Green Turtle Hearing Identification Based on Frequency Spectral Analysis

It is reported that fishing activities equipped with TED (Turtle Excluder Device) able to reduce the number of turtle by catch. This paper aimed to design a TED using sound technology. In order to dispel a turtle using sound, it is necessary to perform on turtle hearing ability. This study focuses on Green Turtle (Chelonia Midas) species. Three turtles have been measured. They have different age ie: 2, 5, and 9 years. A system known as Auditory Brainstem Responses (ABR) has been used on the green turtle detail analysis. The objective of this paper is to identify green turtle hearing ability by analyzing the ABR spectral. The measurement was taken at Turtle Management Centre Kg. Pantai Panjang Segari Perak Malaysia. The duration measurements were 3 days, December 30, 2008 – January 2, 2009. It was conducted out the turtle tank. From the data obtained can be seen that the 2-year old turtle has a hearing range narrower than 5 years old turtle while 9 years old turtle has the widest frequency range. As for the three types of turtle age, has a maximum power at the same frequency of 300 Hz.


Introduction
Sea turtles are unfortunate bycatch in the longline fisheries, mainly because they share the same habitat as fish species targeted by this type of fishing activity.There are seven species of sea turtles around the world whereas five species of them are listed in the official list of endangered species from Brazil (MMA 2003).It is impacted by human activity and need conservation measures.(Parentel, et.al., 2006).The green turtle (Chelonia mydas), the loggerhead turtle (Caretta caratta), and the olive ridley turtle (Lepidochelys olivacea) are classified as endangered species, while the hawksbill-sea-turtle (Eretmochelys imbricata) and the leatherback-turtle (Dermochelys) coriacea) are classified as critically endangered (IUCN 2004).
Fishermen to catch fish and shrimp in accordance with the rules must be equipped with a TED (Turtle Excluder Device).TED that is equipped in the nets and trawls will be able to reduce the number of turtle bycatch.The inventor of TED is Sinkey Boone, a Georgia shrimper in 1969 (Marine Turtle newsletter).Since the TED was first introduced to the U.S. shrimp fishery in the late 1980's, research and development to improve TED performance has continued.Implementation of TED in net and trawl can be seen in Figure 1.
Although the hearing ranges of most species overlap to a large degree, considerable variation occurs in high and low frequency hearing as well as in absolute sensitivity (Heffner, 2007).Lenhardt (2002) suggest that continued exposure to existing high levels sound (noise) in vital sea turtle habitats and any increase in noise could affect sea turtle behavior and ecology.Thus, turtle repelling with sound is possible and can be made practical.(Kuwada S, et.al, 2002) ABR signals have a waveform morphology which typically exhibits five waves (peaks) in the post-stimulus interval.The basic of the ABR signal can be seen in Figure 2. Auditory Evoked Potential (AEP) signals are transient electrical bio-signal produced by various regions of the human brain in response to auditory stimuli (such as a periodic repetition of "clicks").These signals are traditionally categorized into: ABR which occurs during the first 11 ms after the stimulus, followed by a Mid-Latency cortical Response (MLR) which is typically confined to the next 70 ms, followed by a slow cortical response which starts at about 80 ms after the stimulus (Elvir, et.al, 2005).The first description of the human ABR was made by Jewett and Williston in 1971 The ABR responses are acquired from the animal subject in order to asses and determine the cranial auditory nerve function.In humans, similar techniques have been conducted to asses the viability of cochlear implantation, or the existence of any physical conditions that may prevent successful implantation (Intelligent Hearing Systems, 2007).The basic test of hearing consists of determining the ability of an animal to hear pure tones at intervals throughout its hearing range.The testing is done by training an animal to respond to a tone and then reducing the tone's intensity until the animal fails to respond.There is an overlap of those frequencies and the audible frequency range of sea turtles which perceive sounds from 60 to 1000 Hz (Parentel, et.al, 2006).(Sneary, et.al, 2007) have studied that provided a basis for understanding auditory hair cell frequency selectivity in the turtle (Lovell, et.al., 2004), the mechanism of sound reception and the hearing abilities of the prawn (Palaemon serratus) have been studied using a combination of anatomical, electron microscopic and electrophysiological approaches, revealing that P. Serratus is responsive to sounds ranging in frequency from 100 to 3000 Hz.ABR data have been analyzed for 10 turtles (juvenile and subadult C. mydas and juvenile L. kempi) and two of tuna (Thunnus albacares) using a correlation technique to identify the response to sound (Bartol and Ketten, 2006).The AEPs technique is a noninvasive and rapid method to measure the hearing range and temporal resolution of animals.It is a method that requires no training of the subject and is used also to asses hearing response in human infants (Hecox and Galambos R, 1974) The collective knowledge of marine hearing and sound detection capabilities is very limited.Fisheries, which has jurisdiction over most marine mammals and all sea turtles in the region, has not set any standard measurement for safe sound levels.This reason is because limited knowledge of true hearing thresholds of such animals and their sensitivity to various sound levels, frequencies and durations (Peter Howorth, 2003).Amount of 12 hatchling through sub-adult turtles (Lepidochelys kempi, Chelonia mydas, Caretta caretta) show juvenile green turtles have a slightly broader hearing range (100-800 Hz; best sensitivity 600-700 Hz ) than sub-adults (range 100-500 Hz).Kemp's Ridleys had a more restricted range (100-500 Hz) with most sensitive hearing at 100-200 Hz (Ketten, et.al, 2005).
Works describing techniques for extracting the ABR from the EEG are given in Elvir, et.al, (2005).A new algorithm of adaptive filtering in the time-frequency domain using a specific Wavelet Transform and compare it to traditional ABR extraction in the form of compare it to traditional ABR extraction in the form of bandpass filtering followed by averaging as well as adaptive filtering in the Fourier domain proposed by Elvir, et.al.(2005).One of methods to measure the temporal resolution of the auditory system is to estimate the modulation rate transfer function (MRTF) using AEP.Popper, (2000), have used ABR to measure hearing capabilities of several other clupeid species in order to ascertain whether ultrasonic hearing is found in all Clupeiformes, or whether it is only found in a limited number of species.A typical MRTF is low pas in shape, and the corner frequency of that MRTF can be inferred as the temporal resolution of the subject (Supin AY, et.al, 2001).
The extraction of ABR from an EEG signal did by Jacquin et.al.It is based on adaptive filtering of signals in the wavelet domain, where the transform used is a nearly shift-invariant Complex Wavelet Transform (CWT).They compare the algorithm to two existing methods: the first simply consists of band-pass filtering the input EEG signal followed by linear averaging.The second method uses signal-adaptive filtering in the Fourier domain based on phase variance computed at each spectral component of the FFT.Davey, et.al. (2007) classified ABR models by generated using time, frequency and cross-correlation measures.Classification employed both artificial neural networks (NNs) and the C5.0 decision tree algorithm.Moneey Aran, et.al., 2006, using FFT to quantitatively determine the Envelope Following Response (EFR) magnitude.Singleton and Poulter, (2007) the signal analyzed, peaks in the estimated spectrum were observed at each integer multiple of the fundamental frequency.The smoothed spectrum is used as an estimate of the power spectral density function.

Subject and facility
The measurement was taken at Turtle Management Centre Kg.Pantai Panjang Segari Perak Malaysia.The duration times were 3 days, December 30, 2008 -January 2, 2009.It was conducted outside the turtle tank as shown in Figure 3.
The species of the turtle is Green Turtle.The measured Green turtles have different age as shown in table 1.The SmartEP software setting is shown in Table 2.

ABR Recording and measurement
The ABR measurement last for 1 hour (Figure 4).Click stimulus is used in this measurement.The frequency range of ABR analyzer is between 0 to 5 kHz.The species were equipped with ABR system and been hold in stable position.The species was made relaxed in that resting position.The recorded ABR signals saved in rpt format.Later the measured data have been analyzed using SmartEp software.
The click stimulus records were divided into 25 durations.The durations were set from 100 μs up to 2500 μs.The recorded data saved into three modes that are time domain, ASCII code, and data reported (rpt).Furthermore, the data were analyzed in frequency domain using FFT (Fast Fourier Transform) method that included in SmartEP software.

Results and discussions
Green Turtle were equipped with ear electrode (earphones).The earphone delivers the stimulus to the turtle's ear.Four earphones were used to measured turtle hearing threshold.The collected data collected indicates the response of the turtle.
The click stimulus was emitted started in 100 μs duration time.The ABR response signal was obtained in the time domain signal as shown in Figure 5. Later, it was transformed into frequency domain with SmartEp.Frequency spectral was obtained with a stimulus duration 100 μs has maximum power at frequency 300 Hz (Figure 6).The frequency spectral has the harmonic at a frequency of 900 Hz.Later the measurement was repeated until 2,500 μs with 100 μs interval.The frequency spectral is shown in table 4, table 5, Table 6 for 2, 5, 9 years turtles, respectively.

Conclusion
The frequencies spectral of green turtles have been analyzed in frequency domain.Green Turtles hearing ability can be obtained that the 2-year old turtle has a hearing range narrower than 5 years old turtle while 9 years old turtle has the widest frequency range.As for the three types of turtle age, has a maximum power at the same frequency of 300 Hz.Sneary, M.G., Lewis, E.R. (2007).Tuning Properties of Turtle Auditory Nerve Fibers: Evidence for suppression and adaptation.Hearing Research Research, 228 (207), 22-30, Elsevier. Supin AY, Popov VV, Mas Am. (2001).The sensory physiology of aquatic mammals, Kluwer, Dordrecth.
IHS system ABR signal analyzer.

Table 1 .
Green Turtle species for the ABR Measurement

Table 2 .
ABR setting for turtle hearing ability measurement