Acoustic Backscattering Characteristics of Swimbladderred Juvenile Fish Derived from Cage Experiments

Abstract

Acoustic target strengths of captive juvenile fishes of three species were measured at four frequencies using fish cage method. Measurements were conducted on a barge constructed at a fishing port for the experiments. Measured fish were sea smelt Hypomesus pretiosus japonicus (Brevoort), goldeye rockfish Sebastes inermis (Cuvier), and purple puffer Takifugu porphyreus (Temminck et Schlegel). They were caught with a miniature purse seine and maintained in fish breeding tanks before the experiments. Their body lengths ranged from 30 to 65 mm. Each species was classified into three size classes. Specimens were maintained in a cubical cage of 0.5 m each side made of lace cloth during measurement. The echo signals from encaged fish at 25, 50, 100, and 200 kHz were recorded with a data recorder. Echo signals were later digitized and processed by squared integration to calculate the target strengths. The results show that, (1) The backscattering strength of encaged juveniles indicates a linear density dependence for most sample sets, (2) The normalized target strengths 75cm of juveniles ranged from -66 to -55 dB, which is maximum 10 dB higher than that of adults, (3) Target strength changed by the frequency, that is, the TScm for L/λ ≦ 2 was roughly 10 dB higher than the TScm for L/λ ≧ 2.