L'acoustique passive (étude des bruits propres du milieu environnant) constitue une méthode simple, sinon facile, d'étude de l'activité et du comportement de la faune aquatique.
Nous décrivons ici les systèmes de prise de son et d'analyse utilisés à cet effet à l'Institut de Limnologie (Thonon-les-Bains) et rapportons certains résultats d'observations réalisées sur les zones de fraie des corégones (Coregonus lavaretus) du lac d'Aiguebelette et des ombles chevaliers (Salvelinus alpinus du lac d'Annecy : l'activité de fraie des corégones est caractérisée par des bruits brefs (0,5 à 2 s) de type "stridulation"; certains bruits de "fouille" sur substrat d'une omblière semblent directement associés à l'activité prédatrice des lottes (Lota lota) sur la ponte.
Underwater sound detection applied to aquatic ethology: some resuits on coregonids and charr spawing sites in two subalpine lakes
Listening to underwater sound is a simple way of studying the activity and behaviour of aquatic fauna.
The material used in the Institute of Limnology (Thonon-les-Bains) for recording and analysing sounds is described (figure 2). Experiments have been performed on the spawning sites of coregonids (Coregonus lavaretus) in Lake Aiguebelette and of charrs (Salvelinus alpinus) in Lake Annecy. This paper refers mainly to observations made in December 1987 on the site of "Roc de Chère" in Lake Annecy; this is where the charrs spawn and the site is easy to survey (figure 1). Most of the work consisted in continuous visual and acoustical observations, in order to determine the origin of the sounds recorded. A great part of the period was noisy because of water disturbance (wind-induced waves).
The most interesting aspect is the study of sounds produced by fish (figure 3). We could not record the activity of the charrs, because of the inconvenient area studied : the observation field was limited to 1 sq.metre, which is very small in comparison to the spawning zone (figure 1). The presence of burbot (Lota lota) in the field of the video camera suggested that these fish came and ate the charr eggs which had been deposited on the bottom. This hypothesis was confirmed by the continuous digging activity (which was heard and recorded) due to the burbot feeding and by an analysis of the stomacal content of specimens, caught white SCUBA diving, which contained fertile eggs. Though acoustically evident, burbot activity was not analysed.
The coregonids found in Lake Annecy (1987) (C. species) have the same behaviour as those found in Lake Aiguebelette (1986) (C. lavaretus, a different species). Typical, short (0.5-2 s) stridulating noises are produced during courtship, when the male and the female rub against each other; these noises, nearly always the same, have been analysed (figure 4). The loudest sounds emitted are in the 100-300 Hz range.
Another aspect of fish activity has been studied : the noise made by coregonids swimming (figure 4). During courtship, fish is very active and swims energetically. The low frequency of the sound has been recorded by using a special electronic device.
All these observations are in accordance with the results presented in the literature. Acoustical observation is an interesting method which should be developed.
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