Frank Reier Knudsen

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 acceleration. 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 eccentric coupling to an electric motor, with 5 cm p.p. amplitude. The piston movements are 180° out of phase. The piston reaction forces are thus opposed, leading to vibration free operation. The submergible infrasound source is operated freely suspended in the water mass. The emitted sound frequency is 11.8 Hz. The particle acceleration is about 0.01 m s−2 at a distance of 3 m, corresponding to the threshold intensity for deterring effects of infrasound on Atlantic salmon smolts. The sound source was employed to test the effect of intense infrasound on migrating European silver eels. Fish confined in a tank displayed startle behaviour and prolonged stress reactions, telemetrically monitored as tachycardia, in response to intense infrasound. The field tests were carried out in the River Imsa. A trap that catches all the descending eels is installed near the river mouth. The trap was separated in four equal sections. During the periods with infrasound exposure, the proportion of silver eels entering the section closest to the sound source was reduced to 43% of the control value. In the section closest to the opposite river bank, infrasound increased the proportion of trapped eels to 144% of the control values. This shift of the migrating eels away from the infrasound source was highly significant.

Hydroacoustic observations indicating swimbladder volume compensation during the diel vertical migration in coregonids (Coregonus lavaretus and Coregonus albula)

The diel vertical migration of whitefish (Coregonus lavaretus) and vendace (Coregonus albula) was studied in a lake using bottom mounted echosounder transducers beaming upwards. Significant gas release was observed when the fish were ascending towards the surface during the dusk. From mobile hydroacoustic surveys it was found that the target strength (TS) did not decrease with depth, as would be expected if the fish had no possibility to fill the swimbladder. Our observations suggest swimbladder volume compensation during the diel vertical migration in coregonids.

Product development at Kongsberg Maritime related to underwater sensor networks

This paper presents selected product development activities in Kongsberg Maritime related to underwater sensor network nodes. Nodes include tethered fixed seabed nodes, easy deployable wireless nodes, catch monitoring for commercial fishery and Autonomous Underwater Vehicles (AUV). Sensors interfaced to the different nodes are touched upon, including scientific echo sounders. Further, the communication capabilities of the nodes are discussed with emphasis on wireless acoustic communication including PHY, MAC and network layer capabilities of the cNODE.

A Summary Comparison of Active Acoustic Detections and Visual Observations of Marine Mammals in the Canadian Beaufort Sea

Fisheries sonar was used to determine the applicability of active acoustic monitoring (AAM) for marine mammal detection in the Canadian Beaufort Sea. During 170 h of simultaneous observation by marine mammal observers and active acoustic observation, 119 Balaena mysticetus (bowheads) and 4 Delphinapterus leucas (belugas) were visually sighted, while 59 acoustic signals of bowheads were detected by AAM operators. Observations and detection of seals were also recorded. Comparative results indicate that commercially available active acoustic systems can detect seals at distances up to 500 m and large baleen whales at distances up to 2 km.