This work presents a method to observe pelagic fish
around drifting fish aggregating devices (DFADs). A
triple-frequency vertical echosounder was employed to observe fish
distributions in the vicinity of DFADs. Surveys were conducted in a star
pattern that was centred at the DFADs. The objective of the study was to
define a methodology for future acoustic studies. This goal was pursued by
(i) studying the spatial distribution of fish aggregations, (ii) developing
concepts for the grouping of observed aggregations and (iii) developing
specifications for future autonomous acoustic tools. For this purpose 5
cruises were carried out in the western Indian Ocean. The multi-frequency
approach proved useful as a means of separating acoustic detections into
sound-scattering layers (e.g. plankton and micronekton), fish aggregations
and individual fish. Fish target strength (TS) was measured. Four types of
aggregations were found near DFADs: (i) dense structure (ii) medium
structure (iii) loose structure and (iv) structure consisting of separated
targets. More than 90% of these structures were found within a radius of
400 m and about 75% within 200 m of the DFADs. The spatial configuration
of DFAD fish aggregations appeared to be more dynamic compared to
aggregations near moored FADs. The spatial distribution and structure of
DFAD aggregations have direct implications for their catchability by tuna
purse-seiner. We have carried out the first quantitative acoustic recordings
around DFADs, and obtained a better understanding of the spatiotemporal
dynamics of fish aggregations around DFADs in the Indian Ocean. Based on
this knowledge we are now working on specifications for instrumented buoys
that are intended as autonomous data recording observatories for such
pelagic environments.