Introductions have led to major changes in aquatic communities

The introduction and transfer of exotic species of fish have led to major changes in aquatic¾ and human¾ communities and represent a significant threat to aquatic biodiversity. On the other hand, the utilization of exotic species has also resulted in increased production from the aquatic sector, a noted success story being the introduction of the freshwater sardine Limnothrissa miodon into the newly created Lake Kariba, Zimbabwe. The INTRODUCTIONS table is only concerned with movements of aquatic species across international borders. Although within country movements are omitted here, these are also important and hence should be monitored and regulated by national authorities.

In the early 1980s, Robin Welcomme of FAO began to assemble a database documenting the movement of inland fish species between countries (Welcomme 1988; FAO 1997). In 1991, he offered this database for distribution through FishBase. The database on international introductions and transfers has since been expanded through close collaboration between Devin Bartley of FAO and the FishBase staff to also cover marine fish transfers and introductions (based mainly on Walford and Wicklund 1973) and to include non-intentional introductions, such as those resulting from the opening of the Suez Canal and the ensuing Lessepsian migrations (Por 1978).

The current database is an updated version of the original of Welcomme (1988), edited to correspond to current taxonomy and new information. New records were derived from a search of the literature and information was retrieved from FAO questionnaires that were translated into the working languages of the United Nations and distributed internationally to national resource agencies, agriculture-related ministries, scientific institutions and national agriculture research centers. The questionnaires were modelled after the format of the INTRODUCTIONS table of FishBase so that information would be compatible. A listing of each country’s introduced fishes that were already included from Welcomme (1988) was included with the questionnaire so that old information could be checked and new information could be added in the new format.

The 'Top Ten' of introduced fishes

The current database can be analyzed either by predefined or user-specified queries to provide both summary statistics and scientific aspects relating to introductions. There are now over 2,900 records of 530 species from 101 families. The ten species of fish most often introduced or transferred are (in decreasing order): Cyprinus carpio, Oncorhynchus mykiss, Oreochromis mossambicus, Ctenopharyngdon idella, Oreochromis niloticus, Hypophthalmichthys molitrix, Micropterus salmoides, Gambusia affinis, Hypopthalmichthys nobilis, and Carassius auratus. Aquaculture was the most often cited reason for an introduction and national governments were the group most often responsible for an initial introduction. The number of introductions by area (continents), or by reasons can be seen through cumulative graphs such as Fig. 10, inspired from Ruesink et al. (1995).

The 'Top Twenty' of potential pests

Introduced species have been recognized as one of the most effective fishery management tools for increasing production from inland waters (Coates 1995), but they also have been recognized as one of the most significant threats to native aquatic biological diversity (IMO 1994; ICES 1995; FAO 1995, 1996). A list of ‘Adverse introductions’ is available under Reports, Miscellaneous, and under ‘Information by Topic’ on the Internet.

Introductions can be reported on-line

The database still contains many gaps and missing information, especially on the impacts of an introduction, and we acknowledge that the records, especially those derived from the questionnaire may be a biased account of international introductions. If an introduction failed immediately or did not have any significant impact it may have been simply forgotten and not reported. Therefore, in assessing impacts and percent establishment, we should not forget that absence of evidence is not evidence of absence. Users of the database with information on new introductions/transfers or with more complete information on existing records are requested to contact the authors. A version of the INTRODUCTIONS table with an ‘input form’ to enter new data is now available on the Internet on the FAO Fisheries website at http://www/ statist/fisoft/dias/ mainpage.htm.


The INTRODUCTIONS table includes fields indicating from which country the species came, year of introduction, reason for the movement and impact.

From: Refers to the country or geographic area where the species originated. The UN name of the country and the FAO area are also given.

To: Refers to the country into which the species was introduced. The UN name of the country and the FAO area are also given.

Year: Refers to the year of introduction.

Range: Refers to the range of years of introduction.

Period: A multiple choice field which gives a wider range of years of introduction. The choices include: pre-18th century; 18th century; 19th century; 1900-1924; 1925-1949; 1950-1974; 1975-present; unknown.

Fig. 10. Cumulative number of international introductions of freshwater fishes, over time and by FAO inland areas. See Box 8 for discussion of this graph

Box 8. Chronology and success of freshwater introductions.

Fig. 10 shows the cumulative number of freshwater introductions to the different inland FAO areas over the years. The records with unknown dates of introduction were placed before the 18th century mark, together with the early introductions, not only to show the magnitude of these unknown introductions, but also to include them in counts of all introductions. As shown from the graph, Europe and the former USSR combined have the most freshwater introduction records and South America has the least. The graph also shows a steep rise in introductions to Asia from the 1960s to the 1980s, due to the expansion of Asian aquaculture.

Whether an introduced species will become established in the wild is an important concern that is often difficult to predict. Successful establishment will depend on the species’ biological characters and on the environment. To examine the hypothesis of Pimm (1989) that introduction success should be (positively) correlated with a fish’s maximum size, Pullin et al. (1997) plotted percentage of successful introduction, by species, against maximum length from the SPECIES table. The result was that for the overall dataset in FishBase, success rate is negatively correlated with maximum size.

Other factors may also be related to success rate, such as age at maturity, fecundity, mode of reproduction, temperature tolerances, or feeding strategy.


Pimm, S.L. 1989. Theories of predicting success and impact of introduced species, p. 335-367. In J.A. Drake and H.A. Mooney (eds.) Biological invasions: a global perspective. Scientific Committee on Problems of the Environment. John Wiley and Sons, Ltd. Chichester, U.K.

Pullin, R.S.V., M.L. Palomares, C.V. Casal, M.M. Dey and D. Pauly. 1997. Environmental impacts of tilapias, p. 554-570. In K. Fitzsimmons (ed.) Tilapia Aquaculture. Proceedings of the Fourth International Symposium on Tilapia in Aquaculture, Volume 2. Northeast Regional Agricultural Engineering Service (NRAES) Cooperative Extension, Ithaca, New York. 808 p.

Christine Casal and Devin Bartley

Some introduced species are maintained through continuous imports

Reason: A multiple-choice field that states the reason for the introduction. The choices include: aquaculture; fisheries, angling/sport; ornamental; mosquito control; snail control; weed control; phytoplankton control; other pest control; forage; bait; diffusion from introductions in neighboring countries; research; off-site preservation; to fill ecological niche; accidental (alone or together with other species); accidental with ships; Lessepsian migration; removal of natural barrier; other; unknown.

Other reason: Another multiple choice field which gives another or secondary reason for the introduction. The choices are similar to the reason field.

Introduced by: Another multiple-choice field referring to those responsible for introducing the species. The following choices are provided: government; international organization; private sector; individual; other introducer; unknown.

Established in the wild: Refers to whether the species is established in natural water bodies or reservoirs (yes; no; probably yes; probably no; unknown), either self-reproducing, continuous stocking, or both.

Established in aquaculture: Refers to whether the species is currently used in aquaculture (yes/no), and whether that use is wide or rare. Another field states whether the species requires assistance from farmers or breeders to reproduce in aquaculture systems, or whether it is maintained through continuous imports, such as Anguilla anguilla in Israel or Psetta maxima in Spain.

Significant ecological interactions: Refer to the presence of impacts of the introduction on the ecosystem: yes; no; probably yes; probably no; unknown. The available choices for the effects on the ecosystem include: beneficial; adverse; undecided. This refers to effects on the genetic structure, hybridization, stock size, community structure, survival, adaptive behavior, homing accuracy, migration patterns, disease resistance, etc.

Significant socioeconomic effects: Refer to the presence or absence of impacts on the socioeconomic system: see above list. The available choices for effects are: beneficial; adverse; undecided. These refer to effects on the fishing methods, catch per effort, fish consumption, work distribution (equity, gender), income, etc.

Remarks: This field accommodates additional information not found anywhere in the INTRODUCTIONS table. These include data on reintroductions and species that have been affected by the introductions, among others.


Two types of lists can be generated from the INTRODUCTIONS table:

  • a list of all countries or localities to which a given species was introduced, in chronological order (accessed through the Introductions button in Stocks Range window); and

  • a list of all species that have been introduced to a given country, with ancillary information (accessed through the Different Checklists by Country button in the Predefined Reports Menu).


If you click on the Map button in the ‘Introductions as Compiled by FAO window’, FishBase will generate a map that shows the native countries with small dark green boxes and the countries where they have been introduced marked with small light green boxes. Each introduction from one country to another is shown by a straight red line linking central locations in the two countries. Details on the introduction represented by a red line may be obtained by double-clicking on the the small light green boxes at the end of the line, which opens a small window with key information on the introduction.

Many aquarium fishes have established themselves in the wild

The INTRODUCTIONS table is, to our knowledge, the largest global database on international movements of fish by humans, including about 2,900 introductions and transfers of over 530 species which were moved for aquaculture (>1,000 records), angling/sport fishing (>200 records) and for the ornamental trade (>300 records). A large number have unknown reasons for the transfer (>400 records). Over half of the documented introductions have established themselves in the wild.

Note that the INTRODUCTIONS table includes records of the first introduction of a species into a country, but not those that may have followed. Species found in aquarium shops are not considered to be ‘introduced’ into a given country unless they subsequently escaped to and established themselves in the wild (as often happens).


Graphs can be accessed through the Environ. factors & biodiversity button in the graph menu. These are:

  • the cumulative number of freshwater introductions from pre-18th century to the present showing the FAO areas to where they have been introduced (see Fig. 10);

  • the cumulative number of marine introductions from pre-18th century to the present, showing the magnitude of Lessepsian introductions compared to all other marine introductions; and

  • the cumulative number of freshwater introductions from pre-18th century to the present showing the different reasons for the introductions.

The first two graphs can also be accessed from the INTRODUCTIONS table, highlighting individual species.


Information was derived from more than 150 references, e.g., Courtenay and Stauffer (1984), Silva (1989), Crossman (1991), Holcík (1991), Nelson and Eldredge (1991), Ogutu-Ohwayo (1991), Eldredge (1994), Thys van den Audenaerde (1994) and those mentioned elsewhere in this chapter.

Harald Rosenthal of the Marine Science Institute, Kiel, Germany also has a large database with annotated references of transfers of aquatic organisms. We intend to collaborate with him to make this database available through FishBase.

How to get there

Clicking on the Range button in the SPECIES window, then the Introductions button in the RANGE window will give a list of introductions and clicking on a particular item brings you to a specific INTRODUCTION record. Alternatively, select Species from the Main Menu, Topic in the SEARCH BY window, and Introductions in the SEARCH SPECIES BY TOPIC window will give you a list of species with introduction record(s). Double-clicking on a particular species brings you to the SPECIES window. The internal name of this table is INTRCASE table.


On the Internet, click on Introductions in the ‘More information’ section of the ‘Species Summary’ page. Alternatively, you can select a country and the Introductions radio button in the ‘Information by Country/Island’ section of the ‘Search FishBase’ page, to get a list of all species introduced in the respective country. Or you can select the Introductions radio button in the ‘Information by Topic’ section of this page to create a list of all fishes known to be introduced somewhere.


We thank Robin Welcomme of FAO for providing us with the original INTRO database. We thank former FishBase Team member Liza Agustin for her contributions to this table and to a previous version of this chapter.


Coates, D. 1995. Inland capture fisheries and enhancement: status, constraints, and prospects for food security. Thematic paper (KC/FI/95/TECH/3) presented to the Japan/FA) International Conference on Sustainable Contribution of Fisheries Food Security, Kyoto, Japan, December, 1995. 82 p.

Courtenay, Jr., W.R. and J.R. Stauffer, Jr., Editors. 1984. Distribution, biology, and management of exotic fishes. Johns Hopkins University Press, Baltimore. 430 p.

Crossman, E.J. 1991. Introduced freshwater fishes: a review of the North American perspective with emphasis on Canada. Can. J. Fish. Aquat. Sci. 48(Suppl. 1):46-57.

Eldredge, L.G. 1994. Perspectives in aquatic exotic species management in the Pacific Islands. Vol. 1. Introduction of commercially significant aquatic organisms to the Pacific Islands. South Pacific Commission, Noumea, New Caledonia. 127 p.

FAO. 1995. Precautionary approach to fisheries. Part 1. Guidelines on the precautionary approach to capture fisheries and species introductions. FAO Fish. Tech. Pap. 350(1). FAO, Rome. 52 p.

FAO. 1996. Precautionary approach to fisheries, Part 2. Scientific Papers. FAO Fish. Tech. Pap. 350(2). FAO, Rome. 219 p.

FAO. 1997. FAO database on introduced aquatic species. FAO, Rome.

Holcík, J. 1991. Fish introductions in Europe with particular reference to its central and eastern part. Can. J. Fish. Aquat. Sci. 48(Suppl. 1):13-23.

ICES. 1995. Code of practice on the introduction and transfer of marine organisms, 1994, p. 35-40. In FAO. Precautionary approach to fisheries. Part 1. Guidelines on the precautionary approach to capture fisheries and species introductions. FAO Fish. Tech. Pap. 350(1). FAO, Rome.

IMO. 1994. Guidelines for preventing the introduction of unwanted aquatic organisms and pathogens from ships’ ballast water and sediment discharges, p. 41-50. In FAO. Precautionary approach to fisheries. Part 1. Guidelines on the precautionary approach to capture fisheries and species introductions. FAO Fish. Tech. Pap. 350(1). FAO, Rome.

Nelson, S.G. and L.G. Eldredge. 1991. Distribution and status of introduced cichlid fishes of the genera Oreochromis and Tilapia in the Islands of the South Pacific and Micronesia. Asian Fish. Sci. 4:11-22.

Ogutu-Ohwayo, R. 1991. Fish introductions in Africa and some of their implications. Can. J. Fish. Aquat. Sci. 48(Suppl. 1):8-12.

Por, F.D. 1978. Lessepsian migration. Springer Verlag, Berlin, Heidelberg, and New York. 228 p.

Ruesink, J.L., I.M. Parker, M.J. Groom and P.M. Kareiva. 1995. Reducing the risk of nonindigenous species introductions. BioScience 45(7):465-477.

Silva, S.S. 1989. Exotic aquatic organisms in Asia. Proceedings of the Workshop on Introductions of Exotic Aquatic Organisms in Asia. Asian Fish. Soc. Spec. Publ. 3, Asian Fisheries Society, Manila, Philippines. 154 p.

Thys van den Audenaerde, D.F.E. 1994. Introduction of aquatic species into Zambia waters, and their importance for aquaculture and fisheries. Aquaculture for Local Community Development Programme, ALCOM Field Document No. 24, 29 p.

Walford, L. and R. Wicklund. 1973. Contribution to a world-wide inventory of exotic marine and anadromous organisms. FAO Fish. Tech. Pap. 121, 49 p.

Welcomme, R.L. 1988. International introductions of inland aquatic species. FAO Fish. Tech. Pap. 294, 318 p.

Christine Casal and Devin Bartley