Degradability evaluation for natural material fibre used on fish aggregation devices (FADs) in tuna purse seine fishery

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, S-lay), 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.