Target Strength Measurements of Live Golden Cuttlefish Sepia esculenta at 70 and 120 kHz

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  • ABSTRACT

    Cuttlefish Sepia esculenta are commercially important in Korea. Assessments of their biomass currently depend on fishery-landings data, which may be biased. Towards fishery-independent acoustic surveys of cuttlefish, target strength (TS) measurements at 70 and 120 kHz were made of 23 live cuttlefish, in early May 2010. The fish were caught by traps in the inshore waters around Geojedo, Korea. The TS were measured using split-beam echosounders (Simrad ES60 and EY500, respectively). The cuttlefish mantle lengths (L) ranged from 15.6 to 23.5 cm (mean L=17.8 cm) and their masses (W) ranged from 335 to 1020 g (mean W=556.1 g). Their mean TS values at 70 and 120 kHz were -33.01 dB (std=1.39 dB) and -31.76 dB (std=2.15 dB), respectively. The mean TS at 70 kHz was 0.17 dB higher than the TS-length relationship resulting from a least-squares fit to the data (TS = 24.67 log10 L (cm) - 64.03, r2 = 0.52, N=23). The mean TS at 120 kHz was 0.45 dB higher than the fitted TS-length relationship (TS = 40.59 log10 L (cm) - 82.96, r2 = 0.58, N=23). The differences between the mean TS values and an equation regressed from all of the TS measurements at both frequencies (TS = 24.92 log10 L (m) - 4.92 log10 λ (m) - 22.82, r2 = 0.86, N=46) was 0.22 dB at 70 kHz and 0.31 dB at 120 kHz, respectively.


  • KEYWORD

    Sepia esculenta Target strength , Length dependence , Time series , Tilt angle

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  • [Fig. 1.] Diagram of the acoustic-optical apparatus used to measure cuttlefish Sepia esculenta target strength (TS) versus mantle length (L), and acoustic incidence angle (θ). The apparatus comprise: a rectangular (1.2 width × 1.2 length × 1.7 m height), acrylic, saltwater tank; 70 and 120 kHz echosounders (Simrad ES60 and EY500, respectively); two split-beam transducers (Simrad ES70-11 and ES120-7F, respectively); and a closed circuit television (CCTV; MIG system, KJ Tech, Korea) camera for monitoring the fish orientation. Stable orientations of the live cuttlefish were maintained by tying monofilament lines between the cuttlefish mantle and the ends of two tethering rods. This apparatus allowed precision adjustments to the acoustic incidence angle.
    Diagram of the acoustic-optical apparatus used to measure cuttlefish Sepia esculenta target strength (TS) versus mantle length (L), and acoustic incidence angle (θ). The apparatus comprise: a rectangular (1.2 width × 1.2 length × 1.7 m height), acrylic, saltwater tank; 70 and 120 kHz echosounders (Simrad ES60 and EY500, respectively); two split-beam transducers (Simrad ES70-11 and ES120-7F, respectively); and a closed circuit television (CCTV; MIG system, KJ Tech, Korea) camera for monitoring the fish orientation. Stable orientations of the live cuttlefish were maintained by tying monofilament lines between the cuttlefish mantle and the ends of two tethering rods. This apparatus allowed precision adjustments to the acoustic incidence angle.
  • [Fig. 2.] Relationship between the body weight and mantle length of the 23 cuttlefish Sepia esculenta used in the TS measurements.
    Relationship between the body weight and mantle length of the 23 cuttlefish Sepia esculenta used in the TS measurements.
  • [Fig. 3.] A cuttlefish Sepia esculenta swimming freely in a saltwater tank. A cuttlefish suspended ~1 m below the transducers and near their maximum- response axes. Positive θ indicates a head up orientation relative to the center line of the mantle or body, and a negative θ indicates a head down orientation.
    A cuttlefish Sepia esculenta swimming freely in a saltwater tank. A cuttlefish suspended ~1 m below the transducers and near their maximum- response axes. Positive θ indicates a head up orientation relative to the center line of the mantle or body, and a negative θ indicates a head down orientation.
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  • [Fig. 4.] Relationship between mantle length (L) and mean target strength (TS) at 70 kHz for 23 individuals of live cuttlefish Sepia esculenta caught during the spawning season in the waters southwest of Korea.
    Relationship between mantle length (L) and mean target strength (TS) at 70 kHz for 23 individuals of live cuttlefish Sepia esculenta caught during the spawning season in the waters southwest of Korea.
  • [Fig. 5.] Relationship between mantle length (L) and mean target strength (TS) at 120 kHz for 23 individuals of live cuttlefish Sepia esculenta caught during the spawning season in the waters southwest of Korea.
    Relationship between mantle length (L) and mean target strength (TS) at 120 kHz for 23 individuals of live cuttlefish Sepia esculenta caught during the spawning season in the waters southwest of Korea.
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  • [Fig. 6.] Relationship between σ/λ2 and L/λ for the dataset of 46 cuttlefishes Sepia esculenta obtained by combining 120 kHz data (Fig. 5) with 70 kHz data (Fig. 4), where σ is the scattering cross-sectional area (m2), L is the mantle length (m), and λ is the acoustic wavelength (m). An empirical equation showing the variation of TS (or σ) versus L and λ is indicated.
    Relationship between σ/λ2 and L/λ for the dataset of 46 cuttlefishes Sepia esculenta obtained by combining 120 kHz data (Fig. 5) with 70 kHz data (Fig. 4), where σ is the scattering cross-sectional area (m2), L is the mantle length (m), and λ is the acoustic wavelength (m). An empirical equation showing the variation of TS (or σ) versus L and λ is indicated.
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  • [Fig. 7.] (A), (B) Mean tilt-angles (black circles) and ± standard deviations (negative crosses) for each cuttlefish Sepia esculenta obtained at approximately 20 second intervals by analyzing the CCTV camera images acquired independently from 23 cuttlefishes at 70 and 120 kHz, respectively. (C) Distributions of tilt-angles for 23 cuttlefishes at 70 and 120 kHz, respectively.
    (A), (B) Mean tilt-angles (black circles) and ± standard deviations (negative crosses) for each cuttlefish Sepia esculenta obtained at approximately 20 second intervals by analyzing the CCTV camera images acquired independently from 23 cuttlefishes at 70 and 120 kHz, respectively. (C) Distributions of tilt-angles for 23 cuttlefishes at 70 and 120 kHz, respectively.
  • [Fig. 8.] An example echogram showing echoes from gas bubbles, caused the cuttlefish’s sudden expulsion of water through its siphon, rising towards the surface.
    An example echogram showing echoes from gas bubbles, caused the cuttlefish’s sudden expulsion of water through its siphon, rising towards the surface.
  • [Fig. 9.] (A) An example echogram obtained from a cuttlefish Sepia esculenta moving modestly near the axis of sound beam with an echo record for gas release showing as a straight line between the cuttlefish and transmission line, (B) distribution of measured TS values, (C) location of a cuttlefish within the beam, (D) echo signal backscattered from cuttlefish.
    (A) An example echogram obtained from a cuttlefish Sepia esculenta moving modestly near the axis of sound beam with an echo record for gas release showing as a straight line between the cuttlefish and transmission line, (B) distribution of measured TS values, (C) location of a cuttlefish within the beam, (D) echo signal backscattered from cuttlefish.
  • [Fig. 10.] An example of the time series showing the variations in TS values caused by a cuttlefish Sepia esculenta (mantle length 19.4 cm; cuttlebone length 18.5 cm) modestly swimming within the sound beam of a 70 kHz transducer. The mean TS value of such time series was used to derive the TS-length relationship.
    An example of the time series showing the variations in TS values caused by a cuttlefish Sepia esculenta (mantle length 19.4 cm; cuttlebone length 18.5 cm) modestly swimming within the sound beam of a 70 kHz transducer. The mean TS value of such time series was used to derive the TS-length relationship.