Support and reconstruction of fishery in the estuary and coastal area

Representative Organization: Tohoku University

2018/04/01 - 2019/03/31
Monthly

The Natori River Estuary, Sendai Bay and Ishinomaki Bay

Field survey

1. Survey on the environmental conditions of fishery ground of bivalves. Continuous observation of salinity and water temperature. Analyses of grain size distribution and silt -clay % of bottom sediment. Analyze the topographical change of river mouth using aerial photos. 2. Survey on the resources of the bivalves. Collection of Clams at 18 stations in the Natori river estuary. Calculation of Population density and size composition. 3. Study on primary production in the estuary. Development of new device on suction to collect water within 1cm layer on the bottom. Analyses of salinity, nutrients and stable isotope ratios of bottom water. Analyses of C, N- stable isotope ratios of bivalves. 4. Survey on the Ayu population. Collection of Ayu using casting net at 3 stations in the Natori and Hirose River. Analyses of size composition and carbon and nitrogen stable isotope ratios of Ayu. 5. Estimation of the present stock of the Surf clam. Collection of the Surf clams in Sendai Bay, off Yamamoto. Analyses of Population density and shell length composition. 6. Estimation of the stock of the Ark shell. Estimation of population status in the release and restocking are Simulation of fisheries management.

1. Survey on the environmental conditions of fishery ground of bivalves. Low salinity didn't last than ever before due to the river-mouth widening. Silt clay content of bottom sediment in the Natori river became lower overall. Restoration of the river mouth induced regular tidal flow. However, closing of the mouth of Idoura Inlet have been affecting decrease of capacity of seawater within the estuary. 2. Survey on the resources of the bivalves. Brackish water clam in the upper stream, Nuttallia olivacea in the midstream and Manila clam in the downstream are dominant in the Natori River estuary. However successful recruitment of the Manila clam has not yet occurred since 2014. Changes in physical environments closely correlated to the quantity of the Manila clam. Field experiment results showed that environments were not sufficient enough for the Manila clam. 3. Study on primary production in the estuary. New device on suction was developed to collect water within 1cm layer on the bottom. Salinity, nutrients and stable isotope ratios of water fluctuated from ebb to flood. Large amount of organic carbon was supplied into the estuary from terrestrial plants. C, N stable isotope ratios differ among bivalve species and showing food segregation. 4. Survey on the ayu population. In 2018 the catch per unit effort of ayu population showed the same level as before the tsunami. The results show a weak positive correlation between standard length and age in days. The larger fishes had higher deluta 13C values than their smaller siblings. These results suggested that individuals which switch to benthic microalgae earlier take a larger number of benthic microalgae consequently undergo faster growth. 5. Estimation of the present stock of the Surf clam New fishing gears (total 7) for the surf clam were bought out and we accelerated the technical transfer the fishermen. The present stock achieves a restorable level for the fishery. The 2014year class is more abundant than 2009 and 2011year classes. The 2014year class recruiting 2 to 3 years later will grow up to 400 to 500 tons. It is urgent for the fishery to construct a fishing system avoiding rubble. 6. Analyses of the ark shell fishery and Simulation of fisheries management. The meeting and discussions have been conducted about strategy of sustainable management on ark shell. Released clams contribute to fisheries catch. Fishing effort less than 2500 and rotation of 2 years cycle make the ark shell catch sustainable with high level.

FY2018 Support and reconstruction of fishery in the estuary and coastal area

Water temperature and salinity in the Natori River Estuary (FY2018)