It has long been established that seawater varies seasonally in its ability to support the growth and survival of embryos and larvae. This may not happen globally, but adverse conditions do occur on both sides of the Atlantic Ocean, particularly when the sea begins to warm in spring and coincident with periods of intense phytoplankton blooming both in spring and in early autumn. The precise reasons for deterioration in seawater quality during these times are not completely understood and they may not recur annually. Some years are better than others in that respect.