Development and testing of biodegradable gillnets in Norway

Biodegradable gillnets are being tested in Norway since 2016 as an attempt to reduce ghost fishing and marine litter caused by lost gillnets. The mechanical, biological, UV, thermal and chemical degradation as well as the fishing efficiency of biodegradable gillnets are being studied.

Polybutylene succinate co-adipate-co-terephthalate (PBSAT) is a biodegradable plastic that maintains similar properties as a conventional plastic regarding its use, but which can be completely degraded into H2O and CO2 by naturally occurring microorganisms such as bacteria, fungi and algae when disposed in the environment. EnPol (Eco-friendly Polymer) is chemically produced biodegradable plastic developed by S. Korean S-EnPol ( earlier part of Samsung Fine Chemicals Co., Ltd. 

This project is financed by the Research Council of Norway, FHF and the Norwegian Directorate of Fisheries. Research institutions involved are SINTEF Ocean, Arctic University of Norway and S-ENPOL.

Degradation in marine environment

We studied the changes in the mechanical properties of biodegradable PBSAT (polybutylene succinate co-adipate-co-terephthalate) and conventional nylon gillnets over a three-year period. Samples of biodegradable PBSAT gillnets and nylon gillnets were placed inside modified lobster pots and submerged at sea. Samples were retrieved for analyses at 3 to 9 months intervals. Tensile strength testing was performed to determine and compare changes in the mechanical properties. After 25 months, the PBSAT gillnets exhibited a 35% reduction of tensile strength due to degradation and was then 26% weaker than the average strength of the nylon net samples.

Figure 1: Samples of biodegradable (white) and nylon monofilament (yellow) sat in lobsters' pots and submerged at sea
Figure 1: Samples of biodegradable (white) and nylon monofilament (yellow) sat in lobsters' pots and submerged at sea.

Fig 2 ny
Figure 2: Samples of biodegradable and nylon monofilament that have been taken out of the pots to be analysed for strength and elongation. To the right is a H10KT universal tensile testing machine (Tinius Olsen TMC, PA, USA) used to analyse the samples.

Fig 3
Figure 3: Natural bacterial strains degrade PBS monofilaments through an anaerobic digestion process into H2O and CO2 (Source. East Sea Marine Institute)

UV degradation

Two types of monofilament were used in this experiment: Biodegradable PBSAT and nylon. Both materials were aged for 1000 hours in a weathering test, simulating outdoor condition. The degradation of the materials was characterised by FTIR spectroscopy, mechanical testing, light microscopy, and scanning electron microscopy. The analyses reveal that both materials show signs of degradation already after 200 hours of exposure. PBSAT degrades faster than nylon and thus shows a stronger reduction in mechanical strength and material integrity. In addition, PBSAT is changing its chemical structure more significantly during degradation compared to nylon.  

Degradation in controlled laboratory conditions 

Biodegradability testing of PBSAT and nylon monofilaments using bacterial cultures and the use of natural seawater from a Norwegian fjord as a microbial source were carried out in controlled lab conditions (20°C). The focus is on comparing different analysis methods and studying changes in polymers in the material; also, quantification of any detached particles to the seawater (possibly microplastic formation). The trial began in September 2017 and will end in September 2019. Results for the first 12-month period show a 20% reduction of tensile strength in PBSAT monofilament was about. 20% weaker after being a year in the natural seawater (with and without sediments). No traces of microplastics were detected. The experiment is ongoing and the results from chemical (C) and microbiological (M) analyses are not available yet.

Fishing efficiency of PBSAT gillnets

Fishing trials on cod, saithe and Greenland halibut covering fishing seasons in 2016, 2017 and 2018 has shown 10-20% poorer fishing efficiency than traditional nylon gillnets. It seems that PBAST is 10-12% weaker than nylon is one reason for this difference in catch efficiency. The difference in elasticity and breaking strength could explain the major difference in the size structure of fish caught by each type of gillnets, especially for larger fish. Although less catch efficient than nylon gillnets, biodegradable gillnets still show great potential for reduction of ghost fishing and plastic litter at sea caused by this fishery. The daily used and wear of the material throughout the 2-3-month fishing season accelerated their degradation.

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Published: 30.4.2019