Keywords

1 Introduction

The Great East Japan Earthquake 2011 (2011 Tohoku-Oki Earthquake) occurred on the 11th of March 2011. After the earthquake, tsunami waves washed over the Pacific side of the Tohoku area – the northern part of the main island of Japan. Tsunamis destroyed many coastal towns, even those protected by seawalls of 10 m in height, and damaged and disabled a nuclear power plant as well. The Japanese government directed efforts to help the area recover from earthquake and tsunami damage, and several programs were conducted in terrestrial areas. During this time, surveys of marine ecosystems were conducted separately.

An initiative called the Tohoku Ecosystem-Associated Marine Science (TEAMS) project was launched at the end of the fiscal year 2011 with the support of the Ministry of Education, Culture, Sports, Science and Technology. The TEAMS project is required to contribute to the following in the ocean areas off Tohoku: (a) elucidate the effects of earthquakes and tsunamis; (b) with a view to assessing recovery, reveal the dynamics of organisms and their environment; (c) suggest fisheries that could be sustainably exploited; and (d) assess the safety of such products in terms of anthropogenic chemical compound content. As part of this project, multidisciplinary surveys of the environmental conditions and state of the ecosystem have been conducted from coastal areas to the continental slope, where fisheries are conducted, in order to assess the effect of earthquakes and tsunamis.

Previous research suggested heavy damage by the earthquake and tsunami in coastal and shallow ocean areas. Large amounts of sedimentation (Goto et al. 2012), increases in marine debris (Shibata et al. 2013), decrease in ground level/seafloor depth (Ozawa et al. 2011), and changes in the distribution of benthic organisms (Seike et al. 2013; Urabe et al. 2013) are considered to be the main changes in the ecosystem. Although extensive research and cleanup activities have been conducted in shallow waters, research results from deep-sea areas are scarce. A few reports, some sedimentation studies, suggested difficulty in identifying the effects of the earthquake and the tsunami event in the upper part of the deep sea (such as shallower than 750 m) because of bioturbation (Arai et al. 2013). Nonetheless, marine debris is expected to have either positive or negative effects in deep-sea areas (Goto 2012; Ramirez-Llodra et al. 2011).

In the present paper, we introduce preliminary results on the distribution of debris in the datasets collected from deep-sea surveys by TEAMS-related cruises and from archived data from other studies. We focus in particular on the spatial distribution of marine debris and the types of marine debris to assess the positive or negative effects of marine debris originating from the earthquake on ecosystems in the upper part of the deep sea.

2 Methods

2.1 Study Area

The Tohoku offing is located on the northern Pacific side of the main island of Japan and is well-known as a frontal area where the warm, northward-flowing Kuroshio current encounters the cold, southward-flowing Oyashio current. Since 2011, we have surveyed the deep-sea area using a side-scan sonar, deep-tow submergible cameras, and an ROV (remotely operated vehicle) system. The target region for the TEAMS surveys encompassed the area from the coast to the continental slope, and this study focused on the offshore region of Miyagi and Iwate Prefectures. The survey areas and sites are plotted in Fig. 1.

Fig. 1
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Location of study sites for research on species and/or marine debris. Site numbers correspond with those in Table 1

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2.2 Data Collection

We examined the data from five different surveys to evaluate quantitatively the occurrence of marine debris in each survey area or along each survey transect. Details of each survey are as follows: towed camera surveys by the 4KC (4,000 m camera deep-tow camera) and YKDT (Yokosuka deep-tow camera) were conducted from May 16 to May 20 and July 31 to August 7, 2012. The 4KC was equipped with a Super-HARP (high-gain avalanche rushing photoconductor) video camera recording in NTSC (National Television System Committee specification) format for downward-facing seafloor observations, and this allowed observation of an area of approximately 3 × 3 m at an altitude of 3 m above the seafloor. The YKDT was equipped with a similar camera configuration but with a less light-sensitive CCD (charge-coupled device) camera and also recording in NTSC format. An ROV (remotely operated vehicle) survey was carried out during June 30 to July 5 in 2013. The ROV Crambon was equipped with a pan-tiltable, forward-facing, high-definition (1080i) video camera with a 67° view field angle and with video recorded onto a hard disk cartridge (AJA WZZ-KiPro2400) at ProRes 4:2:2HQ resolution via HD-SDI and saved in QuickTime format (.mov). Video and other data were time-stamped and recorded synchronously using an Ethernet-enabled logging/playback system based on that developed by Lindsay et al. (2012). We assumed 2 m as the observation width for the calculation of the approximate number of debris items per km2. Side-scan sonar (EdgeTech 4200MP, 400/100 kHz) was used to obtain topographic data and marine debris occurrence information. In the case of coastal areas, another side-scan sonar system (System 3000, Klein Inc., 500/100 kHz) was deployed from a fishing boat.

3 Results

The deep-tow camera survey from May 16 to May 20, 2012, revealed a broad distribution of the brittle star Ophiura sarsii (Lütken 1855). It was especially dense during transect DT-6C but not during DT-7C (nos. 14 and 15 in Table 1 and Fig. 1). During transects DT-6C to DT-9C, several small items of debris, such as splinters of wood and plastic bags, were found. During transects DT-10C and DT-11C, near a submarine valley off Iwate, many differently sized items of debris were observed clumped together as shown in Fig. 2.

Table 1 Survey point information
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Fig. 2
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Accumulated marine debris in a submarine valley (Deep Tow 10C image)

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During the cruise from July 31 to August 7, 2012, marine debris were found at 3000 m depth in the Japan Trench area, more than 200 km from land, using the YKDT deep-tow camera (nos. 7–13 in Table 1). A fishing net, cans, plastic bags, and sunken wood were scattered over an area of seafloor (Fig. 3). The origin of these debris could not be determined as to whether it was related to the tsunami event or not. At a shallower study site (depth 551 m, no. 12 in Table 2), we discovered high abundances of invertebrates and fish (such as Ophiura sarsii, sea stars, Anguilliformes sp., and Sebastolobus macrochir (Günther 1877)) in a submarine canyon area where much debris had accumulated. An extremely large number of the anthomedusan jellyfish Euphysa japonica were observed near the seafloor at some stations, but the identity and substrate habitat preferences of its benthic polyp stages have not yet been determined (Lindsay et al. 2008), so an analysis of correlations with debris distributions was not possible at the present time.

Fig. 3
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Cans observed near the trench

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Table 2 Properties of marine debris items extracted by the primary analysis
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During the ROV survey in 2013, high densities of brittle stars were observed at stations 1 and 2 (Table 1 and 2) but not as many occurred at station 4. At station 2, the body sizes of brittle stars were significantly different from those occurring at the other stations. At station 4, we found drag marks made by trawl nets and a lower diversity and population size of macrofauna.

Although the area surveyed by the ROV Crambon was limited, detailed information on debris could be collected (Fig. 4). The number of debris items was obviously higher within the bay area (no. 5; the Yamada Bay) on the July of 4th. Eight debris items were observed in an approximately 50 × 50 m area.

Fig. 4
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Wood observed by the ROV

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Surveys by side-scan sonar were able to investigate broader areas of the deep-sea floor, although the resolution of acoustic images was lower than that of the video images obtained during the ROV survey (nos. 21 to 23 in Table 2). The acoustic survey off Miyagi found a large debris item of approximately 10–15 m in length (no. 23). In coastal areas, numerous small debris items, mostly less than 1 m in length, were able to be distinguished in the acoustic images (Fig. 5; nos. 24–26 in Table 2).

Fig. 5
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Marine debris observed by the side-scan sonar

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As shown in Tables 1 and 2, the amount of debris decreased substantially with increasing water depth and/or distance from the coastline. The average number of debris items at the ten main, video-surveyed study sites (1, 2, 4, 12, 14–19) at depths from 289 to 890 m and at distances from 11 to 57 km from the coastline, was 5,195 (standard deviation [SD] = 5,133) per km2 (Tables 1 and 2). It was 1,116 per km2 (SD = 961) at the five sites (7, 9–11, 13) in deeper areas near the trench (>1,000 m depth), which were over 100 km from the coastline. In canyon areas (2, 4, 12, 18, 19), the average number of debris items per km2 was 6,761 (SD = 3,528), and this was higher than at other areas of similar depth that we studied (mean = 3,629, SD = 6,381). Of the observed debris categories, “plastics” were most numerous (113), followed by “wood and paper” (85). A large subset of the debris (over 237 items) were unclassifiable.

4 Discussion

Our preliminary results suggest that the direct impact of tsunami-derived debris in deep-sea areas off the Sanriku region (ca. 200–800 m depth) is not as severe as in coastal environments (Seike et al. 2013; Urabe et al. 2013). Accumulations of tsunami-derived debris were found mainly within submarine canyons in deep-sea areas, while in coastal areas the debris was scattered across the seafloor. The transportation power of tsunamis, turbidity currents, and the geomorphological features of the seafloor appear to greatly affect the distribution pattern of marine debris (Arai et al. 2013).

Our observations also showed that different kinds of debris, such as wood, plastics, and construction materials, still occur in mixed aggregations. These debris items accumulate sessile species that need hard substrates on which to attach. This is assumed to be because of the lack of such surfaces on the largely muddy deep-sea floor, as was observed in the canyon area. Different materials are expected to move and decompose in different ways, and the composition of the debris aggregations is therefore expected to change over time due to transportation and decomposition. For example, plastics can be harmful when accidentally ingested (Ramirez-Llodra et al. 2011). Flustering of plastics is also suspected to contribute to the accumulation of harmful substances in organisms, where micro-plastics are known to be a problem (Andrady 2011). In the case of wood or mixtures of wood and fabric materials, such debris attracts species that can decompose cellulose materials, such as members of the wood-boring mussel subfamily Xylophagidae, which were retrieved in a sample from this area. In the case of nets or wire debris, a negative effect on fisheries has already been reported by Goto (2012). For sustainable fisheries, as a first step the number of ghost fishing nets and plastics of ingestible sizes should be reduced. The next step should be to track the stage/status of flustering in plastics and determine the distribution of various marine debris materials.