Murata Hiroki, Hara Motoyuki, Yonezawa Chinatsu, Komatsu Teruhisa
Graduate School of Agricultural Science, Tohoku University, Sendai, Japan.
Port and Harbor Bureau, City of Yokohama, Yokohama, Japan.
PeerJ. 2021 Jan 14;9:e10727. doi: 10.7717/peerj.10727. eCollection 2021.
Coastal ecosystems are blue infrastructures that support coastal resources and also aquaculture. Seagrass meadows, one of coastal ecosystems, provide substrates for epiphytic diatoms, which are food resources for cultured filter feeder organisms. Highly intensive coastal aquaculture degrades coastal environments to decrease seagrass meadows. Therefore, efficient aquaculture management and conservation of seagrass meadows are necessary for the sustainable development of coastal waters. In ria-type bays, non-feeding aquaculture of filter feeders such as oysters, scallops, and ascidians are actively practiced along the Sanriku Coast, Japan. Before the 2011 Great East Japan Earthquake, the over-deployment of oyster culture facilities polluted the bottom environment and formed an hypoxic bottom water layer due to the organic excrements from cultured oysters. The tsunami in 2011 devastated the aquaculture facilities and seagrass meadows along the Sanriku Coast. We mapped the oyster culture rafts and seagrass meadows in Nagatsura-ura Lagoon, Sanriku Coast before and after the tsunami and monitored those and environments after the tsunami by field surveys.
We conducted field surveys and monitored the environmental parameters in Nagatsura-ura Lagoon every month since 2014. We used high-resolution satellite remote sensing images to map oyster culture rafts and seagrass meadows at irregular time intervals from 2006 to 2019 in order to assess their distribution. In 2019, we also used an unmanned aerial vehicle to analyze the spatial variability of the position and the number of ropes suspending oyster clumps beneath the rafts.
In 2013, the number and distribution of the oyster culture rafts had been completely restored to the pre-tsunami conditions. The mean area of culture raft increased after the tsunami, and ropes suspending oyster clumps attached to a raft in wider space. Experienced local fishermen also developed a method to attach less ropes to a raft, which was applied to half of the oyster culture rafts to improve oyster growth. The area of seagrass meadows has been expanding since 2013. Although the lagoon had experienced frequent oyster mass mortality events in summer before the tsunami, these events have not occurred since 2011. The 2011 earthquake and tsunami deepened the sill depth and widened the entrance to enhance water exchange and improve water quality in the lagoon. These changes brought the expansion of seagrass meadows and reduction of mass mortality events to allow sustainable oyster culture in the lagoon. Mapping and monitoring of seagrass meadows and aquaculture facilities via satellite remote sensing can provide clear visualization of their temporal changes. This can in turn facilitate effective aquaculture management and conservation of coastal ecosystems, which are crucial for the sustainable development of coastal waters.
沿海生态系统是支持沿海资源及水产养殖的蓝色基础设施。海草草甸作为沿海生态系统之一，为附生硅藻提供基质，而附生硅藻是养殖滤食性生物的食物资源。高强度的沿海水产养殖会使沿海环境退化，导致海草草甸减少。因此，高效的水产养殖管理和海草草甸保护对于沿海水域的可持续发展至关重要。在里亚式海湾，日本三陆海岸积极开展牡蛎、扇贝和海鞘等滤食性生物的非投饵养殖。在2011年东日本大地震之前，牡蛎养殖设施的过度部署污染了底部环境，养殖牡蛎的有机排泄物形成了缺氧的底层水层。2011年的海啸摧毁了三陆海岸的水产养殖设施和海草草甸。我们绘制了海啸前后三陆海岸长津浦内湾的牡蛎养殖筏和海草草甸的地图，并通过实地调查对海啸后的这些情况及环境进行了监测。
自2014年以来，我们每月在长津浦内湾进行实地调查并监测环境参数。我们使用高分辨率卫星遥感图像，在2006年至2019年期间不定期绘制牡蛎养殖筏和海草草甸的地图，以评估它们的分布情况。2019年，我们还使用无人机分析了筏下悬挂牡蛎团的绳索位置和数量的空间变异性。
2013年，牡蛎养殖筏的数量和分布已完全恢复到海啸前的状况。海啸后养殖筏的平均面积增加，悬挂牡蛎团的绳索在更广阔的空间附着于筏上。经验丰富的当地渔民还开发了一种减少筏上绳索数量的方法，并将其应用于一半的牡蛎养殖筏上，以促进牡蛎生长。自2013年以来，海草草甸的面积一直在扩大。尽管在海啸前该泻湖夏季经常发生牡蛎大量死亡事件，但自2011年以来这些事件未再发生。2011年的地震和海啸加深了礁槛深度并拓宽了入口，以加强水交换并改善泻湖水质。这些变化带来了海草草甸的扩张和大量死亡事件的减少，从而使泻湖能够可持续地养殖牡蛎。通过卫星遥感对海草草甸和水产养殖设施进行绘图和监测，可以清晰地呈现它们的时间变化。这反过来又有助于有效的水产养殖管理和沿海生态系统的保护，这对于沿海水域的可持续发展至关重要。
