Reconstruction support and planning for the fishery in the estuary

Representative Organization: Tohoku University

2016/04/01 - 2017/03/31
Monthly

The Natori River estuary in Miyagi Prefecture

Field survey

The surveys have been proceeding to clear the status of fishery resources and environmental conditions in the Natori River estuary and support the reconstruction of fishery. 1. Monitoring on the physical environmental conditions (Temperature and salinity) 2. The survey on the Manila clam population status(population density, size distribution) and ecological characteristics 3. The survey on the brackish water clam population status (population density, size distribution) and ecological characteristics 4. The survey on the Ayu population (population density, size distribution) and ecological characteristics

1. Physiological environmental conditions Topographical change, especially forming of sand spit intrusion into the river has been inducing narrower river mouth width. The destruction of training wall in the river mouth induced sedimentation of sand. It is suggested that low salinity has prevented the recovery of the Manila clam population. MLIT of Japan begins to remove the sand spit to recover environmental condition of the Natori River estuary from June 2015. Restoration of river mouth intercepts long-term low salinity condition in 2016. Silty bottom sediments decreased in 2016 compared with those in 2014 and 2015. 2. The Manila clam population Manila clam attained the highest population density in the recent three years in September 2016. Difference of carbon and nitrogen stable isotope ratios among dominant bivalves suggested the difference food source on each species. 3. The Ayu population In 2016, the CPUE of Ayu population showed the same level as before the tsunami. The larvae and juveniles feed on zooplankton in coastal waters. Upstream migration begins in spring, during which it switches its food from zooplankton to benthic microalgae. Growth variation in the Ayu was investigated. To analyze food source utilization, carbon stable isotope ratio measurements were taken from muscle. Hatching date composition of Ayu shows a mode within September and October. The results show a weak positive correlation between standard length and age in days. However, fishes showed large variations in body size (maximum 60mm) among individuals of the same age collected at the same station and the same sampling time. The larger fishes had higher carbon stable isotope ratio values than their smaller siblings. These results suggested that individuals which switch to benthic microalgae earlier take a larger amount of benthic microalgae consequently undergo faster growth.

FY2016 Support of reconstruction and planning for the fishery in the estuary

Water temperature and salinity in the Natori River Estuary (FY2015-2016)
Distribution of bivalves in the Natori River Estuary in 2016