Program

The tsunami caused by the Tohoku Earthquake on March 11, 2011 with an epicenter off the Pacific coast had a significant effect on Manila clam, Ruditapes philippinarum, populations of northeastern Japan. We investigated population dynamics of the clam and evaluated the impact of the 2011 giant tsunami in shallow semi-enclosed bays: Matsukawa-ura Lagoon, Fukushima Prefecture, and Matsushima Bay and Mangoku-ura Inlet, Miyagi Prefecture on the Pacific coast of northeastern Japan. In Matsukawa-ura Lagoon, clam density immediately after the tsunami was markedly lower than before, suggesting substantial impact of the huge tsunami. The impacts on clam population showed some degree of variation depending on the clam size and sampling site. The smaller individuals, which have shallower burrowing depth and lighter individual weight, were swept away by direct tsunami more severely than larger individuals. Larger individuals locally remained with low densities at some sampling sites shortly after the tsunami. Recruitment of early juvenile clams was observed in November 2011, indicating that tsunami surviving large size adult clams obviously contributed to reproduction immediately after the tsunami. Until 2014, the density of juvenile clams rapidly recovered to the equivalent level as before the tsunami. In Matsushima Bay and Mangoku-ura Inlet, the 2011 tsunami inundation heights were decreased by the presence of natural geographical barriers and subsequently the clam populations in both bays were less affected by the tsunami waves. In Mangoku-ura Inlet, artificial tidal flats were constructed in May 2014 as a countermeasure against severe land subsidence up to 1.2 m. In the artificial tidal flats, the clam started to recruit in September 2014 and the density reached up to 4,300 ind m^–2. Shell length growth rates of the clam in Matsukawa-ura and Mangoku-ura were comparable as 15 mm per year after settlement with rapid growth during spring to early-summer.

The Great East Japan Earthquake followed by a large tsunami (3 March, 2011) resulted in significant ground subsidence and deposition of mud in the Natori River (Miyagi Prefecture, northeastern Japan), damaging its brackish water ecosystem and fishing grounds. The Natori River estuary is an important fishing ground for both bivalves and fishes. The Manila clam Ruditapes philippinarum in particular is a commercially valuable bivalve resource. Regular surveys have been in progress to clarify the relationship between the environmental conditions in the fishery ground and the population status of R. philippinarum and to reconstruct the fishery for this clam. The R. philippinarum population in the Natori River recovered temporarily in 2012. However, a marked reduction in the population was recorded after heavy rainfall in the summer of 2013. Heavy rain and dam discharge (to relieve high reservoir levels) induced a longer retention time for fresh water within the river, and the data logger recorded salinity as low as 0 psu during flood tides. Topographical changes, especially the formation of a sand spit intrusion into the river, has been restricting the width of the river mouth, and the destruction of a training wall in the river mouth has resulted in sedimentation of sand. It is suggested that retained water of low salinity has prevented the recovery of the R. philippinarum population. The Japanese Ministry of Land, Infrastructure, Transport and Tourism (MLIT) removed the sand spit during June to October 2015 in an effort to recover the environmental condition of the Natori River estuary. Restoration of the river mouth by MLIT relieved the long-term low salinity condition in 2016. Silty bottom sediments were reduced in 2016 compared with those in 2014 and 2015. The R. philippinarum population attained its highest density in the three years in September 2016.

In 2013, there was a significant decline in the wild seedling yield of the Pacific oyster Crassostrea gigas in Matsushima Bay, Japan. Although higher densities of early stage larvae occurred in 2013 compared with previous years, the pre-attachment stage larvae were rare indicating that the early stage larvae abruptly decreased before they reached the pre-attachment stage. We used hydrographic observational data obtained in 2011–2015 to determine whether hydrographic conditions explained the low yield and the absence of pre-attachment stage larvae. In August 2013, low salinity water was distributed at the surface resulting in the density difference between the surface and near the seafloor of the bay exceeding 10 kg m⁻³. This difference generated a strong outward driving force of 10.0 × 10⁻³ N m⁻³. We hypothesize that the oyster larvae were transferred out of the bay before they could reach the pre-attachment stage. The low salinity distribution resulted from both increased river discharge that flows into the bay and freshwater inputs derived from a river discharging outside a bay. The influence of freshwater from the outside river has likely increased in the bay as a result of topographical changes caused by the 2011 Tohoku earthquake.

In Miyagi Prefecture, total oyster shipment since the earthquake off Tohoku in 2011 has been approximately 20,000 t/year (wet weight with shell), substantially lower than in preceding years (40,000–60,000 t/year). Although oyster culture by non-feeding is a major fishery, the industry has not fully recovered since 2011. Therefore, to aid recovery of oyster production while avoiding overcrowded farming, it was necessary to investigate the feeding environment and estimate a suitable stock density for the region. Specifically, we compared the feeding environments and growth of cultured oyster in Oginohama Bay and Nagatsuraura Bay. We speculate why the culture period is currently shorter at Nagatsuraura (one year) than at Oginohama (two years). Finally, based on our data, we estimated the most appropriate stock density for Nagatsuraura Bay.
We regularly measured the environmental conditions, particularly water temperature and chlorophyll-a (chl-a) concentration, and oyster growth in the culture areas of both bays. We also calculated the total filtration rate of oyster and the growth rate of phytoplankton in the culture areas. The suitable oyster culture quantity was estimated from the phytoplankton quantity in the culture areas minus the total filtration of phytoplankton by oyster, resulting in a value greater than zero.
Maximum chl-a concentration was approximately 100 and 9 µg/L in Nagatsuraura Bay and Oginohama Bay, respectively. Oyster at Nagatsuraura grew faster than at Oginohama. The more rapid growth rate at Nagatsuraura was attributed to higher phytoplankton abundance, because the difference in water temperatures between the two bays was smaller than the difference in chl-a concentrations. By current estimations, we believe that a hanging-culture quantity of 98–120 rafts × 300 ropes × 12.5 masters × 14 ind./master would not constitute overcrowding. We aim to improve the precision of our estimation by increasing the study parameters in future research.

On March 11, 2011, the North Pacific coast of Honshu Island, Japan, was hit by a massive tsunami triggered by a high-magnitude earthquake. In this study, we aimed to determine the impact of the tsunami on the communities of Eisenia bicyclis, a large perennial brown alga, in Tomarihama in Oshika Peninsula, along the Sanriku coast. In Tomarihama, where E. bicyclis is the predominant algal type, the loss of the alga because of detachment from substrata due to the tsunami was limited. However, the previously high population density of sea urchins, which were the primary herbivores and consumers of E. bicyclis in this area, decreased drastically after the tsunami. Owing to a decrease in grazing pressure, numerous juvenile E. bicyclis were observed, some of which survived to grow into adults. As a result, the E. bicyclis beds expanded into deeper waters. The sea urchin population density then increased, leading to a large reduction in adult E. bicyclis density in deeper areas due to the grazing by sea urchins. Meanwhile, there was a constant outflow of soil from landsides caused by land subsidence, causing concerns about the impact on a variety of seaweed communities (including E. bicyclis beds) as well as herbivorous animals that inhabited these areas before the disaster. The degree of damage to E. bicyclis communities was found to vary considerably depending on the sea area concerned. Therefore, it will be necessary to assess the damage and conduct long-term monitoring of the recovery process at as many points as possible.

The tsunami caused by the 2011 Great East Japan Earthquake almost completely destroyed oyster aquaculture facilities in Miyagi Prefecture, which accounted for about 20% of total Japanese oyster production. Although the facilities have been recovering, oyster production in 2016 was only half that before the tsunami, mainly due to decreased numbers of aquaculture farmers and workers. The low production and the labor shortage could further reduce farm activity and market value of Miyagi’s oysters. To address this issue, an empirical research project has been underway since 2013. Here, we introduce the challenges to establishing production systems for two new types of high-quality Pacific oysters. One type is unspawned oysters, shipped within one year of setting, cultivated in cage-culture systems using natural single seed oysters collected on plastic spat collectors. The commercial “Amakoro-kaki” oyster from Shizugawa Bay (central Miyagi) has a clear sweet taste, probably characteristic of sexually immature oysters. Production of this oyster in 2017 is expected to be 100,000 individuals, based on an improved method for collecting natural single seed oyster, and culturing systems using cages treated with “non-stick” material. The other oyster type retains the characteristics of a smooth white shell and condensed umami taste through temporary exposure to air on tidal flats. In shallow areas of Matsushima Bay (south Miyagi), oysters with the commercial name “Atamakko-kaki” are cultivated in mesh bags placed on wooden racks in the intertidal zone. This year, both oyster types will be served in oyster bars and seafood restaurants throughout Japan. We expect that these systems producing these new oyster varieties will become established in the target areas and improve the profitability of local aquaculture. We hope that such improvements in oyster aquaculture will attract young aquaculture farmers.

1.Coho Salmon Aquaculture before the Great East Japan Earthquake.
Coho salmon Oncorhynchus kisutch is an introduced species from North America. There has been net-pen aquaculture of coho salmon around the northeastern Pacific coast of Japan since 1975. Production in 2010 was over 10,000 tons and six billion yen. The tsunami disaster of 11 March, 2011, destroyed all coho salmon net pens, and several million farmed coho salmon escaped into the northeast Pacific Ocean near Japan.
2.Escaped coho salmon from aquaculture facilities.
Escaped coho salmon seemed to remain around the northeast Pacific coast of Japan. In fall of the same year, sexually mature coho salmon migrated up rivers in this area. Only a few 3-year-old coho salmon were found in a survey at a local fish market in 2014, but carefully monitor in the near future is necessary because these individuals may have originated from the coho salmon that escaped during the 2011 earthquake.
3.Recovery of coho salmon aquaculture following the Great East Japan Earthquake disaster.
Fortunately, sea farming of coho salmon was restarted in November 2011 because the juvenile coho salmon at the inland freshwater facilities survived the Great East Japan Earthquake. However, the price of coho salmon in 2012 was declined sharply because of groundless rumors induced by the Fukushima nuclear disaster. There are currently multiple paths being followed to increase the efficiency of the aquaculture system s such as improvement of feed, counter measures for fish disease and selective breeding. Recent production of coho salmon in northeastern Japan is back to pre-earthquake levels but many problems remain.
This work was partially supported by a Grant-in-Aid from the Agriculture Forestry and Fisheries Research Council, Japan.

Chum salmon is an important fisheries resource in northeast Japan; most of the population originates from artificial hatching and stocking. The number of chum salmon returning to the Tohoku Pacific coast has notably decreased in recent years because of the decline in stocked fry after the tsunami disaster of March 2011 and probably also due to changes in the coastal environment. The early life stage when salmon juveniles transition from freshwater to the marine environment is a critical period affecting survival. To facilitate the recovery of the return, we are developing new stocking strategies to improve early survival of released juveniles. We conducted three stocking programs in the Orikasa River and Yamada Bay in Iwate Prefecture from 2013 to 2017. 1) Short term acclimation—Fry were acclimated to seawater in a net pen in the bay for one week before release. 2) Salt supplement diet—To increase seawater adaptation, fry were fed artificial food containing 10% NaCl in the hatchery for two weeks prior to release. 3) Early release—Fry were released in February (earlier than the usual middle March–early May) to reduce the potential risk from warm water during their coastal residence period. Juveniles were collected in the bay for about 40 days after release and otolith marks were checked. We determined early survival and growth, and examined gut contents. Capture rates for fry in the short term acclimation and salt supplement diet groups were higher than for those in the control group (normal river stocking). Growth of juveniles in the early release group was relatively low. The return of adults will be monitored. This study was supported by the “Great East Japan Earthquake Reconstruction Project” sponsored by the Agriculture, Forestry, and Fisheries Research Council, Japan.