WEATHERING EXPERIMENTS WITH NETTING MATERIAL FOR STATIONARY FISHING GEAR. E. Dahm Institut für Fangtechnik,Palmaille 9,D-2000 Hamburg

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1 International Council for the Exloration of the Sea ICES Fish Cature Committee Paer C.M. 1989/B:34. WEATHERING EXPERIMENTS WITH NETTING MATERIAL FOR STATIONARY FISHING GEAR by E. Dahm Institut für Fangtechnik,Palmaille 9,D2000 Hamburg P. Suuronen and E. Lehtonen Finnish Game and Fisheries Research Institute, Fisheries Division, P.O.Box 202, SF00151 Helsinki Abstract: Thirtysix sheets of netting rovided for use in stationary. fishing gear were exosed to the same environmental conditions in Finnish inshore waters. The netting differed in material, construction, yarn size, mesh size and finishing. The exosure eriod was two summers. Changes in the mechanical roerties were tested by determination of the mesh breaking strength according to ISO Standard 1806 four times during the test eriod. The most striking result of the investigation was that the mesh breaking strength of exosed netting made of olyethylene yarn and of olyamide yarn of larger diameter was greater than that of unexosed control material. Treating the netting with a bituminous imregnation agent increased the mesh breaking strength by about 20 %. Knotted netting was markedly less strongly affected by the environmental imact than knotless netting. Netting of the same yarn size showed great differences over the total test eriod in the measured roerty. The results seem to be of some general interest for the choice of the most suitable netting for stationary fishing gear. I '\ 1

2 2 1. Introduction Netting material used for stationary fishing gear, such as ound nets or fish ens, is normally under comletely different stress fram netting used in tawed gears such as trawls. The latter has mainly to bear the towing force of the shi during action. In stationary gear, the material has to be sufficiently strong to resist environmental influences and the ossible damage which can be caused by, for examle, solar radiation, wave action and marine growth. Strong yarns showing a raid decline in breaking strength after rolonged exosure to sunlight or wave action are therefore less suitable for use in stationary gear than weaker yarns which kee their mechanical roerties unchanged over a long eriod. Tests undertaken to comare the "weathering" resistance of different materials are often carried out under which are easy to reroduce and give results with a conditions low degree of deviation. To achieve this, however, they have to be confined to single factors, e.g. solar radiation, or to simulate a situation which is far fram the real conditions at the site of future use (see KLUST 1957a, 1970, HERLINGER et al. 1989). Thus, the results are of little Or no hel in redicting the actual behaviour of the yarn in a given environment. ~ In contrast, rolonged insitu exosure to the natural conditions may give a reliable indication of what to exect from different yarns in this Iace and alication. Disadvantages are the imossibility of searating the different factors resonsible for the result and the difficulty of transferring the results to other laces with a different set of oerating factors. In this context, it is'of interest that recent investigations of roes have shown that reeated dynamic stress caused by forces in an order of magnitude weil below the actual breaking force may cause a relatively raid and unforeseen decline in the strength (Institut für Textiltechnik 1985). The extent to which netting is exosed L

3 3 to wave action may therefore be as imortant for its longevity as resistance to sunlight or deterioration by marine growth. In designing weathering tests, a choice must be made between the aim of indentifying the factors resonsible for the sueriority of one material to another and the aim of giving advice on which material to use in a given situation. The FinnishGerman grou which carried out the exeriments described below"gave riority to the latter alternative. In our oinion, before turning to deeer analysis, alied research in fishery should cover first of all the ractical asects of a scientific roblem (in this case the question of the fisherman regarding the material best suited to the conditions at his articular fishing lace). However, it seems that this investigations also revealed some general rules imortant for stationary gear alications which might" be worthwhile to consider in the choice of materials for the mentioned urose. 2. Material and methods Thirtysix sheets of netting were used in the tests (Table 1). They reresented 19 tyes of material, 17 being made of olyamide (PA) and two of olyethylene (PE). Five of the PA nettings were of the knotless tye. All the PA materials were tested with and without imregnation. The imregnation fluid used was the same in all cases: a black bitumen based dye, diluted with 25 % white sirit of turentine. After 10min exosure to the fluid, the netting was allowed to dry in a dark room. The PE netting was not imregnated. The test site was a sheltered bay near the eninsula of Lauunen, northwest of the city of Turku, in the southwestern Finnish archielago. Throughout the esosure eriod, continuous or at least monthly records were made of the hours of sunshine, temerature, salinity, H, rimary roduction and visibility measured as the secchidisc reading (Table 2).

4 4 As shown in Fig. 1, the test ieces were exosed in a secial order. They were made u into one anel and ket with a system of floats and anchors immediately at the water surface, the layer in which the material was most strongly subjected to unfavourable environmental imact. Stris were cut from all 36 sheets after 30, 62, 123, and 185 days exosure and stored in a dry dark iace for later testing in the laboratory of the Institut für Fangtechnik. The test ieces were taken out of the water in autumn 1987 and ut back after the ice cover had melted. During the storage on shore in winter, the test ieces were ket in an unheated, dark, dry seaside shed. The handling and storing of the test netting thus corresonded to the normal use and storing of stationary fishing gear such as ound nets. tt In ractical situations sheet netting is stressed as a whole. So the conclusion of a good general mesh strength after a longer time of exosure is a convenient mean to control the suitability of a given material for use in stationary fishing gear. The resistance of netting to deformation or ruture can easily be determined by a standardized international test rocedure. The mesh breaking strength was tested in the laboratory in Hamburg in February 1988 and December At least 20 meshes were cut from each of the stri samies taken from the test ieces and subjected to the test rocedure described in ISO standard The tests were carried out after wetting the material for 24 hours. Unexosed control samies of all 36 materials were subjected to the same tests. 3. Results Figs. 2 and 3 show the changes in the mesh breaking load of the different materials during exosure eriod exressed as the ercentage of the mesh breaking load of the control material. (For ease of comarison the results of the single nettings are resented in fig. 2 and 3 according to the criteria material and yarn size. So, for instance the yarns of PA and

5 5 210/12 are ut together in the first column of fig. 3, and all knotless nettings are'to be found in the first column of fig. 2). The most surrising result is the increase in the mesh breaking strength in a range of ca. 20 % for the PE ~arns. It may be noted that the PA yarns of greater diameter show the same trend during the first weeks of exosure. The ossibility of errors of the test machine can be excluded, because i t is calibrated regularly. An exlanation might be that the action of waves and currents tightens the knots in the meshes. As earlier exeriments with stiffening agents have shown, an increase of knot fixation is often accomanied by an increase in total breaking strength, robably due to reduction of the weakening effects of knot sliage. There is little or no doubt in the literature (e. g. KLUST 1957b) that hotodegradation, i.e. deterioration of the material by sunlight is the most imortant cause of damage to stationary fishing gear during longterm oeration. The usual way to revent this is to dye the material. the black dye used.in the in fact the case (Figs. 2 and 3). Imregnation with exerimentcould thus be exected to imrove the final breaking strength of the meshes, tests on, as was From the beginning of the imregnation generally increased the mesh breaking strength by about 20 %~ However, it did not generally change the degradation characteristics of the yarn. The strength of imregnated and unimregnated yarns often showed a similar decreasing strength. The variation coefficient of the 20 tests of mesh breaking strength tests carried out on each of the samies did not vary greatly (average value around 6 %) during the exosure eriod. The action of the environmental factors on tne different arts of the netting might be assumed to be nonuniform and to lead to increasing variation coefficients. As the tests showed, this was not the case.

6 6 Irresective of their origin, the yarns of smaller diameter (210/6) were strongly affected by the exosure, the reduction of mesh breaking strength being in the range of 2030 % (the material falling out of this scheme is 9A and that's why there is two question marks behind the reference value of it). Badly affected were also all the knotless netting samies, which were of the Raschel net tye. There the reduction during the total exosure time often exceeded 30 %. Although yarns of the same yarn number showed more or less similar ercentage strength losses during the exosure eriod, their mesh breaking strength at the start of the exeriment differed considerably, deending on the origin. However, the tt reference value of 15A is so doubtful that we abstained from showing the curve for the unimregnated stri samies of this material (there was an unexected high degree of variability in the tests of this material so that one could think at a hidden damage in the samie). 4. Conclusions The results of the exeriments show that even when the effective environmental factors are not controlled, the exerimental setu may give some useful indications recarding the construction of stationary fishing gear. First of all, the results confirm that it ays to imregnate netting used for stationary fishing gear. The tests showed that this increases the mesh breaking strength and with good imregnation this increased strength is maintained over a long eriod, desite of leaching. Second, they show that knotless netting may not be so ideal for stationary fishing gear or fish enclosures as is generally assumed. Knotless netting is often recommended, since the fish caught inside are in less danger of skinn damage due to chafing by the rotruding knots. These results throw doubt on i ts suitability from a technical oint of view, esecially in resect of weathering endurance. Here, too, imregnation

7 7 increased the strength right from the start and the strength reduction showed the same attern as in the untreated netting ieces. Third, the mesh size has aarently no effect on the weathering endurance. What counts is the yarn size and there the rule is: the thicker the better. However, account must also be taken of water resistance, ease of handling and rice of the gear. As such considerations layamajor role in the choise of yarn size, it ays to look for a good quality yarn. As these tests showed, yarns of the same size will lose their strength in a similar way, but a yarn which had 100 N at the start will still have 92 N after 2 summers, whereas a 80N yarn will retain only 73 N after the same time. Unfortunately, in the case of the PE netting yarns, only two tyes of this material were used in the trials. It would be worthwhile to increase knowledge of this material by including a larger number of PE yarns and mesh sizes in any followu exeriment. Literature HERLINGER, H., KÜSTER, B., ESSIG, H.: Untersuchungen zum 1989 thermo und hotooxydativen Abbau von PolyamidTex tilien in Gegenwart und Abweseheit von Stickoxyd. Textilraxis International 44, 6: INSTITUT FÜR TEXTILTECHNIK: Einfluss von Werkstoffeigen 1985 schaften und Konstruktionsmerkmalen auf das dynamische Festigkeitsverhalten hochbeansruchter schwerer Seile. Denkendorfer Forschungsberichte, AIFForschungvorhaben Nr KLUST, 1957a G.: Zur Wetterfestigkeit von Zwirnen aus einigen syntetischen Faserstoffen. Textilraxis 12: KLUST, G.: Untersuchungen über Eisenlack S als 1957b Präarierungsmittel für PERLONFishnetze. Archiv für Fishereiwissenschaft 7: KLUST, G.: Zur Wetterbeständigkeit gefärbter Schnüre und Seile 1970 aus Polyolefinen. Textilraxis 25:

8 8 Table 1: Materials used and test eriods in the FinnishGerman weathering exeriment from Number Material Meshsize Imregnation 1A/1B PA 210/12 knotless 15 mm no/yes 2A/2B PA 210/24 knotless 18 mm no/yes 3A/3B PA 210/6 knotless 50 mm no/yes 4A/4B PA 210/12 13 mm no/yes 5A PE 400/9 20 mm no 6A PE 380/18 50 mm no 7A/7B PA 210/9 15 mm no/yes 8A/8B PA 210/12 15 mm no/yes 9A/9B PA 210/6 18 mm no/yes 10A/10B PA 210/30 22 mm no/yes 11 A/ 11 B PA 210/60 27 mm no/yes 12A/12B PA 210/15 40 mm no/yes 13A/13B PA 210/6 12 mm no/yes 14A/14B PA 29 TEX 4 knotless 12 mm no/yes 15A/15B PA 210/12 35 mm no/yes 16A/16B PA 210/12 knotless 50 mm no/yes 17A/17B PA 210/12 35 mm no/yes 18A/18B PA 210/15 40 mm no/yes 19A/19B PA 210/12 (93.5 TEX x 3) 50 mm no/yes Test eriod I Test eriod Ir Test eriod 111 : / Test eriod IV : /

9 9 Table 2: Environmental conditions at the water surface during the exercise Date Temerature Salinity Hvalue Primary roduction Secchidisc visibility oe ) 0/00 m 06Jul' Jul' _7AU9' Se' Jun' Jul' Aug' Se'

10 T Fig.1: The anchorage of nettings,r ~ ~... 1"..., ~,. ~~ F "......, ~ II '" I'" r= 1'" I... 1'..7...,... \ ;. loatline. 1 ~ I " I 1 I... '"'... I'" '' '"' " unim regnated 1...,...,. ~ ~... 1 "' "'"...., I... ' '" l I... I... I......,......,..., ' "" I""" 1"" I... 1'' 1'' I... I... l I ''i;7 ~..., l ''...,... " " '"... '"' 1 I, 1 ~O SamIe I SamIe I I Samie I 11 Samie I Samie V lf B \... imregnated \ " v (reserve) o.'.

11 e. ti w, ~ria1 Ho. RIlCl!renc~ Fig. 2: Langter. veathering effects On.esh breaking strength of different netting aaterials 110 PA 3 A H e S Material Ho. Referenc," PE 5 A M ~ Ger.any.... Jaan F.R. 3 B H den19 knotlesa ~ 210/6 60 J" Ja 61 I1J lude Ja 61 I1J 1650E A 100 Material No. Reference 10 PA 16 A 161,05 H ".. Material PE Ho Referenc," 60 Ger.anl', 6 A Portugal H F.R. 16 B H 60 :~~ knot1ess den 210/ Ja 61 I1J 165DE 40 Ja 61 I1J IU OE Ma t e rial No. Reference..._ Haterial No. Reference Iaron 14 A H 10 "A PA 13 A 63.5 H USSR knotless 14 B N "8 Taivan /6 13 B H 2~0/ JO 61 I1J 1850E Ja 61 I1J ISS OE ' ' P Material No. Reference 10 JA '..._..... Msteria1 Ho. Reference PA 1 A N PA 9 A 70.3 H?? Germany 10 9S.t 8 Germanl' F.R. B N knot1ess "'210/12 IA 60 F.R. 9 B N 210/6 10 JO 61 I1J /15 OE JO IISOC 10 Hateria1 No. Reference 18,.. Haterial Ho. Reference PA 2 A N PA 7 A H Germany ~ la F.R. 2 B H 60 knotless ""210/ I1J /l5de JO IISDE 110 Gerllanl' F.R. 210/9 7 B

12 Fig. 3: Longter. veathering eueeta on.eah brealcing strength of different netting.aterials '2 120 Material No. Reference Haterial No. Reference PA 8 A IS3.1 N , PA 12 A I GerlDany 8A Ger.ony F.R. 8 B N F.R. 12 B /15 210/ JO SDE: Ja 62 12J 185 OE: P Haterial No. Reference 10 Materiol No. Reference 100 /8,1. PA 15 A 96.1N11 _158..._ PA 18 A K 188 Germany: 80 10i"on F.R. 15 B 180.7N 210/15 18 B 217,65 I 210/ J 18SDC J 185 OE: P Material No. Reference Materiol No. Reference... 10,1. PA 4 B N PA 10 A ~ Jaan 80 ~~ Germany /12 4 B N F.R. 10 B /3') N J 185 OE I1J 18S0F: 120 /20 I:~ Material No. Reference IIA Material No. Reference PA 17 A PA 11 A H Taiwan Germany 210/12 17 B FoR. 11 B / O 1O JO 61 /2J I8SDE: Ja 62 12J /850F: Material No Reference Koron 19 A N USSR 210/12 19 B N '0 2!B 19A Ja DF: P ercentoge of.esb breoklng streng tb of referenee so.ie OE. days of exosure. ~..