The earthquake and following tsunami mainly hit northeast coast of Honshu Island, Japan in March 2011, heavily damaged the fisheries and aquaculture facilities and the coastal environment, especially in Sanriku Coast located near the epicenter of the earthquake in Northwest Pacific Ocean. Murata et. al, (2021) succeed in revealing the recovery process of seagrass meadows and oyster culture rafts in Nagatsura-ura Lagoon, Sanriku using remote sensing techniques.

The study indicates that seagrass meadows were completely disappeared with the tsunami in Nagatsura-ura Lagoon, but the area of seagrass meadows has been expanding since 2013. Field survey showed that the water quality in Nagatsura-ura Lagoon was improved by increased water exchange through the widened lagoon entrance caused by the tsunami. The study also shows that the improvement of the water quality and introducing a new method to deploy less ropes suspending oyster clumps attached to an oyster culture raft led to higher oyster production and the expansion of seagrass meadows in the lagoon after the 2011 tsunami because decrease in the number of cultured oysters after the tsunami decreased organic loads to the sea bottom. A mass mortality event of cultured oysters had occurred once every few years before the tsunami, whereas it has not occurred after the tsunami. Bottom water hypoxia arising from decomposition of organic loads from oysters by bacteria in the bottom surface layer and/or the transport of high turbidity water from the Shin-Kitakamigawa River is thought to be the cause of the mass mortality event before the tsunami. Ameliorated transparency and bottom environment promote seagrass expansion in the lagoon.

Seagrass provides substrates for attached diatoms, which are detached when they grow. Oysters feed detached diatoms. Seagrass also provides oxygen, absorb nutrients to increase transparency and harbors growth-inhibiting bacteria against the toxic dinoflagellate. Therefore, seagrass has a symbiotic relation with cultured oysters below the number that don’t make eutrophication of the lagoon.

What is interesting is that, without any scientific knowledge, people can get the clear picture of the spatial and temporal changes of lagoon from the satellite and UAV’s images, for example, the distribution of the seagrass meadows and the number and the distribution of oyster culture rafts, even position of the ropes suspending oyster clumps attached to oyster culture raft shot from UAVs. It must help decision makers and stake holders to easily visualize what is happening in the field and to consider future measures.

This study shows us effectiveness of remote sensing and concluded that “remote sensing is a practical and essential tool for the sustainable development of coastal waters by contributing to conserve coastal ecosystems and manage aquaculture” Access to full manuscript is available from here.