Research on Fishery resources and environmental condition on fishery ground in the Natori River

Representative Organization: Tohoku University

2014/04/01 - 2015/03/31
monthly

Natori River

Field survey

1. Fishery resources bivalves; Ruditapes philippinarum and Corbicula japonica, are collected using pumping system in Natori River. Individuals density of bivalves in calculated and shell size compositions are analyzed. 2. Surveys on the environmental conditions; water temperature and salinity, are conducted in the Natori River. Grain size composition, organic carbon and nitrogen contents and stable isotope ratios of the sediments are analyzed to evaluate the condition of fishery ground. 3. Settlement experiments on bivalve using marking and caging method are conducted in Natori River to estimate production rate of bivalve. 4. Survey on the Ayu population in the Natori River and Hirose River.

1. The Manila clam (Ruditapes philippinarum) Two years after the tsunami, the R. philippinarum population temporarily recovered and many juveniles were collected in the spring of 2013. However, a marked reduction in the R. philippinarum population was recorded after heavy rainfall in the summer of 2013 and R. philippinarum recruitment has yet to succeed. It suggested that low salinity induced the damage to the R. philippinarum population. Surveys during the present study using a data logger recorded salinity values close to zero psu during flood tide over a period of several days. Heavy rain and dam discharge induced a longer retention time for fresh water within the river. Topographical change (especially the forming of a sand spit intrusion into the river) has induced a narrower river mouth. It is suggested that low salinity has prevented the recovery of the R. philippinarum population. Successful fisheries recruitment of R. philippinarum has not yet occurred since the tsunami. It will be important to survey biological production systems and the process of physical environmental changes at this river mouth. 2. Brackish water clam (Corbicula japonica) Brackish water clam abundance has returned to approximately the same levels as before the tsunami, but with an upstream shift in the C. japonica habitat of about 1 km. The results on growth experiments in the fields indicated importance of food environment, especially supply of detritus from terrestrial plant is important. It is essential to understand the relation between the growth link to food supply mechanisms for Corbicula japonica and physical environmental conditions. 3. Ayu (Plecoglossus altivelis altivelis) The CPUE of ayu population in 2013 is recovered to the same level as before the Tsunami. The research in 2013 denoted large amount of ayu population running up. Especially, leisure fishing has been progressing in upper stream. The hatch date composition showed a mode between September and October, there is little difference in those before Tsunami. Condition factor and GSI of ayu population indicated high value, and we didn't found the effects of disaster.

FY2014 Research on Fishery resources and environmental condition on fishery ground in the Natori River

Water temperature and salinity in the Natori River Estuary (FY2014)
Distribution of bivalves in the Natori River Estuary in 2014