In addition, the results from such studies are a useful instrument for validating the tuna forage abundance prediction models. Unless every single fish is caught, any attempt to estimate the abundance of a fish population will necessarily be biased. The abundance measurements carried out with passive gear such as the longline are particularly biased. Until now, tuna observations were only possible when they were actually caught or present in the form of a surface aggregation. The only source of data to study distribution and estimate scattered tuna abundance over a range of depths up to 500 m was longline catches.
Acoustic methods make it possible to observe fish echoes throughout the water column.
No research had been carried out on measuring tuna TSs. A summary of the two methods used to measure the TS of free-swimming adult tunas will be presented. This method is appropriate for field measuring the target strength of large pelagic fish. TS variability can be studied with reference to the behaviour of a clearly identified fish swimming freely in its environment. Such a method is particularly useful for scattered deep-swimming large fish.