Composition et répartition spatiale des déchets sur le sol marin

Litter on the seafloor has been studied in both coastal and deep sea waters using techniques that include snorkelling, SCUBA diving, trawl surveys, sonar and the use of manned and unmanned submersibles (Spengler and Costa, 2008; Miyake et al., 2011 Watters et al., 2010; Bergmann and Klages, 2012; Galgani et al., 2013; Schlining et al., 2013). The presence of large amounts of plastic litter has been reported in European continental shelf seas (Galgani et al., 2000; Pham et al., 2014), including in the Baltic Sea, the North Sea, the Celtic Sea, the Bay of Biscay (Galgani et al., 1995a), the Mediterranean Sea (Galgani et al., 1995b, 1996; Galil et al., 1995; Stefatos et al., 1999), the Adriatic Sea (Bingel et al., 1987) and the Black Sea (Loakeimidis et al., 2014). On continental shelves, trawl surveys are a practical way to monitor seafloor litter because they are already used for fish stock assessments, cover a wide area of the seafloor and collect a sufficient quantity of litter for analysis.

Benthic trawls (Figure a) are designed to capture marine biota on or near the seafloor over a range of different seabed types. As a result, some trawl designs will plough through the seafloor while others roll over the seafloor. This interaction with the seafloor together with the mesh size will influence the amounts of litter captured during a survey.

As the trawl passes across the seafloor, litter is ‘kicked up’ into the water. Because plastics are prone to drifting they are more likely to remain suspended long enough be retained in the cod-end (see Figure a). In contrast, metals, glass and ceramics and other heavier materials are more likely to drop out through the mesh before reaching the cod-end. As a result, different litter types have different catchabilities and so are differently represented in the catch (Moriarty et al., 2016). However, because these differences in catchability are relatively constant and should show little change from survey to survey, the survey results can still be compared. Furthermore, plastics (which have the highest catchability) are of greatest concern in terms of harm.

Thus, although the sampled quantities are ‘relative’ rather than absolute numbers, they can be used to compare regions sampled with similar gear. The number of stations monitored will determine the confidence that can be applied to the assessments and define the time (number of years of survey data) that will be needed to obtain an acceptable confidence in the assessment.

Data analysis

The distribution and abundance of marine litter on the seafloor in the OSPAR Maritime Area was investigated on the basis of data generated by trawl surveys from seven countries (Denmark, France, Germany, the Netherlands, Spain, Sweden and the United Kingdom). Two separate assessments were made: one using Grande Ouverture Verticale (GOV) trawl data and one for data collected by BAK trawl (Figure 1). The GOV assessment was based on 670 hauls in 2012, 790 hauls in 2013 and 794 hauls in 2014. These were undertaken across three OSPAR regions: the Greater North Sea, the Celtic Seas and the Bay of Biscay. For the Spanish BAK assessment there were 182 hauls in 2012, 197 hauls in 2013 and 160 hauls in 2014 across two sub-regions: the Iberian Coast and the Gulf of Cadiz. For GOV trawls, the numbers of hauls by region and year are summarised in Table a.

Widespread distribution of litter items, especially plastics, was discovered on the seafloor of the Greater North Sea, the Celtic Seas, the Eastern Bay of Biscay, the Iberian Coast and the Gulf of Cadiz. Table b shows the proportion of all litter items that are plastic by year and region for the GOV data. The Celtic Seas have a particularly high proportion of plastic items. Ideally, the number of items found would be standardised to the area of the haul. This is effectively the distance of the haul multiplied by the cross-sectional area of the net (Figure a). Although it would be better to use wing spread to determine the cross-sectional area of the net, due to missing data values this was not possible and door spread was used instead, with the mean of the door / wing ratios (4.18) used as an approximate conversion factor to generate wing spread values. Table c, Table d and Table e present data showing how the litter items collected in the GOV trawls changed over the three years of the study for each region. Although there appear be differences between years, at least five years of data are required to be able to make confident statements regarding trends.

There is wide variation in terms of abundance, with higher amounts of litter and plastic per km² in the Eastern Bay of Biscay. Figure 2 andTable c show the number of litter items per km²for all GOV trawls. The Eastern Bay of Biscay is the area of most litter, particularly for plastic items. Figure c and Table d and give similar information, except that the figures are based on the number of items caught per haul only, Figure d shows also show just the total number of plastic items caught per haul. Not weighting by haul area does not have a major effect on the assessment outcomes because most hauls are of similar area. Table e gives the percentage of hauls in which each of the litter types was found at least once. The results for the Eastern Bay of Biscay are notable with nearly all of the trawls in the Eastern Bay of Biscay contained at least one plastic item (Figure b).

The statistical power (the likelihood that a study will detect an effect when there is an effect there to be detected) was calculated with total litter counts for each of the three areas: Celtic Seas, Greater North Sea and the eastern Bay of Biscay. The sample size for each year was fixed at the following values (which correspond with the number of stations surveyed each year), 225 stations in the Celtic Seas, 500 stations in the Greater North Sea and 75 stations in the eastern Bay of Biscay. A starting mean for each region corresponding to the observed 2012 mean was assumed. The statistical power was then calculated for 10%, 20% and 30% increases over 5, 10, 15, 20, 25 and 30 years. A negative binomial distribution was assumed for the distribution of numbers of items per haul. The results are shown in Figure e. The Greater North Sea is the only region where there are sufficient stations to detect 20% or 30% changes with a statistical power of more than 90% (Figure e).

A separate assessment was undertaken, using only the Spanish BAK hauls. The sampling locations are shown in Figure 1. There were 182 hauls in 2012, 197 hauls in 2013 and 160 hauls in 2014. Similar summary results to those provided for the other regions are shown in Table f, using a door / wing spread ratio of 5.00. There is quite a lot of litter, but not as much as in the eastern Bay of Biscay. The proportions of litter items monitored that were plastic were 60% (2012), 50% (2013) and 65% (2014). The statistical power results for Spain, assuming a sample size of 182 are shown in Figure f. The statistical power results for the BAK trawls indicate that in order to detect changes of 30%, with more than a 90% statistical power, a further 15 years of data are required (Figure f).

There is moderate confidence in this methodology. There is consensus within the scientific community and the methodology has been used before for this type of assessment and has been published in scientific journals. However it is acknowledged that some aspects require further development, such as inter-comparison between trawl types used in different regions (BAK – GOV) to allow a complete assessment of the relative amounts of litter per km² to be undertaken.

There is low to moderate confidence in the data availability. The data are a by-product generated by fish stock assessment surveys. There is currently no quality assurance and quality control procedures in place for this data stream, although this will be addressed in the future, and therefore there is some variability in the data quality across some assessment areas (e.g. the English Channel). Also, data have only been collected for the full spatial area covered by the assessment since 2012 so temporal coverage is currently limited, although this will improve with time. 

A longer time series or more survey stations are required to detect trends in individual items or types of litter, therefore these will only be included in future assessments. However, for the Greater North Sea there are sufficient monitoring stations to detect changes of 20% or 30% in seafloor litter abundance, as a result of implemented litter reduction measures. For the Celtic Seas, the Eastern Bay of Biscay, the Iberian Coast and Gulf of Cadiz, there are fewer monitoring stations. An additional 15 to 30 years of monitoring data will be required to predict similar changes (20% or 30% in seafloor litter abundance) in these areas, unless more monitoring stations are added.

To assess the effectiveness of measures through changes in seafloor litter abundance will require more monitoring stations and/or longer data series. Future litter assessments should include modelling to determine sources and pathways. Better knowledge of the fragmentation rates and mechanisms is needed to understand seafloor litter trends and behaviour.

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