Detailed in situ target strength measurements of 0-group cod

Abstract

Target strength measurements in situ of cod fry have been made using 38 and 120 kHz split beam echo sounders. Two data sets are sampled in essentially pure, evenly sized, cod concentrations, one in a Ullsfjord, Northern Norway, and the other one in Parisvatnet, a closed 270.000 m3 saltwater basin used for production of cod fry. The average length of the two cod populations were 5.1 and 3.1 cm, respectively. The target strengths measured are significantly lower than the values obtained if extrapolating the target strength to length relation used for larger cod towards small fish. The target strength measured at 120kHz is lower than at 38 kHz. Details on measurement procedures, biology and analysis are reported.