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

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 log 10 L (cm) -64.03, r 2 = 0.52, N=23). The mean TS at 120 kHz was 0.45 dB higher than the fitted TS-length relationship (TS = 40.59 log 10 L (cm) -82.96, r 2 = 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 log 10 L (m) -4.92 log 10 λ (m) -22.82, r 2 = 0.86, N=46) was 0.22 dB at 70 kHz and 0.31 dB at 120 kHz, respectively.

The influence of cuttlebone on the target strength of live golden cuttlefish (Sepia esculenta) at 70 and 120 kHz

“…These characteristics-such as length, weight, width and thickness-were measured following completion of the acoustic experiments. The mean tilt angles (<θ>) with standard deviations (S θ ) measured for 19 live cuttlefish at 70 and 120 kHz in our earlier study (Lee and Demer 2014) are shown in Table 2. The TS and θ for these live cuttlefish were measured simultaneously using a split-beam echo sounder and a CCTV camera system, respectively and the (<θ>) and S θ values were used in estimating the tilt-averaged TS from the TS functions of the corresponding cuttlebone.…”

“…The biological and morphological characteristics of 19 specimens of cuttlebone and the corresponding live cuttlefish (Lee and Demer 2014) used in the TS experiments are shown in Table 1. These characteristics-such as length, weight, width and thickness-were measured following completion of the acoustic experiments.…”

“…1 (Lee and Demer 2014). It comprised two split-beam echosounders (ES60 and EY500, Simrad, Norway) operating at 70 and 120 kHz, two split-beam transducers (half-power beamwidths = 11°a nd 7.1°, respectively) and a DC servomotor system (motor: BG90, Sung Shin, Korea; driver: KDC248H, KScontrol, Korea) to control the tilt angles of the cuttlebone during each acoustic transmission.…”

“…554 issued from Pukyong National University, Busan, Korea. Nineteen cuttlebones directly extracted from 19 of the 23 live cuttlefish individuals (with the exception of four broken cuttlebones) tested in our earlier study (Lee and Demer 2014) were used as specimens for the TS measurements. Each cuttlebone was carefully extracted from each live cuttlefish specimen in sea water just before the experiment, and then immediately moved to a small tank filled with seawater at 18°C.…”

“…These facts suggest that the contribution of cuttlebone to the TS of cuttlefish may be larger than that of the swimbladder to the TS of fish, if the volume of cuttlebone does not change with depth. Other than our earlier study (Lee and Demer 2014), there has been no systematic attempt to determine the relationship between the TS and size of cuttlefish, especially the importance of cuttlebone to cuttlefish TS.…”