This project has developed knowledge and methods to enable the estimation of the length of medium sized pelagic fish prior to them being caught. The project uses broadband echosounders to detect echoes from parts of individual fish and from this estimates their length. The project has demonstrated the method using fish-like artificial targets, dead tethered fish, free-swimming fish in large cages, and wild fish in the open ocean. The aim of the project was to develop methods and knowledge that can improve the size selectivity of commercial fishing operations and hence reduce fish discarding and unwanted by-catch.
The measurements on artificial targets and dead fish showed that the method can measure the length of fish that are longer than about 20 cm with an accuracy of about 2 cm when using a broadband signal of 160 to 260 kHz. Some types of acoustic pulse are better than others at measuring fish size and the project has developed recommendations around this for use in the field. A procedure for automating the processing of the acoustic data has been developed that will facilitate the use of acoustic remote sizing in operational situations. A procedure that removes the need to know the orientation of fish relative to the echosounder has also been developed - this eases the implementation of the sizing method in operational situations.
The project has raised competency in the use of broadband acoustics and has the desirable outcome of providing a method to directly, remotely, and non-destructively estimate the size of pelagic fish in their natural environment. The impact of this will be to reduce the need to catch fish during research surveys and to reduce unwanted catches in commercial fishing.
Recent discoveries made during analysis of broadband echoes from resolved single targets of commercial fish indicate that the Fast Fourier Transformed echo spectrum contains more information than presently utilized in standard fishery acoustic analysis. The project will through controlled experiments investigate how the echo formation is affected by the physical size of the target, and the internal organs of the target. Similar to ultrasound imaging, the echo formation in complex elastic targets is quite complex, and not fully described or understood. In narrowband acoustics, as utilised by present echo sounders, this information is hidden in the echo summation process. The project will first describe the echo formation of fish and artificial targets in laboratory conditions and then do field trials with special transducers. Sizing of the fish remotely is the primary goal of the project, but measurements on body elements like gonads and fat may also be within reach.