Understanding the sources and effects of abandoned, lost, and discarded fishing gear on marine turtles in northern Australia

Abstract

Globally, 6.4 million tons of fishing gear are lost in the oceans annually. This gear (i.e., ghost nets), whether accidently lost, abandoned, or deliberately discarded, threatens marine wildlife as it drifts with prevailing currents and continues to entangle marine organisms indiscriminately. Northern Australia has some of the highest densities of ghost nets in the world, with up to 3 tons washing ashore per kilometer of shoreline annually. This region supports globally significant populations of internationally threatened marine fauna, including 6 of the 7 extant marine turtles. We examined the threat ghost nets pose to marine turtles and assessed whether nets associated with particular fisheries are linked with turtle entanglement by analyzing the capture rates of turtles and potential source fisheries from nearly 9000 nets found on Australia's northern coast. Nets with relatively larger mesh and smaller twine sizes (e.g., pelagic drift nets) had the highest probability of entanglement for marine turtles. Net size was important; larger nets appeared to attract turtles, which further increased their catch rates. Our results point to issues with trawl and drift-net fisheries, the former due to the large number of nets and fragments found and the latter due to the very high catch rates resulting from the net design. Catch rates for fine-mesh gill nets can reach as high as 4 turtles/100 m of net length. We estimated that the total number of turtles caught by the 8690 ghost nets we sampled was between 4866 and 14,600, assuming nets drift for 1 year. Ghost nets continue to accumulate on Australia's northern shore due to both legal and illegal fishing; over 13,000 nets have been removed since 2005. This is an important and ongoing transboundary threat to biodiversity in the region that requires attention from the countries surrounding the Arafura and Timor Seas.

Imaginary sea monsters and real environmental threats: Reconsidering the famous Osborne, ‘Moha-moha’, Valhalla, and ‘Soay beast’ sightings of unidentified marine objects

“…98 Today, one widely recognised 'monstrous act' of human modernity is the serious threat posed to the abundance and biodiversity of marine animals by entanglement in maritime debris and active or abandoned ('ghost') fishing gear. More than 200 species, 99 including six of the seven species of sea turtles, 100 are caught in such material. Due to their life cycle and their feeding, swimming and migratory behaviour, chelonians are particularly susceptible to becoming entangled.…”

Status of international monitoring and management of abandoned, lost and discarded fishing gear and ghost fishing

“…The catching process of active gear typically ceases when the gear collapses upon detachment from the vessel [22,[27][28]. However, ghost fishing has also been observed in ALDFG from active gears, including in seine nets, trawl net fragments, and Fish Aggregating Devices used in purse seine and other fisheries [22,25,26,[29][30][31]. Long-term 'automated re-baiting', where moribund and decomposing organisms caught in the derelict gear attract scavengers, feeding by the scavengers releases odors that attract more organisms, and some of the scavengers get caught and eventually decompose, providing a continual source of 'bait' until the ALDFG loses its fishing efficiency, can augment ghost fishing efficiency of some gear types for some species [13,15,21,22,28].…”

“…Long-term 'automated re-baiting', where moribund and decomposing organisms caught in the derelict gear attract scavengers, feeding by the scavengers releases odors that attract more organisms, and some of the scavengers get caught and eventually decompose, providing a continual source of 'bait' until the ALDFG loses its fishing efficiency, can augment ghost fishing efficiency of some gear types for some species [13,15,21,22,28]. Furthermore, some species may be attracted to live conspecifics caught in the derelict gear, and the structure of ALDFG has been observed to aggregate marine organisms, increasing the local abundance of organisms, contributing to augmented ghost fishing efficiency [9,14,26].…”

Regulation and Management of Marine Litter

“…hazards to navigation) (Taylor et al 2014), entailing economic costs to coastal towns/communities, fisheries, tourism, and other maritime industries (Ballance et al 2000;Mouat et al 2010;Jang et al 2014;Newman et al 2015). For instance, the total number of turtles entangled by the 8,690 derelict fishing nets sampled in northern Australia was estimated to be between 4,886 and 14,600 (Wilcox et al 2014). The estimate of damage cost from marine litter across the 21 Pacific Rim economics is €949 million annually in total, €273 million for the fishing industry, €209 million for the shipping industry and €467 million for marine tourism (Mcllgorm et al 2011).…”