Fishing, although one of the oldest human occupations, is carefully monitored and controlled in most modern societies in order to use natural resources most efficiently. Fishing generally requires a license or permit and is allowed only on certain bodies of water at certain times of year. Only certain types of fish and fish of certain sizes can be legally caught. The gear and equipment used to fish is also closely regulated. A breach of any of these regulations constitutes illegal fishing.
A common variety of illegal fishing involves fishing without a permit. Most states and countries require that would-be fishermen purchase licenses before fishing. This serves to limit the number of people engaged in fishing but is primarily a means of generating revenue, which is often used to fund the governmental agencies responsible for overseeing and managing fish and wildlife. States that rely heavily on tourism will often charge more for fishing licenses that are issued to visiting tourists than those issued to residents. A fishing license typically grants the right to catch a specified number of fish of certain types in a given period of time, and exceeding this bag limit constitutes illegal fishing.
The fish population varies from lake to lake, and environmental restrictions are often different for different bodies of water. Lakes with many visitors or fragile ecosystems are often more carefully monitored, and fishing on these lakes may be further restricted. In some cases, no fishing is permitted whatsoever. In others, fishing is permitted during only certain parts of the year.
The species and size of fish caught is also regulated. Popular game fish, such as northern pike, can be caught in limited numbers, and only fish of a certain minimum size can legally be kept. This type of prohibition ensures that fish are able to reach reproductive age and allows the fish stocks to maintain themselves. Catching smaller fish is obviously permitted, but keeping them is a form of illegal fishing. Invasive species and nuisance fish can generally be caught in unlimited numbers and at any size.
Illegal fishing can also involve the use of prohibited fishing techniques. Spear fishing is not generally permitted although exceptions are made for certain native peoples with a long history of fishing in this fashion. A more extreme example of an illegal fishing technique is the use of explosives. Water transmits concussive force very well, and even small explosives can stun or kill fish. Needless to say, this practice is almost universally prohibited.