Typology and behaviour of tuna aggregations around fish aggregating devices from acoustic surveys in French Polynesia

Eighty-seven two-hour acoustic surveys (radius 0.8 nautical mile, vertical range 0-500 m) around 17 fish aggregating devices (FADs) were conducted in French Polynesia between December 1995 and February 1997. Associated tuna d ensities were calculated using two different techniques: echo counting when the fish had sufficient distances from each other and echo integration whe n the fish swam close together tin schools). No acoustic detection of tuna was observed during 27 of the 87 surveys, representing 81 % of all the noct urnal surveys and 15 % Of the diurnal ones. The 60 other surveys showed thr ee different classes of aggregations: (1) 'deep scattered fish', observed 4 5 times, (2) 'intermediate scattered fish', observed 16 times, and (3) 'sha llow schooling fish', observed 16 times. Sometimes aggregations of differen t classes were observed beneath the same FAD. The size of the fish inside t he aggregations (determined from target strength values), the distance betw een the individuals, and the depth of the fish all decreased from 'deep sca ttered fish' to 'shallow schooling fish' (100-300 m for 'deep scattered fis h', 50-150 m for 'intermediate scattered fish', and above the depth of 50 m for 'shallow schooling fish'). Fish densities also varied according to the class of aggregations: 7.3, 26, and 801 fish pel km(3) on average for 'dee p scattered fish', 'intermediate scattered fish', and 'shallow schooling fi sh', respectively. The highest densities were observed during daytime, whil e night-time observations indicated a variety of situations, from the absen ce of individuals to large amounts of fish. (C) 2000 Ifremer/CNRS/INRA/IRD/ Cemagref/Editions scientifiques et medicales Elsevier SAS.