Yuchng Wng, Chng Zhou,b,c,＊, Liuxiong Xu,b,c, Rong Wn,b,c, Jingo Shi,Xufng Wng,b,c, Ho Tng,b,c, Lumin Wng, Wnwn Yu, Ki Wng

aCollege of Marine Sciences, Shanghai Ocean University, Shanghai, 201306, China

bNational Engineering Research Center for Oceanic Fisheries, Shanghai, 201306, China

cKey Laboratory of Sustainable Exploitation of Oceanic Fisheries Resources, Ministry of Education, Shanghai, 201306, China

dEast China Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences, Shanghai, 200090, China

eCollege of Marine Ecology and Environment, Shanghai Ocean University, Shanghai, 201306, China

Keywords:

ABSTRACT Purse seiners deploy large amounts of drifting fish aggregation devices (FADs) in all tropical oceans to catch tunas. These FADs are constructed with porous synthetic fibre netting, which are responsible for incidental mortality of sea turtles and sharks through entanglement, and can engage in ghost fishing if they are lost and abandoned. The use of natural or biodegradable materials to build FADs can effectively mitigate marine pollution and bycatch issues, and as such they are currently promoted by fisheries management organizations. This study evaluated the degradability of ropes made of common natural fibres; cotton rope (3-strand, 96-thread, twisted, Slay), jute rope (3-strand, 13-thread, twisted, Z-lay) and sisal rope (3-strand, 8-thread, twisted, Z-lay). Degradability was evaluated by measuring the breaking strength (cN/dtex) and retention ratio of rupture elongation.Results showed that jute rope and sisal rope experienced rapid reduction of strength which over the first month soaking in the sea fell by 66.8% and 46.9%, respectively. Cotton rope exhibited the most inert degradation behavior, with breaking strength maintained at half of the initial value at 10 months. The retention ratio of rupture elongation for jute ropes and sisal ropes fluctuated considerably in the following months after deployment, compared to the relatively stable change for cotton rope. Results suggested that among these congeners,cotton rope would be most preferable component for Bio-FADs with a 10-month minimum durability that matches the required lifetime of FAD fishing strategies.

1.Introduction

Many pelagic species, including some tunas, exhibit associative behavior underneath floating objects (e.g. logs, seaweed or palm fronds)on the ocean surface (Castro et al., 2002; Hall, 1998). Since the 1980s,artificial fish aggregation devices (FADs) have been developed and used for more efficient capture of tunas (Scott & Lopez, 2014; Ushioda, 2015).Since FADs with buoys are easier to locate and track than free swimming fish schools, search time can be significantly reduced and catch success improved. In the early 1990s, FAD-associated fishing operations began to expand rapidly (Dagorn et al., 2013; Escalle et al., 2018; Forget et al.,2015; Orue et al., 2019), and so far, nearly half of the tropical tuna caught worldwide are caught using this method (He & Suuronen, 2018;Miyake et al., 2010, p. 125).

Despite the advantage of reducing fisheries’ “carbon footprint”(Dagorn et al., 2013), massive deployment of FADs (100,000 estimated),made of synthetic materials has aroused global concern regarding the possible negative impacts on marine ecosystems including: (i) entanglement of sharks and turtles (Filmalter et al., 2013); (ii) accumulation of synthetic marine debris (Maufroy et al., 2015; Escalle et al., 2019);(iii) ghost fishing and (iv) damage to vulnerable ecosystems (e.g. reefs)(Cillari et al., 2018; Moreno et al., 2018a, 2018b, 2018c). With the purpose of alleviating these detriments, the EU Common Fishery Policy and the Marine Strategy Framework Directive proposed environmentally friendly fishing methods (Zudaire, 2017; Zudaire et al., 2019). To date, four tuna regional fisheries management organizations (tRFMOs),including Inter-American Tropical Tuna Commission (IATTC), International Commission for the Conservation of Atlantic Tunas (ICCAT),Indian Ocean Tuna Commission (IOTC), and Western and Central Pacific Fisheries Commission (WCPFC), have adopted several relevant resolutions that are committed to reducing the entanglement of marine animals or fauna associated with FADs (IATTC, 2019; ICCAT, 2016; IOTC,2019; WCPFC, 2018). A solution on the outset proposed by International Seafood Sustainability Foundation (ISSF) is using small meshes size (less than 70 mm) or tying up netting into bundles like “sausages” for submerged appendages, i.e. so-called Non-Entangling FADs (NE-FADs)(ISSF, 2019). However, observation in practice revealed that, after a period of time, the small meshes split into larger holes, and the bundle of netting untie. In order to eliminate the risk of entanglement, the use of ropes or canvas suspending into the water substituting for netting should be suspended. Moreover, tRFMOs encourage the vessels to use biodegradable/natural fibre materials in the construction of FADs (i.e.Bio-FADs) to reduce the amount of synthetic marine debris (Murua et al.,2016, 2017; Pilling et al., 2017). Scientists from AZTI, IEO and IRD research centers, participating in the EU funded BIOFAD project with the collaboration and support of ISSF FAO-GEF Common oceans,compared the Bio-FADs with NE-FADs (hanging with netting panel or bundles) by comprehensively evaluating the lifetime, drift performance and tuna aggregation (Santiago et al., 2019; Zudaire et al., 2019). Preliminary results of the project suggested that all prototypes of both FADs types had a lifespan of more than 1 year, and the highest biomass aggregated by NE-FADs and Bio-FADs occurred in the first and ninth month at sea, respectively.

A competent Bio-FAD should have a lifespan ranging from 5 to 12 months (Moreno et al., 2016), and thereafter biodegrades as fast as possible. Although the most reliable material used for the construction of FADs has not been identified, several investigations, based on the linear relationship between breaking strength and soaking time at sea,have reported the degradation of ropes manufactured from natural fibre materials and suggested that some of them are potentially good alternative (Araya-Schmidt & Queirolo, 2019; Lopez et al., 2019; Moreno et al., 2018; Winger et al., 2015). However, the degradability of ropes may vary depending on natural fibre properties, production technology and specifications (diameter, material, lay length, construction and structure, etc.). Moreover, these previous studies ignore the potential effect of diameter on breaking force when evaluating the degradability of ropes. To this end, this study aimed to test the degradability for natural fibre ropes made by domestic manufacturers in China. We thus collected common natural fibre ropes made by cotton, jute, and sisal,and measured the breaking strength and retention ratio of rupture elongation varying with time of soaking in the sea, in order to provide references for the material selection of Bio-FADs.

2.Materials and methods

2.1.Test materials

Three common natural fibre ropes (cotton 3-strand twisted rope, 96-thread; jute 3-strand twisted rope, 13-thread; and sisal 3-strand twisted rope, 8-thread) with different physical and mechanical properties were examined for degradation. The samples selected in this study referred to the specification (e.g. diameter) of ropes used in Moreno et al. (2018a)and Zudaire (2017) that reported the degradation characteristic of Bio-FADs at sea trials. All of the ropes were fabricated by the same company (Runya Chemical Fibre Rope Company, Shandong, China). The specifications of structural parameters are shown in Table 1.

2.2.At-sea deployment for natural degradation

The decay or degradation rate of the vegetable fibre is a function of the type of fibre, water temperature, rotting power of the water, and soaking time (Kim et al., 2016; Klust, 1982, p. 177). Due to the difficulties associated with real fishing conditions (e.g. Most FADs are retrieved by other vessels or sink to the seabed) that prevent samplesfrom being tracked by a given vessel over the lifetime, we deployed samples near the shore of the East China Sea (3048N, 12236E)(Fig. 1). We included 16 pieces of 5 m long samples for each of the three rope types in the experiment, totaling 48 samples. The samples were tied on the floating collar of a net cage and sinkers were attached to the end of ropes to keep suspension in the water.

Table 1 Specifications of structural parameters for three types of ropes.

The total soaking time lasted for 302 days with the initial deployment in November, 2018 and the last retrieval in September, 2019.Depending on the sea conditions, the samples were retrieved once per cycle ranging from 28 to 31 days (mean30.2 days) for laboratory measurement. We also monitored environmental conditions at the deployment site using the Aqua TROLL 100/200 conductivity, temperature and depth sensor (CTD). Over the period of study, sea surface temperature varied from 9.8C to 27.0C and salinity from 30.7‰ to 33.5‰.

2.3.Laboratory measurement

Two indices associated with physical and mechanical properties,breaking strength and retention rate of the percentage of breaking elongation, were used as the indicators of material degradability. The breaking strength (cN/dtex) is expressed as the ratio of tension force when the rope breaks under stress (i.e. breaking force) to the linear density. The retention rate of the percentage of breaking elongation at the

n

-

th

month is defined as the ratio of breaking elongation at the

n

-

th

month to that of samples without soaking (Shi et al., 2006). The breaking elongation is the ratio between increased length after breakage and initial length of the samples.

For the initial values, we tested the breaking strength and breaking elongation of virginal samples soaked in fresh water for 24 h in advance.Samples obtained after soaking in the sea, were immediately pre-treated in the laboratory by hanging them for draining until water no longer dropped. Measurements were then made with a tensile test machine(HY-5080, made in China) with the maximum capacity of 50 kN and precision of 0.1 N (Fig. 2). All samples to be tested shall be exposed to the atmosphere for testing of 20C±4C and relative humidity 65%±4% until they have reached equilibrium. Each of retrieved samples was cut into three small sections with each one 150 cm long. These tested samples were mounted in holding devices, the distance between the two holding devices is of 263 mm, which then moved at a constant stretching speed 100 mm/min until breaking. We repeated every test three times to reduce accidental error. Finally, the degradation ability was determined based on the monthly variation of breaking strength and retention rate of percentage of breaking elongation with soaking time.

Due to possible effects of bio-fouling on tuna aggregation as well as degradability, we observed the presence of micro and macro organism colonization on the rope and developed preliminary taxonomy.

Fig. 1.Experiment site and snapshot of in-situ deployment.

Fig. 2.Schematic drawing of tensile testing machine. (a): HY-5080 tensile testing machine; (b): holding device; (c): tested rope; (d): center console.

3.Results

The validity of measurement results were evaluated by examining the fracture position of samples. As shown in Fig. 3, the positions for three ropes consistently fell into the span between two holding devices,suggesting that the breaking was aroused by tension stress rather than the attrition with holding devices.

Three types of ropes presented comparable initial breaking strength around 0.80 cN/dtex (Fig. 4). Jute rope and sisal rope experienced rapid reduction of strength in the following months. In particular, the strength of jute rope and sisal rope were reduced by almost half during the first month soaking in the sea (66.8% and 46.9%, respectively) and lost all strength after three months. There was an almost coincident degradation trend between jute rope and sisal rope, although the strength of sisal rope was greater than jute rope at the same soaking time. Cotton rope exhibited the most inert degradation behavior, with the reduction ratio at 56% of initial strength by the third month. Hereafter, the strength remained stable in the following months before suddenly declining to 0.41 cN/dtex during month 7. During month 10, the breaking strength achieved its minimum, corresponding to 46.4% of the initial strength.

The relationship between breaking strength and the soak time of the ropes was fitted using the power function regression, the formula and parameters are shown in Table 2.

The variation of the retention ratio of rupture elongation is shown in Fig. 5. The retention ratio of rupture elongation for all three ropes showed an overall increased trend during the first two months, especially for sisal rope which had the most significant increment in month 1.During month 3, the retention ratio for both jute ropes and sisal ropes declined sharply to 39.6% and 63.9%, respectively. Cotton ropes showed relatively stable retention ratio around 80% during months 3–6,and gradually declined after month 7. A minimum retention ratio of 42.7% was observed during month 9.

Table 3 summarizes the presence of microorganism colonization on the rope, mainly including green, red and brown algae, mussels, and barnacles. Bio-fouling observation suggested that none of sample was colonized during the first month of soaking. Jute ropes and sisal ropes exhibited attributes of rapid colonization within 2 months by microorganisms representative of green algae. For cotton ropes, the first presence of colonization by green algae was observed during month 4. In the following months, red and brown algae started to appear, though red algae was observed only in month 5. During months 7 and 8, mussels and barnacles were also observed growing in the strands.

4.Discussion

Fig. 3.Fracture position of three types of ropes (referring to the left sketch).

Fig. 4.Variation of breaking strength (cN/dtex) with soak time (months) for three types of ropes over a total periods of 10 months. Shadow zones denote 95% confidential interval.

Natural fibres are the more competitive candidates for constructing Bio-FADs compared to biodegradable synthetic fibres; they are more commonly available and at low-cost. Although evaluating degradation in real fishing conditions is necessary to explore the most appropriate materials to use, the difficulty of revisiting and monitoring experimental FADs prevents this information from being consecutively collected(Davies et al., 2017). In the present study, we investigated the evolution of Bio-FAD submerged appendages decomposing in sea water under controlled conditions. We used breaking strength and retention rate of the percentage of breaking elongation as the representative indices to reflect the mechanical properties and aging of materials, respectively(Shi et al., 2006). The results showed that cotton ropes were most robust compared with jute ropes and sisal ropes, and could maintain durability,for a minimum of 10 months. Jute ropes and sisal ropes had equivalent high decay rates and were unable to maintain strength for 3 months.

Our results were in substantial agreement with previous experiments with cotton rope deployed offshore or in a test tank. We consolidated the assertion by Moreno et al. (2018a), who reported that 100% cotton rope exhibited a lifetime of approximately one year. The breaking strength variation was also coincident with that of Lopez et al. (2019). Our results also showed that, even for the same material, the potential variations in the degradation rate may depend on the specifications and/or manufacturing processes. For the ropes in same manufacturing processes, the initial value of breaking strength was positively correlated with the number of strands and threads (Xu & Zhang, 2004). Of the ropes with the same diameter and threads, braided ropes have a higher initial breaking strength value than twisted ropes, and the lay length of twisted ropes is positively related to the breaking strength (Meng &Chen, 2012). Just for examples, Winger et al. (2015) predicted that sisal ropes possessed a lifetime of at least 142 days which was far greater than ours. Araya-Schmidt and Queirolo (2019) also exhibited lower decay rates of jute than our results at the same soaking time.

The determination of the lifetime required for a FAD can provide the judgement criterion for how well the material is used in a biodegradable FAD. Although the required lifetime for a FAD was ocean-specific, given the complexity of FAD fishing strategies and the “maturing time” in different ocean, it is generally between 5 and 12 months. These experiments suggest that cotton rope is the preferred component in terms of its fulfillment of fishery standards. However, blended fabric behaves more robust than pure one. Moreno et al. (2018a) reported that the cotton and sisal mixed rope showed more resistant than 100% cotton rope, while Lopez et al. (2019) suggested that regenerated cotton and linen rope was weaker than 100% cotton rope. Therefore, it isreasonable to call for more extensive exploration of these novel functional materials resulting from the blending process as potentially suitable options.

Table 2 Formula and parameters results for three types of rope.

Fig. 5.Variation of retention ratio of rupture elongation (%) with soak time(months) for three types of ropes over a total periods of 10 months. Error bars denote standard deviation of the mean.

Table 3 Observation of bio-fouling on three types of ropes at different soak times.

Recently, trials using FADs made from diverse, natural fibre materials have been conducted under actual fishing conditions. Since the collaboration vessels were not equipped with scientific instruments to quantify the mechanical properties of materials, the degradability of the natural fibre materials was evaluated on the basis of the qualitative description from observation reports. Santiago et al. (2019) found that cotton ropes showed less degradation until the fifth month. This was different from our finding that cotton had a reduction ratio at 56% of initial strength by the third month. Considering the difference in experimental sites and the resulting sea conditions, our results may be biased to some extent. For example, these samples were not exposed to actual wave/current field to drift in this case the material fatigue was unrecognized. Moreover, the samples were deployed at higher salinity and nutrient-rich environment relative to real pelagic ocean, which might lead to an over estimation of the degradability.

Our observation suggested that all tested samples allowed biofouling. Among the microorganisms, green algae were the most prominent phylum parasitizing the samples and existing throughout the degradation process, which was in line with Lopez et al. (2019).Although there is very little scientific information to prove the significance of bio-fouling on fish aggregation (Lopez et al., 2019; Moreno et al., 2007, 2018a), some fishermen are more inclined to use those FADs with easy organism colonization. They believed that it may play important role in attracting non-tuna species and subsequent tuna species (Franco et al., 2009). However, another group of fishermen took the opposite views and thought that bio-fouling may weaken FADs due to the increase in weight, eventually causing the FADs to sink or the ropes to break (Moreno et al., 2018).

A competent Bio-FAD should be ecologically and economically compatible. Other than the fulfillment of criteria in degradation and durability, they should be at least as efficient as traditional FADs in aggregating fish so as to be easily accepted and popularized by fishermen. Therefore, it is necessary to deploy different prototypes Bio-FADs in actual fishing conditions to evaluate the catch performance. At present, such field trials have been undertaken with preliminary results on tuna aggregation and drift performance of specific Bio-FADs (Baidai et al., 2019; Lopez et al., 2019; Zudaire, 2017). Future work should involve collaboration with fishermen and modify the Bio-FADs prototypes based on their knowledge (Moreno et al., 2018; Orue et al.,2019; Zudaire et al., 2019). Moreover, in conjunction with catch,acoustic signatures derived from echo-sounder buoy can be used to provide information on tuna biomass associating with Bio-FADs. Finally,optimization design should rely on substantial volume of experiments to support the application and popularization of Bio-FADs, which ultimately promotes sustainable exploitation by way of reconciling profitability with ecological preservation.

Acknowledgments

This research was done through financial support by National Key R&D Program of China (No.2019YFD0901502); National Natural Science Foundation of China (No.41806110, 31972844 and 31902426);and Shanghai Sailing Program (19YF1419800). Special thanks go to skipper Zhiguo Zhang for his assistance in the deployment, retrieval, and transportation of samples during the field trial. Our thanks also extend to Mattew Damiano from North Carolina State University for his English edits.