Acoustic approach for estimation of Skipjack ( Katsuwonus pelamis ) abundance in Bone Bay

Abstract

Skipjack is one of the most important economical fish. As a result, skipjack stocks are vulnerable to over-exploitation. Therefore, to anticipate and to set sound management plan, it is necessary to monitor from time to time the abundance of these fish to ensure the sustainability of its resources. The purpose of this study was to determine the abundance distribution of skipjack fish based on acoustic method in Bone Bay waters. The abundance estimate of the skipjack was conducted using Split Beam Echosounder Simrad EY 60 with frequency 120 kHz. The results of this study indicated that the range of skipjack target strength (TS) was -67.0 dB to -55.3 dB, or equivalent to fish length of 8 cm to 31 cm, detected from surface to a depth of 40 m. Temporally, skipjack density during the day was more towards the surface with an average of 43 ind/1000m³ and at night in deeper waters with an average density of 36 ind/1000m³. Vertically, the highest density was detected at a depth of 5 m - 15 m with a density of 82 ind/1000m³. While horizontally, the highest skipjack density of 188 ind/1000m³ was found around Kolaka waters.

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IOP Conference Series: Earth and Environmental Science
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Acoustic approach for estimation of Skipjack (
Katsuwonus pelamis
)
abundance in Bone Bay
To cite this article: R S Putri et al 2018 IOP Conf. Ser.: Earth Environ. Sci. 176 012033
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MSTBIHO IOP Publishing
IOP Conf. Series: Earth and Environmental Science 176 (2018) 012033 doi :10.1088/1755-1315/176/1/012033
Acoustic approach for estimation of Skipjack (Katsuwonus
pelamis) abundance in Bone Bay
R S Putri1, I Jaya2*, S Pujiyati2, A Priatna3, A Makmun3 and A Suman3
1 Department of Marine Technology, Graduate School of Bogor Agricultural University,
Indonesia
2 Faculty of Marine Science and Technology, Bogor Agricultural University, Indonesia
3Research Institute for Marine Fisheries, Ministry of Marine and Fisheries Affairs, Indonesia
*e-mail: indrajaya@ipb.ac.id
Abstract. Skipjack is one of the most important economical fish. As a result, skipjack stocks are
vulnerable to over-exploitation. Therefore, to anticipate and to set sound management plan, it is
necessary to monitor from time to time the abundance of these fish to ensure the sustainability
of its resources. The purpose of this study was to determine the abundance distribution of
skipjack fish based on acoustic method in Bone Bay waters. The abundance estimate of the
skipjack was conducted using Split Beam Echosounder Simrad EY 60 with frequency 120 kHz.
The results of this study indicated that the range of skipjack target strength (TS) was -67.0 dB to
-55.3 dB, or equivalent to fish length of 8 cm to 31 cm, detected from surface to a depth of 40 m.
Temporally, skipjack density during the day was more towards the surface with an average of 43
ind/1000m³ and at night in deeper waters with an average density of 36 ind/1000m³. Vertically,
the highest density was detected at a depth of 5 m - 15 m with a density of 82 ind/1000m³. While
horizontally, the highest skipjack density of 188 ind/1000m³ was found around Kolaka waters.
1. Introduction
Skipjack is one of fishery commodities in Indonesia which has important economic value. This makes
this fish as one of the main target in fishing activities. One of Skipjack producing areas in Indonesia is
Bone Bay waters. Bone Bay is a potential waters for Skipjack fishing in Eastern Indonesia. The potential
abundance of Skipjack is related by the spatial and temporal pattern of biophysical distribution of the
oceanographical condition in Bone Bay. The oceanography condition near the mouth of the bay (Flores
Sea) is certainly different from the other area [1]. It is suspected as a trigger of the high abundance of
Skipjack in this waters.
Skipjack fishing mostly use pole and line, purse seine, gillnet and handline. The skipjack behaviour
that form a schooling gives the opportunity to catch Skipjack in large quantities. As the result, Skipjack's
resource is vulnerable to over-exploitation. Although fishery resources are a renewable resource, the rate
of recovery may be unbalanced with the rate of utilization.
In 2005-2014, Skipjack production in WPP 713 experienced an annual increase of 11.40%, with
production in 2014 was 56.299 tons. Skipjack production in Indonesia in 2014 was 496.682 tons,
meaning that 11% of it came from WPP 713 [2]. With the increasing of skipjack production, it is
necessary to have information about the status of Skipjack’s stock. The estimation of Skipjack stock is
important so that the potential of Skipjack resources can be utilized sustainably and to avoid the
overfishing. One of the most effective methods for estimating fish stocks is by using the acoustic method.
The principle of acoustic method is based on the echo system, that is the reflection of the emitted sound
by something that prevents it [3]. The acoustic method has several advantages in estimating schooling
of fish and its abundance, including information that can be generated faster and covering larger areas,

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stock estimation can be done in real time and in situ regardless of data fishery statistics, have high
accuracy, can be used when other methods are unusable, and are not harmful or destructive because the
frequency of sound used is not harmful to the users or the target of the survey [4].
The existence of Skipjack’s schooling can facilitate the detection of Skipjack using acoustic method.
The estimation of abundance of fish resources through acoustic survey generally has the same condition
with other fishery survey, that is to gather data about a fisheries resource in an area. This analysis can
produce information about the conditions of a fisheries resources [5]. One approach to estimate the fish
abundance with acoustic methods is by measuring the target strength (TS). Measurement of fish target
strength is the main parameter in estimating fish stock with acoustic method [6]. Generally, the higher
the target strength of fish, the larger the size of the fish, but not always so. In addition to size, TS values
are also influenced by the swim bladder in the fish, fish body shape or type of fish itself.
The purpose of this research was to estimate the density of Skipjack temporally (day and night),
vertically (depth) and horizontally (spatial) using the acoustic method in Bone Bay waters, and to
validate the results with the results from fishery survey. It was expected that the results of this study
could provide information about the existence and abundance of Skipjack. This information could be
used to support the sustainable management of Skipjack fisheries resources in the waters of Bone Bay.
2. Methods
The acoustic survey was conducted in April 2014 in Bone Bay waters and was validated using a survey
on landing site in September - October 2016. The survey on landing site was conducted to collect data
about the size of Skipjack in two fishing port with high Skipjack production, namely Sinjai and Luwu.
The acoustic survey track that was done in Bone Bay waters was shown in figure 1.
Acoustic data was collected using Split Beam Echosounder Type Simrad EY 60 with frequency 120
kHz. Software Echoview, R, and ArcGIS 10.3 were used for acoustic data analysis. Fork Length (FL)
data were obtained by following the Skipjack fishing operation by using pole and line in Bone Bay
waters. In addition, FL measurement was also conducted at the fishing port in Sinjai and Luwu. FL
measured using roll meter.
Acoustic data processing followed the method in [7] with the following processing steps :
Target Strength analysis and average TS are calculated by equations (1) and (2).
ܶܵ ൌ ͳͲ  ݐݏ (1)
ݐݏ
ഥൌσ௧௦
୬ (2)
where TS is acoustic reflection strength of fish as a single target (dB).
Scattering volume (SV) and average SV are calculated by equations (3) and (4).
ܸܵ ൌ ͳͲ  ݏݒ (3)
ݏݒത
ത
ത
ൌσ௦௩
୬ (4)
where SV is volume backscattering strength (dB).

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.
Figure 1. Figure 1 Research location and track of acoustic survey.
To obtain fish density value (ind / 1000m3), equation (5) is used.
ܸܵ ൌ ͳͲ  ߩ ൅ ܶܵ
ത
ത
ത
ത
(5)
So that:
ߩൌͳͲ
ೄೇష೅ೄ
ത
ത
ത
ത
భబ (6)
where  is density of fish (ind/1000m3).
The prediction of TS value can be known based on the length of fish caught by using target strength
of Skipjack in [8]:
 ൌ ʹͲ݈݋݃ െ ͺͷǡͲͺ (7)
3. Results and discussion
3.1.Fisheries survey
The fishery survey was conducted to obtain the fork length data which was then was used in the
estimation of the target strength (TS) of Skipjack. The total of samples used were 626 fishes, which were
obtained from two different locations, Sinjai (406 fishes) and Luwu (220 fishes). Figure 2 showed that
the dominant of fish length was ranged from 40 to 49 cm, with the total of sample was 188 and the
lowest size was 70 cm. The catchable size of Skipjack in the waters of Bone Bay is > 47 cm in TKG
IV [9]. Based on this result, it could be concluded that from the total samples, 65% of them was not
eligible to catch. Skipjack production in South Sulawesi in 2015 was 21038.8 tons, 32% comes from
Sinjai and Luwu [10].

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Figure 2. Fork Length composition of Skipjack from the fisheries survey.
3.2.Estimation of target strength (TS)
The Acoustic method was not able to provide definitive information on detected fish species, so the TS
were predicted. Target Strength (TS) describes the ability of a target to reflect an incoming sound wave.
TS is expressed in decibels (dB) [5]. TS was a function of size, shape, and presence or absence of swim
bladder [11]. Based on the size of Skipjack catch in fishing port, it could be known that the range of
Skipjack TS was -59 dB to -47 dB (figure 3).
Figure 3. The relationship between FL (Fork Length) and estimated TS of Skipjack using the equation
(TS=20 log FL – 85.08) [8].
The graphic relationship between FL (Fork Length) and estimated TS of Skipjack in this study
indicated that the longer size of the fish, the higher its target strength (figure 3). Measuring the target
strength of the same species, the target strength had a linear relationship with the length of the fish [12-
13].
The length of fish is one factor that was very influential on the TS. In general, the relationship
between TS and the fork length of the fish is positive, which means that the longer the fish, the higher
the TS. In addition, TS values are also influenced by swim bladder, behavior and the instrument factors.
Skipjack tuna is a fish that does not have a swim bladder, unlike other types of tuna fish, so the TS value
of skipjack is lower than other tuna species [14].

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3.3.Acoustic survey
Based on the results of acoustic survey, the obtained acoustic data was in the form of echogram. Figure
4 was an example of an echogram that indicated the existence of a fish schooling and vice versa. This
reference was used in performing acoustic data analysis.
Figure 4. Echogram with an indication and no indication of fish schooling.
3.3.1. Temporal distribution. Distribution of fish in night and day tend to be different. Pelagic fish during
the day is more often to form schooling as an effort to facilitate foraging, mating and spawning, to avoid
or defend themselves from predators while at night is more distributed in the water column [15]. Figure
5 showed that the temporal distribution of Skipjack during the daytime was in the water column with a
depth range of 7.1 m to 38.9 m, density 1 to 188 ind/1000m
3
and estimated FL was ranged from 4 cm to
76 cm. The central of Skipjack distribution was at 21.4 m depth, with a density of 43 individual and an
estimated FL of 29 cm.
In the evening, Skipjack was distributed in the depth range of 11.8 m to 39.3 m with a range density
4 to 141 ind/1000m
3
and estimated FL range of 4 to 73 cm. The central of Skipjack distribution at night
time was at a depth of 29.1 m with a density of 36 ind/1000m
3
and an estimated FL of 27 cm. In general,
the distribution of Skipjack at night time located in deeper water while in the day was located closer to
the surface of the waters. Skipjack is more active during the day time than at night because they are
looking for food, its peak occurence is in the morning and mid of noon and afternoon [3].
Figure 5. Skipjack distribution in the day and night.

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3.3.2. Vertical distribution. Skipjack tuna is often referred as surface tuna [16], because Skipjack
generally distributes in surface waters. Skipjack performs repetitive ups and downs movement in the
waters, but the main distribution is in the upper layer where the water temperature is greater than 15qC
[17]. Skipjack was distributed in the depth range up to 40 m, so the echo analysis was limited to a depth
of 40 m. Skipjack was indicated high concentrated in the depth range up to 40 m, and used as a reference
in the analysis of acoustic data conducted in this study [18].
The average target strength and density of Skipjack was then plotted in a graphic to evaluate their
differences in the depth of the waters. Generally, the deeper the water, the higher the target strength due
to the longer of fish size. However, in Figure 6, the target strength values was fluctuated to a depth of
40 m, this might be due to the calculated target strength was come from the average of the target strength
of schooling fish that might had varying sizes. Skipjack is a tuna that has no swim bladder, so that their
target strength was highly influenced by the size their body. The highest average of target strength was
-55.3 dB and was occurred in the depth range of 20 m - 25 m, with the estimated FL was 31 cm.
The highest density of skipjack was 82 fish/1000m³, and was occurred in the depth of 5 m - 15 m or
closer to the surface of the water (figure 6). It could be assumed that these fishes were generally small
fish because their average of target strength was small (TS was -66.8 dB and estimated FL was 8 cm).
Generally, the deeper the waters, the lower density of fish [19]. The highest average of target strength (-
55.3 dB) as previously mentioned was estimated to be the longest fish that was detected by acoustics,
with density of 29 ind/1000 m³.
Figure 6. The relationship of depth with target strength and Skipjack density.
The estimated FL of Skipjack based on TS was then presented in table 1 to give an information
about the estimated of Skipjack size vertically. Based on TS average from acoustic survey, an average
of estimated FL was ranged from 8 cm to 31 cm. The overall data from estimated FL showed that the
catchable fishes according to their size were distributed in the depth range of 10 m - 35 m, with size of
48.1 cm - 76.0 cm.
Table 1. Depth, ܶܵ
ത
ത
ത
ത
, estimated ܨܮ
ത
ത
ത
ത
and density of Skipjack.
Depth (m) ܶܵ
ത
ത
ത
ത
(dB) Estimated ܨܮ
ത
ത
ത
ത
(cm) Density (ind/1000m³)
5 - 10 -66.8 8 82
10 - 15 -63.3 12 81
15 - 20 -65.9 9 45
20 - 25 -55.3 31 29
25 - 30 -55.5 30 27
30 - 35 -60.4 17 26
35 - 40 -67.0 8 47

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3.3.3. Horizontal distribution. The horizontal distribution of fish in aquatic ecosystems is complex.
There are several factors that influence the distribution of fish populations, such as migration, feeding
behaviour, patterns of predation, reproduction and habitat selection [20]. The skipjack density was
showed horizontally (figure 7) to determine the position and the abundance of Skipjack in Bone Bay
waters. Figure 7 showed that the highest density of Skipjack in Bone Bay waters was located in Kolaka
waters precisely at 121q10'84,00 "E and 4q55'11,56"S, with the total density of 188 ind/ 1000m³. This
might be due to high disribuition of small pelagic fish in these waters . Small pelagic fish is food source
for Skipjack, thus inviting Skipjack to concentratein that waters. Skipjack generally distributed in areas
with high productivity. This is due to the presence of small pelagic fish that is high in these water
condition for feeding [21].
Figure 7. Map of skipjack density in Bone Bay.
4. Conclusion
The range of skipjack target strength was -67.0 dB to -55.3 dB, or equivalent to fork length of fish of 8
cm to 31 cm that was detected from the surface to a depth of 40 m. Temporally, the Skipjack density
during the day was more toward the surface at 21.4 m depth with average density of 43 ind/1000m³ and
at night in deeper waters at 29.1 m depth with average density of 36 ind/1000m³. Vertically, the largest
density at the time of the acoustic survey was conducted was at a depth of 5 m - 15 m with a density of
82 ind/1000m³. While horizontally, the highest Skipjack density, with the total density was 188
ind/1000m³, was located in Kolaka waters or in the southern part of Bone Bay.
Acknowledgments
We acknowledge the support of DITJEN DIKTI for the scholarship of PMDSU under the contract
number: 330SP2H / LT / DRPM / IX / 2016 dated September 8, 2016 on behalf of Prof. Indra Jaya.
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