Seasons are usually set to protect fish during the spawn. During closed seasons it's illegal to fish for specific species, and those caught accidentally while fishing for other kinds of fish must be released immediately. Seasons vary widely in different areas and for different kinds of fish.
When freshwater fish spawn they're often very easy to catch because they move into shallow water and many try to eat anything that comes near their nest. If enough spawning fish are caught, they might not produce enough young to replenish the population. Even if the fish are released after they're caught they might not spawn successfully.
If there are huge numbers of fish in a body of water there may be no closed seasons because enough young are produced no matter how many adults are caught. For example, in the South, fishing for bream while they're spawning is a common activity. Although prohibited in many northern states, fishing for bass on the spawning beds is also allowed in the South where growth rates and survival rates are higher.
Seasons on saltwater fish are also set to protect them during the times when they are spawning. Many kinds of saltwater fish have no closed season but some of the more desirable fish do. These seasons are in place to allow them to spawn and produce future catches.
Endangered species may not have any open season at all, so they are protected year round.
Fish are usually divided into two groups: game fish and nongame fish. Nongame fish may include fish that are caught commercially and fish that are undesirable for eating or for sport. Limits are different for different kinds of fish.