Avoidance responses to infrasound in downstream migrating European silver eels, Anguilla anguilla

In an attempt to develop an efficient acoustic fish fence, we have designed an infrasound source able to generate large nearfield particle acceleratio n. The source generates water movements by means of two symmetrical pistons in an air-filled cylinder with 21 cm bore. The pistons are driven by eccen tric coupling to an electric motor, with 5 cm p.p. amplitude. The piston mo vements are 180 degrees out of phase. The piston reaction forces are thus o pposed, leading to vibration free operation. The submergible infrasound sou rce is operated freely suspended in the water mass. The emitted sound frequ ency is 11.8 Hz. The particle acceleration is about 0.01 m s(-)2 at a dista nce of 3 m, corresponding to the threshold intensity for deterring effects of infrasound on Atlantic salmon smolts. The sound source was employed to t est the effect of intense infrasound on migrating European silver eels. Fis h confined in a tank displayed startle behaviour and prolonged stress react ions, telemetrically monitored as tachycardia, in response to intense infra sound. The field tests were carried out in the River Imsa. A trap that catc hes all the descending eels is installed near the river mouth. The trap was separated in four equal sections. During the periods with infrasound expos ure, the proportion of silver eels entering the section closest to the soun d source was reduced to 43% of the control value. In the section closest to the opposite river bank, infrasound increased the proportion of trapped ee ls to 144% of the control values. This shift of the migrating eels away fro m the infrasound source was highly significant.