Seasonality and occurrence of the dominant mysid Neomysis awatschensis (Brandt, 1851) in the Yura River estuary, central Sea of Japan

Abstract Mysids play an essential role as the dominant prey species for coastal fishes. In Japan, the mysid Neomysis awatschensis (Brandt, 1851) is commonly distributed in estuarine and shallow coastal waters with high productivity due to supply of nutrients from the watershed. The present study investigates the distribution and life history of the mysid N. awatschensis in relation to biotic and abiotic factors by monthly sampling from March 2014 to July 2015 at stations along the Yura River estuary, the central Sea of Japan. The Yura River physical conditions change seasonally and are affected by its discharge. Salt-wedge-intrusions were frequently observed and found up to 18 km from the river mouth from April to December 2014 when river discharge was low, whereas in 2015 intrusions were less pronounced, possibly due to river mouth clogging. Clear seasonality was observed in mysid density, showing highest densities (up to 202 inds/m 2 ) in spring and early summer in contrast with lowest densities ( 2 ) from late summer to winter. Neomysis awatschensis appeared to change its reproduction characteristics in response to changes in the environment, large-sized females occurred in spring to produce large numbers of eggs and larvae during the high primary production period. In late summer, the population declined due to increasing mortality of juveniles at high temperatures. In autumn and winter, high river discharge and low temperature suppressed reproduction and survival in the mysid population. It is notable that N. awatschensis occurred within a salinity range of 0–32 and no clear influence of salinity on the mysid density was found. Those results suggest that reproduction is partially suppressed by variable and oligotrophic conditions in the Yura River estuary. Combined with the flexibility to seasonal changes in temperature and food availability, the great tolerance to acute salinity fluctuations allows N. awatschensis to maintain its population in the estuary.