The life-saving effect of biodegradable cotton thread in lost fishing traps: from a cleanup-project in Raet National Park, Norway
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The fishing gear used today is primarily made of synthetic materials, which gives the gear a long breakdown time. This means that Abandoned, Lost, and Discarded Fishing Gear (ALDFG) can continue fishing for a long time in a process called ghost fishing. Ghost fishing happens when animals are trapped in gear and die, which then attract new animals into the trap, and a vicious circle of entrapment and death continues. In 2018 it became mandatory to use a biodegradable thread, called a cotton thread, on lobster traps in Norway. The purpose of the cotton thread is to dissolve within a certain amount of time, and for the gear to open and stop ghost fishing. In the subsequent years it has become mandatory to use a degradable thread in several more fisheries in Norway. There is limited information on breakdown time of cotton thread in an actual ghost fishery, and how many lives that are saved in a ghost fishery using cotton thread. In Raet National Park, on the South coast of Norway, a clean-up project has been completed between June 29th, 2020, and May 1st, 2022, with the goal of mapping and estimating the extent of ghost fishing in the national park. The data collected during the clean-up project was used to estimate mortality in Raet National Park, and to make an assessment on the breakdown time of cotton thread in a ghost fishery. The cotton thread installed on the retrieved gear had a 50% chance to dissolve within 4.7 months after gear loss, but this estimate was based on a limited number of observations. Most animals are found in parlour traps, wrasse traps, and crayfish traps, which by current regulations all are required to have cotton thread. A high percentage of animals are caught in gear without cotton thread (80%), which is likely due to the high number of old gear that is still fishing from before cotton thread became mandatory. However, newer traps are also found without cotton thread indicating that the use of cotton thread could still be improved. The yearly mortality of animals caught in ghost gear, compared to a worst-case scenario where no gear has cotton thread, is high for the same reason. If the cotton thread is used consistently in the required gear, the yearly mortality will be lowered considerably in the future once still actively fishing gear without cotton thread stops fishing. To reduce the yearly mortality as quickly as possible, gear that is still fishing but has no cotton thread must be removed, and further clean-up actions are therefore recommended.