Fisheries Acoustic Studies on the Purpleback Flying Squid Resource in the South China Sea

The fisheries acoustic technique, which is capable of directly assessing underwater populations over a large volume of waters in a short time, makes it a powerful tool for fisheries scientists. This paper dealt with the application of such technique to investigate the unexploited Purpleback Flying Squid (Sthenoteuthis oualaniensis) resource in the South China Sea. The results summarized as follows:
(1) Mantle Length (ML) survey using especially designed squid jigger, which can cast down to a specific depth with notification of catching a squid specimen, as well as concurrent collection of in situ Target Strength (TS) using EK500 scientific echosounder were carried out and the results show : (i) the larger the ML the stronger the in situ TS; (ii) seasonal mean in situ TS of squid and its corresponding ML are -51.2dB and 15.0cm, in spring (February-April); -48.2dB and 18.9cm appeared in summer (May-July), -49.3dB and 17.1cm in autumn (August-October), -54.1dB and 13.8cm in winter (November-January), respectively.
(2) A searching with validation algorithm was developed in this study to acquire the logarithm-linear relationship of in situ TS versus ML of the squid. The results are as follow: for ML from 10cm to 32cm. According to the depth range dispersion echoed by in situ squid TS, squid concentration of nighttime appeared in water column depth of 10-210m in spring and autumn; appeared in depth of 10-160m in summer and winter.
(3) A procedure to extract the echo energy of squid signals from the total echo energy was developed, as to filter noises that mainly composed of background noise and. The precision improvement in Volume backscattering Strength (SV) due to such procedure is estimated about 11%.
(4) Seasonal mean SV, which is essential for estimating abundance, of the squid obtained in this study show that: -77.9 dB in spring 1999, -70.6 dB in summer 1998, -75.8 dB in autumn 1999 and -72.2 dB in winter 1999. Seasonal mean squid abundance thus derived appeared to be 4.2 in spring; 8.9 in summer; 5.1 in autumn; and 26.1 in winter.
(5) Female Gonad Index (GI), which is the ratio of gonad weight to total body weight multiplied by 100, is analyzed in this study. The results show: that GI is a good indicator for female maturation; GI larger than 3 (ML larger than 20cm) indicating a matured female, smaller than 2 (ML smaller than 15cm) indicating immature ones. Squids caught by in situ biological sampling device appeared that all matured in summer and all immatured in winter while both matured and immatured squid present in spring and autumn.
(6) Assumed that the population of the Purpleback Flying Squid in the South China Sea was an enclosed stock and under a steady population condition in recent years, the basic population characters of this squid was postulated as follow: the main spawning season occurs in summer; the major detectable recruitment (ML large than 9 cm) occurs in winter with rather high density; yet the survival rate of this recruitment in next spring appeared to be low (about 20%).
(7) The total surveyed one-degree square block area, within which biological survey conducted in this study has captured Purpleback Flying Squid, is about . The abundance of Purpleback Flying Squid in the South China Sea is estimated as individuals in summer 1998, which is also the spawning season of this species. The mean body weight of the squid in this season is about 0.3 kg, thus the estimated total standing stock size of this resource was about 1500 thousand metric tons in the offshore regions of the South China Sea.