The Simrad SN90 sonar with its high resolution, showed good results for counting individuals in schools, and methods for automatic and objective counting individual fish are investigated. It was further clear that visual monitoring provides useful extra information but, is inefficient in monitoring the stock in a routine survey as daylight, good weather conditions and fish close to surface are required. It was possible to identify bluefin tuna schools based on the sonar signature and swimming speed at a range up to 800 m.
The results indicate that the omnidirectional medium frequency sonar (Simrad CS90) was the best sonar for detecting and inspecting the schools out of the sonars used in the survey. Only operational at low vessel speed < 5 knots Only short range (< 100 m) detection and small sampling volume. Iv) Kongsberg M3 (500 kHz high frequency, 120 deg sector): Best for fish counting. Limited sampling volume compared with omni medium frequency Iii) Simrad SN90 (75 kHz medium frequency, 160 deg sector): Good medium range detection (400 m), good for identifying BFT school, suitable for fish counting in medium and small size schools. Ii) Simrad CS90 (82 kHz medium frequency, 360 deg omni): Good for long range detection (800 m), good for identifying BFT school, suitable for fish counting in medium and small size schools I) Simrad SU90 (26 kHz low frequency, 360 deg omni): Good for long range detection (800 -1000 m), Not suitable for fish counting (except few fish in soldier formation) and not the best for identifying BFT schools
The advantages and disadvantages of the different sonars used can be summarized as: Ytterstad (75 m long) with promising preliminary results for acoustic methods for monitoring the distribution, abundance and behaviour of bluefin tuna in Norwegian waters. The major aim of this survey was to map and quantify distribution, school size and behaviour of Atlantic bluefin tuna ( Thunnus thynnus) migrating to and feeding in Norwegian waters.The survey covered the region between Bergen and Molde, between 4 th to 15 th October, onboard hired vessel M.
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