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Introduction
More than half of all vertebrate species are fishes, having 800 species in over 100 families described to produce sounds which also include many commercial species.
Fish Sound Charecteristics
usually of lower frequency (<2000Hz)
short duration (<5 seconds)
poor frequency modulation
relatively limited acoustic repertoires
The study was conducted in an intertidal region of the Tagus estuary. The species being studied were the Lusitanian Toadfish (Halobatrachus didactylus), weakfish (Cynoscion regalis) and meagre (Argyrosomus regius)
Lusitanian Toadfish
Why do fish produce sounds ?
Fishes produce noise for multiple reasons
Territorial defense
Feeding competition
Mate attraction
Courtship
Spawning
Why do we need to study fish sounds?
Studying fish sounds can provide us with some valuable information like
biological and ecological processes
spatial and temporal variability
ecosystem health
monitoring of key species
How do fish produce sounds?
Fishes produce sounds in a number of different ways some of which are shown here along with sonograms which show the different sounds produced by the different mechanisms
(a) Intirinsic sonic muscles (SMi) attached to swim bladder lobes (SL) for vibration in the Lusitanian Toadfish (Halobatrachus didactylus)
(b) Extrinsic sonic muscles (SMe) originating at the second rib (2R) and inserting on a broad tendon (BT) ventrally of the swim bladder in the black piranha (Serrasalmus rhombeus)
(c) Stridulatory mechanism in catfish a ridged dorsal process (DP) of the pectoral spine (PS) rubs in a groove of the shoulder girdle (SG)
(d) Enhanced pectoral fin tendons (ETs) are plucked similar to guitar strings in the croaking gourami (Trichopsis vittata)
(e) Pharyngeal teeth (PT) stridulation in damselfish
(Ladich, 2014)
Professional Practice Tasks
Software
There are multiple sound analysis softwares available but the one I used was Raven Pro 1.6.1. To visualise the sounds the different views can be used which are the waveform or oscillogram, the spectrogram and the power spectrum. There is also a table of measurements that provides different values.
Raven Pro
Distinguishing between species
The first part of what I had to do involved analysing files from loggers that were setup in the Tagus estuary in Lisbon, Portugal. The aim of the task was to check the occurence of weakfish (invasive species) in space and time and compare how much it overlaps with meagre (native species).
I had to distinguish between the two different species. This was be done by visualing the sounds on the spectrogram and they both appear different as seen in the image. For the most part the vocalisations are not as clear as shown in this image due to anthropogenic noises and various other disturbances so one would have to rely on visualising as well as listening to the recordings.
Sound Analysis
Next I learnt how to measure the frequency values like peak frequency, frequency 25% and frequency 75% (which divides the selection into 2 frequency intervals that contain 25% and 75% of the energy) which were obtained using the power spectrum. Other values measured included pulse period, number of pulses which were obtained using the waveform.
Field Setup
I assisted in setting up of an ongoing playback experiment the aim of which is to look at if boat noises have any effect on toadfish eggs and if male toadfish calls can counter that effect. Also assisted in collection of toadfish eggs for the analysis of the embryos.
Conclusion
I thoroughly enjoyed and learnt a lot during this professional practice and was able to gain a lot of knowledge in the field of bioacoustics. At the same time got to interact with different people working on different projects which further enhanced my knowledge of the subject.
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