In situ target strength of bigeye tuna (Thunnus obesus) associated with fish aggregating devices

Bigeye tuna (Thunnus obesus) is an important commercial fish species, which aggregates around fish aggregating devices (FADs) together with other tropical tuna species. Acoustics is the main technology used by fishers and scientists for the location and quantification of tunas at FADs. However, currently it is not possible to reliably discriminate between the different tropical tuna species that are found together at FADs using acoustic methods, which hampers the development of selective fishing needed to preserve some of the tropical tuna species for which overfishing is occurring. One of the prerequisites for species discrimination is to know the target strength (TS) of each species at different frequencies. This paper measures in situ TS values and explores the frequency response of bigeye tuna at FADs in the central Pacific Ocean using three different acoustic frequencies. For the range of body length caught (40-100 cm), the obtained b20 values were -65, -66, and -72 dB for 38, 120, and 200 kHz, respectively. The decreasing frequency response pattern obtained for this swimbladder bearing species contrasts with the opposite pattern previously observed for skipjack tuna (bladder-less), the most abundant tuna species found at FADs, hence allowing the potential for discrimination between the two species.