Split Beam Echo Sounder (Acoustic Systems) to Determine Abundance of Fish in Marine Fisheries

Acoustic waves are transmitted into the subsurface ocean will experience scattering (scattering) caused by marine organisms; material distributed in the ocean, the structure is not homogeneous in seawater, as well as reflections from the surface and the seabed. Estimation of fish stocks in the waters wide as in Indonesia have a lot of them are using the acoustic method. Acoustic method has high speed in predicting the size of fish stocks so as to allow acquiring data in real time, accurate and high speed so as to contribute fairly high for the provision of data and information of fishery resources. Split beam echo sounder comprises two aspects, and a transducer. First aspect is the high-resolution color display for displaying echogram at some observations and also serves as a controller in the operation of the echo sounder. The second aspect is transceiver consisting of transmitter and receiver. Echo sunder divided bim first inserted into the ES 3800 by SIMRAD beginning of the 1980s and in 1985 was introduced to fishermen in Japan as a tool for catching up. Split beam transducer is divided into four quadrants. Factors that contribute affect the value of Target Strength (TS) fish Strength target can generally be influenced by three factors: a target factor itself, environmental factors, and factors acoustic instrument. Factors include the size of the target, the anatomy of fish, swim bladder, and the behavior of orientation.


Introduction
Acoustic waves are transmitted into the subsurface ocean will experience scattering (scattering) caused by marine organisms; material distributed in the ocean, the structure is not homogeneous in seawater, as well as reflections from the surface and the seabed. Part of the initial acoustic energy on an object and is reflected back to the source called backscattering [1].
According in [2], a good fisheries resource management must control the number of catches in conjunction with the number of stocks that can be exploited. It required an estimate of the number of fish stocks at the time and acoustic survey techniques can be used to estimate the abundance of fish at a time and under certain conditions. The use of echo sounder and echo integrator for the purposes of exploration of fishery resources today is growing rapidly.
Hardware echo integrator aims to get the echo signal integration. The accuracy of this method is very high so it can be applied as estimate the abundance of fish in the waters [2].According in [3], hydro acoustic method with detection backscatter value of mangrove crab [4] using crushproof fish finder pcff-80 hydro acoustic instrument. According in [5] hydro acoustic method is an underwater detection method that uses acoustic devices, among others: echo sounder, fish finder, sonar, and Acoustic Doppler Current Profiler (ADCP). According in [6] a good fisheries resource management must control the number of catches in conjunction with the number of stocks that can be exploited. It required an estimate of the number of fish stocks at the time and acoustic survey techniques can be used to estimate the abundance of fish at a time and under certain conditions.
The use of echo sounder and echo integrator for the purposes of exploration of fishery resources today are growing rapidly. Hardware echo integrator aims to get the echo signal integration. The accuracy of this method is very high so it can be applied as an estimate abundance of fish in the waters [2].
Estimation of fish stocks in the waters wide as in Indonesia have a lot of them are using the acoustic method. Acoustic method has high speed in predicting the size of fish stocks so as to allow acquiring data in real time, accurate and high speed so as to contribute fairly high for the provision of data and information of fishery resources [6].
The second aspect is transceiver consisting of transmitter and receiver. Echo sunder divided bim first inserted into the ES 3800 by SIMRAD beginning of the 1980s and in 1985 was introduced to fishermen in Japan as a tool for catching up. Split beam transducer is divided into four quadrants [7] in which the transmitting wave conducted by the merger of four full beam. The signal reflected by the target is received by each quadrant and reassembled to form a full beam. Gilihat of direction on the ship split beam is divided into four (4) ie Fore, Aft, Port and Starboard. While in principle Split Beam is divided into four quadrants that FP.FS.AP and AS.   Split beam echo sounder has the function of Time Varied Gain (TVG) in acoustic data acquisition system serves as reliever TVG attenuation (Amplifier) whether caused by geometrical spreading and absorbs noise as it propagates into the water. There are two types of functions, namely TVG function that works to echo a single fish called TVG 40 log R and a function for a group of fish that TVG 20 log R.  In ( Figure 6) by Simrad, fish axis A located right above the maximum transducer gain, while fish B is located at the end (edge) transducer beam where the gain is lower. A fish echo thus more likely to result stronger than the backscatter echo in fish B. Although both of these fish are at the same depth and the same size.
To determine the size of the fish from the echo strength alone is not enough, however, knowledge about the pattern beam transducer and the fish in the beam position is very important to correct transducer gain strength and determining the target value of real fish.  An estimate obtained approximate angle of incidence and factors beam pattern in the acoustic signals can be obtained by using a processor of the split beam which has a signal source X leads to Phase detection and will produce energy or power by means of calculating the results of input and will generate output waveform display in figure 8.

Information:
TSi: Intensity target of strength Ii: Intensity of sound on targets Ir: Reflected sound intensity targets TSE: Energy Target Strength Ei: Energy sound on targets Er: Energy reflection sound at a distance of 1 meter from the target According in [11]stated that the Target Strength (TS) is a measure decibel sound that is returned by the target as measured on a standard distance of 1 meter from the acoustic center of the target is located, relative to the intensity of sound that hit the target. A simple model to estimate the back scattering cross section based on the size of fish referred by [6]: Then according in [12] introduced the equation which connects the backscattering cross section (σ bs ), fish length (L) and wavelength (λ) by the following equation: σ bs / λ 2 = a(L/ λ) b (dB) where A and B are constants that depend on the anatomy, fish size and wavelength. Equation (4)  Factors that contribute affect the value of Target Strength (TS) fish Strength target can generally be influenced by three factors: a target factor itself, environmental factors, and factors acoustic instrument. Factors include the size of the target, the anatomy of fish, swim bladder, and the behavior of orientation [15]. Factors such targets are:

Size of fish
There is a relationship between the sizes of the fish with a value of TS, but the relationship varies greatly depending on the species. Generally for fish species, the larger the fish the greater its value TS. This is especially true for the region of the graph geometrical relationship between the size of the target and TS, for the region, resonance, resonance region and the transition region, the tendency of the relationship is not valid [11]. Anatomy such as the head, body, tail and fins have a different sound reflections. Likewise, stomach, intestine, liver, bones, flesh and gills have a specific gravity = (ρ) and the speed of sound = (c) different so acoustically will have the ability to reflect a different sound.

Swim bladder of fish
Acoustically fish and marine organisms are divided into two major groups, namely blader fish (have a swim bladder). Fish that have a swim bladder generally do not have the right maximum TS on the dorsal aspect, while fish that do not have a swim bladder with a maximum value of TS is generally right on the dorsal aspect. TS value of fish that have a swim bladder [16]. With deformed-cylinder model (DCM) with Approximation of> 5 and the value of Tilt Angle was not until (<40°) according to [17]. Results from the resultant corner of a fish that has swim bladder that is: Figure 9. Swim bladder Geometry for Soft spheroid models, Source in to [17,18].

Behavior / Orientation Fish
Results of a previous study conducted by [19,20] states that the value of Target Strength (TS) is determined by the orientation of the fish, especially the slope of the body to a line connecting between the head and tail. Fish orientation will include tilting, yawing and rolling along. Yawing no effect because generally spherical transducer position so that the fish does not cause changes in the angle when viewed from the transducer, for Rolling no real effect because the fish have a swim bladder due partly reflected energy is derived from the swim bladder did not come from the dorsal aspect. Tilting lead to a change in angle position transducer is good for fish that have a swim bladder or not [8].

Instrumental factor
The small big factor value Beam pattern depending on the extent of the transducer will be greater the beam angle of the transducer, and vice versa. Large beam angle changes cause TS great value, separately it is better to use a relatively narrow beam.
Acoustic reflections of fish and plankton that are returned in the form of echo is detected by the receiver has an appeal. Estimation of biomass can be seen from how much force the target and how to interpret it. TS plankton is numbers that indicate the size of the echo. The larger the value, the greater echo energy is returned to the receiver by the target. Unit of measure Standard International (SI) for the TS expressed in decibels (dB). The decibel is a logarithmic form of a comparison or ratio of the two intensities due to the values involved can be very large or very small. According in [21] TS formulated as backscattering cross-section of the target which returns a signal and is expressed in the equation: TS = 10 log (σ / 4π) (14) Then the value of TS theoretical spherical object is: TS = 10 Log a 2 /4 (15) Where σ = Target strength individual or backscattering cross-section (σ bs) with TS according in [21,22] with equation: TS = 10 log σ BS (16)

Volume Backscattering Strength (SV)
Volume Backscattering Strength (SV) is defined as the ratio between the intensity reflected by a group of single targets (target located at a water volume of certain diinsonifikasi instantaneously measured at a distance of 1 m from target with the intensity of sound that hit the target. Definition Volume Backscattering Strength (SV) has the same meaning as the target strength for a single target, while Volume Backs catering strength (SV) for a group of fish.
Each individual targets is the source of the reflected sound wave, so that the output of the integration will be proportional to the quantity of fish in the group. Echo integration methods used to measure Volume Backscattering Strength (SV) based on the measurement of the total power backscattered on the transducer [8].
Volume Backscattering Strength (SV) is the ratio between the intensity reflected by single group targets where the target is located at a water volume [23]. This is similar to the definition of TS where TS value is the result of the detection of a single organism, while SV is the value for mendetaksi organism groups in [24] states SV is defined into the equation: SV = 10 log (Is i/ I) (17)

Information
Is: Intensity scattering volume measured 1 m from the center of the acoustic waves.

Fish Density (Abundance Fish)
To date research on fish stock estimates done by cruise track using a SIMRAD EK 60 Scientific split beam echo sounder system with a frequency of 70 kHz and acoustic data acquisition is performed continuously during the day and night during the period boat cruise at speeds ranging between 7-8 knots. Trails include a data acquisition area of an area that allows the analysis of spatially made with zig-zag shape according to in [25,26] with the length of each transect approximately 12 NMI of bounds islands outwards. Density values for fish processing performed on Ms. Excel. The treatment may be carried out after the integration process SV and TS. Density is generated by using the formula [27]. SV (dB) = 10 log (N τbs) = 10 log N + TS (18) Assuming the numerical density is proportional to the density of individuals, and then the equation (1) can be rewritten as follows: SV (dB) = 10 log ρ + A Where SV: Volume strength (dB) ρ : Abundance / density of organisms (Ind / m 3 ) A: Target average strength (dB)