Multibeam omnidirectional sonars are tools currently used by
fishers, but also allow the monitoring of pelagic fish schools surrounding a
platform. Multibeam processing methods now offer improved capacities for raw
data storage. The Simrad SP90 sonar was used for the detection of fish
schools associated with drifting fish aggregating devices (FADs), and
digital systems developed for the acquisition and processing of volume
backscattering echoes and position data. Data sampling methods were defined
based on two modes: one for periods searching for FADs and associated
schools, and one for school monitoring in drifting mode. Validation of the
detection of several FAD-associated schooling species was made by
simultaneous visual observations or/and cross-checking with echosounder
recordings. The characteristics of schooling behaviour in the targeted fish
species are fundamental for the correct interpretation of acoustic data.
Sonar detection threshold is the result of a compromise between fish number,
size, species and the nearest neighbour distance (NND) of individuals per
dynamic structure (school or shoal). Tuna schooling dynamics mean that NND
can sometimes be too large to allow the presence of these fish to be
detected, despite their number. Sonar data should be analysed and
interpreted in a holistic manner, in combination with behaviour pattern and
dynamics of all species around the drifting FADs. An autonomous sonar buoy
prototype equipped with 360° scanning sonar coupled to video cameras
will increase our understanding of tuna behaviour around drifting or
anchored objects. A similar methodology can be applied to different kinds of
platforms, either anchored or in permanent positions. This would improve the
monitoring of fish schools around artificial reefs, open sea aquaculture
farms, and across estuaries, channels or straits; applications which are
undoubtedly essential for progressive fisheries management.