To clean or not to clean- if, when and how should plastic clean-up technologies be applied?
Jannike Falk-Andersson, Marthe Larsen Haarr, Vilma Havas.
There have been numerous initiatives to develop and implement clean-up technologies to remove floating litter from the ocean and rivers. The most famous one is the Ocean Clean-up initiative that aims at cleaning up plastics floating in the North Pacific Gyre. While removal of plastics benefits the environment, the effectiveness of these solutions as well as potential costs should be accounted for when evaluating how and if plastics should be cleaned up.
The authors argue that the cost-benefit of clean-ups can be evaluated using principles well-known from fisheries management. These are catch-per-unit effort (CPUE), where the density of litter is important, and impact on non-target species, where the overlap between plastics and biota is key.
To secure efficient implementation of clean-up technology, these should target areas and times where the densities of plastics are high. A number of factors determine the density of plastics including where they enter the river or ocean, how fast they sink and how fast currents and wind result in plastics concentrations decreasing.
There are few studies on macroplastic densities, with only 19 publications on ocean surface density and six publications on river density since 2015. These studies show large variation in densities and that distribution is patchy. In other words, we do not have sufficient knowledge on where and when high densities of plastics exists and what these densities are. The few studies reporting floating macroplastics shows that the density of plastics is very low on the ocean surface. Thus, we can expect that it will be very inefficient to apply clean-up technologies in the open ocean, even in areas known for high plastic pollution densities such as the Pacific Gyre. There may be areas of higher plastic densities, such as close to the coast, harbour basins and river mouths, and times of the year when densities are higher, for example during rainy season when larger amounts of litter may be washed out. Evaluation of how efficient implementation of clean-up technologies may be, is dependent on new knowledge on where and when high densities of plastics occur.
Marine litter clean-up technology will not only interact with its target plastics, but also with life in the ocean and rivers, as well as habitats. There is little knowledge of the overlap between floating plastics and life in the oceans and rivers. In many areas, we can expect a high overlap in time and space with organisms, from zooplankton to whales, as well as floating habitat. An example of the latter is the Saragassum algae mats that are regarded Essential Fish Habitat by the US Marine National Fisheries Service and therefore protected. We can also expect a high overlap between plastics and organic matter in rivers. In the few river studies that did report organic debris, this made up 10-90 % of the total debris. Given the importance of organic matter in rivers and coastal habitats, up-scaling of clean-up technologies can also be expected to have ecological impacts.
The authors conclude that there is a need to evaluate the density and availability of litter in order to secure that the implementation of clean-up technology is efficient and fit for its purpose. Furthermore, in development and implementation of the clean-up technology it is important to evaluate potential negative impacts to limit by-catch and damage to habitat. Currently the existing research is insufficient to evaluate the potential efficiency and damage of clean-up technologies.
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