dB) and ML (cm) is given by TS Z 20 log 10 (ML) ÿ 75.4 (r Z 0.81) at 38 kHz or TS Z 20 log 10 (ML) ÿ 73.5 (r Z 0.64) at 120 kHz. Based on the tethered method for the anesthetized squid, the mean standardized TS values (b 20 ) were found to be highly correlated with the tilt angle, and the resultant fitted equations for b 20 were expressed as: b 20 Z ÿ73.3 C 0.48 ! q C 0.0122 ! q 2 C 0.00016 ! q 3 for 38 kHz and b 20 Z ÿ72.6 C 0.53 ! q C 0.0134 ! q 2 C 0.00014 ! q 3 for 120 kHz, where q is the negative tilt angle in degrees. The mean TS based on the measurements using live squid was higher than that of tethered measurements, i.e., 2.6 dB at 38 kHz and 4.0 dB at 120 kHz. The higher mean TS in the ex situ measurements for the live squid can be explained by the influence of the low tilt angle on the overall TS data. The results can be used to understand the influence of tilt angle on the TS of Todarodes pacificus and thus improve the accuracy of biomass estimates.

This study determined the ex situ target strength (TS) of rockfish (Sebastes schlegeli) and red sea bream (Pagrus major) in an artificial seawater tank as a means of helping to estimate fishery resources in coastal areas. TS experiments were conducted at frequencies of 38 kHz (split beam), 120 kHz (split beam), and 200 kHz (dual beam). The species were examined under two conditions: first, live fish confined to a small, net cage; and, second, as free-swimming fish inside a large tank. The study examined 21 rockfish and 20 red sea bream. The data were used to obtain expressions for TS against length and weight for the two species. The relationships between TS and fish length were as follows: for rockfish, TS38 kHz = 20 log10(L) − 67.7 (r = 0.80), TS120 kHz = 20 log10(L) − 74.3 (r = 0.61), TS200 kHz = 20 log10(L) − 72.8 (r = 0.41); and for red sea bream, TS38 kHz = 20 log10(L) − 66.8 (r = 0.86), TS120 kHz = 20 log10(L) − 74.0 (r = 0.65), TS200 kHz = 20 log10(L) − 74.1 (r = 0.83). The TS equations for rockfish and red sea bream as a function of fish weight at 38 kHz were TS38 kHz = 6.75 log10(W) − 56.0 (r = 0.78) and TS38 kHz = 4.08 log10(W) − 49.9 (r = 0.89), respectively. For comparison, calculations using the Helmholtz–Kirchhoff ray-approximation model based on swimbladder morphology were compared with the measured TS. When the tilt angle of the fish is zero, the mean TS from the model is 3–10 dB higher than the experimental results, although the maximum TS values were only 3–4 dB different.

Black porgy are hatched and released in marine ranching areas in Japan, Korea and Taiwan to maintain fisheries resources in coastal areas. If acoustic methods are used for field surveys, target strength ( TS ) information on the species is needed to convert acoustic data to biomass density. Target strength of black porgy ( Acanthopagrus schlegeli ) 15.5-32.9 cm fork length (nine individuals) was measured at 38 and 120 kHz (split beam) by ex situ measurements (cage and fishhook methods) of living fish, a tethered method with shock-frozen fish, and an acoustic scattering model, the Kirchhoff ray mode (KRM) model based on fish morphology. The ex situ TS -to-fork length ( FL , cm) relationships were TS 38kHz = 20log 10 FL − 64.6 ( r = 0.77) and TS 120kHz = 20log 10 FL − 65.2 ( r = 0.70). The values for the KRM model were TS 38kHz = 20log 10 FL − 64.8 ( r = 0.88) and TS 120kHz = 20log 10 FL − 65.4 ( r = 0.80). The agreement between the ex situ measurements and the KRM model for the mean TS was very good. In contrast, the results for the tethered method ( TS 38kHz = 20log 10 FL − 69.3 [ r = 0.42]) differed markedly from the other two methods, perhaps because of the effects of the shock-freezing procedure in air. The results of the ex situ measurements and scattering model provide basic information for studying TS and conducting acoustic surveys of black porgy, even though the tethered method must be modified.KEY WORDS: black porgy, ex situ measurements, Kirchhoff ray mode model, target strength, tethered method.

. 1996. Relationship between acoustic backscattering strength and density of zooplankton in the sound-scattering layer. -ICES Journal of Marine Science, 53: 507-512.Hydroacoustic sampling in the ocean is a useful technique for studying the biomass and structure of the distribution of marine organisms, including zooplankton. Acoustical and biological sampling of zooplankton in the sound-scattering layer (SSL) off the east coast of Oshima Peninsula, northern Japan has been conducted during the last 10 years to obtain the scale factor for converting the backscattering strength to biological density. The volume backscattering strength (SV) was measured at 25, 50, 100, and 200 kHz, while an IKMT (Isaacs-Kidd Midwater Trawl) and a Norpac (North Pacific standard net) were used to sample the biological organisms in the SSL. During drifting observation at dusk, the maximum speed of the upward migration of the SSL toward the surface was 3 m min 1 , and the SV changed over a range from 80 dB to 50 dB. Vertical tows of the Norpac indicated that euphausiids were the major zooplankton species of the migrating SSL. For quantitative analysis, the IKMT was towed horizontally about one hundred times, and the calculated density of zooplankton showed a maximum of 10 g m 3 . We assumed that the acoustic reflectivity of the individual plankton was related to the size of the plankton squared, and that the acoustic backscattering strength of biomass was proportional to the distribution density. Results of regression analysis showed a linear relationship between the log of zooplankton density (mg m 3 ) and the acoustic volume backscattering strength SV (dB), with correlation coefficients greater than 0.5 at all frequencies.1996 International Council for the Exploration of the Sea

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