Long-term monitoring of zooplankton and temperature has been carried out at the nuclear power station of Flamanville (west coast of Cotentin, English Channel, France) since 1977. Zooplankton sampling and temperature measurements took place at fortnightly intervals, particularly during the spring and summer. In addition, daily records of water temperature were recorded since 1986. The seasonal appearance of Maja brachydactyla Balss, 1922 first stage zoeae in the plankton outside the thermal plume near to the power station can greatly vary from year-to-year. Earliest observations have occurred in early June whilst latest observations were made at the end of July. During the period of study, temperature records display a warming trend whilst hatching is gradually recorded earlier in the season. An analysis of year-to-year variability in temperature and hatching date reveals that early hatching is related to higher winter-spring sea temperature. Cumulated temperature (degree-days) has been calculated for different periods (during winter and spring) to detect the time at which temperature may influence the date of initial occurrence of first zoeal stage. There is no relationship with January temperatures whilst a strong correlation is observed for the period ranging from early February to the end of May. Based on regression analysis, a predictive model of timing of zoeae appearance in the plankton is constructed. The model can predict timing of zoeae appearance from mean temperature recorded from February 1st to March10th.