RESEARCH ON THE QUALITY OF TUNA CAUGHT BY HANDLINES WITH ARTIFICIAL LIGHT IN VIETNAM NGHIÊN cứu ĐÁNH GIÁ CHẤT LƯỢNG cá NGỪ đại DƯƠNG BẰNG NGHỀ câu TAY kết hợp ÁNH SÁNG ở VIỆT NAM

Tạp chí Khoa học - Công nghệ Thủy sản

Số 3/2014

THOÂNG BAÙO KHOA HOÏC

RESEARCH ON THE QUALITY OF TUNA CAUGHT BY HANDLINES
WITH ARTIFICIAL LIGHT IN VIETNAM
NGHIÊN CỨU ĐÁNH GIÁ CHẤT LƯỢNG CÁ NGỪ ĐẠI DƯƠNG
BẰNG NGHỀ CÂU TAY KẾT HỢP ÁNH SÁNG Ở VIỆT NAM
Trần Đức Phú1
Ngày nhận bài: 30/7/2014; Ngày phản biện thông qua: 11/8/2014; Ngày duyệt đăng: 13/8/2014

ABSTRACT
In Vietnam, the quality of tuna caught by handlines with artificial lights seemed to have declined compared with those
caught by longlines. In order to determine the main causes of such phenomenon, surveys were conducted by interviewing
concerned fishers while four trials were also conducted on tuna fishing boats, namely: absolute blooding out of catch,
changing lights power, changing lines hauling speed, and fishers’ normal practice of handling (as control). Catch from
the first three trials were compared with the control using chemical indicators as criteria, such as total protein and NH3
contents, protein oxygenation level, total acid and lactic acid contents, and amino acid level. The paper suggested to reduce
line hauling speed, which may improve the quality of tuna caught by handlines with artificial light.
Keywords: quality of tuna, handliner with artificial light, Vietnam tuna

TÓM TẮT
Kết quả nghiên cứu của bài báo nhằm xác định nguyên nhân chất lượng cá ngừ đại dương bằng nghề câu tay kết hợp
ánh sáng giảm so với cá ngừ được khai thác bằng nghề câu vàng truyền thống. Việc xác định nguyên nhân ban đầu được
tiến hành bằng cách phỏng vấn ngư dân, những người thu mua, cơ quan quản lý và các nhà khoa học. Nghiên cứu đã tiến
thành thực nghiệm trên bốn phương án (1) xả máu triệt để; (2) thay đổi tốc độ thu câu; (3) thay đổi công suất nguồn sáng
và (4) khai thác bình thường để đối chứng. Các chỉ số hóa học được phân tích bao gồm hàm lượng đạm tổng số, NH3, tổng
a xít, a xít Lactic, Peroxide, TBARS và Histamine. Bài báo cũng đã đề xuất phương pháp làm chậm tốc độ thu câu để cải
thiện chất lượng cá ngừ đại dương bằng nghề câu tay kết hợp ánh sáng

Từ khóa: chất lượng cá ngừ, câu tay kết hợp ánh sáng, cá ngừ Việt Nam

I. INTRODUCTION
The tuna handline fisheries was developed in the central provinces of Vietnam sometime in 2010, especially
in the Provnces of Binh Dinh, Phu Yen, and Khanh Hoa where more than 1,000 handliners have been
documented (DECAFIREP, 2012). Although tuna handline fishing with artificial lights has been recently practiced,
the catch using such gear accounted for a large portion of the total oceanic tuna catch in Vietnam (Phu Yen &
Binh Dinh Sub-DECAFIREP, 2012). However, the real situation which should be gleaned is on the quality of the
tuna caught by handlines which appeared to be lower than those caught by traditional longlines. As a result,
the price of tunas caught by handlines is only one-half of the tunas caught by longliners, while the ratio of tuna
reaching the sashimi standards in handline-caught tuna accounted for only 1% compared with 70% of those by
traditional longlines (DECAFIREP, 2012). Fisheries researchers, managers and fishers tried to adapt advance
methodologies and share experiences to address this concern, but the quality of tuna caught by handlines has
not improved so far. Therefore, the Marine Science and Fishing Technologies Institute of Nha Trang University
conducted a case study to investigate the major reasons that led to reduced quality of handline-caught tuna.
II. METHODOLOGIES
Based on interviews with fishers and stakeholders, and from the initial results of fishing trips onboard
handline vessels, the main causes of reduced quality of handline-caught tuna could be deduced from the current
1

TS. Trần Đức Phú: Viện Khoa học và Công nghệ khai thác thủy sản - Trường Đại học Nha Trang

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practices of fishers such as: (1) absolute blooding out of catch not completely carried out before preserving

the catch; (2) speed of hauling lines very fast; and (3) intensity of artificial lights is too much for the tuna catch.
Based on such hypotheses, trials were conducted onboard handline vessels at sea, the results of which were
analyzed at the Institute’s Laboratory to compare these current practices with those of the proper handling
procedures of tuna catch. The suppositions are described as following:
Absolute blooding out: Local fishers normally kill the fish immediately after these are hooked and hauled.
The fish should be completely bled out, disemboweled (internal organs removed) and cleaned before preserving
the catch (figure 1).
Reducing the lights power: After tuna catch is hauled by local fishers on board, artificial lights near the
hauling areas should be turned off or intensity reduced, and as soon as half of the line is hauled, all lights should
be turned off before taking all the catch onboard.
Changing the hauling speed: In normal practice which had been adapted from in tuna longline fisheries, as soon
as tunas are hooked these are immediately hauled onboard the fishing vessels. Research team were conducted
when tunas are hooked, fishers drop an extra buoy to the sea and then connect the tuna line to this buoy, in order
to link the extra buoy with the vessel, and control the speed of tuna fishing boat. This should reduce or limit hauling
effort of fishers, but only one effort exists during such time that is the tuna line dragging the extra buoy (figure 2).

Figure 1. Preparing tuna catch for preservation: (left) letting the blood out and (right) gutting

Samples were taken from the tuna catch corresponding to each current practice, and analyzed at the
Laboratory of the Marine Science and Fishing Technologies Institute of Nha Trang University. The samples are
analyzed based on their: (1) total protein and NH3 contents; (2) protein oxygenation level; (3) total acid and
lactic acid contents; and (4) amino acid contents. The samples were compared with tunas immediately collected
onboard after hauling and those collected from fishing ports. The sample treatments had been correspondingly
marked as: TUNA1 - absolute blooding out; TUNA2 - changing light powers; TUNA3 - normal practice; TUNA4 changing hauling speed; Moreover, samples collected immediately after hauling were marked as D, while those
collected from fishing ports by C. For example, TUNA1-D indicates that samples have been taken onboard
vessels and bled out; while TUNA1-C indicates samples taken from fishing ports and completely bled out.

Figure 2. Changing of hauling speed by dropping an extra buoy (left) and connecting tuna line
with the extra buoy (right)

III. RESULTS
1. Protein and NH3 contents
Results of the laboratory analysis indicated no significant difference in the total protein contents among
the samples analyzed. In terms of NH3 contents, samples taken onboard fishing vessels did not exhibit any

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significant difference, but those collected from fishing ports had shown significant differences. The total protein
contents from samples collected onboard fishing vessels fluctuated from 23.27 to 23.50 g/100 g tuna meat, those
collected from fishing ports had protein contents that range from 23.13 to 23.40 g/100 g tuna meat (figure 3).
As for the NH3 contents (figure 4), although samples collected onboard did not have significant differences with
values that range from 18.09 to 18.66 mg/100 g tuna meat, those collected from fishing ports had high contents
of NH3 that were significantly different from those collected onboard fishing vessels.

Figure 3. Total protein contents of tuna samples

Figure 4. NH3 contents of tuna samples

The results also indicated that among the samples collected onboard vessels, TUNA4-C (changed
hauling speed) had the lowest NH3 contents (40.35 mg/100g tuna meat) followed by TUNA1-C (43.08),
TUNA3-C (47.58), and TUNA2-C (48.36). Considering that NH3 is the toxic form of ammonia in fish and the
number 1 killer of fish, it can be concluded that TUNA4 experiments could have possibly produced better quality
handline-caught tuna followed by TUNA1 experiments, while results from TUNA2 and TUNA3 experiments did
not show any significant differences.
2. Protein oxygenation level

Peroxide and TBARS (Thiobarbituric Acid Reactive Substances) are two of the most commonly used
indicators to evaluate protein oxygenation level of seafood. Results of the case study showed that the Peroxide
and TBARS values corresponding to the samples collected for the experiments had declining oxygenation levels
as follows: TUNA3 > TUNA2 > TUNA1 > TUNA4. This result tends to imply that TUNA4 (changed hauling speed)
had lower oxygenation level than the other experiments (figure 5 and figure 6), and point to the possibility that
TUNA4 experiments could attain increased shelf life.

Figure 5. Peroxide levels of tuna samples

Figure 6. TBARS levels of tuna samples

3. Total acid and lactic acid contents
As shown in Figure 7, the total acid contents from TUNA1-D, TUNA2-D, TUNA3-D and TUNA4-D were not
significantly different, with values that fluctuated from 1.023% to 1.043%. However, for the samples taken from
fishing ports, TUNA4-C had the lowest total acid contents (1.044%) followed by TUNA1-C (1.157%), TUNA2-C
(1.193%) and TUNA3-C (1.231%). Similar trend was observed for the lactic acid contents of the tuna samples
(Figure 8). While the samples collected onboard fishing vessels did not show significant differences in terms
of lactic acid contents, but those collected from fishing ports had significantly different lactic acid contents, i.e.
TUNA4-C had the lowest lactic acid contents (0.980%), TUNA3-C had the highest lactic acid contents (1.133%)
followed in descending order by TUNA2-C (1.113%), and TUNA1-C (1.077%).

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Figure 7. Total acid contents in tuna samples

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Figure 8. Lactic acid contents in tuna samples

Based on the aforementioned analysis, the total acid and lactic acid contents in the tuna samples
corresponding to TUNA-4 was the lowest compared with the other trials, although only three (3) experiments per
trial could be conducted thus, the results are only preliminary. In order to obtain more conclusive results, more
samples should be collected from many vessels, more trials should be conducted, and the time of trial fishing
trips also should be made much longer.
4. Histamine contents
Histamine content is one of the most important indicators to evaluate the quality of tuna. Results of the
analysis on histamine contents from collected samples have shown that those collected onboard did not differ
significantly in terms of histamine contents, although the values fluctuated from 24.3 to 25.7 mg/kg (figure 9).
However, samples collected from fishing ports showed high histamine contents that were significantly different
from those samples that were collected onboard fishing vessels.

Figure 9. Histamine contents in tuna samples

The results therefore showed that TUNA4-C had the lowest histamine contents (34.1 mg/kg tuna meat)
followed by TUNA1-C (38.5 mg/kg tuna meat), TUNA2-C (44.4 mg/kg tuna meat) and TUNA3-C (45.0 mg/kg
tuna meat). These results also suggested that tunas produced through speed changing trials and collected
onboard vessels were of better quality compared with those produced through the other trials.
IV. CONCLUSION AND RECOMMEMDATIONS
1. Conclusion
Based on the abovementioned results of the experiments, initial conclusions were reached on the possible
reasons for reduced quality of tuna caught by handlines. In general, based on the chemical contents of the
samples, namely: NH3, total acid, lactic acid, peroxide, TBARS, and histamine, tuna caught through reduced
hauling speed gives better quality tuna compared with the other trials. This also implies that a very quick hauling
speed is the main factor that affects the quality of tuna caught by handlines since the quality is reduced
immediately as soon as tuna is taken onboard the vessels. Moreover, another factor that should also be
considered is the method used to kill the fish. As practiced by handline fishers, once tuna is hauled out of the water,

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the killing process is not completely accomplished. In addition, when the fish is knocked against the floor of
wooden vessels, the quality is reduced. Therefore, the two major reasons responsible for the declining quality
of tuna catch are the fast hauling speed and killing methods.
2. Recommendations
Based on the results of the case study, initial solutions are proposed in order to improve quality of tuna
caught by handlines in Vietnam.
- Fishing technologies
From the aforementioned analysis, the quality of tuna could be improved when the hauling speed is
reduced. As observed during the actual sea trips, huge numbers of hooked tuna escape from the handlines,
easily accounting for 40% of total hooked tuna, of which 80% was due to jumbled handlines. Therefore, the
fishing technology could be improved by adapting two requirements: reducing the hauling speed and limiting the
number of tuna that escape. As soon as tuna is hooked, an extra buoy is dropped and then tuna line is linked
to this extra buoy which is connected to the vessel. The buoy float will slower the speed and keep tuna during
hooking time. This extra buoy also reduces the hauling efforts of fishers and minimizes the interaction between
tuna and fishers. This requirement also limits sudden change of sea water pressure on tuna which is usually the
case for tunas caught from traditional longline fisheries.
- Preservation technologies
The seafood preservation technology on wooden vessels makes use of freeze cellars that use ice to
maintain the quality of seafood while the temperature inside the cellars is maintained to minimize the melting
of ice. These freeze cellars are used on many tuna fishing fleets, especially in Taiwan. This technology could
be applied for tuna fleets in Vietnam but the investment for cellars could be high depending on the structure of
fishing vessels. In this regard, tuna fleets in Vietnam should be organized into groups, with the big vessels to be
equipped with freeze cellars and serve the grouped fleets, while all products from smaller vessels in the group

will be maintained and preserved with the big vessel.
3. Way forward
The results presented are initial findings based on limited conditions (time, quantity of experiments and
laboratory conditions). Further research should be conducted with time, number of sea trips as well as number
of experiments increased so that detailed and applicable solutions can be attained, which could be applied in
real situations and transferred to fishers. Another aspect for future study is the use of electric shock on tuna
immediately after the fish is hooked. This way, tuna is killed at the hooking time instead of after hauling. As a
proposed research area, which had not been conducted in any trials, it however important that the use of electric
shock is safe for fishers, targeted fish, and juvenile fishes.

REFERENCES
1.

DECAFIREP, 2012. Estimate tuna catches in Vietnam - The 1st technical workshop Tuna Fisheries Management in Western
Pacific and Eastern Asia Projects - In Vietnamese.

2.

Binh Dinh Sub-DECAFIREP, 2012. Report on survey results of longlines, purse seine and gillnet in Phu Yen province since
2010 up to now. The 1st technical workshop Tuna Fisheries Management in Western Pacific and Eastern Asia Projects - In
Vietnamese.

3.

Phu Yen Sub-DECAFIREP, 2012. Report on survey results of longlines, purse seine and gillnet in Phu Yen province since
2010 up to now. The 1st technical workshop Tuna Fisheries Management in Western Pacific and Eastern Asia Projects - In
Vietnamese.

4.

SEAFDEC, 2003. Handbook for Pelagic longline. Training Department. November, 2003.

5.

SEAFDEC, 2005. Onboard fish handling and preservation technology. Training Department. September, 2005.

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