Anadromous fish, like shad and herring, spend most of their lives in the ocean, but each spring enter the Bay and migrate into freshwater to spawn. These are just a few of the seasonal variations that occur. Some Bay communities are prone to rapid population fluctuations of one or more species. This is particularly true of plankton. Rapid changes in plankton diversity and abundance may occur hourly or daily due to the interaction of biological, physical and chemical -factors.
Many species exhibit long-term patterns in population abundance and distribution.
For example, croakers suffer high mortalities during exceptionally cold weather. This fish was abundant in the Bay during the late 1930s and early 1940s. It is believed that relatively mild winters in the late 1930s and early 1940s promoted the high numbers of croakers. Striped bass declined rapidly in the late 1970s and through the 1980s due to overharvesting and subsequent reproductive failure. However, successful management measures led to a restored stock in 1995.