How do we know if a fish species is native or not?

Understanding which fish species are native to a particular watershed is important for development of conservation and fishery management plans. A species is considered native to an area if it naturally occurs there without human intervention. Conversely, species that are non-native to an area are transported from their native range either intentionally or unintentionally by humans or human activity. Non-native species introductions could occur due to physically moving organisms or removing natural barriers, which could permit colonization (Sea Lampreys colonizing upstream Great Lakes from Lake Ontario following completion of shipping channels around Niagara Falls is an example). Some readers have asked, How do we know if fish species are native or non-native to a particular water body? Although zoogeography (the study of the geographical distributions of animals) is not my area of expertise, I attempt to answer that question in this post.

A Redbreast Sunfish (Lepomis auritus) from the New River, Virginia where the species is non-native. The species has been introduced from its native range (mostly watersheds draining to the Atlantic Ocean) to systems flowing to the Missississippi River, including the New River and Tennessee River systems. Photo: Corbin Hilling

Determining native status is not always clear cut due to uncertainty in defining where a species naturally occurs due to introductions. Dr. Robert E. Jenkins, author of Freshwater Fishes of Virginia, stated that answers to zoogeographical questions “often constitute hypotheses, qualified conclusions, and predictions, some highly speculative.” Dr. Jenkins acknowledges here that we aren’t always sure how or why species exist where they do and sometimes don't know with certainty if species are native to a particular watershed. Zoogeography is a science and in any science we gather evidence to explain the natural world. In many cases, that body of evidence may have contradictory components or missing pieces of evidence. Zoogeographers attempt to piece together the history of the animals on Earth by pulling information from various fields including, biology, geology, literature, and archeology. I outline ten pieces of information below (in no particular order) that may help scientists assess native status with a focus on freshwater fishes in North America.

Early literature, including writings by European colonists such as Captain John Smith (above), provide insights into what aquatic communities may have been like prior to European colonization of North America. Photo: Public Domain.

1. Historic Biological Collections – Naturalists have gathered and cataloged the organisms of North America since Europeans arrived. Often our best information to assess native status comes from biological collections by early naturalists/scientists. A species' presence in one of these early expeditions provides support for native status. Early fish collections were not conducted with the “sophisticated” equipment aquatic scientists use today, so early surveys could miss species in remote waters or difficult to sample habitats (such as turbulent rivers). The timing of the first record for a species is also important. If a species was not collected during multiple intensive historic fish inventory expeditions, but is collected in modern times, scientists will likely question its native status.

2. Archeology – Support for native status can come from archeological sites. For instance, the native status of fishes can gain support (or not) based on the presence (or absence) of fish bones from a particular species in excavated stream-side Native Americans settlements.

3. Recent Population Trends – If scientists discover a new species in a particular stream, native status can be supported based on population size stability. If population size increases rapidly after discovery in a new river system, it would suggest the species is non-native (assuming there aren’t other detectable changes in the ecosystem). If the species remains rare, it could be a native species that naturally exists at low population levels in the system.

4. Range Expansions – A newly discovered species that rapidly expands its range post-discovery supports that it was an introduction. Presumably, if the species existed in the area previously, it would have been capable of colonizing available habitats.

5. Presence in Adjacent Watersheds – Native status could be supported or not based on the distance to other known native populations. This could provide support that an introduction was possible as fish could be kept alive long enough to move them from stream to stream. However, it could support that the species is close enough to disperse by natural means or the two streams were connected at some point in geological time.

6. Genetics – Genetics is a relatively new tool that can provide evidence for or against native status. For instance, Walleye have been heavily stocked throughout the US, often from parent fish originating from the Great Lakes. If Walleye genetics in a Virginia river are more similar to Great Lakes fish than native Virginia Walleye (native to Big Sandy and Tennessee River watersheds, maybe New River), it may support that Walleye were of stocked origin. We would expect populations located close together to be more genetically similar as the populations could have once been connected by ancient rivers that have since changed course.

7. Human Uses – Species that have value to humans may be more likely to draw suspicion as introductions. Many popular sportfish have been introduced to new systems, often before detailed sampling records were available. Consequently, the native range of Largemouth Bass is frequently in question due to humans introducing the species in the 1800s. In addition, the native range of many baitfish comes into question as species are relocated to enhance forage for sportfish or dumped from bait buckets.

8. Geological Processes – Geological processes change the landscape and connect or disconnect stream networks. For instance, a process called stream capture can occur where a stream erodes a bank that divides it from another stream. The two streams become connected and offers an opportunity for species to colonize. A stream capture event can help scientists understand otherwise puzzling distribution patterns. Click here for an example of stream capture.

9. Stocking Records – Ideally, natural resource management agencies have stocking records to help zoogeographers understand the sanctioned stocking history and distribution for a species. However, these can add confusion as common names vary by region and may not be specific enough to discern which species was actually stocked. For instance, “black bass” could be listed as a stocked species, but could refer to several species, including Largemouth, Spotted, or Smallmouth Bass.

10. Habitat Similarity – Habitat characteristics at a collection site could support an introduction if a fish is collected in unusual habitats for that species, but has never been collected in its preferred habitats up or downstream. For instance, a species that lives in fast, flowing rivers is collected for the first time in a lowland swamp. If the species has been collected in fast-moving water upstream it may be considered native, but would be suspected as non-native if absent from its preferred habitat in the watershed.

After assembling the available information (note that not everything above may be available and additional information not discussed here may be useful), zoogeographers make the best possible determination on whether a species is native or not. In some cases, all the evidence points in one direction, but sometimes contradictory evidence can lead to heated debates among scientists. Based on the evidence available, the consensus is that Blue, Channel, and Flathead Catfish are all non-native to Chesapeake Bay, primarily because stockings or pond escapes are known for each species prior to population establishment. Although Channel Catfish are often considered naturalized in Chesapeake Bay tributaries, contention remains on how to manage more recently introduced Blue and Flathead Catfish.

Virginia Tech Scientists present Blue (left) and Flathead Catfish (right) from the tidal James River during a spring electrofishing survey. Both species are considered non-native to the James River watershed and the Chesapeake Bay watershed. Photo: Corbin Hilling