Litter Windrows in the South-East Coast of the Bay of Biscay: An Ocean Process Enabling Effective Active Fishing for Litter

Abstract

Large scale convergence regions of floating marine litter are commonly observed in semi-enclosed seas as the Bay of Biscay. However, clean-up activities on such accumulation regions are limited by the spread of the large-size floating litter on the sea surface. Data gathered by a small-scale fishing vessel devoted to active fishing for floating litter activities during the spring and summer of 2018 reveals that the linear streaks of high concentration of floating litter (so-called litter "windrows") are common accumulation structures in the south-east coast of the Bay of Biscay. The random search of litter windrows for their collection through surface tows of macro-nets was proved to be an effective action for floating litter mitigation. A total of 196 tows collected 16.2 tons of floating marine litter in 68 working days. Most of the litter windrows were around 1 km length and, on average, accumulated 77.75 kg of floating marine litter. Fishing, shipping and aquaculture sectors were the source of 35% of the 4,130 litter items analyzed (55% in weight of the sourced items), and plastic was the most common type of material (96% in terms of items). A better understanding of the phenomenon of the litter windrows, capable to guide clean-up efforts in space and time, would provide a considerable improvement in the efficiency of mitigation actions to reduce the marine litter pollution. The observations of litter windrows in the coastal area of the south-east of the Bay of Biscay demonstrate the key role of submesoscale processes in the distribution of FML. The present work provides a thorough description of floating litter windrows in nature, which it was non-existent to date. The results are the kind of proof necessary to boost the research addressed on the submesoscale aggregations of FML. Coupling litter windrows observations with remote-sensing technology and high-resolution modeling techniques offer great opportunities for the mitigation actions against marine litter.

Observational Evidence and Open Questions on the Role of Internal Tidal Waves on the Concentration and Transport of Floating Plastic Debris

“…In both cases the floating surface drifters, what might be considered models of floating plastic debris, were pulled into the surface convergences. A companion paper in this special issue (Ruiz et al, 2020) tested if "naturally" occurring floating plastic debris could be collected and removed from the ocean by "fishing" linear accumulations of plastic. The linear accumulations of plastic debris depicted in figures in this paper look strikingly like internal wave convergences and in the Internal Wave Atlas (see text footnote 1, Bay of Biscay, Figure 7) there is a satellite image of numerous internal wave in the area in which this study took place.…”

“…This limited set of observations begs the question: we have demonstrated that the convergence over tidally generated internal waves can concentrate and transport plastic surface drifters, but how much floating plastic does one find in internal wave convergences and is this a regular occurrence? If internal waves regularly concentrate plastic debris, could this be exploited; could one "fish" internal wave slicks to remove plastic debris (Ruiz et al, 2020;Cozar et al, 2021)?…”

“…From the perspective of transport of floating plastic debris, ignoring wind driven transport of this material, which is clearly important, more onshore transport by internal waves may occur in the spring/summer when pycnoclines are shallower than during the winter when they are deeper. This hypothesis has not been tested, however, Ruiz et al (2020) observed linear plastic debris lines primarily in the spring consistent with these linear features being caused by internal wave convergences.…”

“…Zooplankton was sampled with a neuston net, a floating zooplankton net, that fished the waters within about 30 cm of the surface. The collection of plastic debris from convergence zones described in Ruiz et al (2020) used a nearly identical technique. Replicate neuston net tows were made in the slicks, between the slicks, and, if the wind remained light for a long enough period, in front of and behind the entire set of slicks.…”

Towards Detecting Floating Objects on a Global Scale With Learned Spatial Features Using Sentinel 2

“…This makes the UAV data of floating objects difficult to acquire. In our work, we focus on Sentinel 2 imagery as it is globally available and free of charge which is essential for a remote sensing technology to guide clean-up operations of plastic with dedicated ships, as done by Ruiz et al (2020). Compared to the UAV-driven approaches for targeted detection of plastic litter, we aim at detecting the general class of floating objects on the sea surface using globally available medium-resolution Sentinel 2 imagery.…”