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ISSN : 2671-9940(Print)
ISSN : 2671-9924(Online)

Journal of the Korean Society of Fisheries and Ocean Technology Vol.52 No.1 pp.1-8
DOI : https://doi.org/10.3796/KSFT.2016.52.1.001

Dynamic analysis and improvement of the sinking performance of the Vietnamese tuna purse seine using numerical methods

Dinh Xuan HUNG^,, Chun Woo LEE¹, Subong PARK²

Fishing Technology and fishery Oceanography Department, South Research Sub-Institute for Marine Fisheries, Vungtau city, Vietnam
¹Department of Marine Production System Management, Pukyong National University, Busan 48513, Korea
²Department of Fisheries Physics, Pukyong National University, Busan 48513, Korea

cwlee@pknu.ac.kr, Tel: +82-51-629-5891, Fax: +82-51-629-5886

Received January 11, 2016 Review February 28, 2016 Accepted February 29, 2016

Abstract

In this study, numerical method was used to assess technical properties and improve the Vietnamese tuna purse seine. Thedata were extracted from the two national level projects. The study results showed that average lead-line sinking speed reached 0.139 m/s and 0.143 m/s and maximum sinking depth was 61.6 m and 65.8 m for The gears 2003 and 2014 respectively. The maximum tension on ring line of The gear 2003 was 4,742 kgf and 2014 was 4,219 kgf. The improved tuna purse seines I, III and IV showed similar sinking speed results with 0.220 m/s, 0.219 m/s and 0.219 m/s respectively. The average lead-line sinking speed of the improved gear II was lowest with 0.215 m/s. The maximum lead-line sinking depth of the four improved gears I, II, III and IV were 116 m, 112 m, 115 m and 114.9 m respectively. Maximum tension on ring line of the improved gears I, II, III and IV were 5,657 kgf, 5,406 kgf, 5,645 kgf and 5,654 kgf respectively. The improved tuna purse seine IV is the most suitable for Vietnamese tuna purse seine fishery, Which corresponds with tuna purse seiner scale and its fishing supporting equipment at the present.

Key Words : Vietnamese tuna purse seine , Sinking depth , Sinking speed , Tensional force , Improved tuna purse seine

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Introduction

Vietnam had 592 tuna purse seiners. Two kinds of tuna purse seine were being used in Vietnam. The purse seines fishing in night time, have float line of less than 700 m length, stretch height of less than 90 m, small buoyancy and sinking force. These purse seines are used on the purse seiners of less than 20 m of length and main machine of under 300 Hp (Nguyen, 2002). The fishermen use high voltage lights (total of 10 kw to 20 kw per a boat) at night and/or other FADs as bamboo, coconut leaves to concentrate fish. Production and species component referred to small pelagic fish such as anchovy, mackerel, scad, etc. Tuna species (mostly juvenile tuna) just reaches less than 40% in total of catch. Two other purse seines fishing tuna schools in day time, have large float line of between 800 m and 1,500 m length, stretch height of 100 to 150 m; maximum mesh size of 80 mm and main engine of the purse seiner is over 300 Hp (Nguyen, 2003). Production in each setting was all of tuna; skipjack tuna reaches over 40% of all that is main target species. Using bamboo, coconut leaves, adrift things concentrate tuna too. The both kinds of the purse seine operate mainly in middle and south-east water areas of Vietnam.

Domestic tuna purse seines of Vietnam were developed from small pelagic purse seine by a traditional method and fishermen’s experiences with low technical design. The seines were only changed some parameters as increase of length of net, depth and sinker weight, but no change of mesh size, material, hanging ratio, knot type, etc. The designer has not based on physiology and morphology characteristic of major target species. Thus, the seines have operated non-effectively in new fishing ground or new target species as fast swimming tuna species. The development policy of tuna purse seine fishery has not yet invested pertinently in Vietnam.

In this study, we analyze the technical properties ofthe Vietnamese tuna purse seines and improve Vietnamese tuna purse seine.

Materials and methods

In this study, the data are collected from two tunapurse seines: The first tuna purse seine (called The gear 2003 here after) from the Project of “Research on development in tuna fishing by purse seine” was implemented in 2003. This project only based on three purse seine samples of fishermen. This gear has float line length of 1,550 m, lead-line length of 1,948 m and stretch depth of 150 m, total of 2,464 kgf sinking force and 5,426 kgf buoyancy force.

The other tuna purse seine (called The gear 2014 hereafter) from the Project of “Research building of tuna fishing technology process by purse seine that set from stern in Vietnam” was implemented from 2013. This gear has flat-line length of 1,098 m, lead-line length of 1,538 m and stretch depth of 150 m, total of 4,522 kgf sinking force and 7,784 kgf buoyancy force.

Assessing technical properties and improving the tunapurse seines is implemented by numerical method that was described by LEE et al. (2005). Simulation is implemented on SimuPurse software (MPSL, Korea).

Simulation process is implemented at pursing speedof 1.7 m/s, shooting speed of 5 m/s and currents condition of 0.11 m/s; 0.34 m/s and 0.29 m/s at three water levels of 0~60 m (180°); 60~130 m (210°) and 130~200 m (250°) respectively (the currents was collected from data of the Project of Fishing boat, Oceanography and Resource Monitoring System in Vietnam).

Improving the tuna purse seine gear

Base on the current Vietnamese tuna purse seine 2014 to increase 1500 m length, stretch depth 200 m; to use minimum mesh size of 70 mm at bunt part and maximum mesh size of 254 mm at body part and ratio of twine diameter per mesh size is 0.016 at bunt part and 0.005 at the body part.

And the difference of PES material and netting knottype distribution is implemented:

The improved tuna purse seine I: Using PES materialand knotless netting at all of parts (except bunt part) showed in Fig. 1.

The improved tuna purse seine II: Using only PESmaterial netting at upper part and knotless netting at all parts (except bunt part) showed in Fig. 2.

The improved tuna purse seine III: Using PES materialnetting at under part and knotless netting at all of parts (except bunt part) showed in Fig. 3.

The improved tuna purse seine IV: Only using PESmaterial and knotless netting at under part (except bunt part) showed in Fig. 4.

Results

Simulation results of the Vietnamese tuna purse seines

Average lead-line sinking speed reached 0.139 m/sand 0.143 m/s for the gears 2003 and 2014 respectively (Fig. 5). Maximum sinking depth of lead-line reached 70 m and 60 m for the seines 2014 and 2003 respectively (Fig. 6). Sinking performance of the gear 2014 was higher than of the gear 2003.

Under actual winch speed of 0.85 m/s, then tensionon pursing line of the Vietnamese tuna purse seines had small value of under 5,000 kgf (Fig. 7).

Simulation results of the improved tuna purse seines

Comparison of sinking performance

The lead-line of improved gear II had the lowestsinking depth, the lead-line depth of improved gear I, III and IV reached the same amount by time. The maximum lead-line sinking depth of four improved gears I, II, III and IV were 116 m, 112 m, 115 m and 114.9 m respectively (Fig. 8).

The average lead-line sinking speed of four improvedgears I, II, III and IV were 0.22 m/s, 0.215 m/s, 0.219 m/s and 0.219 m/s respectively. The lead-line sinking speeds of improved gear II was the slowest in all of the other improved gears. The lead-line sinking speeds of the improved gear IV approximated to of the improved gears III and I (Fig. 9).

The improved gear I, III and IV reached the sameamount of tension with maximum value of around 5,600 kgf. The tension value on ring line of the improved gear I, III and gear IV was not large different comparing with of the gear II (Fig. 10).

Comparison of the improved and the present tuna

Lead-line sinking depth of the improved tuna purseseine increased significantly. In detail, at intermediate part, the depth rose to 100 m after 10 minutes of sinking process while the gear 2003, the gear 2014 only reached 50 m and 62 m respectively. The lead-line sinking depth was smaller than sinking depth of the Korean gears (Fig. 11).

Although sinking speed of the improved tuna purseseine was slower than the speed of the Korean gears, it was great increased comparing with the present tuna purse seines of Vietnam (Fig. 12).

The maximum tension of the Vietnamese gear 2003reached 4,742 kgf, of the Vietnamese gear 2014 was 4,212 kgf and of the improved gear IV was 5,654 kgf while the Korean gear 2013 reached over 25,000 kgf of tensional force (Fig. 13). The maximum tension on ring line of the improved gear IV was very small comparing with of the Korean tuna purse seine.

Discussion

Hosseini et al. (2011) showed that the lead-line averagesinking speed of Korean tuna purse seine is over 0.2 m/s at three parts. To compare with tuna purse seines of Korean, the Vietnamese tuna purse seines had very low sinking speed, of course, including sinking force and its scale. Although the gear 2014 increased sinking depth and speed of lead–line but it was not enough to catch tuna efficiently. To use small mesh size arbitrarily and this not only decrease sinking performance of the gear, it also increase by-catch rate in catch. The maximum 30 mm mesh size at bunt part of the Vietnamese tuna purse seines, so juvenile tuna even small pelagic juveniles cannot escape. Moreover, sinking performance of the Vietnamese purse seines has not been assessed yet until present. The small sinking speed is results of (1) very small mesh size and unsuitable mesh size distribution, of (2) using PA material and knotted netting, (3) small total of sinking force. Small mesh size will make increase of total twine area and net weight; it results in rise of resistance force. Thus, small mesh size does not only cause decline of sinking speed, it also makes increases of tensional force on pursing line.

Mesh size affects sinking performance of purse seine.Sinking speed of lead-line will increase when using large mesh size of the web (Hosseini et al., 2011). The tuna purse seine of Vietnamese distributed mesh size via regulation of the smallest at bunt and the largest at end part (horizontal direction) and keeping the same mesh size via vertical direction. As we known, the mesh always forward to closed when the winch hauled pursing line. The Food Agriculture Organization (FAO, 1990) also reported that Skipjack species usually encountered in epipelagic waters (from the surface to the thermocline 0~50m depth) and this species also often surfaces. So, increasing mesh size at under part of net does not make escape of fish, it will make increase of sinking performance of net. Consequently, mesh size of Vietnamese tuna purse seines should increase via vertical direction to bring out more sinking performance.

The depth of the Vietnamese tuna purse seine reachedmaximum 70 m while Korean tuna purse seines achieved over 150 m of lead-line depth. In the fishing ground, Skipjack tuna (Katsuwonus pelamis) often creates schools with mass radius round maximum of 30 m (the Vietnamese fishermen’s report) and Skipjack tuna swims fastly and actively. Clearly, the sinking depth of the Vietnamese tuna purse seines has not corresponded to catch tuna like Skipjack species.

Comparing simulation results of sinking performance and tensions of 4 improved gears, we found that, theimproved tuna purse seine IV with using PES material and knotless type netting at lower part (near lead-line) have the good technical quality and is suitable for Vietnam condition at present. The improved gear IV saved huge PES material and knotless netting but lead-line sinking depth and speed was high, tension value on pursing line was small. Using PES netting at upper part (near float line) did not increased sinking performance significantly as in the improved gear II. To distribute PES material netting at lower part, it plays a role as sinker weight for both upper net part and lower part but, otherwise, PES netting distributes at upper part, then the sinker weight do not influence low part. To use only PES material netting at lower part resulted sinking performance is similar in term using PES material for all part. Using PES material and knotless type netting at lower part of the net is not only decrease product cost and tension force on ring line, it also increases significant sinking performance.

The PES netting recorded the deeper depth and fastersinking speed than PA netting (KIM et al., 2007; Hosseiniet al., 2011). Aris Widagdo (2014) indicated that increasing the mesh size of net will increase the sinking speed. In this study, PES material netting was used and meshes size also was increased remarkably, so sinking performance of the tuna purse seine was improved significantly. From average sinking speed of lead line of the prototype gears were 0.13 m/s to reached value of 0.22 m/s of the improved gears. The depth of lead-line of the improved gears also achieved double value comparing with the old Vietnamese tuna purse seines.

The sinker weight per meter of the Vietnamese tunapurse seine 2014 and of the improved tuna purse seines was kept the same amount of 3 kg/m, this value equal only a half of the Korean tuna purse seines (6.6 kg/m). Consequently, Korean tuna purse seines achieved more sinking depth, speed and more tensional force on pursing line. Simulation results showed that, tension on ring line of Korean tuna purse seines reached over 25,000 kgf while tension on ring line of the Vietnamese tuna purse seines and the improved tuna purse seine achieved small value of under 6,000 kgf. Increasing length, depth and sinking force of tuna purse seine is necessary for every designed gear, but this increase have to be limited. Because, In Vietnam, small tuna purse seiner size is maximum 25 m length, gross weight of 100 tones; the power of winch machine (friction drum) fluctuates from 80 Hp to 120 Hp, and other vessels use friction drum with power extraction from main engine (maximum 500 Hp). These present machines are not enough ability to purse tuna purse seines that exceeding length or/and sinking force like Korean tuna purse seines. Moreover, increasing the parameters of the gears also depends on target species, fishing ground, fishery characteristics and in term of economic benefit.

According to the FAO document, suitable relationshipbetween the diameter of twine and the mesh size is from 0.01 to 0.06 at bunt part and from 0.005 to 0.03 at body part for large pelagic fish. In this improved gear, diameter of twine per mesh size was from 0.011 to 0.017 at bunt part and was from 0.0057 to 0.007 at body part, it fell in the range. Vietnamese tuna purse seines are also attached PP (Polypropylene) material ropes at net wall from float-line to lead-line to ensure more endurance of net twine.

Horizontal hanging ratio of net affected strongly tovertical shape of net wall in water. The Vietnamese tuna purse seine used low hanging ratio of 0.64~0.71 along bunt to end part of float-line, it caused the shape like bell shape (Fig. 14). This is bad shape, thus, it made small float-line area and trends to pulling net wall to surround center. Hanging ratio increased over 0.7 like Korean Tuna purse seines created an advantageous shape to keeping upright net wall. Purse seines must have extra netting strung on a shorter float-line to allow the net to form the rounded bowl shape. The hanging ratio is generally in the range of 23 to 26%. These are actually average figures as the ratio may be as much as 30% in the center and taper to only 15% on either end. Differences in the hanging ratio will affect the rate of net sinking, pursing and the maximum pursing depth (South Pacific Commission Twenty First RegionalTechnical Meeting On Fisheries, Technical Aspects ofTuna Purse Seine Operations, 1989).

Conclusion

The Vietnamese tuna purse seines have low sinkingperformance; Base parameters like hanging ratio, mesh size, material, depth of net have not appropriated to catch tuna efficiently, especially, fishing free swimming fish school in day time.

The lead-line sinking speed and depth of the improvedtuna purse seines were higher 1.5 times than of the present Vietnamese tuna purse seines.

Using PES material at lower part is better than atupper part of purse seine. Sinking performance of net with PES at lower part is similar to of net with all of PES material.

Vietnamese tuna purse seines should use PES materialand knotless type at lower part (vertical direction) of the net; using minimum mesh size of 70 mm at bunt part and maximum mesh size of 254 mm at body part; distribute maximum mesh size at body part (horizontal direction) and middle part (vertical direction). Beside, using hanging ratio is minimum value of 0.71 at float line middle and increases at either end also are better for Vietnamese tuna purse seines.

To increase float line length, net depth and sinkerweight of Vietnamese tuna purse seine should be corresponded with the tuna purse seiner scale and fishing supporting engine equipment, and the target species.

Acknowledgments

This work was supported by a Research Grant of Pukyong National University (2015 year).

Figure

Fig. 1.

The improved gear I.

Fig. 2.

The improved gear II.

Fig. 3.

The improved gear III.

Fig. 4.

The improved gear IV.

Fig. 5.

The sinking speed of the lead-lines at intermediate part.

Fig. 6.

The depths of the lead-lines at intermediate part.

Fig. 7.

The tensional force on pursing line.

Fig. 8.

The depth by time at intermediate part of the improved gears.

Fig. 9.

The speed by depth at intermediate part of the improved gears.

Fig. 10.

Tension on ring line of the improved gears.

Fig. 11.

The speed by time of the improved and present gears.

Fig. 12.

The depth by time at intermediate part of the improved and the present gears.

Fig. 13.

Tension on ring line under the currents.

Fig. 14.

The net wall shapes.

Table

Table 1..

Specifications of the two Vietnamese tuna purse seines

No.	Mesh size (mm)	Hanging ratio (%)	Length (m)	Vertical mesh number	Material	Twine diameter (mm)
The gear 2003
1	80	65	20	1,350	PA	1.500
2	35	65	50	2,000	PA	0.887
3	50	65	50	600	PA	0.887
4	80	65	50	100	PA	0.887
5	50	75	50	2,700	PA	0.887
6	50	75	300	2,880	PA	0.887
7	45	65	50	400	PA	1.130
8	45	65	50	400	PA	1.120
9	45	65	50	800	PA	1.050
10	50	65	50	1,440	PA	0.887
11	35	65	50	800	PA	1.130
12	35	65	50	800	PA	1.120
13	35	65	50	800	PA	1.050
14	50	65	50	1,200	PA	0.887
15	50	75	300	2,780	PA	0.887
16	50	75	150	600	PA	0.887
17	60	75	150	1,400	PA	0.887
18	50	75	150	600	PA	0.887
19	80	75	950	1,800	PA	0.887
20	100	75	150	1,200	PA	0.887
The gear 2014
1	30	65	50	800	PA	1.122
2	30	65	50	800	PA	0.972
3	30	65	50	800	PA	0.887
4	30	65	50	1,200	PA	0.794
5	30	83	50	100	PA	0.794
6	30	71	50	4,000	PA	0.794
7	30	83	50	100	PA	0.794
8	30	71	50	4,400	PA	0.794
9	30	83	50	100	PA	0.794
10	50	71	150	2,800	PA	0.887
11	50	83	150	100	PA	0.887
12	50	71	100	3,000	PA	0.794
13	50	83	100	100	PA	0.794
14	50	71	750	2,800	PA	0.794
15	50	83	750	100	PA	0.794
16	60	71	200	2,200	PA	0.794
17	60	83	200	100	PA	0.794
18	80	71	100	1,500	PA	0.794
19	80	83	100	100	PA	0.794
20	80	71	50	1,400	PA	0.794
21	80	83	50	100	PA	0.794
22	80	71	50	1,200	PA	0.794
23	80	83	50	100	PA	0.974

Reference

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