Fisheries Research 66 (2004) 115–119

The effects of solar radiation upon breaking strengthand elongation of fishing nets

H. Al-Oufi a,∗, E. McLeanb, A.S. Kumara, M. Claereboudta, M. Al-Habsica College of Agricultural and Marine Sciences, Sultan Qaboos University, P.O. Box 34, Al-Khod 123, Oman

b Department of Fisheries and Wildlife Sciences, Virginia Polytechnic Institute and State University,100 Cheatham Hall, Blacksburg, VA 24061, USA

c Ministry of Agriculture and Fisheries, P.O. Box 467, Ruwi, Oman

Received 17 May 2002; received in revised form 21 March 2003; accepted 29 March 2003

Abstract

The effect of solar radiation upon netting twines maintained under ambient conditions was investigated. Netting twinesexamined during the study were polyamide (PA) continuous filaments. These included 40 untreated grey twines (controls), 40that were coated with a coal tar film to reduce UV radiation penetration of the fibres (treated), and 40 untreated green colouredtwines. The last acted as a secondary control group. All twines were exposed to 780 h of direct solar radiation after whicheach was examined for breaking strength and extensibility. Results indicated that exposure of netting twines to solar radiationdecreased breaking strength over time(P < 0.05), with the rate of deterioration being three times higher for untreated twines.In addition, coal tar treatment reduced elongation for up to 5 weeks following exposure.© 2003 Elsevier Science B.V. All rights reserved.

Keywords: Oman fisheries; Fishing twines; Solar radiation; Breaking strength

1. Introduction

In Oman, as in other regions of the world (Nielsenand Johnson, 1992; Tait, 1981), synthetic fibres areemployed in drift gillnets, set gillnets, and amongstothers, beach seines. Synthetic netting twines werefirst actively marketed nationally in Oman duringthe early 1970s as a substitute for cotton fibre. Fol-lowing their introduction, synthetic fibres spreadrapidly across the country and today, they representan essential component of fishing gear technologiesnation-wide. Their use has played an integral role in

∗ Corresponding author. Tel.:+968-515-246;fax: +968-513-418.E-mail address: hamdoufi@squ.edu.om (H. Al-Oufi).

the advances that have triggered the development ofthe fisheries sector. However, synthetic twines havebeen associated with net deterioration caused by en-vironmental factors, including ultra-violet radiation,which is known to impact breaking strength.1 Netsaffected by ultra-violet radiation exhibit reduced op-erational life and decreased catchability and therebyresult in a general reduction in income. Despite pos-sessing superior rot-proof properties when comparedto natural fibres, synthetic twines are however, moreapt to exhibit decreased strength when exposed tointense sunlight (JICA, 1986; Dahm, 1992).

1 Breaking strength can be expressed as the maximum forceapplied to a specimen in a tensile test carried to rupture in kgforce (kgf).

0165-7836/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0165-7836(03)00103-6

116 H. Al-Oufi et al. / Fisheries Research 66 (2004) 115–119

Many fishermen remain unaware of the importanceof net storage to gear longevity and efficiency andmost remain uninformed of the potential damage thatprevailing environmental conditions may have. More-over, fishers do not possess appropriate informationregarding the characteristics of different twines and fi-bres. Many of the synthetic netting twines that havebeen introduced have been developed for temperatefisheries, which calls into question their validity fortropical waters. A wide variety of factors, including,but not limited to wind, water, sand, chemical andindustrial smoke affect the strength and longevity ofnet fibres (Dahm et al., 1990). For synthetic fibressuch as polyamide (PA), which dominates the tradi-tional fishing industry in Oman, ultra-violet solar radi-ation is believed to compromise net breaking strength.This reduction in strength may be augmented by thefisherman’s practice of exposing nets to sunlight fordrying after returning from fishing. The high illiteracylevel (Al-Oufi et al., 2000), and lack of knowledge offishermen relating to rot proof properties of syntheticfibres, results in fishermen believing that net dryingextends the lifespan of gears.

Weathering2 has greatest negative impact uponfishing nets, with strongest deterioration being causedby ultra-violet radiation (Klust, 1959, 1982). In ad-dition, there are variations in the amounts of damagecaused to synthetic fibres according to season andlocation (i.e. tropical versus temperate). For example,Sengbusch (1971, cited inKlust, 1983) illustratedthe decisive importance of geographical location byconducting weathering tests on identical twine types(polyethylene (PE), orange monofilament) in north-ern Europe and in the tropics. These nets maintainedinitial breaking strength after exposure to sun radi-ation for 12 months in northern Europe, whereas inthe West Africa, 80% of initial breaking strength waslost over the same period.

Protection of synthetic fibres from solar radiationcan be achieved by incorporating radiation-absorbersduring and after the manufacturing process. For ex-ample the use of different dyes, such as catechu, mayimprove a twine’s resistance to solar radiation. Nettreatment with tar not only increases twine resistance

2 In this paper the term weathering refers to the damage causedby light, rain, wind, industrial smoke and gases on the propertiesof textile materials.

against radiation, but also increases stiffness, sinkingspeed, abrasion resistance and knot stability. A ma-jor disadvantage of tarring however is increased gearweight. Fishermen themselves however, may play animportant role in reducing net deterioration. For ex-ample, proper handling and provisions for shaded stor-age may dramatically reduce damage caused by directexposures to solar radiation (Warenzeichenverband,1959).

2. Materials and methods

The present investigations were restricted to exam-ining the most commonly used synthetic fibre (PA ornylon) in the Omani fishing industry. In this exper-iment, 120, 1 m long twines were employed. Of the120 twines, 40 (grey colour) acted as controls, whilsta further 40 were coated with a film of coal tar. Coaltar was diluted in heavy oil as described byTakayamaand Shimozaki (1959)and Shimozaki (1959)beforeuse. Traditionally, Omani fishermen use coal tar asa protectant, and because it is the only dying mate-rial readily available in Oman, it was selected for thepresent study to demonstrate the effect of anti-UVcoating on the breaking strength and extensibility offishing twines.

Forty untreated green-coloured twines were usedto determine whether pigmentation affected UV re-sistance. Results from breaking strength and elonga-tion of test twines (solar irradiated) were examinedand compared to twines maintained in shaded con-ditions (controls). Samples were suspended, withouttension, on nails set 1 cm apart on the frame (Brandtand Carrothers, 1964). Frames were sun-exposed andplaced on a roof to minimise the effect of other weath-ering factors (i.e. they were shielded from dust andsmoke). The mean annual temperature in this area is28.6◦C, relative humidity is 58%, sunshine hours is9.7 h per day and rainfall 81.4 mm per year (basedon 18 years of climate data at the weather station atSeeb Airport). Although considerable care was takenin ensuring that all environmental factors other thansolar irradiation were taken into account during thetrial, it remains impossible to exclude the possiblenegative effects of other weathering conditions, suchas warm air (Takase, 1972) upon the experiment’soutcome.

H. Al-Oufi et al. / Fisheries Research 66 (2004) 115–119 117

Fifteen test samples (n = 5 per experimentalgroup) were examined weekly for breaking strengthand extensibility. The measured result was related tothe total time of exposure (total number of hours ofsunlight) measured during the exposure period. Thetreated twines were exposed to direct solar radiationfor 8 weeks during the summer months between Julyand September. Over this period, the photophase inOman averages 14 h per day. Hence, throughout thestudy period, twines were exposed to approximately780 h of direct solar irradiation.

Breaking strength was measured with tensilestrength testing device (dynamometer; INSTRON,Model 1011, Instron Corporation), which progres-sively increased tension on a test specimen untilbreaking. The twines were fastened at each end ofthe instrument’s grips which moved apart at a setspeed. Increased distance between the grips repre-sented a measure of extensibility. Ideally, wet break-ing strength and extension are generally considered asbeing of primary importance since this represents netoperating conditions. However, for synthetic fibres,such as the ones used herein, wet stress properties donot differ from those measured under dry conditions,such that all tests were undertaken using dry twines(Wijngaarden, 1959). Extensibility of netting twineswas also recorded during the tensile test to determinethe effect of treatment and exposure to solar irradi-ation on the extensibility properties of the twines.Elongation at breaking strength was recorded by thetesting device. These extensibility values provided anestimate of the effect of treatment on the extensibilityproperties of netting twines.

Data were analysed by analysis of variance(ANOVA) for the three treatments for 8 weeks. Leastsquare means (LSM) for the treatment× time inter-action was generated. These LSM were used for re-gression analyses. All statistical treatments employedSAS software (SAS/STAT, 1993).

3. Results and discussion

Net weathering studies under normal environmen-tal conditions have not been previously undertakenin Oman. Furthermore, over the last two to threedecades, only limited research has considered thistopic; particularly in arid regions (e.g.Botros and

Table 1Least square mean breaking strength for the three treatments (kgf)

Week Treated grey Untreated grey Untreated green

0 17.09 17.58 15.711 17.25 14.69 14.152 15.76 13.42 14.443 15.21 12.38 13.114 15.16 11.89 13.715 14.94 10.51 10.616 14.34 8.69 8.527 14.17 5.36 6.238 13.90 4.86 5.81

Al-Sayes, 1973). At the start of the experiment, break-ing strength of treated and untreated grey twines wasdetermined to be higher(P < 0.01) than untreatedgreen twine. Following a 1-week exposure to solarirradiation, breaking strengths declined (Table 1). Themean breaking strength for coal tar treated grey twineswas 15.30 kgf (range 17.09–13.90). This was greaterthan that observed for either untreated grey twines(11.04 kgf; range 17.58–4.86) or untreated green PA(11.36 kgf; range 15.70–5.81). For the following 7weeks of the trial, coal tar treated twine returnedsuperior (P < 0.01) breaking strength characteris-tics than non-treated twines, irrespective of colour(Fig. 1). The results of the present study therefore, aresimilar to those reported byKenehiro et al. (1988),who observed loss of breaking strength in twines ex-posed to photo-irradiation over time, but differ fromthe observations ofBotros and Al-Sayes (1973)intheir studies upon coal tar treated cotton twines.

Fig. 1. Mean breaking strength for the three treatments (kgf).

118 H. Al-Oufi et al. / Fisheries Research 66 (2004) 115–119

Table 2Regression equations describing the relationship between breakingstrength of treatments to the length of exposure to sun radiation

Treatment Intercept,a Regressioncoefficient,b(±S.E.M.)

R2 (%)

Treated grey 16.99 −0.42 (0.05) 91Untreated grey 17.06 −1.50 (0.11) 96Untreated green 16.55 −1.29 (0.15) 91

Regression equations describing the relationship be-tween breaking strength and time of solar exposureare summarised inTable 2. The regression coefficient(b) describes the changes in breaking strength (depen-dent variable) with increasing period of solar expo-sure for each of the different treatments (independentvariable). Thus, a 1-week increase in solar exposurereduced breaking strength by 0.42 kgf in the case oftreated grey twine and by 1.5 and 1.3 kgf for the un-treated grey and untreated green twines, respectively.While not significant, the twine colour appeared to beprotective, with grey pigments apparently being moreable to resist the effects of solar irradiation than greentwines (Table 2). Similar effects have been reportedfor other synthetic twines coloured differentially butchallenged by artificial irradiation (carbon-arc light;Kenehiro et al., 1988). The partial slope of untreatedgrey twine did not differ from that recorded for un-treated green samples (−1.5 versus−1.3), but was sig-nificantly higher(P < 0.01) than the coal tar coatedtwines (−0.42; Table 2). These results indicate thatexposure of netting twines to solar irradiation clearlyincreases the rate of deterioration as this pertains tobreaking strength. From a practical perspective, if suchweathering impacts were duplicated during beach-sidenet storage, then their half-life would be substantiallyreduced and thus decrease profitability of artisanalfishers. At the very least the negative effects of irradi-ation would increase time for net repair and mainte-nance.

The least square mean values of elongation forthe three treatments are presented inTable 3. After1-week exposure, treated grey twines exhibited least(P < 0.01) elongation, with the elongation curvebeing cubic in nature with the prediction equation ofY = 129.58 − 42.11X + 9.37X2 − 0.61X3, whereX is weeks(R2 = 0.952). In contrast, the predictioncurves for untreated controls were linear (Table 4).

Table 3Least square mean elongation at breaking strength for the threetreatments (%)

Week Treated grey Untreated grey Untreated green

0 132.9 201.8 174.81 90.2 160.2 131.92 78.3 141.0 135.13 71.6 122.3 94.724 75.7 140.8 112.85 76.4 99.8 115.86 84.3 99.6 80.37 76.7 78.6 77.08 83.0 84.0 81.41

The elongation slopes were similar for the un-treated grey(−13.69± 1.9) and the untreated green(−10.45± 2.07). While elongation was lower for thecoal-tar treatment during the first 5 weeks of datacollection, by trial end, similar values were recordedto those of the untreated controls (Table 3).

The present results suggest that appropriate dyeingof net twines could extend the useful half-life of net-ting materials. While coal tar, which has been usedby fishermen of Oman to protect cotton nets from rot-ting, was employed as a net protectant by the presentstudy, its application should be restricted due to en-vironmental concerns. Other methods of protectingtwines from solar radiation, as illustrated by the con-temporary practice of incorporating antioxidants andradiation-absorbers during the polymer extrusion pro-cess, are now commonly used by the netting indus-try, but these materials are more expensive. Neverthe-less, since these protective pigments are incorporateddirectly into the fibre matrix, they cannot be washedout and hence do not require reapplication during theuseful life of nets. In addition, the chemical industryprovides a large range of UV coating materials withdifferent protection properties for the textile industry.Selection of appropriate dyes for fishing twines should

Table 4Regression equations describing the relationship between elonga-tion and types of treatment applied to PA fishing twines

Treatment Intercept,a Regressioncoefficient,b(±S.E.M.)

R2 (%)

Treated grey 129.58 See text 95Untreated grey 180.12 −13.69 (1.90) 88Untreated green 153.3 −10.44 (2.07) 78

H. Al-Oufi et al. / Fisheries Research 66 (2004) 115–119 119

be further evaluated for their specific use in high tem-perature countries. Further research is needed to testthe effect of existing and new generation UV coat-ings on breaking strength and extensibility of twines.Preferably, such studies should be undertaken in col-laboration with twine manufacturers.

While weathering tests tell little about the lifetimeof fishing nets, they are able to illustrate the differentabilities of various fibre types and treatments to with-stand the effects of solar irradiation. Much is depen-dant upon the standard operating procedures employedby fishers and how they treat and store their nets. Un-der normal circumstances, there is no need to exposenets to sunlight. Synthetic nets do not rot, thus they donot need to be dried out, and can be stored even whenwet. Fishing nets should always be protected from ex-posure to direct sunlight. This would reduce deteriora-tion in net breaking strength and would have benefitsto the fishermen including extension of net half-life,and reduction in the capital cost of fishing and timespent upon net maintenance and repair. Stationary fish-ing gears, on the other hand, such as traps and bottomset gillnets, because they are set for a considerabletime immediately below the water surface and hencefoul rapidly need to be regularly sun dried to removedebris and other fouling organisms before they can bestored. Application of appropriate dyes in such gearswill extend their useful life considerably. This aspectof fishing technology is demanding of further study.

References

Al-Oufi, H., McLean, E., Palfreman, A., 2000. Observations uponthe Al-Batinah artisanal fishery, the Sultanate of Oman. Mar.Policy 24, 423–429.

Botros, G.A., Al-Sayes, A.A., 1973. Impregnation of rot-retardingpreservatives by Egyptian cotton netting twines. Bull. Inst.Oceanogr. Fish. Cairo 3, 419–426.

Brandt, A.V., Carrothers, P.G., 1964. Test methods for fishing gearmaterials (twines and netting). In: Modern Fishing Gear of theWorld, 2. FAO, Rome, pp. 9–49.

Dahm, E., 1992. Reduction of Mesh Size of Netting afterLong-time Storage Under Atmospheric Conditions. ICES CM1992/B:21.

Dahm, E., Suuronen, P., Lehtonen, E., 1990. Weatheringexperiments with netting material for stationary fishing gear.Finn. Fish. Res. 11, 17–24.

JICA (Japan International Cooperation Agency), 1986. GillnetFisheries. Outline of Fishing Gears and Methods. Kanagawa-Ken, Japan.

Kenehiro, H., Kasu, J.E., Suzuki, M., Inada, H., 1988. The effectof additives on the photodegradation of nylon netting twines.Bull. Jpn. Soc. Sci. Fish. 54, 1769–1776.

Klust, G., 1959. The efficiency of synthetic fibres in fishingespecially in Germany. In: Kristjonsson, H. (Ed.), ModernFishing Gear of the World, 1. FAO, Rome, pp. 139–146.

Klust, G., 1982. Netting Materials for Fishing Gear, 2nd ed. FishingNews Books Ltd., Fraham, Surrey, UK.

Klust, G., 1983. Fibre Ropes for Fishing Gears. Fishing NewsBooks Ltd., Fraham, Surrey, UK.

Nielsen, L.A., Johnson, D.L., 1992. Fisheries Techniques. SouthernPrinting Company Inc., Blacksbury, VA, USA.

SAS/STAT, 1993. User’s Guide, vols. 2 and 6. SAS Institute Inc.,Cary, NC, USA.

Shimozaki, Y., 1959. Characteristics of synthetic twines used forfishing nets and ropes in Japan. In: Krjstjonsson, H. (Ed.),Modern Fishing Gear of the World, 1. FAO, Rome, pp. 19–29.

Tait, R.V., 1981. Elements of Marine Ecology, 3rd ed.Butterworths/Heinemann Ltd., London, UK.

Takase, M., 1972. On change in some properties of netting twines(nylon, vinylon-spun and multifilament and polyethylene)treated with heated dry air. J. Shimonoseki Univ. Fish. 20, 55–63.

Takayama, S., Shimozaki, Y., 1959. Development of fishing net andrope preservation in Japan. In: Kristjonsson, H. (Ed.), ModernFishing Gear of the World, 1. FAO, Rome, pp. 113–122.

Warenzeichenverband, P., 1959. The technological characteristicsof perlon for fishing equipment. In: Krjstjonsson, H. (Ed.),Modern Fishing Gear of the World, 1. FAO, Rome, pp. 34–42.

Wijngaarden, J.K., 1959. Testing methods for net twines and nets,especially those manufactured from synthetic materials. In:Krjstjonsson, H. (Ed.), Modern Fishing Gear of the World, 1.FAO, Rome, pp. 75–81.