Effect of wind blows on the transport and settlement of brown sole (Pleuronectes herzensteini) larvae in a shelf region of the Sea of Japan: numerical experiments with an Euler–Lagrangian model

The transportation of eggs and larvae of brown sole in a shelf region of the Sea of Japan was explored using a 3D Euler–Lagrangian model, with special interest in the effect of wind blows on the larval retention. The general pattern of egg and larval transport from the spawning ground to the nursery was well reproduced under a constant westerly wind (4 m s−1), by assuming vertical movement of the eggs and larvae (upward in the earlier phase and downward in the later phase of the drift period). Using this model, the effect of westerly wind speed on egg and larval retention in the shelf was evaluated, and it was shown that when the wind speed exceeded a critical value (>5 m s−1), the retention rate could be rapidly reduced. An exploratory simulation of the effect of a one-day westerly blow with a speed of 15 m s−1 on the retention further indicated that the westerly blow induced strong offshore drift of the shelf water at the subsurface, and that it could markedly reduce the retention rate in the earlier phase of the drift period when eggs stay in the lower layer, and in the final phase when larvae stay near the bottom before settlement. mbers of juveniles which successfully settled on the nursery in the study area showed a significant negative correlation with the frequency of strong westerlies in April. This suggests that wind blows could seriously reduce the retention of the larvae just before settlement, providing field evidence to support the model results. Recent declines in the fishery catch can therefore be partly explained by recruitment failure due to the increase in the frequency of strong westerly blows.