Munechika Ishizaki

Sound generated by a payao and comparison with auditory sensitivity of jack mackerel Trachurus japonicus

Sound generated by a payao, an anchored bamboo fish aggregating device, is believed to be attractive to fish; but until now, there is no available record of payao-generated sound. This study presents payao-generated sound recorded by a hydrophone at water depths of 5, 10 and 15 m from a fixed distance of 3 m relative to the payao, and compares the sound with the auditory sensitivity of jack mackerel Trachurus japonicus measured at discrete frequencies from 100 to 2000 Hz using the auditory brainstem response protocol. A consistent peak appeared in the sound spectrum at 49 Hz and showed an increasing sound pressure level with depth, which suggests that payao sound may come from the anchor rope. However, the contribution of the bamboo raft can not yet be discounted. The hearing threshold curve indicated that the most sensitive frequency range in jack mackerel is from 92.1 dB at 800 Hz to 111.0 dB at 200 Hz. These results show that the dominant frequency range of payao sound does not correspond with the high sensitivity frequency range of fish hearing.

Characteristics of periodical movement of live squid (Todarodes pacificus) hooked on the branch line of a red sea bream long line

In Jeju Island, red sea bream is mainly caught by long line with a live squid as bait. The movements of artificial jigs in the fishery are supposed to be an important factor for catching the target organism as well as bait. To develop new fishing method for long line fishery, movements of the bait which was squid were tried to be characterized. In a water tank experiment, a live squid was hooked by a fish hook attached to a model long line. And then movements of squids in the water tank were recorded for 20 minutes by a video camera. Recorded movement of the squid was just periodically up-and-down moving, with a dominant frequency of 0.125Hz, and amplitude of 10.8cm.

COMPARISON OF AUDITORY SENSITIVITY OF PELAGIC FISH WITH THE SOUND GENERATED BY A FISH AGGREGATION DEVICE

Anchored bamboo fish-aggregating devices (FADs) are used to catch tuna and small pelagic fish. Several hypotheses had been advanced previously to explain the association of fish with FADs (Castro et al. 2002; Freon & Misund 1999), but the processes responsible for fish attraction and aggregation remain unknown. There is no previous research on fish behaviour related to sound near FADs (Dempster & Tarquet 2004) or actual records of sound from FADs.

Shape and Hydrodynamic Characteristics of Canvas Kite for Midwater Trawl.

キャンバスカイトの流体特性を明らかにするために, 形状の異なる5種類のキャンバスカイトの模型を用いて, 0.2から0.5までの4段階の異なるキャンバーと迎角30°から130°の範囲で水槽実験を行った。実験で得られた揚力と抗力から求めた最大揚力係数と最大揚抗比は, I型の反り比0.2と迎角度50°で, それぞれ1.37と2.00であった。また, 簡単な構造の拡網装置としてキャンバスカイトを用いるには, I型が最適形状であるといえた。

Auditory Threshold of Black Rockfish.

The auditory threshold of the black rockfish Sebastes schlegeli was determined by condition-ing method using a sound coupled with bait in a tank. The audio-singnals used were pure tones of 70 to 1400Hz. The mean size of the fishes used in this experiment was 6.53g in body weight and 75.3mm in body length. A positive response was defined by analyzing the swimming performance of the experimental fishes. The fish learned to come to a feeding point in response to the stimulus sounds after 7 to 14 days training. The threshold in sound pressure was 90.6 dB (re 1μPa) at 100Hz which was most stimulative to the fish.