Target strength measurements of tethered live purpleback flying squid (Sthenoteuthis oualaniensis) at sea

Knowledge of target strength (TS, dB) is essential for accurate assessment of fish abundance using acoustic methods. In order to investigate acoustic scattering characteristics of purpleback flying squid Sthenoteuthis oualaniensis, 25 squid specimens were collected by jig in the open sea in the northern South China Sea in September 2017 near geographical position 18°42.9′N 113°05.5′E. The TS of 25 specimens were measured respectively one by one using Simrad EY60 split-beam scientific echosounder with 120 kHz and self-made tethering-controling device. In this paper, we primarily explored the impact of different parameters on the single target detection for S. oualaniensis, and the parameters included pluse length determination level (PLDL, dB), min normalized pluse length (min NPL), max normalized pluse length (max NPL) and maximum standard deviation of minor-axis angles (MIA). Then we analyzed comprehensively the stochastic nature of single TS which was highly variable, and concluded the mathematical regression relation between TS of S. oualaniensis and mantle length (ML, cm). The results showed: (1) As the PLDL increased, the number of single target of squid detected increased firstly and then decreased, and the number was the largest when PLDL was 6 dB, while the average TS of all single targets increased monotonously. With increase of min NPL, the number of single squid target decreased; in the range of min NPL<0.7, the average TS of all single targets presented increasing obviously. When the max NPL was less than 1.2, the number of single squid target increased as the max NPL increased, but the average TS decreased; when the max NPL fell between 1.2 and 1.8, the number of single squid target presented increasing slowly, but the average TS remained relatively stable. As the MIA increased, the number of single squid target increased, but the average TS decreased; (2) In this study, the optimum parameters settings of single target detection were as follows: PLDL=6.00 dB, Min NPL=0.7 dB, Max NPL=1.8, MIA=0.8°; (3) The maximum and minimum values of the average single TS of live Sthenoteuthis oualaniensis specimens were respectively −48.6 dB and −63.63 dB, and the corresponding mantle length of Sthenoteuthis oualaniensis were respectively 25.2 cm and 12.4 cm. The mathematical regression relation between TS of Sthenoteuthis oualaniensis and mantle length (ML) was concluded as follows: TS=34.22 lg (ML)−98.23 (N=16, R²=0.603). This TS measurement was the first attempt to investigate the acoustic scattering characteristic and TS of live S. oualaniensis using tethering controlling method at sea by field work. The results of this paper not only accumulated important information and experience for further investigating the acoustic scattering characteristics and improving the acoustic estimation precision of S. oualaniensis, also provided the reference for measuring TS of other squids or fishes by tethering method at sea.