| Abstract |
The aim of this work is to develop a novel experimental technique to investigate the effect of sound on the behavior of various cultured species of fish, at first, and subsequently to demonstrate the usefulness of this effect. Three species are investigated: gilthead seabream (Sparus aurata), seabass (Dicentrarchus labrax), and red porgy (Pagrus pagrus).
A digital imaging system was developed to monitor fish position in 2 dimensions as a function of time. The system includes 2 dedicated software packages for the storage and analysis of the data directly on a laptop and can be connected to a sound production system including a sound generator, an amplifier and a water-proof loudspeaker set in the water. Altogether, these constitute a portable, water-proof, easily-used instrument for the study of the effect of sound on fish behavior, and it was successfully tested in a small tank, large tank, sea cage, and in the field. The contrast of the image can be adjusted according to lighting conditions. The data-collection software records the positions of the fish as a function of time (every 1/15 of a second). The instrument can also operate with a portable, rechargeable battery.
The experiments were conducted in a) a small tank (110 l) with 2 populations of S. aurata (gilthead seabream) weighing 5.2 g and 12.0 g respectively, 1 population of D. labrax (seabass) (∼1.6 g) and 1 population of P. pagrus (red porgy) (∼1.7 g) b) a large tank (10 m3) with a population of S. aurata (∼250 g) c) in a sea cage of an aquaculture establishment with a population of S. aurata (∼150 g), and d) the Amvrakikos gulf (Menidi) after the release of the population of S. aurata being in the sea cage. The method of the conditioned response was applied, whereby the new stimulus, the sound, is followed by a reward: feeding. In the small tank, experiments were conducted at 6 different frequencies (100, 200, 500, 800, 1250, and 2500 Hz), and additionally also at 5000 Hz for the gilthead seabream, and at 5 levels of sound (-62, -42, -32, -20 και -10 dBu). In the large tank, experiments were conducted with 3 different frequencies (200, 500, and 800 Hz), and at 1 level of sound (-20 dBu), but also using sound with the timbre from 4 different groups of instruments: strings, woodwind, percussion, and electric instruments. In the sea cage, experiments were conducted with 5 different frequencies (100, 200, 500, 800, and 1250 Hz) and at 1 level of sound (-10 dBu), whereas in the gulf 3 frequencies were used (100, 200, and 500 Hz) at -10 dBu.
The response to the sound was judged by the location of the fish with respect to the sound source before, during, and after production of sound. A positive response was observed for all three species of fish and in all developmental stages studied. The aggregation of the fish at the point of feeding is judged as a positive response. In the experimental setup used for most measurements, the feeding point and the sound source were located closely to each other. However, when the sound source was moved to other locations within the tank, the gilthead seabream located it both optically and by sound (during operation). A positive response was obtained more often when the feeding was accompanied or immediately followed by the sound and in that case a sound burst lasting 15-30 s was ideal.
We determine that D. labrax and S. aurata are “hearing generalists” (non-specialists), that is they respond positively at low frequencies until 800 Hz. P. pagrus responded positively only at 500 Ηz. The responses are independent of the sound
levels in the small tank. In the large tank we chose to use our loudest setting only. Sounds from musical instruments produce a positive response on fish which have been previously conditioned at a single frequency. However, fish did not respond, when music was played in the air (not transmitted with the underwater loudspeaker). The response to the sound is affected by abiotic factors (especially in the field), but not by the time of day.
The change in behavior caused by the application of the sound pulses in the small tank appears between the 2nd and the 4th day of the experiments, with a progressive improvement of the response as long as the conditioning is going on. For the gilthead seabeam positive responses can be observed even after 1 month without any stimulation. The change in behavior caused by sound stimulus in the large tank appears on the 4th day of the experiment and is not affected by the change to the other two frequencies tested. In the sea cage a weak response was observed on the 3rd day, which, however, was not repeated on subsequent days, likely due to the abiotic factors.
The sound produces a positive response even in the presence of high levels of noise (caused by water pumps, water intake of the tank, generators, and low-flying aircrafts) as long as feeding is offered.
The movement of the 3 species in the small tank shows some differences. The seabasses move quickly, mainly horizontically and they move vertically during stimulation (either sound or feeding). Once the sound begins they form a school and they move together from the sound source to the feeder. The red porgies move quickly up and down and they tend to form a school when the sound is produced. The gilthead seabreams move about the tank without a repeated up-down movement, and they show a definite preference to reside in the central regions of the tank. When the sound is produced, they gather at the sound source without forming a school.
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