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6.4. INVESTIGATION OF THE PROPERTIES OF MATERIALS AND TECHNOLOGIES USED IN AQUACULTURE

Two thirds of all fish escapes, in terms of both incidents and number of fish, occur through a hole in the net (Jensen et al. 2010). Holes form due to a number of reasons, the most common being: biting of the net by predators or the caged fish; abrasion; boat collisions; contact with flotsam, and cage handling procedures such as lifting.

There are currently no written standards specifying the properties of the materials used in aquaculture, or explaining how the materials might degrade with time and cumulative wear and tear. This is particularly problematic for non-metallic materials such as polyethylene, polyamide and polypropylene; their functional properties are known to change with temperature conditions and over time.

INTRODUCTION

Authors: Østen Jensen1, Per Christian Endresen1, Heidi Moe1, Diego Mendiola2, Gorka Gabiña2, Leire Mendia Huarte3 & Jose Miguel Iturrioz3

1 SINTEF Fisheries & Aquaculture, Norway2 AZTI-Tecnalia, Spain3 INASMET-Tecnalia, Spain.

Cite this article as: Jensen Ø, Endresen PC, Moe H, Mendiola D, Gabiña G, Mendia Huart L, Iturrioz JM (2013) Investigation of the properties of materials and technologies used in aquaculture. In: PREVENT ESCAPE Project Compendium. Chapter 6.4. Commission of the European Communities, 7th Research Framework Program. www.preventescape.eu

OBJECTIVE

We aimed to determine the functional characteristics of commonly used aquaculture materials and establish objective methods for testing the materials used in sea-cage aquaculture nets.

ISBN: 978-82-14-05565-8

METHODS

New and used net materials were collected from a number of net producers and fish farmers. Unfortunately, the history of the used net samples was generally not known; farmers do not keep fully detailed records about when they wash their nets (whether in-situ or onshore), whether or not the nets are coated, or details of any operations performed on the net during use that could influence the strength of the material. Some variation in the results is, therefore, to be expected.

Strength reduction of new and used nets, with different mesh sizes and thread numbers, were compared. A number of factors can reduce the strength of the nets used in fish-farm cages, such as: onshore washing; coating; in-situ washing using high-pressure disks; creep during lifting operations; ultra-violet radiation, and abrasion. We tested how some of these factors would affect net strength.

Nets may also be subject to extra loads from the weights or sinker-tube, which are used to maintain the shape and volume of the cage, plus any water currents. Continuous loads can introduce creep effects, especially on plastic materials such as nylon and Dyneema (http://en.wikipedia.org/wiki/Creep_(deformation)). In addition, waves often introduce motions to the floating collar that propagate as dynamic tensions in the net. To investigate how this may affect the break-strength of the net, nylon and Dyneema samples were tested at increasing rates of deformation.

The effect of onshore machine washing was investigated by washing net samples together with net cages that had been used to farm salmon for one production cycle. After washing the net panels 1 – 5 times the mesh strength was tested. During summer and early autumn months the net cages, especially in mid- and southern parts of Norway are washed, using high pressure disks, as often as once per week to remove biofouling. Nylon and Dyneema net samples were washed using industry standard equipment to investigate the effect on mesh strength.

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Testing procedure

Numerous net materials were tested in a uni-axial tensile test machine (Figure 6.4.1). Two different test types were used to test the strength of the material; mesh strength tests (Figure 6.4.2) and uni-axial tensile tests to determine the tensile strength of the thread as well as the elasticity of the material (Figure 6.4.3). Tensile tests were conducted on 50 cm lengths of net. Pure nylon netting was cut while wet, whereas netting containing Dyneema fibres were cut while dry, using a butane powered cutting tool, to prevent filaments from slipping through the knots at the thread ends. Unless otherwise stated, samples were immersed in tap water at 20 – 23°C for at least 24 h prior to testing. Nylon and Dyneema specimens were subjected to a permanent load for 30 minutes. After seven days of relaxation the break-strength of the specimen was determined to investigate whether the 30 minutes creep-load had weakened the material. Quasi-static mesh-strength tests (ISO 1806:2002; ISO 2002) were performed on new and used material to serve as a reference for other tests. Equivalent test settings were adopted for the uni-axial tensile tests.

Test methods were developed to investigate the influence from washing in-situ with high pressure cleaning disks (Figure 6.4.4), and load duration – both creep and dynamic tests. Two different nylon materials (N10 and N11), with (C) and without coating (U), as well as coated Dyneema (N12_C) net were subjected to one hour of controlled abrasion (Figure 6.4.5), and the strength of the material tested using uniaxial tensile tests.

Figure 6.4.1. Test machine used to determine net strength and elongation.

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Figure 6.4.3. Test specimen used in uni-axial tensile test.

Figure 6.4.2. Test specimens for mesh strength testing.

Figure 6.4.4. High pressure disks used to clean net cages.

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Figure 6.4.5. Equipment used to inflict controlled abrasion damage to net materials.

Tests were performed on samples of the type of new materials that are supplied to the Spanish fish farming industry (Figure 6.4.6), and on used net samples from European seabass (Dicentrarchus labrax) and gilthead seabream (Sparus aurata) cages (Figure 6.4.7). A 30% decrease in the break-load and a 15 – 40% decrease in the length of elongation at rupture were found in nets that had been used for one production cycle (Figure 6.4.6). The Dyneema nets were found to have much higher break-load and stiffness compared to the nylon nets. The difference between nets from different producers was relatively small. The mesh strength of nets used to farm Atlantic cod (Gadus morhua) and Atlantic salmon (Salmo salar) were determined (Figures 6.4.8, 6.4.9 and Figure 6.4.10). There was no obvious correlation between production year or the number of wash cycles and degradation in the strength of the net.

Figure 6.4.6. Mesh strength and elongation at break - virgin and used Spanish net materials.

RESULTS AND DISCUSSION

Net type

Figure 6.4.7. Mesh strength and elongation at rupture - new nets from different producers.

Figure 6.4.8. Mesh strength for nets of varying age.

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Figure 6.4.9. Changes in net strength due to washing of the net.

Figure 6.4.10. Force vs strain curve for different net materials.

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Tests showed that a 30 minute creep-load had a positive effect on the break-load for the Dyneema material (N6; Figure 6.4.11). For the C90 case, (i.e. where the creep-load is equal to 90% of the break-load), the break-strength increased by close to 20%. An opposite effect was observed for nylon nets (N7 and N8) where the break-strength was reduced by 5 – 10%.

Figure 6.4.11. Stress at break divided by stress at break for virgin material.

The relative mesh strength, normalized against the mesh strength reported in ISO 1806:2002 (ISO 2002), of one of the two nylon nets tested, increased by 10% as the rate of deformation increased (Figure 6.4.12). However, no significant change was detected for the second nylon sample and the Dyneema mesh strength was reduced by up to 10%. The highest rates of deformation tested gave a total load time of 1.2 second for the Dyneema net, 1.6 seconds for one of the nylon nets, and 2.7 seconds for the other. These values are within the range of

Creep loads

MESH STRENGTH AND DEFORMATION RATES

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loads expected due to wave action. Snap loads will have even shorter load times. These results indicate that short periods of extra load bearing, such as from waves, should not be a problem for nylon nets, but suggest that for Dyneema nets these loads may be problematic, particularly if snap loads with even higher rates of deformation occur.

Washing of nets led to a distinct drop in strength after only one wash (Figure 6.4.13). A reduction in strength of 20% was observed after the net panels had been washed four times. According to the Norwegian technical standard NS9415 (Standard Norge 2009) net cages must be discarded if the remaining strength is reduced by 35%. Net washing therefore accounts for more than half of the allowable strength reduction. In comparison, there was no reduction in strength even after 40 wash cycles of in-situ high pressure cleaning (Figure 6.4.14). However, escape incidents have occurred during high pressure cleaning, especially in connection with the use of cranes, but these incidents have occurred due to the incorrect use of equipment by their operators.

Figure 6.4.12. The effect of rate of deformation on relative mesh strength.

WASHING

Figure 6.4.13. How the washing of the net effect the mesh strength.

Figure 6.4.14. Effect of high pressure claning on the mesh strength.

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RECOMMENDATIONS

Used net materials showed a significant reduction in strength, even after just one production cycle. However, no clear relationship was found between the duration of net use and the subsequent reduction in strength.

High pressure cleaning had no effect on mesh strength.

Cleaning of nets in washing machines lowered the strength of the nets by up to 20% after 4 wash cycles.

Abrasion significantly reduced the strength of the nets. Coating limited the reduction in strength, but could not eliminate it.

Following abrasion testing, the reduction in strength was significant for all of the tested nets (Figure 6.4.15). The coating had a positive effect on the abrasion resistance of nylon nets. Even though the tensile strength of new Dyneema net is higher than that of nylon, the relative reduction following abrasion was far larger than for the nylon nets. This result may reflect the fewer filaments in the Dyneema thread.

ABRASION

Figure 6.4.15. The effect on abrasion on tensile strength.

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REFERENCES CITED

Jensen Ø, Dempster T, Thorstad E, Uglem I, Fredheim A (2010) Escapes of fish from Norwegian sea-cage aquaculture: causes, consequences and prevention. Aquaculture Environment Interactions 1:71-83

The International Organization for Standardization, 2002. ISO 1806. Fishing nets -Determination of mesh breaking force of netting.

Standard Norge (2009) Norwegian standard NS 9415.E:2009 Marine fish farms—requirements for site survey, risk analyses, design, dimensioning, production, installation and operation. Standard Norge, Lysaker