Vertical distribution of Pacific oyster Crassostrea gigas larvae and modeling larval transport in Hiroshima Bay, Japan

Abstract

ABSTRACT: Understanding vertical distribution of planktonic larvae is essential for elucidating larval dispersal and recruitment processes. We investigated the vertical distribution and horizontal transport of Pacific oyster Crassostrea gigas larvae by field observations and numerical simulations during their main spawning season in Hiroshima Bay, Japan. In field observations, despite horizontal differences and slight diurnal/semi-diurnal changes depending on larval sizes, most larvae were distributed in the upper 3 m layer. The relationship between C. gigas larvae and environmental conditions revealed that larval density increased with increasing temperature and chlorophyll a concentration, and the density peaked at salinity of approximately 20 for all larval sizes. The observed results suggest that the distribution characteristics of C. gigas larvae are suitable for survival in an estuarine area, where environmental conditions are potentially favorable but hydrodynamic conditions can drastically change over the short term due to variations in river discharge. To examine the effect of high river discharge on larval transport, numerical simulations were conducted using a particle-tracking model incorporating the vertical motion of C. gigas larvae. The simulation results reproduced the spatio-temporal dynamics of planktonic and settled larvae after the high river discharge. Although most particles simulating larvae outflowed from the main spawning area, an area of high particle density at the end of simulation corresponded with the offshore area for seedling collection. The present study suggests the role of vertical distribution of C. gigas larvae for recruitment, and the prospect of sustainability in oyster aquaculture with respect to seedling collection despite the frequent heavy rainfall associated with climate change.