Ecological investigation on ayu population at the Sanriku, after the Great East Japan Earthquake

Representative Organization: Atmosphere and Ocean Research Institute The University of Tokyo

2013/04/01 - 2014/03/31
once or twice a month

Unosumai River
0020_R01.pdf

Field survey

To reveal the effect of the disturbance by the Great East Japan Earthquake on resources of ayu, ecological investigations are performed in the river located in Iwate prefecture. Ascending population of ayu and downstream larvae are collected from April to August, and from September to March, respectively. Riverine conditions including water temperature and volume flow rate is also collected.

After the Great East Japan Earthquake, it was reported that the hatch-date composition of ayu population in the Sanriku region significantly shifted later season compared to the result before the tsunami. This ecological change was probably due to the selective mortality depending on hatch-date caused by the huge tsunami associated with the earthquake. It was concerned that the timing of life history events are subsequently shifted and it probably causes a negative influence on resource abundance of the species. The purpose of this research was to clarify the ecological influences of the tsunami on the local population of ayu and its subsequent recovery process. A total of 226 individuals were collected in the lower part of the Unosumai River from May to July 2013. Of the individuals morphologically identified as wild fish, 14 individuals collected in June were subjected to the otolith increment analysis and electron microprobe analysis (EPMA) for Sr/Ca ratio. Hatch-date ranged from September 4 to October 27 (average September 29), and the age at migration into the river was 211 to 276 days (average 244 days). Those results were similar with the result obtained in 2010, before the earthquake. The body size at migration into the river was 82.6 [7.5] mm (mean [standard deviation]) and was significantly smaller than 2010 (p < 0.05). The growth rate during their oceanic period (0.34 [0.02] mm/day) was also significantly lower (p < 0.05) than in 2010. Thus, it was considered that the timing of hatch and migration has recovered to the original states, but their growth in the sea has adversely affected by the long-lasting changes in the coastal environment. The temporal distribution of drifting larvae of ayu was also investigated in the Unosumai River from September to December 2013. The drifting larvae were collected from the early September to the end of December with the peak in early October. Because this result was same as 2010, it was suggested that delay in spawning season did not occur despite the smaller size of migration individuals. Considering these results, it can be supposed that water temperature shapes the life history of ayu and influenced the recovery process of hatch-date composition and spawning season through selective mortality in the sea and regulation of maturation timing.

FY2013 Ecological investigation on ayu population at the Sanriku, after the Great East Japan Earthquake