Hydroacoustic ex situ target strength measurements on juvenile cod (Gadus morhua L.)

Most TS-measurements on fish have been carried out for 38 kHz, and the exis ting TS algorithm for 120 kHz on cod is based on measurements on stunned fi sh. The main objective of these experiments was to establish an empirical e stimate of the relation between acoustic reflection (target strength, TS) a nd length of live juvenile cod (7-10 cm and 15-20 cm) at 120 kHz. This was done by recording the variation in TS of freely swimming cod tracking singl e fish targets for the two size groups within the acoustic beam held. The e xperiment was set up in an open air 2000 m(3) tank where the small 5-10 cm long fish were swimming freely during measurement in cages (1 x 1 x 3 m) wi thin the acoustic beam under natural conditions in seawater with a salinity of 30 and a temperature of 11 degrees C. An EY500 split-beam acoustic syst em was used to detect single fish passing through the acoustic beam field, which was Video recorded in order to isolate the measurements on single tar gets and to get an indication of their angle. A mean target strength-to-siz e relation was calculated for small cod based on single fish tracks with to tal acoustic angles below 3.5 degrees off axis in the beam field. This rela tionship is compared to other TS measurements on juvenile cod in literature . TS at 120 kHz for the investigated cod size range seems to decrease faste r by length than the 20 logL relation used for larger cod. The results were used to check the expected range limits of TS for juvenile cod during surv ey, and are expected to be taken into consideration in density estimation o f juvenile cod during acoustic surveys targeting young gadoids in general. (C) 1999 International Council for the Exploration of the Sea.