Abstract
Broadband echo sounders are known to provide fine spatial image of each target fish in a dense
group. The total number of fish in an acoustic beam can be estimated by counting the number of
separated echograms. The target strength of an individual fish and beam attack angle to the fish body
provided the body length of each fish. However, counting the total number of fish and body size
distribution in a net enclosure or in the wild is difficult mission. Because double counting and missing are
unavoidable. Here we propose the acoustic re-capture method to count the number of fish including
missed ones in a net enclosure. The mark recapture is well known method for fisheries resource
management. An idea is to use Sonar Bell as an individual tag. The Sonar Bell has extremely high target
strength comparing with its size. Once it will be introduced in several tuna for example, high spatial
resolution sonar will be used to count the number of tagged and intact individuals in the sonar beam
remotely. The numbers of tagged and intact fish provide the virtual capture rate, which can be used to
calculute the total number of individuals including missed fish. Multiple acoustic recaptures such as every
10 minutes will provide precise estimation of total number of fish in a net enclosure.
