A method for controlled target strength measurements of pelagic fish, with application to European anchovy (Engraulis encrasicolus)

Measuring fish target strength (TS) in the wild is challenging because: (i) TS varies versus physical (orientation relative to the incident sound wave, size, and depth) and physiological fish attributes (maturity and condition), (ii) the target species and its aforementioned attributes are difficult to assess in near real time, and (iii) in the case of packed fish schools, accepted echoes may originate from multiple unresolved targets. We propose a method for controlled TS measurements of densely packed small pelagic fish during daytime, based on the joint use of a Remotely Operated Towed Vehicle, “EROC”, with a pelagic trawl fitted with a codend opening system, “ENROL”. EROC, equipped with a 70-kHz split-beam echosounder (Simrad EK60) and a low-light black and white camera, can be moved inside the fishing trawl. Pelagic fish are funnelled into the open trawl and their TS is measured in the middle of the net, where small groups actively swim towards the trawl mouth. The swimming behaviour allows for near-dorsal TS to be measured, minimizing the large effect of incidence angle on TS variability. The EROC camera, located near the open codend, provides optical identification of the species. This method was used to measure the TS of European Anchovy, Engraulis encrasicolus in the Bay of Biscay during 2014. The mean, near dorsal TS was −43.3 dB, for a mean fork length of 12.5 cm. This value is compared to published values of clupeiforms mean TS obtained for a range of natural incidence angles and discussed in the light of TS modelling results obtained for E. encrasicolus.

[1]  C. S. Wardle,et al.  Limit of fish swimming speed , 1975, Nature.

[2]  Ices Wgacegg.  Report of the Working Group on Acoustic and Egg Surveys for Sardine and Anchovy in ICES Areas VIII and IX (WGACEGG) , 2010 .

[3]  Effects of in situ target spatial distributions on acoustic density estimates , 2001 .

[4]  E. Ona.  An expanded target-strength relationship for herring , 2003 .

[5]  E. John Simmonds,et al.  Fisheries Acoustics: Theory and Practice , 2005 .

[6]  Manuel Barange,et al.  Acoustic identification, classification and structure of biological patchiness on the edge of the Agulhas Bank and its relation to frontal features , 1994 .

[7]  Mathieu Doray,et al.  Joint use of echosounding, fishing and video techniques to assess the structure of fish aggregations around moored Fish Aggregating Devices in Martinique (Lesser Antilles) , 2007 .

[8]  Heikki Peltonen,et al.  The acoustic target strength of herring (Clupea harengus L.) in the northern Baltic Sea , 2005 .

[9]  M. Barangé,et al.  Evidence of bias in estimates of target strength obtained with a split-beam echo-sounder , 1995 .

[10]  Yong Wang,et al.  Depth-dependent target strength of anchovy (Engraulis japonicus) measured in situ , 2008 .

[11]  Manual of fisheries survey protocols , 2014 .

[12]  Manuel Barange,et al.  Performance of a new phase algorithm for discriminating between single and overlapping echoes in a split-beam echosounder , 1997 .

[13]  K. Foote.  Fish target strengths for use in echo integrator surveys , 1987 .

[14]  R. F. Coombs,et al.  A re-evaluation of relationships between fish size, acoustic frequency, and target strength , 1996 .

[15]  Timothy K. Stanton,et al.  Sound scattering by cylinders of finite length. III. Deformed cylinders , 1989 .

[16]  Kouichi Sawada,et al.  Target-strength, length, and tilt-angle measurements of Pacific saury (Cololabis saira) and Japanese anchovy (Engraulis japonicus) using an acoustic-optical system , 2009 .

[17]  J. Jech,et al.  Boarfish (Capros aper) target strength modelled from magnetic resonance imaging (MRI) scans of its swimbladder , 2013 .

[18]  Rudy J. Kloser,et al.  Measurement and visual verification of fish target strength using an acoustic-optical system attached to a trawlnet , 2009 .

[19]  Timothy K. Stanton,et al.  Sound scattering by cylinders of finite length. II. Elastic cylinders , 1988 .

[20]  Mathieu Doray,et al.  Acoustic characterisation of pelagic fish aggregations around moored fish aggregating devices in Martinique (Lesser Antilles) , 2006 .

[21]  Pierre Fréon,et al.  Dynamics of pelagic fish distribution and behaviour : effects on fisheries and stock assessment , 1999 .

[22]  J. Oeffner,et al.  In situ target strength estimates of optically verified southern blue whiting (Micromesistius australis) , 2013 .

[23]  M. Barangé,et al.  Empirical determination of in situ target strengths of three loosely aggregated pelagic fish species , 1996 .

[24]  Sébastien Bourguignon,et al.  Overview of recent progress in fisheries acoustics made by Ifremer with examples from the Bay of Biscay , 2009 .

[25]  John K. Horne,et al.  Acoustic approaches to remote species identification: a review , 2000 .

[26]  T. Stanton.  Sound scattering by cyclinders of finite length. I: Fluid cylinders , 1988 .

[27]  K. Foote.  Importance of the swimbladder in acoustic scattering by fish: A comparison of gadoid and mackerel target strengths , 1980 .

[28]  E. Ona,et al.  Target strength and tilt-angle distribution of the lesser sandeel (Ammodytes marinus) , 2012 .

[29]  K. Foote.  Calibration of acoustic instruments for fish density estimation : a practical guide , 1987 .

[30]  G. Rose,et al.  A review of problems and new directions in the application of fisheries acoustics on the Canadian East Coast , 1992 .