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Rapid assessment of marine debris in coastal areas using a visual scoring indicator
Marine Pollution Bulletin 149 (2019) 110552
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Rapid assessment of marine debris in coastal areas using a visual scoring indicator
T
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Jongmyoung Lee, Sunwook Hong , Jongsu Lee Korea Marine Litter Institute, Our Sea of East Asia Network, #101-210, 23-57 Jukrim 3-ro, Gwangdo, Tongyeong, Gyeongnam 53020, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Rapid assessment Visual scoring indicator Marine debris Standing stock Citizen science Plastic pollution
Information regarding the spatial distribution and standing stock of marine debris in coastal areas is a prerequisite for efficient cleanup and management. We conducted a rapid assessment of marine debris on the coasts of South Korea using a visual scoring indicator. The indicator consisted of a table and photographs representing nine pollution levels that were quantitatively tested. Locations at every 10 km were selected along the natural coastline for a total of 382 locations, and a length of 100 m at each location was assessed. Approximately 40 participants were trained and assessed the pollution levels using a smartphone application. The surveys were conducted four times in 2017, in April, June, August, and October. The total amount of marine debris stock in the natural coastal areas was estimated to be approximately 17 thousand tons. It suggests that approximately 60% of the marine debris can be cleaned from 10% of the coastline.
1. Introduction Marine debris has recently been recognized as an urgent global pollution issue (Jambeck et al., 2015; Rochman et al., 2013; Vince and Hardesty, 2017). It can be found on the coast, at the sea surface, on the seabed, and even in remote areas (Eriksen et al., 2014; Hirai et al., 2011; Ivar do Sul et al., 2009). In particular, plastic debris poses serious threats to fish, seabirds, and mammals through entanglement and ingestion (Boerger et al., 2010; CBD, 2016; Hong et al., 2013; Jacobsen et al., 2010; Laist, 1997; van Franeker et al., 2011). From the socioeconomic aspect, it damages the tourism industry (Jang et al., 2014a), threatens navigational safety (Hong et al., 2017), and reduces the revenue of fisheries (Donohue et al., 2000; Gilardi et al., 2009). Moreover, its removal incurs high costs (Ballance et al., 2000). Several studies have focused on estimating the abundance or density of marine debris on the coast, at the sea surface, and on the seabed (Eo et al., 2018; Galgani et al., 2015; Hardesty et al., 2017), and considerable efforts have been made to reduce the debris input to the marine environment. These efforts include beach cleanups, monitoring programs, countermeasure policies, and legislations (Schuyler et al., 2018; Xanthos and Walker, 2017). For establishing these measures, a quantitative assessment of marine debris is a prerequisite, and the majority of these measures rely on marine debris monitoring in various compartments, especially beaches (Ariza et al., 2008; Bravo et al., 2009; Ribic, 1998; Ribic et al., 2010). However, monitoring marine debris
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requires extensive efforts, and often incurs high costs because the monitoring requires detailed information such as number/weight, types, and sources of debris. It is likely that this fact limits the number of sampling sites and impedes the comprehensive quantification of marine debris over a broad area or along numerous coasts. Several studies have adopted rapid assessment of marine debris (Alkalay et al., 2007; Cheshire et al., 2009; De Araujo et al., 2006; Lippiatt et al., 2013; Moore et al., 2007). However, these studies were mostly based on counts of debris items, and still required extensive efforts in terms of time, manpower, and survey costs and were limited to assessing geographically broad areas within a short period. A visual scoring indicator can be an alternative tool for surveying marine debris along coasts. In particular, a visual scoring indicator was originally developed by the Japanese government and NGOs (MLIT, 2007; Table 1). The indicator includes nine pollution levels, and each level has a standard photograph that the surveyors can compare with in situ pollution. Using this method, a surveyor can assess the degree of debris pollution scattered in a designated area. The method is easy to follow and does not require special techniques, and therefore, once accustomed to it, many people can conduct the survey. Furthermore, the method enables the coverage of extensive areas and numerous sites in a relatively short time. With this survey, researchers can obtain valuable information on marine debris pollution that can be used for planning and conducting cleanup projects (MLIT, 2007). This study provides a guide to obtain baseline information using a
Corresponding author. E-mail address: [email protected] (S. Hong).
https://doi.org/10.1016/j.marpolbul.2019.110552 Received 7 May 2019; Received in revised form 26 August 2019; Accepted 26 August 2019 Available online 05 September 2019 0025-326X/ © 2019 Elsevier Ltd. All rights reserved.
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Table 1 Visual scoring indicator. Higher levels indicate a higher amount of marine debris per 100 m2. Source: (MLIT, 2007) Level
Debris amount (L/100 m2)
0 1
2.5 5
2
10
3
20
4
40
5
80
6
160
7
320
8
640
Table 2 Categories of dominant debris and land use types.
Description (reference for estimation)
Categories
Description
Dominant debris
Fishing gear Household waste
Net, rope, buoy, trap Plastic bag, food container, beverage bottle, utensil Appliance, tire, masonry Foreign aquaculture buoy, foreign food container Other debris Aquaculture farm Hotel, accommodation facility, yacht deck Residence Field, rice paddy Store, restaurant Other
Dumped waste Foreign debris
No litter Rare litter (Two 2 L beverage bottles) Only a few litter (Four 2 L beverage bottles, 15,500 mL water bottles) Moderately scattered litter (Eight 2 L beverage bottles, 30,500 mL water bottles) Extensively scattered litter (15 2 L beverage bottles, Two 20 L containers) Considerable amount of litter (Four 20 L buoys) Huge amount of litter (One 44-gal drum, Eight 20 L buoys) Almost covered by litter (Five 60 L buoys) Fully covered by litter (Three 200 L drums)
Land use
Other Aquaculture Tourism Village Agriculture Commercial Other
coast was not accessible, the site was replaced with the closest accessible coast. The survey included a total of 382 coasts, and one transect of 100 m was selected on each coast. We designated a serial number from 1 to 382 to each survey site. For smooth field research and record collection, we created a database by combining information such as address, GPS location, and aerial photographs. Then, we provided this database to the surveyors through a mobile application. The information regarding the debris in the target sites was recorded by the surveyors (Table 2). Surveyors walked along the target site and chose the most visible type of debris among the five types that were used in previous studies (Hong et al., 2014; Maharani et al., 2018). The land-use type of the background of the sites was also recorded as “aquaculture”, “tourism”, “village”, “agriculture”, “commercial”, or “other” (Hong, 2013).
rapid assessment tool for the management of marine debris in coastal areas. The objectives of this research are to 1) introduce a visual scoring indicator as a tool for the rapid assessment of coastal debris, 2) depict the distribution of coastal debris as a snapshot at a certain time of year, and 3) estimate the standing stock of marine debris in natural coastal areas in South Korea. 2. Methods 2.1. Visual scoring indicator
2.3. Field surveys
The visual scoring indicator table consists of nine reference photographs that show the debris scattered in a quadrat (10 m × 10 m, 100 m2), according to the pollution level, and indicates the amount of much debris that is likely to be found at each level (Table 1). Each photograph was taken after debris was scattered in the designated area and shows an in-situ image of debris (gomi-map.org/manual_gomichosa.pdf). For example, pollution Level 1 indicates that there are 5 L of debris in 100 m2, while Level 2 indicates the double amount of debris in the designated area (10 L/100 m2). The highest level is Level 8, in which the amount of debris is 640 L/100 m2. During the survey, the surveyors find the reference photograph that most closely resembles the appearance of debris on the beach to determine the beach's debris level. The standing stock of debris within the surveyed area, which was 100 m in length, was calculated by multiplying the debris level by the width of the coast. The standing stock (S) of debris in the entire surveyed area was calculated as follows:
For the visual scoring survey, we recruited 42 surveyors who had previously participated in marine debris monitoring and related activities in organizations, universities, and other institutions, and conducted two half-day training workshops. In the first workshop, on April 15, 2016, we trained the surveyors on assessing the pollution level in situ. As a pilot study, we tested the method at 40 sites selected every 100 km along the natural coastline. Twenty groups, with two surveyors each, participated in the pilot survey at the assigned sites in order to get accustomed to the indicators and fieldwork. We analyzed the results of the pilot survey, corrected common errors in the pollution assessment, and developed a smartphone application based on the revised versions of the data card and categories. In the second workshop, held on August 19, 2016, we delivered the results of the first pilot study, and provided instructions to the participants to reduce the potential errors owing to subjectivity in the assessments. Another pilot survey of all the 382 sites was conducted from August 25 to September 1, 2016. Directions for improving credibility and consistency of the assessment were once again delivered to the surveyors after the survey. In particular, we unified the width of the surveyed area to 10 m as much as possible in order to reduce variability and to facilitate the estimation of the standing stock of debris. Initially, we decided to survey the coast from the water edge to the vegetation area or sea wall. However, this approach was modified because the debris primarily accumulated within 10 m of the coast, mainly along the high strandline. When the surveyors encountered a different width of the accumulated area, the width was recorded and adjusted to the calculation of the standing stock. We guided the surveyors to visit the assigned sites; assess the in situ pollution level over 100 m of the coastline using a smartphone; record the width of the surveyed beach section, major debris type, land use type, and accessibility for cleanup; and take photographs to represent their assessment. Actual field surveys were conducted by surveyors four times during April, June, August, and October 2017. The assessment
n
Standing stock of debris in full surveyed area =
Information
∑ 2.5 × 2Li × Wi i=1
(1)
where Li is the level of the ith coast and Wi is the average width of the ith coast. The constant values 2.5 and 2 are derived from a calculation in which the amount of debris allocated to Level 1 was 5 L and its amount doubles as the level goes up. 2.2. Site selection and recording of information about sites To select the survey sites, we chose one coast every 10 km of natural coastline from the entire South Korean coast that extends to 3770 km (www.data.go.kr). Artificial and insular coastlines were excluded from the selection process. Initially, 377 coasts were selected, to which 5 more coasts were added to include the coastal counties that were not originally selected because of short coastal lengths. When the selected 2
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Fig. 1. Distribution of marine debris along the natural coastline of South Korea in April, June, August, and October 2017. The colors of the circles represent the level (0–8) and their size indicates the volume of marine debris (L/100 m2).
majority of them had low levels (SI Tables 1–4). We estimated the total standing stock of coastal debris of South Korea for the end of 2017 based on our results (Table 3). The amount of debris in the surveyed coasts was calculated by multiplying each level with beach width. We surveyed a stretch of 38.2 km, which represents 0.25% of the total coastline of Korea. We assumed that the distribution of debris is similar all over the coastline, including the artificial and insular coastlines that were not covered by our survey. We obtained an average of 220,000 L of debris from the calculation and then extrapolated the value to the total length of the coastline. Thus, we estimated the total amount of coastal debris at the end of 2017 to be 86,158 L scattered in about 15,000 km of coast. This would represent 17,318 tons of debris, considering the ratio of weight to volume obtained from the average of the results in the recent 5-year Korea National Marine Debris Monitoring Program (0.201 kg/L) (MOF and KOEM, 2017). The standing stock was estimated to be higher after the summer.
results and photographs that were saved in the app were automatically collected in the database. We used these photographs and information to verify the results of the surveys and understand the situation regarding marine debris pollution on the shore. Finally, the results were compiled into a map using the QGIS software (Quantum GIS Development Team, 2017). 3. Results 3.1. Pollution level and standing stock of marine debris The results of the survey, which included pollution level and standing stock, are shown on the map (Fig. 1). From the map, we determined the coasts that were highly polluted, as well as the amount of debris on the coasts. The map could be enlarged to a metropolitan or a regional scale. The results revealed that 60% of the debris was concentrated on 10% of the surveyed coasts. This pattern can be observed on the graph depicting the cumulative proportions of the number of coasts and the debris amount at each level (Fig. 2). Among the 382 surveyed coasts, only a small number of coasts had high debris pollution levels, while
3.2. Dominant debris types and land use around the surveyed coasts The dominant debris types and the land use around the surveyed coasts were investigated in this research (Fig. 3). Fishing gear was the 3
Marine Pollution Bulletin 149 (2019) 110552
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Cumulative ratio of number of coasts (%) Fig. 2. Cumulative ratio of debris and coastal length (number of coasts) along the natural coastal areas of South Korea.
concentrated on 10% of the Japanese coasts (MLIT, 2007). The results can be used to enhance the efficiency of coastal cleanup projects, considering that if 10% of the coasts that are measured to be debris hot spots are cleaned, more than half of the debris accumulated along the natural South Korean coastline can be removed. The prevention of the input of marine debris to the ocean from its sources is an important strategy to combat this issue (Vince and Hardesty, 2017). However, with regard to the existing debris in the marine environment, its removal is one of the most common measures employed by municipalities and authorities. The removal of debris from coastal areas is relatively easy compared to its removal from the sea surface and bottom due to the feasibility and cost-efficiency (Hong et al., 2015). The removal of debris from beaches can contribute to a reduction in other marine compartments since marine dynamics move the debris between these compartments (Hidalgo-Ruz et al., 2012). Particularly, plastic debris on beaches can be easily fragmented, yielding a large amount of microplastics that cannot be collected and can pose significant threats to the marine environment (Andrady, 2017; Song et al., 2017). Thus, the removal of large-sized plastic debris from coastal areas is the most practical approach to reduce the accumulation of microplastics. Therefore, if the exact spots where urgent debris removal is needed could be determined, a more efficient cleanup and management of hot spot areas would be possible. In particular, these results are useful for coastal managers to plan coastal cleanup, allowing them to pinpoint the heavily polluted areas on the pollution map obtained from this research and concentrate their workforce and budget on cleanup activities in those areas. Moreover, records regarding area accessibility can also help determine the possibility and feasibility beach cleanup. However, beach cleanup is not the ultimate solution to the marine debris issue. Visual scoring indicator can only contribute to the efficiency of cleanup operations with quick assessment; however, in-depth understanding of the causes and impacts of marine debris is required to address the issue at the source (Hong et al., 2013, 2014, 2017). The major pollution sources identified in the existing studies and the
Table 3 Estimated standing stock of coastal debris in South Korea. Estimation range
Length of coast
Estimated standing stock
Surveyed coast Coasts of mainland (including natural and artificial coastline) Coasts of islands Total
38.2 km 7752.5 km
219,961 La 44,639,991 Lb
7210.3 km 14,962.8 km
41,517,927 Lc 86,157,918 L (in volume) 17,317,741 kg (in weight)d
a b c d
Average value of 4 times of survey. a × 7725.5 km/38.2 km. a × 7210.3 km/38.2 km. (b + c) × 0.201 kg/L.
most abundant type, which ranged from 56.5% to 61.0%, followed by household wastes (22.0–36.9%) and other wastes (4.5–11.5%). The land use around the surveyed coasts showed that aquaculture farm (40.8–45.3%) was the most common, followed by tourism (10.2–19.6%) and village (10.7–12.8%). The majority of the debris was fishing gear, and aquaculture was the most used form among land use. The coastal areas are widely used for fisheries in Korea, and it can be seen that the debris generated there has been polluting the coast. 4. Discussion 4.1. Debris concentrated on a small number of coasts The results of this research showed that the marine debris on the coasts of the Korean Peninsula was concentrated on a small number of coasts, with 60% of the marine debris accumulated on 10% of the coasts. The results were consistent with a study in Japan wherein the same indicators were used. In that case, 70% of the coastal debris was 4
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Fig. 3. Dominant debris types (A) and land use (B) of the surveyed coasts.
(40.8–45.3%; Fig. 3). This indicates that aquaculture is the main source of marine debris polluting the coastal areas in South Korea. Previous studies have also revealed that derelict fishing gears, such as fishing ropes, nets, and buoys from aquaculture, were the main components of the coastal debris in South Korea (Hong et al., 2014; Jang et al., 2014b; Lee et al., 2013). Several projects aimed at reducing derelict fishing gear debris have been developed in South Korea. The South Korean government supports fishermen with subsidies on durable aquaculture buoys in order to replace styrofoam buoys (Lee, 2017). The installation of reception facilities and the establishment of transportation and treatment systems for used fishing gear have already been implemented in the pilot projects. The change in the ratio of derelict fishing gear in major debris types in
approximate sources identified in this study were similar, indicating that more efforts are needed to reduce fishing gears from aquaculture and capture fisheries in South Korea. 4.2. Fishing gear and aquaculture debris The results of this study can also contribute to the establishment of coastal debris prevention policies. Surveyors recorded dominant debris types and land use (potential debris sources) around the surveyed coast, that can help establish prevention plans focusing on the sources of coastal debris in the region. In this research, the Korean coasts were heavily polluted by fishing gear (56.5–61.0%), and the predominant land-use around the coasts was in the form of aquaculture farms 5
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Seasonal variations should be considered in future studies in order to determine if seasonality can affect the accumulation of marine debris in coastal areas. Considering the results of the Korea National Marine Debris Monitoring Program (Hong et al., 2014), a trend showing that the amount of debris accumulated bi-monthly on the South Korean coast is relatively high in the summer. Our survey shows a similar pattern, with the highest standing stock observed in August. More information covering several years could provide a good-quality baseline for better decision making. A simple linear extrapolation was applied in order to simplify the calculation process and make the result easier to understand. Further approaches should be conducted to estimate the total quantity of the debris more accurately and reflect its spatial variation.
future surveys could be an indicator of the effectiveness of these policies. 4.3. Efficiency of the visual scoring as a rapid assessment tool This research revealed that surveying coastal debris using the visual scoring indicator is an efficient and rapid assessment method. In this research, 37 surveyors among the trained 42 people surveyed 382 coasts in 8 days. By adopting the visual scoring indicator, the surveyors only had to look around the designated survey area in order to determine its pollution level. The survey takes a relatively short amount of time (less than 30 min for each survey) compared with other survey methods. This study was not designed to investigate major debris types and sources in detail which have been surveyed in a separate program, ‘Korea National Marine Debris Monitoring Program’ since 2008 (Hong et al., 2014). The results of the rapid assessment of coastal debris are very useful for the planning of coastal debris cleanup. Information on the areas where coastal debris is concentrated enables its efficient cleanup (Critchell et al., 2015; Martens and Huntington, 2012). Furthermore, knowing the total amount of debris in the area where cleanup is needed can be used to determine the budget, workforce, and resources that need to be invested. In particular, rapid assessment reduces the time between the investigation and cleanup because the survey is quick, thereby reducing further impacts of wind and ocean currents on the debris. The average pollution level can be a measurable goal for management by local authorities (i.e., Alkalay et al., 2007) as well as by the central government. In this study, the average pollution level in the Gyeongnam Province was 3.23 as of October 2017. The rapid removal of coastal debris from hot spots identified by the survey might lower this level. Local authorities could set a goal for the average level to be 3.00 for the following year. Furthermore, the average level in each province can be used to evaluate management achievements. The use of visual scoring indicators can also represent a very good method for citizen science, as the method is easy, safe, and low-cost (Huddart et al., 2016). Marine debris surveys are one of the most active areas in environmental studies for citizens (Hidalgo-Ruz and Thiel, 2015; Hyder et al., 2015). Using visual scoring indicators, in addition to existing monitoring and survey methods, will further strengthen citizen science in this field. The local residents, being more knowledgeable about marine debris pollution in their communities, can be good potential surveyors. Providing the results to the public can also motivate and encourage the local authorities and enhance general awareness. The smartphone application developed in this study represents a good example of ICT (Information & Communication Technology) in citizen science, as was the case in Jambeck and Johnsen (2015).
4.5. Limitations and directions for future research One limitation of the visual scoring indicator is the potential errors during the debris level evaluation, which is strongly dependent on the surveyors. In fact, in this study, when three researchers reevaluated the photographs taken during the pilot survey, there were several cases in which the results were significantly different from those produced by the field surveyors. However, this difference mainly occurred when the pollution level was relatively low (mainly Levels 3 and 4). On the coasts with high pollution levels (above Level 6), the deviations between the evaluators decreased significantly. In other words, this method is very useful for locating marine debris hot spots. In addition, as photographs are saved, the results can be objectively verified afterward, and there is a chance to correct potential errors. To develop and use this method in the field, it is important to enhance the objectivity of the evaluation indicators. In this study, reference photographs were taken in Japan and South Korea, where the standing stocks of debris were measured. It is necessary to take reference photographs that can estimate the standing stocks as accurately as possible, and the selected photographs that are used as evaluation indicators should reflect the characteristics of the debris in each region. It is also necessary to develop an ‘index’ that can be used to provide information on management priorities, even though the results of the survey focused on the standing stock of the debris. Coastal managers need to determine the urgency of cleanup considering the debris type and the land use around the coasts. It is necessary to determine which factors should be included in this index and what weights should be applied to these factors. Acknowledgments This study was conducted with financial support from the Ministry of Oceans and Fisheries of Korea and administrative support from the Korea Marine Environment Management Corporation (Nationwide Survey of Marine Debris on Korean Coasts in 2017). We thank our colleagues from the Japan Environmental Action Network, who provided insights and expertise that greatly assisted the research. We truly appreciate the participants of the surveys. We express thanks to Ms. Shin Yeong Park for helping us organize the data and three anonymous reviewers for their comments and suggestions.
4.4. Comparison of the estimations of coastal marine debris stock The standing stock that was estimated in this study turned out to be slightly higher than the value reported by Jang et al. (2014c). In this study, the total standing stock of marine debris on the coastline was estimated to be 86,158 L, which is equivalent to 17,318 tons. Jang et al. (2014c) estimated the stock to be 12,029 tons as of the end of 2012. They reached this value by multiplying the average weight of debris per 100 m of the eastern, western, and southern coasts provided by the Korea National Marine Debris Monitoring Program by the coastal length (MLTM and KOEM, 2012). Considering the fact that the monitoring sites had been regularly cleaned by surveyors since the launching of the program in 2008, the estimation of Jang et al. (2014c) could represent an underestimation. In addition, the sample size (382 coasts) of this study is much larger than that of Jang et al. (2014c; 20 coasts). Therefore, the estimation obtained in this study could be closer to reality. Variation over time series can also affect the difference between two estimations.
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Ariza, E., Jiménez, J.A., Sardá, R., 2008. Seasonal evolution of beach waste and litter during the bathing season on the Catalan coast. Waste Manag. 28, 2604–2613. Ballance, A., Ryan, P.G., Turpie, J.K., 2000. How much is a clean beach worth? The impact of litter on beach users in the Cape Peninsula, South Africa. S. Afr. J. Sci. 96, 210–213. Boerger, C.M., Lattin, G.L., Moore, S.L., Moore, C.J., 2010. Plastic ingestion by planktivorous fishes in the North Pacific Central Gyre. Mar. Pollut. Bull. 60, 2275–2278. Bravo, M., de los Ángeles Gallardo, M., Luna-Jorquera, G., Núñez, P., Vásquez, N., Thiel, M., 2009. Anthropogenic debris on beaches in the SE Pacific (Chile): results from a national survey supported by volunteers. Mar. Pollut. Bull. 58, 1718–1726. CBD, 2016. Marine Debris: Understanding, Preventing and Mitigating the Significant Adverse Impacts on Marine and Coastal Biodiversity. Cheshire, A., Adler, E., Barbière, J., Cohen, Y., Evans, S., Jarayabhand, S., Jeftic, L., Jung, R.T., Kinsey, S., Kusui, E.T., et al., 2009. UNEP/IOC Guidelines on Survey and Monitoring of Marine Litter. United Nations Environment Programme, Regional Seas Programme. Critchell, K., Grech, A., Schlaefer, J., Andutta, F.P., Lambrechts, J., Wolanski, E., Hamann, M., 2015. Modelling the fate of marine debris along a complex shoreline: lessons from the Great Barrier Reef. Estuar. Coast. Shelf Sci. 167, 414–426. https://doi.org/10. 1016/j.ecss.2015.10.018. De Araujo, M.C.B., Santos, P.J.P., Costa, M.F., 2006. Ideal width of transects for monitoring source-related categories of plastics on beaches. Mar. Pollut. Bull. 52, 957–961. Donohue, M.J., Brainard, R., Parke, M., Foley, D., 2000. Mitigation of Environmental Impacts of Derelict Fishing Gear through Debris Removal and Environmental Monitoring. Issues Pap. 5. pp. 6–11. Eo, S., Hong, S.H., Song, Y.K., Lee, Jongsu, Lee, Jongmyoung, Shim, W.J., 2018. Abundance, composition, and distribution of microplastics larger than 20 μm in sand beaches of South Korea. Environ. Pollut. 238, 894–902. https://doi.org/10.1016/j. envpol.2018.03.096. Eriksen, M., Lebreton, L.C.M., Carson, H.S., Thiel, M., Moore, C.J., Borerro, J.C., Galgani, F., Ryan, P.G., Reisser, J., 2014. Plastic pollution in the World’s oceans: more than 5 trillion plastic pieces weighing over 250,000 tons afloat at sea. PLoS One 9, e111913. https://doi.org/10.1371/journal.pone.0111913. Galgani, F., Hanke, G., Maes, T., 2015. Global distribution, composition and abundance of marine litter. In: Marine Anthropogenic Litter. Springer, pp. 29–56. Gilardi, K., Good, T., June, J., Broadhurst, G., 2009. Puget Sound Derelict Fishing Gear: Assessing the Problem and Prioritizing Solutions. Hardesty, B.D., Lawson, T.J., van der Velde, T., Lansdell, M., Wilcox, C., 2017. Estimating quantities and sources of marine debris at a continental scale. Front. Ecol. Environ. 15, 18–25. https://doi.org/10.1002/fee.1447. Hidalgo-Ruz, V., Thiel, M., 2015. The contribution of citizen scientists to the monitoring of marine litter. In: Marine Anthropogenic Litter. Springer, pp. 429–447. Hidalgo-Ruz, V., Gutow, L., Thompson, R.C., Thiel, M., 2012. Microplastics in the marine environment: a review of the methods used for identification and quantification. Environ. Sci. Technol. 46, 3060–3075. Hirai, H., Takada, H., Ogata, Y., Yamashita, R., Mizukawa, K., Saha, M., Kwan, C., Moore, C., Gray, H., Laursen, D., Zettler, E.R., Farrington, J.W., Reddy, C.M., Peacock, E.E., Ward, M.W., 2011. Organic micropollutants in marine plastics debris from the open ocean and remote and urban beaches. Mar. Pollut. Bull. 62, 1683–1692. https://doi. org/10.1016/j.marpolbul.2011.06.004. Hong, S., 2013. Assessment of Marine Debris Pollution and Emergy Evaluation of Management Measures in Korea. Ph. D. Dissertation of Graduate School. Pukyong National University. Hong, S., Lee, J., Jang, Y.C., Kim, Y.J., Kim, H.J., Han, D., Hong, S.H., Kang, D., Shim, W.J., 2013. Impacts of marine debris on wild animals in the coastal area of Korea. Mar. Pollut. Bull. 66, 117–124. https://doi.org/10.1016/j.marpolbul.2012.10.022. Hong, S., Lee, J., Kang, D., Choi, H.-W., Ko, S.-H., 2014. Quantities, composition, and sources of beach debris in Korea from the results of nationwide monitoring. Mar. Pollut. Bull. 84, 27–34. https://doi.org/10.1016/j.marpolbul.2014.05.051. Hong, S., Lee, J., Kang, D., 2015. Emergy evaluation of management measures for derelict fishing gears in Korea. Ocean Sci. J. 50, 603–613. https://doi.org/10.1007/s12601015-0055-8. Hong, S., Lee, J., Lim, S., 2017. Navigational threats by derelict fishing gear to navy ships in the Korean seas. Mar. Pollut. Bull. 119, 100–105. https://doi.org/10.1016/j. marpolbul.2017.04.006. Huddart, J.E.A., Thompson, M.S.A., Woodward, G., Brooks, S.J., 2016. Citizen science: from detecting pollution to evaluating ecological restoration: detecting pollution to evaluate ecological restoration. Wiley Interdiscip. Rev. Water 3, 287–300. https:// doi.org/10.1002/wat2.1138. Hyder, K., Townhill, B., Anderson, L.G., Delany, J., Pinnegar, J.K., 2015. Can citizen science contribute to the evidence-base that underpins marine policy? Mar. Policy 59, 112–120. Ivar do Sul, J.A., Spengler, A., Costa, M.F., 2009. Here, there and everywhere. Small plastic fragments and pellets on beaches of Fernando de Noronha (equatorial Western
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Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul
Rapid assessment of marine debris in coastal areas using a visual scoring indicator
T
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Jongmyoung Lee, Sunwook Hong , Jongsu Lee Korea Marine Litter Institute, Our Sea of East Asia Network, #101-210, 23-57 Jukrim 3-ro, Gwangdo, Tongyeong, Gyeongnam 53020, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Rapid assessment Visual scoring indicator Marine debris Standing stock Citizen science Plastic pollution
Information regarding the spatial distribution and standing stock of marine debris in coastal areas is a prerequisite for efficient cleanup and management. We conducted a rapid assessment of marine debris on the coasts of South Korea using a visual scoring indicator. The indicator consisted of a table and photographs representing nine pollution levels that were quantitatively tested. Locations at every 10 km were selected along the natural coastline for a total of 382 locations, and a length of 100 m at each location was assessed. Approximately 40 participants were trained and assessed the pollution levels using a smartphone application. The surveys were conducted four times in 2017, in April, June, August, and October. The total amount of marine debris stock in the natural coastal areas was estimated to be approximately 17 thousand tons. It suggests that approximately 60% of the marine debris can be cleaned from 10% of the coastline.
1. Introduction Marine debris has recently been recognized as an urgent global pollution issue (Jambeck et al., 2015; Rochman et al., 2013; Vince and Hardesty, 2017). It can be found on the coast, at the sea surface, on the seabed, and even in remote areas (Eriksen et al., 2014; Hirai et al., 2011; Ivar do Sul et al., 2009). In particular, plastic debris poses serious threats to fish, seabirds, and mammals through entanglement and ingestion (Boerger et al., 2010; CBD, 2016; Hong et al., 2013; Jacobsen et al., 2010; Laist, 1997; van Franeker et al., 2011). From the socioeconomic aspect, it damages the tourism industry (Jang et al., 2014a), threatens navigational safety (Hong et al., 2017), and reduces the revenue of fisheries (Donohue et al., 2000; Gilardi et al., 2009). Moreover, its removal incurs high costs (Ballance et al., 2000). Several studies have focused on estimating the abundance or density of marine debris on the coast, at the sea surface, and on the seabed (Eo et al., 2018; Galgani et al., 2015; Hardesty et al., 2017), and considerable efforts have been made to reduce the debris input to the marine environment. These efforts include beach cleanups, monitoring programs, countermeasure policies, and legislations (Schuyler et al., 2018; Xanthos and Walker, 2017). For establishing these measures, a quantitative assessment of marine debris is a prerequisite, and the majority of these measures rely on marine debris monitoring in various compartments, especially beaches (Ariza et al., 2008; Bravo et al., 2009; Ribic, 1998; Ribic et al., 2010). However, monitoring marine debris
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requires extensive efforts, and often incurs high costs because the monitoring requires detailed information such as number/weight, types, and sources of debris. It is likely that this fact limits the number of sampling sites and impedes the comprehensive quantification of marine debris over a broad area or along numerous coasts. Several studies have adopted rapid assessment of marine debris (Alkalay et al., 2007; Cheshire et al., 2009; De Araujo et al., 2006; Lippiatt et al., 2013; Moore et al., 2007). However, these studies were mostly based on counts of debris items, and still required extensive efforts in terms of time, manpower, and survey costs and were limited to assessing geographically broad areas within a short period. A visual scoring indicator can be an alternative tool for surveying marine debris along coasts. In particular, a visual scoring indicator was originally developed by the Japanese government and NGOs (MLIT, 2007; Table 1). The indicator includes nine pollution levels, and each level has a standard photograph that the surveyors can compare with in situ pollution. Using this method, a surveyor can assess the degree of debris pollution scattered in a designated area. The method is easy to follow and does not require special techniques, and therefore, once accustomed to it, many people can conduct the survey. Furthermore, the method enables the coverage of extensive areas and numerous sites in a relatively short time. With this survey, researchers can obtain valuable information on marine debris pollution that can be used for planning and conducting cleanup projects (MLIT, 2007). This study provides a guide to obtain baseline information using a
Corresponding author. E-mail address: [email protected] (S. Hong).
https://doi.org/10.1016/j.marpolbul.2019.110552 Received 7 May 2019; Received in revised form 26 August 2019; Accepted 26 August 2019 Available online 05 September 2019 0025-326X/ © 2019 Elsevier Ltd. All rights reserved.
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Table 1 Visual scoring indicator. Higher levels indicate a higher amount of marine debris per 100 m2. Source: (MLIT, 2007) Level
Debris amount (L/100 m2)
0 1
2.5 5
2
10
3
20
4
40
5
80
6
160
7
320
8
640
Table 2 Categories of dominant debris and land use types.
Description (reference for estimation)
Categories
Description
Dominant debris
Fishing gear Household waste
Net, rope, buoy, trap Plastic bag, food container, beverage bottle, utensil Appliance, tire, masonry Foreign aquaculture buoy, foreign food container Other debris Aquaculture farm Hotel, accommodation facility, yacht deck Residence Field, rice paddy Store, restaurant Other
Dumped waste Foreign debris
No litter Rare litter (Two 2 L beverage bottles) Only a few litter (Four 2 L beverage bottles, 15,500 mL water bottles) Moderately scattered litter (Eight 2 L beverage bottles, 30,500 mL water bottles) Extensively scattered litter (15 2 L beverage bottles, Two 20 L containers) Considerable amount of litter (Four 20 L buoys) Huge amount of litter (One 44-gal drum, Eight 20 L buoys) Almost covered by litter (Five 60 L buoys) Fully covered by litter (Three 200 L drums)
Land use
Other Aquaculture Tourism Village Agriculture Commercial Other
coast was not accessible, the site was replaced with the closest accessible coast. The survey included a total of 382 coasts, and one transect of 100 m was selected on each coast. We designated a serial number from 1 to 382 to each survey site. For smooth field research and record collection, we created a database by combining information such as address, GPS location, and aerial photographs. Then, we provided this database to the surveyors through a mobile application. The information regarding the debris in the target sites was recorded by the surveyors (Table 2). Surveyors walked along the target site and chose the most visible type of debris among the five types that were used in previous studies (Hong et al., 2014; Maharani et al., 2018). The land-use type of the background of the sites was also recorded as “aquaculture”, “tourism”, “village”, “agriculture”, “commercial”, or “other” (Hong, 2013).
rapid assessment tool for the management of marine debris in coastal areas. The objectives of this research are to 1) introduce a visual scoring indicator as a tool for the rapid assessment of coastal debris, 2) depict the distribution of coastal debris as a snapshot at a certain time of year, and 3) estimate the standing stock of marine debris in natural coastal areas in South Korea. 2. Methods 2.1. Visual scoring indicator
2.3. Field surveys
The visual scoring indicator table consists of nine reference photographs that show the debris scattered in a quadrat (10 m × 10 m, 100 m2), according to the pollution level, and indicates the amount of much debris that is likely to be found at each level (Table 1). Each photograph was taken after debris was scattered in the designated area and shows an in-situ image of debris (gomi-map.org/manual_gomichosa.pdf). For example, pollution Level 1 indicates that there are 5 L of debris in 100 m2, while Level 2 indicates the double amount of debris in the designated area (10 L/100 m2). The highest level is Level 8, in which the amount of debris is 640 L/100 m2. During the survey, the surveyors find the reference photograph that most closely resembles the appearance of debris on the beach to determine the beach's debris level. The standing stock of debris within the surveyed area, which was 100 m in length, was calculated by multiplying the debris level by the width of the coast. The standing stock (S) of debris in the entire surveyed area was calculated as follows:
For the visual scoring survey, we recruited 42 surveyors who had previously participated in marine debris monitoring and related activities in organizations, universities, and other institutions, and conducted two half-day training workshops. In the first workshop, on April 15, 2016, we trained the surveyors on assessing the pollution level in situ. As a pilot study, we tested the method at 40 sites selected every 100 km along the natural coastline. Twenty groups, with two surveyors each, participated in the pilot survey at the assigned sites in order to get accustomed to the indicators and fieldwork. We analyzed the results of the pilot survey, corrected common errors in the pollution assessment, and developed a smartphone application based on the revised versions of the data card and categories. In the second workshop, held on August 19, 2016, we delivered the results of the first pilot study, and provided instructions to the participants to reduce the potential errors owing to subjectivity in the assessments. Another pilot survey of all the 382 sites was conducted from August 25 to September 1, 2016. Directions for improving credibility and consistency of the assessment were once again delivered to the surveyors after the survey. In particular, we unified the width of the surveyed area to 10 m as much as possible in order to reduce variability and to facilitate the estimation of the standing stock of debris. Initially, we decided to survey the coast from the water edge to the vegetation area or sea wall. However, this approach was modified because the debris primarily accumulated within 10 m of the coast, mainly along the high strandline. When the surveyors encountered a different width of the accumulated area, the width was recorded and adjusted to the calculation of the standing stock. We guided the surveyors to visit the assigned sites; assess the in situ pollution level over 100 m of the coastline using a smartphone; record the width of the surveyed beach section, major debris type, land use type, and accessibility for cleanup; and take photographs to represent their assessment. Actual field surveys were conducted by surveyors four times during April, June, August, and October 2017. The assessment
n
Standing stock of debris in full surveyed area =
Information
∑ 2.5 × 2Li × Wi i=1
(1)
where Li is the level of the ith coast and Wi is the average width of the ith coast. The constant values 2.5 and 2 are derived from a calculation in which the amount of debris allocated to Level 1 was 5 L and its amount doubles as the level goes up. 2.2. Site selection and recording of information about sites To select the survey sites, we chose one coast every 10 km of natural coastline from the entire South Korean coast that extends to 3770 km (www.data.go.kr). Artificial and insular coastlines were excluded from the selection process. Initially, 377 coasts were selected, to which 5 more coasts were added to include the coastal counties that were not originally selected because of short coastal lengths. When the selected 2
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Fig. 1. Distribution of marine debris along the natural coastline of South Korea in April, June, August, and October 2017. The colors of the circles represent the level (0–8) and their size indicates the volume of marine debris (L/100 m2).
majority of them had low levels (SI Tables 1–4). We estimated the total standing stock of coastal debris of South Korea for the end of 2017 based on our results (Table 3). The amount of debris in the surveyed coasts was calculated by multiplying each level with beach width. We surveyed a stretch of 38.2 km, which represents 0.25% of the total coastline of Korea. We assumed that the distribution of debris is similar all over the coastline, including the artificial and insular coastlines that were not covered by our survey. We obtained an average of 220,000 L of debris from the calculation and then extrapolated the value to the total length of the coastline. Thus, we estimated the total amount of coastal debris at the end of 2017 to be 86,158 L scattered in about 15,000 km of coast. This would represent 17,318 tons of debris, considering the ratio of weight to volume obtained from the average of the results in the recent 5-year Korea National Marine Debris Monitoring Program (0.201 kg/L) (MOF and KOEM, 2017). The standing stock was estimated to be higher after the summer.
results and photographs that were saved in the app were automatically collected in the database. We used these photographs and information to verify the results of the surveys and understand the situation regarding marine debris pollution on the shore. Finally, the results were compiled into a map using the QGIS software (Quantum GIS Development Team, 2017). 3. Results 3.1. Pollution level and standing stock of marine debris The results of the survey, which included pollution level and standing stock, are shown on the map (Fig. 1). From the map, we determined the coasts that were highly polluted, as well as the amount of debris on the coasts. The map could be enlarged to a metropolitan or a regional scale. The results revealed that 60% of the debris was concentrated on 10% of the surveyed coasts. This pattern can be observed on the graph depicting the cumulative proportions of the number of coasts and the debris amount at each level (Fig. 2). Among the 382 surveyed coasts, only a small number of coasts had high debris pollution levels, while
3.2. Dominant debris types and land use around the surveyed coasts The dominant debris types and the land use around the surveyed coasts were investigated in this research (Fig. 3). Fishing gear was the 3
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Cumulative ratio of number of coasts (%) Fig. 2. Cumulative ratio of debris and coastal length (number of coasts) along the natural coastal areas of South Korea.
concentrated on 10% of the Japanese coasts (MLIT, 2007). The results can be used to enhance the efficiency of coastal cleanup projects, considering that if 10% of the coasts that are measured to be debris hot spots are cleaned, more than half of the debris accumulated along the natural South Korean coastline can be removed. The prevention of the input of marine debris to the ocean from its sources is an important strategy to combat this issue (Vince and Hardesty, 2017). However, with regard to the existing debris in the marine environment, its removal is one of the most common measures employed by municipalities and authorities. The removal of debris from coastal areas is relatively easy compared to its removal from the sea surface and bottom due to the feasibility and cost-efficiency (Hong et al., 2015). The removal of debris from beaches can contribute to a reduction in other marine compartments since marine dynamics move the debris between these compartments (Hidalgo-Ruz et al., 2012). Particularly, plastic debris on beaches can be easily fragmented, yielding a large amount of microplastics that cannot be collected and can pose significant threats to the marine environment (Andrady, 2017; Song et al., 2017). Thus, the removal of large-sized plastic debris from coastal areas is the most practical approach to reduce the accumulation of microplastics. Therefore, if the exact spots where urgent debris removal is needed could be determined, a more efficient cleanup and management of hot spot areas would be possible. In particular, these results are useful for coastal managers to plan coastal cleanup, allowing them to pinpoint the heavily polluted areas on the pollution map obtained from this research and concentrate their workforce and budget on cleanup activities in those areas. Moreover, records regarding area accessibility can also help determine the possibility and feasibility beach cleanup. However, beach cleanup is not the ultimate solution to the marine debris issue. Visual scoring indicator can only contribute to the efficiency of cleanup operations with quick assessment; however, in-depth understanding of the causes and impacts of marine debris is required to address the issue at the source (Hong et al., 2013, 2014, 2017). The major pollution sources identified in the existing studies and the
Table 3 Estimated standing stock of coastal debris in South Korea. Estimation range
Length of coast
Estimated standing stock
Surveyed coast Coasts of mainland (including natural and artificial coastline) Coasts of islands Total
38.2 km 7752.5 km
219,961 La 44,639,991 Lb
7210.3 km 14,962.8 km
41,517,927 Lc 86,157,918 L (in volume) 17,317,741 kg (in weight)d
a b c d
Average value of 4 times of survey. a × 7725.5 km/38.2 km. a × 7210.3 km/38.2 km. (b + c) × 0.201 kg/L.
most abundant type, which ranged from 56.5% to 61.0%, followed by household wastes (22.0–36.9%) and other wastes (4.5–11.5%). The land use around the surveyed coasts showed that aquaculture farm (40.8–45.3%) was the most common, followed by tourism (10.2–19.6%) and village (10.7–12.8%). The majority of the debris was fishing gear, and aquaculture was the most used form among land use. The coastal areas are widely used for fisheries in Korea, and it can be seen that the debris generated there has been polluting the coast. 4. Discussion 4.1. Debris concentrated on a small number of coasts The results of this research showed that the marine debris on the coasts of the Korean Peninsula was concentrated on a small number of coasts, with 60% of the marine debris accumulated on 10% of the coasts. The results were consistent with a study in Japan wherein the same indicators were used. In that case, 70% of the coastal debris was 4
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Fig. 3. Dominant debris types (A) and land use (B) of the surveyed coasts.
(40.8–45.3%; Fig. 3). This indicates that aquaculture is the main source of marine debris polluting the coastal areas in South Korea. Previous studies have also revealed that derelict fishing gears, such as fishing ropes, nets, and buoys from aquaculture, were the main components of the coastal debris in South Korea (Hong et al., 2014; Jang et al., 2014b; Lee et al., 2013). Several projects aimed at reducing derelict fishing gear debris have been developed in South Korea. The South Korean government supports fishermen with subsidies on durable aquaculture buoys in order to replace styrofoam buoys (Lee, 2017). The installation of reception facilities and the establishment of transportation and treatment systems for used fishing gear have already been implemented in the pilot projects. The change in the ratio of derelict fishing gear in major debris types in
approximate sources identified in this study were similar, indicating that more efforts are needed to reduce fishing gears from aquaculture and capture fisheries in South Korea. 4.2. Fishing gear and aquaculture debris The results of this study can also contribute to the establishment of coastal debris prevention policies. Surveyors recorded dominant debris types and land use (potential debris sources) around the surveyed coast, that can help establish prevention plans focusing on the sources of coastal debris in the region. In this research, the Korean coasts were heavily polluted by fishing gear (56.5–61.0%), and the predominant land-use around the coasts was in the form of aquaculture farms 5
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Seasonal variations should be considered in future studies in order to determine if seasonality can affect the accumulation of marine debris in coastal areas. Considering the results of the Korea National Marine Debris Monitoring Program (Hong et al., 2014), a trend showing that the amount of debris accumulated bi-monthly on the South Korean coast is relatively high in the summer. Our survey shows a similar pattern, with the highest standing stock observed in August. More information covering several years could provide a good-quality baseline for better decision making. A simple linear extrapolation was applied in order to simplify the calculation process and make the result easier to understand. Further approaches should be conducted to estimate the total quantity of the debris more accurately and reflect its spatial variation.
future surveys could be an indicator of the effectiveness of these policies. 4.3. Efficiency of the visual scoring as a rapid assessment tool This research revealed that surveying coastal debris using the visual scoring indicator is an efficient and rapid assessment method. In this research, 37 surveyors among the trained 42 people surveyed 382 coasts in 8 days. By adopting the visual scoring indicator, the surveyors only had to look around the designated survey area in order to determine its pollution level. The survey takes a relatively short amount of time (less than 30 min for each survey) compared with other survey methods. This study was not designed to investigate major debris types and sources in detail which have been surveyed in a separate program, ‘Korea National Marine Debris Monitoring Program’ since 2008 (Hong et al., 2014). The results of the rapid assessment of coastal debris are very useful for the planning of coastal debris cleanup. Information on the areas where coastal debris is concentrated enables its efficient cleanup (Critchell et al., 2015; Martens and Huntington, 2012). Furthermore, knowing the total amount of debris in the area where cleanup is needed can be used to determine the budget, workforce, and resources that need to be invested. In particular, rapid assessment reduces the time between the investigation and cleanup because the survey is quick, thereby reducing further impacts of wind and ocean currents on the debris. The average pollution level can be a measurable goal for management by local authorities (i.e., Alkalay et al., 2007) as well as by the central government. In this study, the average pollution level in the Gyeongnam Province was 3.23 as of October 2017. The rapid removal of coastal debris from hot spots identified by the survey might lower this level. Local authorities could set a goal for the average level to be 3.00 for the following year. Furthermore, the average level in each province can be used to evaluate management achievements. The use of visual scoring indicators can also represent a very good method for citizen science, as the method is easy, safe, and low-cost (Huddart et al., 2016). Marine debris surveys are one of the most active areas in environmental studies for citizens (Hidalgo-Ruz and Thiel, 2015; Hyder et al., 2015). Using visual scoring indicators, in addition to existing monitoring and survey methods, will further strengthen citizen science in this field. The local residents, being more knowledgeable about marine debris pollution in their communities, can be good potential surveyors. Providing the results to the public can also motivate and encourage the local authorities and enhance general awareness. The smartphone application developed in this study represents a good example of ICT (Information & Communication Technology) in citizen science, as was the case in Jambeck and Johnsen (2015).
4.5. Limitations and directions for future research One limitation of the visual scoring indicator is the potential errors during the debris level evaluation, which is strongly dependent on the surveyors. In fact, in this study, when three researchers reevaluated the photographs taken during the pilot survey, there were several cases in which the results were significantly different from those produced by the field surveyors. However, this difference mainly occurred when the pollution level was relatively low (mainly Levels 3 and 4). On the coasts with high pollution levels (above Level 6), the deviations between the evaluators decreased significantly. In other words, this method is very useful for locating marine debris hot spots. In addition, as photographs are saved, the results can be objectively verified afterward, and there is a chance to correct potential errors. To develop and use this method in the field, it is important to enhance the objectivity of the evaluation indicators. In this study, reference photographs were taken in Japan and South Korea, where the standing stocks of debris were measured. It is necessary to take reference photographs that can estimate the standing stocks as accurately as possible, and the selected photographs that are used as evaluation indicators should reflect the characteristics of the debris in each region. It is also necessary to develop an ‘index’ that can be used to provide information on management priorities, even though the results of the survey focused on the standing stock of the debris. Coastal managers need to determine the urgency of cleanup considering the debris type and the land use around the coasts. It is necessary to determine which factors should be included in this index and what weights should be applied to these factors. Acknowledgments This study was conducted with financial support from the Ministry of Oceans and Fisheries of Korea and administrative support from the Korea Marine Environment Management Corporation (Nationwide Survey of Marine Debris on Korean Coasts in 2017). We thank our colleagues from the Japan Environmental Action Network, who provided insights and expertise that greatly assisted the research. We truly appreciate the participants of the surveys. We express thanks to Ms. Shin Yeong Park for helping us organize the data and three anonymous reviewers for their comments and suggestions.
4.4. Comparison of the estimations of coastal marine debris stock The standing stock that was estimated in this study turned out to be slightly higher than the value reported by Jang et al. (2014c). In this study, the total standing stock of marine debris on the coastline was estimated to be 86,158 L, which is equivalent to 17,318 tons. Jang et al. (2014c) estimated the stock to be 12,029 tons as of the end of 2012. They reached this value by multiplying the average weight of debris per 100 m of the eastern, western, and southern coasts provided by the Korea National Marine Debris Monitoring Program by the coastal length (MLTM and KOEM, 2012). Considering the fact that the monitoring sites had been regularly cleaned by surveyors since the launching of the program in 2008, the estimation of Jang et al. (2014c) could represent an underestimation. In addition, the sample size (382 coasts) of this study is much larger than that of Jang et al. (2014c; 20 coasts). Therefore, the estimation obtained in this study could be closer to reality. Variation over time series can also affect the difference between two estimations.
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