Feasibility study on the use of scientific multibeam sonar to characterize the Atlantic Bluefin Tuna spawning stock
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The Institute of Marine Research (IMR) and the Polytechnic University of Valencia (UPV) were invited by ICCAT Atlantic-Wide Research Program for Bluefin Tuna (GBYP) to carry out a feasibility study on the use of scientific multibeam sonar, Simrad MS70, to characterize the Atlantic Bluefin Tuna (BFT) spawning stock in the Balearic Sea. The study was carried out on board the research vessel B/O Ángeles Alvariño owned by the Spanish Institute of Oceanography. The aim was to map the distribution of BFT along a pre-defined track (part of an ictyoplankton survey) in the first part of the cruise and in the second part of the cruise do detailed measurements of BFT schools detected and measured by an aircraft. In the first part of the cruise 12 schools were detected with the MS70, but none of these were likely to be BFT schools. In the second part of the survey no BFT schools were detected by the aircraft in the coordinated search. There may be several reasons for the lack of BFT schools; too little time dedicated to school search, wrong area was covered or BFT avoided the research vessel. There was also a problem with electrical noise in the sonar, which reduced the effective range and made data processing more challenging. However, BFT were successfully measured with the MS70 sonar inside and near a transport cage. These data indicate that the MS70 is suitable for detailed measurements of BFT schools. However there is a need to validate estimates made with the sonar and there is a need for lateral target strength estimates of tuna at the frequency range of the MS70 sonar. We recommend that the project is continued, the experience from the current study used to improved survey design and focus is aimed at basic studies on the acoustic properties of tuna.