Comparative analysis of physical performance and fishing efficiency between biodegradable PLA gill net and conventional PA gill net

Abstract

With the development of fishery economy, fishing gear materials, such as cotton, which are generally biodegradable, have been replaced by synthetic materials, such as polyethylene (PE), polypropylene and polyamide (PA), which are difficult to decompose. Synthetic fiber materials exhibit the advantages of high strength, wear resistance and corrosion resistance. Currently used fishing net materials can be classified into PA, PE and polyester, among which PA is the most commonly used and has been used for the longest time. However, fishing nets made from synthetic fibers, such as PA, are not able to degrade in marine environment for decades. When they are lost or abandoned at sea, they will continue to trap fish and other animals, becoming “ghost fishing” gears. Fish and other marine organisms trapped in the abandoned fishing nets will eventually die and become baits to lure other creatures, forming a vicious circle. To conserve the marine environment and mitigate the threats of “white pollution” and “ghost fishing”, the use of environment-friendly degradable materials instead of conventional materials has become one of the major innovations in the sustainable development of modern fisheries. In this study, we assessed the physical performance of monofilaments made from a biodegradable resin, polylactic acid (PLA), in the laboratory, and compared and analyzed the fishing efficiency of gill nets made of PLA monofilaments and conventional PA monofilaments in sea trials conducted on Langya Island from September to October 2019. Results showed that the biodegradable PLA gill net captured 75.0% less <italic>Portunus trituberculatus</italic> but 90.0% more <italic>Pneumatophorus japonicus</italic> than PA gill net. The total catch number by PLA gill net was slightly lower than that by PA gill net, and the fishing efficiency of PA gill net was approximately 60.0% higher than that of PLA gill net. Moreover, mechanical tensile tests showed that the mesh breaking strength of the two gill nets was both significantly reduced after the fishing operation. In addition, PLA meshes were more vulnerable to damage than conventional PA gill net due to the poor toughness and lack of elasticity and flexibility of PLA monofilaments. At the same time, the microscope picture showed that the damage of PA gill net appeared in the middle of the mesh, while PLA gill net in the knot. Furthermore, the breaking strength of PLA monofilaments was significantly lower than that of PA, so PLA gill net meshes were more easily to be broken. Although PLA gill net is not as efficient as PA gill net in terms of fishing performance, it exhibits a potential advantage in reducing “ghost fishing” and plastic pollution in the ocean, so the usage of PLA gill net can be extended with improvement on strength performance.