This project gave us insight into the acoustic communication of gurnards by recording their sounds by hydrophones
Duration: 3 Months and more (project-specific)
Location: Helgoland
Scientists: AG Prof. Fischer / Dr. Emanuel Hensel
In the last few decades, the investigation of inter- and intraspecific communication between organisms has increased in importance, because the central priority of optimal adaptation and thus survival in the environment has become increasingly evident. This applies to both sound-specific communication and other forms, such as the chemical (with fragrances), or the tactile (by touch) etc. From a human point of view, conducting acoustic research in the medium of water did not appear attractive due to the impractical possibility there to efficiently exchange information by means of sounds. However, beyond the human auditory spectrum (in the air), nature has found ways to communicate acoustically in the physically much denser medium of water, in which other frequency ranges have established themselves in the course of evolution. Some species of whales communicate via infrasound (such as fin whales), others use ultrasound (such as dolphins). Not only marine mammals, but also fish exchange information in this way. The red gurnard (Trigla lucerna), native to the North Sea, produces sounds with its swim bladder and was therefore selected as a promising species for further studies. In the following, an insight into the process of concrete research is given on the basis of a model bioacoustic investigation. The basic question is: can fish talk? The hypothesis to be tested is: Yes, fish can speak!
Because the previously widespread attribution “as mute as a fish in water” is no longer tenable according to current scientific knowledge, as it has been found that more than 800 – 1,000 species of fish communicate with sounds. In order to analyze the tones, they have to be recorded, evaluated, explained and interpreted. How do you record acoustic signals underwater? With hydrophones – microphones that, due to their technical nature, can record sounds underwater.
But “simple” recording alone did not lead to usable results in the first test phase, because due to the relatively small area (1.80 m * 1.80 m = 3.24m2) of the aquarium tank which was used as a test tank, and the fact that if there was a hydrophone in each corner for optimal recording, there was annoying interference.
In addition, the fish sounds were overlapped by disturbing ambient noises, i.e. pump noise. Therefore, the audio technology and the detector software had to be adapted accordingly. In order to enable a simplified and improved evaluation, not only acoustic but also visual recordings were implemented. This made it possible to record situations as a whole and to assign certain behaviors to sounds (and vice versa). In addition to the display tank, a holding tank was also used for the experiments. The display tank was optimized for the experiments by appropriate modifications. This included the installation of a lightproof covering above and around the pool in order to avoid unwanted light reflections, as well as being able to regulate the lighting conditions in the tank independently of the ceiling lighting.
The daylight was generated by two halogen spotlights that shone into the tank from above.
A second light source above the tank was used for lighting at night; it provided a weak light and was additionally muted by a transmission filter. This also ensured that the lighting conditions corresponded to the natural in situ conditions and that sufficient light was still available for video recordings at night. The video recordings were made using a video camera installed above the tank. The recordings were recorded on a computer around the clock. The water level in the test tank was 40 cm. The soil substratum consisted of fine sediment, which corresponds to the natural habitat of the gurnard. Both this tank and the holding tank were supplied with filtered seawater from the North Sea in the through flow (approx. 2 L / min.), so that almost the same conditions prevailed as in nature. However, since the animals migrate to warmer areas in their natural habitat at temperatures below 6 ° C or become increasingly lethargic, the water temperature was kept at at least 11°C. The animals used for the experiments were weighed and measured before, so that they could be differentiated afterwards. If animals could not be distinguished by significant differences in size, they were color-coded subcutaneously. The animals were kept in a holding tank measuring 80 x 120 x 100 cm, in which the same water conditions (composition, temperature) were implemented as in the test tank. The animals were fed ad libitumin the holding tank on the 1st, 3rd and 5th day with live North Sea shrimp Crangon crangon. After moving into the experimental tank, a day of acclimatization was allowed. From the second day onwards, the animals were fed every day between 10:00 and 14:00. The feeding took place via an inlet pipe into the aquarium (= feeding pipe) by adding the North Sea shrimp one by one into the pipe and flushing it into the tank with 1 liter of seawater. The waiting time between each shrimp was 5-10 minutes, a total of five individuals were placed in the experimental tank. In this way, direct access to the experimental tank was avoided, which could have influenced the animals in their behavior or lead to habituation. According to this feeding method, the gurnards also had to actively compete for food. This corresponds more to the natural conditions than pure ad libitumfeeding and should lead to natural acoustic communication.
As can be seen from this brief description of a potential experimental setup and the corresponding implementation, a large amount of motivation, energy, meticulousness, flexibility, perseverance, as well as biological, technical and statistical expertise are necessary in order to achieve usable and meaningful results, and thus to be able to meet generally valid statements. However, since this form of knowledge acquisition and the associated transfer of knowledge contribute to the preservation of nature, the survival of species and thus ultimately human existence, the Aquarium Research Foundation e.V. has set itself the goal of promoting research in the aquarium.
Author: Dr. Emanuel Hensel