Ex situ echo sounder target strengths of ice krill Euphausia crystallorophias

Abstract

Ice krill is the keystone species in the neritic ecosystem in the Southern Ocean, where it replaces the more oceanic Antarctic krill. It is essential to understand the variation of target strength (TS in dB re 1 m2) with the different body size to accurately estimate ice krill stocks. However, there is comparatively little knowledge of the acoustic backscatter of ice krill. The TS of individual, formalin-preserved, tethered ice krill was measured in a freshwater test tank at 38, 120, and 200 kHz with a calibrated split-beam echo sounder system. Mean TS was obtained from 21 individual ice krill with a broad range of body lengths (L: 13–36 mm). The length (L, mm) to wet weight (W; mg) relationship for ice krill was W=0.001218×103 × L3.53 (R2 =0.96). The mean TS-to-length relationship were TS38 kHz =−177.4+57log10 (L), (R2 = 0.86); TS120 kHz = −129.9+31.56log10 (L), (R2 =0.87); and TS200 kHz =−117.6+24.66log10 (L), (R2 =0.84). Empirical estimates of the relationship between the TS and body length of ice krill were established at 38, 120, and 200 kHz and compared with predictions obtained from both the linear regression model of Greene et al. (1991) and the Stochastic Distorted Wave Born Approximation (SDWBA) model. This result might be applied to improve acoustic detection and density estimation of ice krill in the Southern Ocean. Further comparative studies are needed with in situ target strength including various body lengths of ice krill.