Target strength and tilt angle distribution of lesser sandeel (Ammodytes marinus)

Lesser sandeel (Ammodytes marinus) was investigated acoustically as well as using video camera in two field experiments in 2007 and one in 2008, both during North Sea sandeel surveys by the Norwegian research vessel “Johan Hjort”. Specially designed cubic cage was put on the sea bottom trapping sandeels inside in 2007, while in 2008 sandeels were caught by trawl and dredge and investigated acoustically in enclosed cage hanging in mid-water. In total 3 successful experiments conducted. Approximately 22 hours of acoustical and 11 hours of video data was available. Lesser sandeel target strength measurements extracted using LSSS acoustical data post- processing software by manual handpicking. Pictures from video data grabbed with Vegas Pro8.0 computer program and later carefully analyzed using ImageJ photo editing software. The way of calculating sandeel body tilt angle from data, collected with tilted camera, was developed by author. In total 5 mean TS values were calculated and presented separately by discussing strengths and possible error sources in each of them. One of the calculated means was decided to be possibly erroneous and was not used for calculating target strength-length relationship equation. Resulted sandeel target strength-length relationship equation is TS=20*log10(L) – 99.7. Sandeel tilt angle measurements from two experiment video data (2007 and 2008) differ from each other. 1.8º (±3.1º with 95% confidence interval) mean sandeel body tilt angle (as mean of all single tilt angle measurements) was calculated from 2007 video data and 23.3º (±3.0º with 95% conf.int.) from 2008 video data. Strong dispersion of single tilt angle measurements around the means was recorded (standard deviation of 24.1 and 25.4 for 2007 and 2008 data respectively)