Hostname: page-component-848d4c4894-nmvwc Total loading time: 0 Render date: 2024-06-19T14:00:32.858Z Has data issue: false hasContentIssue false

An acoustic approach to study tuna aggregated around fish aggregating devices in French Polynesia: methods and validation

Published online by Cambridge University Press:  15 September 1999

Erwan Josse
Affiliation:
Centre IRD de Bretagne, BP 70, 29280 Plouzané, France
Arnaud Bertrand
Affiliation:
Centre IRD de Bretagne, BP 70, 29280 Plouzané, France
Laurent Dagorn
Affiliation:
Centre IRD de Montpellier, BP 5045, 34032 Montpellier cedex 1, France
Get access

Abstract

The behaviour and spatial distribution of tuna, aggregated beneath fish aggregating devices (FADs), have been studied through ultrasonic tagging experiments but, surprisingly, very few studies on FADs have used underwater acoustic devices. We present techniques, and their limits, incorporating a scientific echo sounder connected to a split-beam transducer to observe and characterise tuna aggregations around FADs, and propose a general approach for future studies. Experiments were conducted in French Polynesia between December 1995 and February 1997. Two methods, echo-counting and echo integration, were used. Echo-counting is possible when individual fish are sufficiently scattered so that each target can be discerned. On the other hand, echo integration can be used with both scattered and aggregated fish schools. The knowledge of tuna target strength is useful for separating targets for echo-counting, and essential for obtaining absolute estimates of densities by echo integration. Sonar performances and settings should be considered when choosing the most suitable method to determine fish density or assessing spatial structure of a tuna aggregation. These techniques allow one to study an entire tuna aggregation, its behaviour in space and time at very fine time–space scales (about a nautical mile and over a few hours), and open up a new scientific field to study the spatial structure and behaviour of tuna aggregations around anchored or drifting FADs.

Type
Research Article
Copyright
© Elsevier, IRD, Inra, Ifremer, Cemagref, CNRS, 1999

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)