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11-08-2010, 01:03 PM #1
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Hydroacoustic monitoring of fish in sea cages: target strength (TS) measurements on A
By F.R. Knudsen, J.E. Fosseidengen, F. Oppedal, O. Karlsen, E. Ona
Fisheries Research, Volume 69, Issue 2, September 2004, Pages 205-209, ISSN 0165-7836, DOI: 10.1016/j.fishres.2004.05.008.
The aim of the present study was to establish the relationship between target strength (TS) and body length of Atlantic salmon (Salmo salar) in sea cages. Five size-groups (mean total length 20, 25, 55, 67 and 78 cm, n = 6-17 fish per group) of Atlantic salmon were used in the experiment. The fish were monitored in a sea cage (12 m x 12 m x 20 m) using an echosounder with two split-beam transducers (120 and 200 kHz). The two transducers were mounted side by side in the centre of the cage. Measurements were made with the transducers in the bottom of the cage transmitting upwards (ventral recordings), and with the transducers at the surface transmitting downwards (dorsal recordings). Underwater video was used to observe the tilt angle of the fish.
A bi- and tri-modal TS distribution was found in dorsal recordings of the smallest size-groups (20, 25 and 50 cm), but this was not evident for the two largest groups (67 and 78 cm). As a result, the TS-to-length relationship for dorsal recordings was rather poor. However, when the recording was made ventrally, the groups showed a unimodal TS distribution. A good correlation was then found between TS and fish length, both for 120 and 200 kHz.
Keywords: Atlantic salmon; Target strength; Sea cages