Preliminary evaluation of degradability for natural material ropes potentially used on fish aggregating devices (FADs) in tuna purse seine fishery

Purse seiners deploy thousands of drifting fish aggregation devices (DFADs) in all tropical oceans to catch tropical tunas. Nowadays these FADs were constructed with synthetic netting, which are explicitly considered responsible for incidental mortality of sea turtles and sharks through entanglement, even causing ghost fish if they are lost and abandoned. The use of natural and/or biodegradable materials to build FADs can effectively mitigate marine pollution and bycatch issues so that they are currently made efforts to promote by fisheries management organizations. This paper presents the degradability of three natural material ropes (3-ply 96-thread cotton, 3-ply 13-thread jute, and 3-ply 8-thread sisal) on the basis of an experiment measurement on breaking strength (N/ktex). These samples were deployed in China’s nearshore waters attached to the floating frame of net cage in Dec 2018 and retrieved per month for testing breaking strength in the laboratory. Results showed that jute and sisal experienced rapid reduction of strength in the following months, in particular over the first month soaking in the sea fallen by almost half and lost all strength after three months. Cotton rope appeared to be more preferable component for building Bio-FAD among these congeners which could be durably used, minimally, in seven months, and matches the lifetime of FAD serving fishing for 6 to 12 months.