Database Characterization

The Nonindigenous Aquatic Species (NAS) Database contains (as of April 01, 2024) 718,735 spatially-referenced biogeographic records of introductions and/or observations of 1,406 species of aquatic organisms, including fishes, mammals, reptiles, amphibians, invertebrates, and plants. Each specimen record within the database represents an individual observation of a nonindigenous aquatic species in the wild (e.g., angler catching a red-bellied pacu Piaractus brachypomus in Missouri) or documented release/introduction (e.g., intentional stocking of Brown Trout Salmo trutta by natural resource agencies for fisheries enhancement). Species introductions come in a variety of forms including accidental (e.g., transport in ballast tanks of shipping vessels; escape from pond or enclosure during flooding) and intentional releases, both authorized (e.g., stocking of fishes for sport) and illegal (e.g., dumping of aquaria into local waterways). Specimen records are derived from a variety of sources, including (but not limited to): scientific publications, news reports, museum collection databases, partnerships/data exchanges with other researchers or state/federal agencies, agency stocking reports, and direct reports of observations through a sighting report form.

The information and charts on this page were generated from specimen records originating within the United States between 1800 and 2023, and do not include Lionfish (Pterois volitans and P. miles) records from other countries or specimen records of species formerly tracked by the NAS Database (i.e., marine invertebrates).

Database overview

Figure 1 shows the number of individual specimen records, and the number of unique species comprising those records, recorded for each decade since 1800:

Increasing number of unique species and total specimen reports for each decade since 1800.

Figure 1

Although the NAS database contains specimen records dating back to 1790, the majority of records are reports of introductions within the last ~40 years. This does not indicate an increase in the rate of species introduction in recent years: species introduction, detection/observation of non-native species, and documentation of a non-native species occurrence are all independent events, and it is difficult to derive or estimate introduction rates from rates of observation or documentation. Increased numbers of reports within the last ~40 years could result from an increased awareness of impacts associated with non-native species and the growth of research in invasion biology, and/or increasing rates of communication of non-native species occurrences. Similarly, a low number of specimen records prior to ~1900 could be due difficulty in documenting historical records of species introductions and observations of non-native species (e.g., no available documentation or problems accessing literature sources; missing or ambiguous species or locality descriptions).

Temporal Accuracy of Specimen Records

During entry of specimen records into the NAS database, any temporal information contained within source material is used to identify or assign a date of observation/introduction to a record. Additionally, each record is scored on the degree of uncertainty in the assigned date (to year), using the following categories of temporal accuracy:

  • Actual: Record indicates actual year of introduction/observation.
  • Estimated: Actual year of introduction/observation is unknown, but approximate range is given. Year is estimated as midpoint of approximate range. For example, if a report indicates that a species was introduced “in the 1980s”, the year of introduction would be estimated as 1985.
  • Publication Year: No year given in record source. Publication date of record source is used as a proxy for introduction/observation year

Figure 2 shows the number of specimen records within the NAS database assigned to each class of temporal accuracy, for each decade. Figure 3 shows an expanded view of the temporal accuracy of specimen records in the NAS database with an introduction date within the last 50 years:

Records with 'Actual' year of introduction largest portion of records for each decade. Generally more 'Publication Year' records than 'Estimated', especially since 1930.

Figure 2

Graph of NAS specimen observation date accuracy from previous 50 years. Records with 'Actual' observation year generally most abundant, and those with 'Estimated' least abundant.

Figure 3

A majority of the specimen records within the database include the actual year of observation/introduction (i.e., when a species was collected or reported), including most records for observations within the last 50 years. Publication of books of state fish distributions (e.g., Rohde et al.’s Freshwater fishes of South Carolina), and fish stocking information released by state agencies (e.g., Washington Department of Fish and Wildlife’s Warmwater fishes of Washington) or collated by other sources (e.g., DeLorme state gazetteer for Arizona), containing many reports lacking an observation/introduction year resulted in large spikes in specimen records with a temporal accuracy of “Publication Year” in 1998-2001, 2005, and 2009.

Geographic Accuracy of Specimens

Specimen records within the NAS database are also classified by the degree of spatial uncertainty in the assigned geographic location associated with each specimen record. Spatial uncertainty is represented by three separate categories:

  • Accurate: Locality description indicates a specific site, and/or site is unique. Geographic coordinates can be easily inferred from locality description (e.g., Santa Fe River, 100 m downstream of boat ramp at US 27, High Springs, FL), or are directly provided by the source. No other water body with the same name exists in the state and/or county. Size of water body is small relative to dispersal capabilities of reported species.
  • Approximate: Locality description indicates a general area, and/or site is potentially ambiguous. Geographic coordinates cannot be easily inferred from locality description (e.g., near Gainesville, FL), or locality description only indicates the name of a large body of water (e.g., Lake Okeechobee).
  • Centroid: Geographic location is the calculated center point of a specified geographic polygon. Used for records indicating presence within or throughout a specific county or HUC (e.g., Alachua County, FL).

Figure 4 examines the geographic accuracy of specimen records within the NAS database. Numbers above the points indicate the number of specimen records in each category:

Geographic accuracy of NAS specimen records: 'Accurate': 73% (517918/707035), 'Approximate': 19% (133799/707035), 'Centroid': 8% (54741/707035), 'Other': 0% (577/707035).

Figure 4

Most records within the NAS database are assigned to a specific geographic location, due to either direct reporting of geographic coordinates or a robust description of the collection location that allows for geolocation. “Other” on this graph represents specimen records that do not fall into any of the three defined classes, and are primarily historic records or those that are extremely ambiguous in their geographic description, including some state non-specific records or records where geographic coordinates could not be assigned. We are currently working to assign a more specific class to these records.


The NAS reference database comprises all literature sources (including books, news stories, articles from scientific journals, agency reports, etc.) that are used as documentation for specimen records, as well as references containing supplementary information used to create species-specific fact sheets: written summaries of a species’ biology, ecology, current known native and introduced range, impacts, and management. Fact sheets exist for the majority of species within the NAS database, and are continually updated and expanded. Species-specific fact sheets can be found through a search page.

Figure 5 shows the distribution of references in the NAS reference database by publication date:

Number of references per year sporadic before ~1900, but generally increasing trend in number of references each year since.

Figure 5

A large increase in the number of references after ~1990 is due to the increasing importance and use of internet-associated sources (e.g., news websites, online state agency stocking reports, museum collection databases) of species occurrence data, and the increase of research efforts in invasion biology.


The data represented on this site vary in accuracy, scale, completeness, extent of coverage and origin. It is the user's responsibility to use these data consistent with their intended purpose and within stated limitations. We highly recommend reviewing metadata files prior to interpreting these data.

Citation information: U.S. Geological Survey. [2024]. Nonindigenous Aquatic Species Database. Gainesville, Florida. Accessed [4/22/2024].

Contact us if you are using data from this site for a publication to make sure the data are being used appropriately and for potential co-authorship if warranted.

For general information and questions about the database, contact Wesley Daniel. For problems and technical issues, contact Matthew Neilson.