A fish species is considered non-native to a location if it does not occur naturally there and only is present as a result of direct or unintentional human introduction. This does not necessarily mean that these fish cannot thrive in their non-native habitat. In fact, some fish, such as Sea Lamprey (Petromyzon marinus), can be in decline in their native range but can be considered invasive and even imperil native species in their non-native locations.
A fish species is considered native to a location if it occurs naturally there. It may have evolved in that region or dispersed and become established there without human assistance. For example, Lake Trout (Salvelinus namaycush) is native to some locations in North America but is considered non-native in other locations where it has been introduced by people.
Holomictic references the most common type of lake which turns over at least once per year (as opposed to meromictic lakes which are constantly stratified). This mixing is an important process for maintaining fish and aquatic communities by distributing nutrients and oxygen throughout the lake before stratification occurs again. There are four categories of holomictic lakes:
- Oligomictic: mixing is irregular,
- Monomictic: turnover occurs once a year (most common in polar areas)
- Dimictic: turnover occurs twice a year (most common in temperate areas), and
- Polymictic: frequent turnover (most common in tropical areas).
Meromictic references a type of lake which is constantly stratified. The surface and bottom waters do not ever mix. In most cases, the bottom layer has very low oxygen levels, where few fish and other organisms can live, restricting them to the surface layer. Meromictic lakes are uncommon (most lakes are holomictic and turnover at least once per year) and may be formed because they are:
- small and unusually deep, or
- the lower layer is denser and more saline than the surface layer.
Lotic refers to freshwater ecosystems involving flowing water, such as a river, stream, brook or creek. Certain fish species are lotic specialists and have evolved to live in higher flow water, such as darters which prefer to live in swift-moving riffles.
Lentic describes freshwater ecosystems characterized by still water and low flow, such as a lake or pond. Certain fish species have evolved to become lentic specialists, such as the high diversity of cichlid species found in Lake Victoria.
A scute is an external bony plate on the surface of a fish. Scutes serve a protective function, acting as a body armor for fish against environmental abrasions and even predation. In some fishes, such as Alewife (Alosa pseudoharengus), scutes are a row of scales modified into sharp, protective plates. In other fishes, like Shovelnose Sturgeon (Scaphirhynchus platorynchus), scutes serve a homologous function but are derived from ossified deposits in the dermis (essentially bone over skin).
Ampullae of Lorenzini are a network of electroreceptors, sensory organs that detect electric fields in water, found in chondrichthyes (sharks, rays, and chimaeras). The ampullae are a series of symmetrical pores, concentrated around the snout and nose, connected by gel-filled canals. They can conduct electrical impulses so small, that chondrichthyes are likely to be more sensitive to electric fields than any other group of animals. Because all muscle contractions produce a weak electrical field, these electroreceptors make sharks, rays, and chimaeras highly capable of detecting other organisms, such as prey, nearby in water.