What are exotic species? Exotic species are species that are not native to the environment. They can be plants, fish, or animals. The reason they are dangerous is that they can invade a native environment and take over because the native species, whether plant, fish, or mammal, do not have the means to defend themselves against the exotic introduced species.
One exotic species, that is not really so exotic, but comes under the heading of harmful or potentially harmful and can overtake an environment and affect the ecosystem is the common goldfish. Goldfish are plentiful. They are easily obtained. And they may be dumped into a local pond or lake where they can grow and crowd out native species as they eat food meant for the native species. Another fairly well-known example is how pythons have been taking over the Everglades in Florida, even able to consume alligators.
Potentially harmful plants include water hyacinths, giant salvinia, and hydrilla. Exotic fish include Asian carp which go by the names of grass, silver, and bighead; tilapia, and zebra mussels. Harmful plants and animals are regulated by strict state laws. Permits are required to transport them. There are fines if these species are found. There are some permits given to zoologists or for research purposes, but outside of that there are strict controls against exotic and potentially harmful species. Texas alone has a 370 page book that presents information on the various species and similar species. If you do a Google search for "exotic species regulated", in conjunction with a specific state's name, such as "Texas exotic species regulated", you will find the different states have information and links specific to their state regulations. The point is, the issue of exotic species and dangerous species is an important topic.
For example, in the case of the zebra mussel, it is required that boats be drained of water as they leave or approach public fresh water. Plus, these mussels can not be found clinging to the vessel. Other regulations cover the clinging of vegetation to the boat, or regulations on live bait. The only way to know the regulations for the particular state where you live or vacation is to search out the regulations of that particular state. People in tournaments are covered by special rules and documentation that the tournament organizers possess.
Another form of regulation concerns transporting live, non-game fish in specific geographic locations. In other words, certain geographical locations have specific rules covering only them. These will be better known by the locals, but if you are going into a certain location, it is worth searching out these regulations so as not to be in violation. The purpose of these regulations is to keep the native species alive and well, and keep the non-native species from overcoming the native species and destroying the natural ecosystem.