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Latin american journal of aquatic research

On-line version ISSN 0718-560X

Lat. Am. J. Aquat. Res. vol.47 no.1 Valparaíso Mar. 2019 

Short Communication

Breaking strength evaluation of biodegradable twines to reduce ghost fishing in the pot and trap fisheries of Chile

Tomás Araya-Schmidt1  2 

Dante Queirolo2 

1Fisheries and Marine Institute of Memorial University, St. John's, Canada

2Escuela de Ciencias del Mar, Facultad de Ciencias del Mar y Geografía Pontificia Universidad Católica de Valparaíso, Valparaíso, Chile


Passive fishing gears such as pots and traps can be constructed with a section of netting made from natural fibers. In the event the fishing gear is lost or abandoned, these fibers deteriorate which effectively disables the fishing gear and allows organisms which accidentally enter the gear, an opportunity to escape without harm. This study evaluated several twines manufactured from natural fibers for their suitability to reduce ghost fishing in pots and traps. Nine cotton and jute twines of different diameters and construction were selected from Valparaiso local market, Chile. The twines were submerged for 56 days in seawater under laboratory conditions. Measurements of breaking strength were performed every 14 days. A decrease in the mean breaking strength was observed after 56 days for all twines, ranging from 0.5 to 29.6% for jute twines and 12.8 to 71.7% for cotton twines. The time until total degradation was predicted to be 113-230 days for the twisted jute twines, 68-234 for the twisted cotton twines, and 108-205 for the braided cotton twines. Our results suggest that cotton twines are a more suitable and feasible option to be implemented. The need to perform trials of biodegradable devices under commercial fishing conditions is warranted.

Keywords: ghost fishing; twine; biodegradation; escape; trap; pot

Ghost fishing is defined as the continued capture of organisms in lost, abandoned or discarded fishing gears (Macfadyen et al., 2009). The phenomenon is particularly prevalent in passive fishing gears such as pots and traps (Kim et al., 2016). Unaccounted fishing mortality can be high in cases where the captured organisms are unable to escape. These animals die and perform the function of bait, attracting more organisms and continuing the cycle of self-baiting for years or even decades (e.g., Hébert et al., 2001; Brown et al., 2005). In Chile, pots and traps are used mainly in the lobster (Jasus frontalis) and golden crab (Chaceon chilensis) fishery of Juan Fernández Archipelago (Arana, 2012), in the mixed fishery of king crabs (Lithodes santolla and Paralomis granulosa) at the southern tip (Lovrich, 1997), and in the crab fishery along the coast of the country (Retamal et al., 2009). No devices are currently used to mitigate or reduce ghost fishing in these fisheries. While there is no documented evidence on the degree of impact of ghost fishing in Chile, many agree that it is an unnecessary ecological impact and that efforts to reduce such effects are consistent with the precautionary approach to fisheries management.

Several technological innovations exist to disable fishing gears in the event they are lost or abandoned (Matsuoka et al., 2005). Biodegradable twines, galvanic time releases, and corrodible hog rings are among the most commonly used devices to deactivate pots and traps (e.g., Scarsbrook et al., 1988; Kimker, 1990; Gagnon & Boudreau, 1991; Kruse & Kimker, 1993; Selliah et al., 2001; Redekopp et al., 2006; Barnard, 2008; Kim et al., 2014). Degradable devices are important for species conservation and produce economic benefits, considering that natural twines are a lower cost measure available to accomplish these objectives (Kruse & Kimker, 1993). Biodegradable twines are constructed of vegetable fiber thus water temperature, fiber type, rotting power of the water, and duration of immersion are the main factors that determine degradation rates (Klust, 1982; Kim et al., 2016).

Several shellfish fisheries require the use of biodegradable netting/twine as a means to reduce or mitigate the negative effects of ghost fishing (Macfadyen et al., 2009). Examples include the spiny lobster (Panulirus argus) fishery in Florida, that requires the use of a degradable device since 1982 (Matthews & Donahue, 1996), the king crab (Paralithodes camtschaticus) and the tanner crab (Chionoecetes bairdi) fisheries in the Bering Sea, that require an escape mechanism (Macfadyen et al., 2009), the Dungeness crab fishery (Cancer magister) in Alaska, that requires the use of number 60 thread cotton twine or less as an escape cord in traps (Redekopp et al., 2006), and the snow crab (Chionoecetes opilio) fishery in eastern Canada, that requires the use of 96 thread cotton twine installed in a special zipper (Winger et al., 2015).

This study aimed to evaluate the degradation of different twines manufactured from natural fibers under a controlled laboratory experiment. We focused on twines that were readily available through the local market of Valparaíso, Chile, with potential application to local fisheries employing pots and traps.

A total of nine (n = 9) twine types manufactured from natural fibers were sourced from the local market of Valparaíso, Chile. These included twisted jute, twisted cotton, and braided cotton. Three diameters of each twine type were evaluated, ranging from 2.0 to 6.0 mm for jute, 2.5 to 5.0 mm for twisted cotton, and 2.0 to 5.2 mm for braided cotton: the twines varied in their type of construction (twisted or braided), in the number of cords, in the number of yarns per cord, and the number of total yarns (Table 1).

Table 1 Biodegradable twines selected from the local market of Valparaíso Region. Where “S” and “Z” indicate the direction of the twist, from left to right and right to left, respectively. 

Twine Diameter (mm) Construction N° cords N° yarns per cord N° yarns in the core N° total yarns
Twisted jute 6 6.0 ± 0.4 Twisted “Z” 3 7 / 21
Twisted jute 3 3.0 ± 0.2 Twisted “S” 1 7 / 7
Twisted jute 2 2.0 ± 0.1 Twisted “S” 1 5 / 5
Twisted cotton 5 5.0 ± 0.5 Twisted “Z” 3 15 / 45
Twisted cotton 3 3.3 ± 0.2 Twisted “Z” 3 20 / 60
Twisted cotton 2 2.5 ± 0.1 Twisted “Z” 3 10 / 30
Braided cotton 5 5.2 ± 0.3 Braided 16 4 24 88
Braided cotton 3 3.0 ± 0.2 Braided 16 2 8 40
Braided cotton 2 2.0 ± 0.1 Braided 16 1 4 20

Twine samples were submerged in a flowing seawater tank (0.5 m3) located at Escuela de Ciencias del Mar, Pontificia Universidad Católica de Valparaíso. The twines were suspended on frames (0.5 × 0.64 m) to maintain complete immersion and effective contact with the seawater which ranged from 14.5-15.5°C. The experiment was carried out from October 22 to December 17 of 2012. In order to quantify the loss of breaking strength (kgf) over time, a frame with a total of 10 samples of each twine type was removed at regular intervals (14, 28, 42 and 56 days), dried at room temperature for five days and assessed. Samples of the dried twines were tested using a dynamometer (Buraschi Dyna 400DP). All measurements were performed under ISO1805-2006. Due to the heteroscedasticity and non-normality observed in the exploration of the data, and heteroscedasticity shown in the residual versus fitted values of the Gaussian GLM model, it was decided to use the Poisson GLM, which does not assume normality or homogeneity of variance of the data. Poisson GLM (P = 0.05) was used to evaluate the effect of soak time, twine material (cotton or jute), twine construction (twisted or braided), twine diameter and a number of total yarns in the response variable breaking strength. Assumptions of independence, fixed X, no overdispersion and no patterns in the residuals were met. ‘Stats’ package of R statistical software (R Core Team, 2014) was used to perform the statistical analysis with the glm function. Bivariate linear regression was used to predict the value of the dependent variable (breaking strength) based on the value of the independent variable (soak time), as well as extrapolate the time until failure. In cases where breaking strength increased initially, we initiated the regression from that time forward to failure. Sigmaplot 12.0 was used to create the bivariate linear regression plots.

Initial measurements of breaking strength were collected before submersion of the twines in seawater (Day 0). Mean breaking strengths at that time ranged from 11.8 to 66.3 kgf (Table 2). Once submerged, many of the twines exhibited an initial increase in breaking strength on days 14 and 28. This increase would be due the day 0 samples were not soaked in seawater prior testing, and may have affected the compression and rigidity of the fibers in the twines.

Table 2 Average breaking strength (kgf), the coefficient of variation and total loss of average breaking strength of the twines. Values on parenthesis are coefficients of variation. 

Twine Average breaking strength at day Total loss of average breaking strength (%)
0 14 28 42 56
Twisted jute 6 66.3 (0.13) 81.7 (0.15) 75.4 (0.16) 82.2 (0.09) 66.0 (0.10) 0.5
Twisted jute 3 33.5 (0.10) 31.0 (0.14) 35.1 (0.15) 33.3 (0.13) 29.9 (0.15) 10.4
Twisted jute 2 18.9 (0.11) 17.5 (0.17) 17.2 (0.17) 17.0 (0.2) 13.3 (0.21) 29.6
Twisted cotton 5 39.3 (0.11) 41.0 (0.17) 42.5 (0.05) 28.2 (0.14) 17.6 (0.15) 55.2
Twisted cotton 3 26.5 (0.05) 28.6 (0.04) 24.6 (0.09) 23.7 (0.05) 23.1 (0.07) 12.8
Twisted cotton 2 16.6 (0.09) 19.9 (0.05) 15.5 (0.09) 8.80 (0.12) 4.70 (0.15) 71.7
Braided cotton 5 42.4 (0.09) 42.9 (0.04) 40.2 (0.06) 36.5 (0.04) 33.6 (0.05) 20.8
Braided cotton 3 18.6 (0.02) 18.5 (0.12) 16.4 (0.07) 13.8 (0.05) 11.1 (0.03) 40.3
Braided cotton 2 11.8 (0.04) 12.3 (0.04) 10.5 (0.04) 8.60 (0.05) 6.80 (0.08) 42.4

All of the twines experienced a reduction in breaking strength throughout the study (Tables 2-3). After 56 days in seawater, the mean reduction in breaking strength ranged from 0.5 to 71.7%. Poisson GLM results indicated that for every additional day of soak time there was a 0.5% (95% C.I. 0.4-0.6%) reduction in breaking strength. For every additional mm/unit of diameter, there was a 29% (95% C.I. 2534%) increase in breaking strength. For every additional yarn in the twine, there was a 0.007% (95% C.I. 0.006-0.008%) increase in breaking strength.

Table 3 Initial, maximum and final average breaking strength (kgf), breaking strength loss rate and estimated time for the twine to break. *Calculated from the maximum average breaking strength value by a linear fit of the data. 

Twine Initial average breaking strength (Day 0) Maximum average breaking strength Final average breaking strength (Day 56) Breaking strength loss rate (kgf day−1)* Twine broke estimated time (days)*
Twisted jute 2 18.9 18.9 13.3 0.08 230
Twisted jute 3 33.5 35.1 30.0 0.18 221
Twisted jute 6 66.3 82.2 66.0 1.16 113
Twisted cotton 2 16.6 19.9 4.7 0.37 68
Twisted cotton 3 26.5 28.6 23.1 0.13 234
Twisted cotton 5 39.3 42.5 17.6 0.89 75
Braided cotton 2 11.8 12.3 6.8 0.13 108
Braided cotton 3 18.6 18.6 11.1 0.18 119
Braided cotton 5 42.4 42.9 33.6 0.22 205

As we move from cotton to jute twine material, we experienced a 94% (95% C.I. 80-107%) increase in breaking strength. As we move from braided to twisted twine construction, we experienced a 23% (95% C.I. 16-30%) increase in breaking strength (Table 4).

Table 4 Estimated regression parameters, standard error, Z value and P-value for the Poisson GLM. SE: standard error. 

Estimate SE Z value P-value
Intercept 1.670 0.043 39.139 < 0.001
Soak time -0.005 0.0004 -10.939 < 0.001
Twine material Jute 0.661 0.035 19.068 < 0.001
Twine diameter 0.293 0.008 36.206 < 0.001
Twine construction twisted 0.206 0.028 7.430 < 0.001
Total yarns 0.007 0.0007 9.622 < 0.001

Linear regression from maximum breaking strength was used to predict the time to failure in the manner like Winger et al. (2015). Extrapolating the x-intercept (i.e., when breaking strength reaches 0 kgf) produced predictions of 113-230 days for the twisted jute twines, 68-234 for the twisted cotton twines, and 108-205 for the braided cotton twines (Fig. 1, Table 3). Linear regressions produced the following equations that relate breaking strength and soak time for the different twines tested:

Breaking strength (Twisted jute 2)=0.083×soak time+19.088R2=0.248Breaking strength (Twisted jute 3)=0.1829×soak time+40.453R2=0.178Breaking strength (Twisted jute 6)=1.16×soak time+130.96R2=0.617Breaking strength (Twisted cotton 2)=0.3746×soak time+25.36R2=0.961Breaking strength (Twisted cotton 3)=0.1254×soak time+29.39R2=0.548Breaking strength (Twisted cotton 5)=0.89×soak time+66.913R2=0.922Breaking strength (Braided cotton 2)=0.1313×soak time+14.15R2=0.957Breaking strength (Braided cotton 3)=0.1774×soak time+21.14R2=0.836Breaking strength (Braided cotton 5)=0.2249×soak time+46.18R2=0.818

Figure 1 Breaking strength (kgf) of the biodegradable twines with increasing soak time (days) and the linear fit of the data from the maximum average breaking strength. 

The results revealed that soak time had a significant effect on breaking strength. Twine properties included in the analysis (diameter, material, construction and some yarns) were significant predictors of breaking strength and affected the degradation rates. Braided and twisted cotton twines exhibited significant decay rates and low variability in breaking strength, which makes them ideal candidates for further experiments as escape mechanisms in pots and traps to reduce ghost fishing. Twisted jute twines, by comparison, exhibited lower decay rates and higher variability in breaking strength, making them poor candidates for implementation into fisheries.

In conclusion, the findings of this study suggest that braided and twisted cotton twines have the appropriate properties for further testing as a biodegradable escape mechanism in pot and trap fisheries in Chile, while twisted jute twines are unlikely to offer a solution to reduce ghost fishing. This study represents a preliminary evaluation of potential biodegradable twines to reduce ghost fishing in Chilean fisheries. Results are based solely on laboratory observations, which is a common starting point for such investigations (e.g.,Barnard, 2008; Matsushita et al., 2008). We recommend further experiments be conducted to evaluate the decay of twines under in situ field conditions. Similar to Winger et al. (2015), the twines should be tested in the fishery in which they want to be implemented, deploying them on real fishing grounds, mounted in the traps or pots, and during the fishing season.


The authors would like to especially thank Dr. Paul Winger for his comments and suggestions in this work. This work was supported by Escuela de Ciencias del Mar, Pontificia Universidad Católica de Valparaíso.


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Received: July 21, 2017; Accepted: August 21, 2018

Corresponding author: Tomás Araya-Schmidt (

Corresponding editor: Antonio Avila

Creative Commons License This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.