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The water temperature influence on timing of spawning and the spat settling is considered for yesso scallop (Mizuhopecten (=Patinopecten) yessoensis Jay, 1857) in the Minonosok Bay (Posyet Bay, Peter the Great Bay) on the data of observations in 1970–2011. The date of the spawning start varied from May 1 to June 9 (within 40 days) and the start of the scallop spat settling onto collectors of marine farm varied from June 4 to July 5 (within 32 days). Usually the spawning was delayed in the years with long cold season and big sum of daily temperatures below zero or low mean temperature in winter months and April, when the pre-spawning period with the temperature above zero before May 1 was too short. The spat settling was delayed in the years with severe winter or low mean temperature in the pre-spawning period (r = –0.46); the early settling was observed in the years with early warming above 0o C. The higher sum of positive daily temperatures accumulated till May 1 and till June 1, or within April and May, the earlier the spat settling on collectors (r = –0.58; –0.60; –0.55; –0.42, respectively). Prognostic equations for 10-day and 15-day forecasting of the dates of scallop spawning start and spat settling start are proposed and tested; accuracy of these forecasts is 70% for the spawning and 60 % for the spat settling.