TRENDS IN INTRODUCTIONS

A total of 1 354 introductions of 237 species into 140 countries are recorded in this document. These consist of records of the first introduction of any species into a country. Because many introductions of a species into any one country have been carried out on a repeated basis these represent only a part of those reported. Furthermore, these records do not include the sometimes extensive transport of a species within a country once it had been introduced.

Trends in time in the numbers of introductions are summarized in Table 1, from which it is apparent that the spread of exotic fish species around the world essentially dates from the middle of the nineteenth century. A few introductions, termed ancient or historical had occurred before this period. Typical of these is the history of the common carp which, perhaps, was spread to a limited extent by the Romans and, certainly, diffused throughout continental Europe in the 13 - 15th centuries along with monastic fish farming. Such movements, however, were isolated. Since 1850 the pattern of introductions has shown a steady increase, until 1960, when, at 41 introductions in one year, a peak in movements occurred.

After 1960 the rate of new introductions has apparently declined. This slackening in the rate of the international transport of species has several possible origins. Firstly, legislation in many areas of the world, which has arisen from a growing awareness of the possible negative consequences of species introductions, has slowed if not stopped the flow. Secondly, there may be a saturation effect whereby a species reaches a level where it has been introduced to all suitable recipient areas. These effects can be detected in Table 2 where the introduction rate of successive groups of species attains a peak and then diminishes relative to other groups.

Table 1
Number of Introductions per decade for which date of introduction is known

Date	Number of introductions	Date	Number of introductions
Ancient	7	1910 - 1919	28
1800-1850	13	1920 - 1929	59
1850-1859	4	1930 - 1939	65
1860-1869	9	1940 - 1949	53
1870-1879	20	1950 - 1959	166
1880-1889	33	1960 - 1969	200
1890-1899	38	1970 - 1979	175
1900-1909	51	1980 - 1985	62

Table 2
Percentage by major species groups introduced during each decade since 1900

Decade	Salmonids	Centrarchids	Tilapias	Carps	Crustacea	Other
Pre 1900	45.9	9.9		2.7	0.9	40.5
1900	50.0	14.6		8.3	2.1	18.8
1910	15.4	7.7				76.9
1920	14.5	9.1			1.8	74.5
1930	21.9	21.9	1.6			54.7
1940	16.7	20.4	16.7	3.7	1.9	40.7
1950	11.8	15.5	46.6	1.2	6	24.2
1960	12.5	4.0	23.5	16.5	5.5	38.0
1970	9.8	2.4	7.3	28.0	2.4	50.0
1980	6.5		17.4	13.0	23.9	39.1
Total	18.6	9.4	16.4	10.3	3.3	41.7

The frequency of introduction of individual species varies considerably (Table 3) and the majority of species have only been introduced to few countries. Eighty six percent of the species that have been introduced have been recorded from 10 countries or less and 40% have been recorded from only one country. At the other extreme, 9 species have been introduced into more than 30 countries. Many of these species are very widespread throughout the world, and three in particular, Cyprinus carpio, Oreochromis mossambicus and Salmo gairdneri, each with over 50 host countries, have become pan-global within the limits set by their thermal tolerance.

A country or region may be also expected to attain a state where it has received all suitable species, and perhaps many that are not suitable. Such effects at the national or regional level are more difficult to detect because of the bias introduced by the large number of introductions that were carried out between 1950 and 1980.

Table 3
List of species and the number of countries into which they have been introduced listed in numerical order

Species	Number of countries
96 species	1
34 species	2
20 species	3
15 species	4
12 species	5
8 species	6
12 species	7
3 species	8
2 species	9
1 species	10
Oncorhynchus tshawytscha; Osphronemus gourami;
Procambarus clarkii; Salvelinus namaycush;
Xiphophorus helleri	11
Ictalurus melas	13
Lepomis cyanellus; Lepomis gibbosus;
Oreochromis macrochir	14
Micropterus dolomieui; Oreochromis hornorum;
Tinca tinca	15
Ictalurus punctatus; Tilapia zillii	16
Oreochromis aureus	17
Lepomis macrochirus	18
Poecilia reticulata	24
Aristichthys nobilis; Carassius auratus;
Tilapia rendalli	26
Salmo trutta	29
Gambusia affinis	32
Hypophthalmichthys molitrix	38
Oreochromis niloticus	38
Salvelinus fontinalis	39
Ctenopharyngodon idella; Micropterus salmoides	49
Cyprinus carpio	59
Oreochromis mossambicus	66
Salmo gairdneri	82

Table 4, which shows the number of species introduced into different continental areas each decade, gives no indication that this process occurred. On the contrary, the impression is that the rate of introductions was sustained in all areas until the 1960's and declined thereafter. Interesting details are that Europe had an early history of introduction in the later decades of the 19th century and maintained a high rate of introductions until the 1970's. Introductions into Africa started later and reached a peak in the 1950's. The rate of introductions into South and Central America on the other hand continued to increase until the present, and this region is now the major importer of fish species.

Table 4
Number of species introduced into different continents by decade

Decade	Africa	Asia/ USSR	Europe	Middle East	North America	Oceania	South America	Total
Ancient		1	4		1	1		7
Pre 1850		4	9					13
1850	2						2	4
1860	1		2			5	1	9
1870		3	9			6	2	20
1880		2	27		3		1	33
1890	2	1	27		4		4	38
1900	3	6	14		1	10	17	51
1910	6	11	5		1		5	28
1920	19	6	14	1	2	9	8	59
1930	16	11	14	1	1	5	17	65
1940	12	10	4	1		5	21	53
1950	67	21	11	1	9	13	44	166
1960	44	37	40	7	10	21	41	200
1970	35	20	43	1	2	3	71	175
1980	1	7	14		1	2	37	62
Total	208	140	237	12	35	60	271	983
Unknown	48	45	57	4	54	80	72	360
TOTAL	256	185	294	16	89	160	343	1 343
Species/ Country	5.0	8.4	10.5.	1.24	4.5	16.0	10.7	8.5

This Table provides information on the different intensities of introductions in various regions. North America, South and Central America, Europe and Oceania are all areas where the number of introductions per country is high relative to the mean, whereas in Africa and the Middle East this index is low and Asia is almost at the average level. This situation is confirmed when the analysis is carried out by individual country (Table 5).

Table 5
List of countries and the number of species they have received, listed in numerical order

Country	Species
29 countries	1
6 countries	2
14 countries	3
6 countries	4
15 countries	5
5 countries	6
4 countries	7
5 countries	8
10 countries	9
5 countries	10
Congo, Costa Rica, Korea, Thailand	11
Bolivia, Finland, Ivory Coast, Morocco	12
Hungary, Poland	13
Argentina, Denmark, Guam, India, Yugoslavia	14
Japan	15
Canada, Indonesia, Kenya	17
Zimbabwe	18
Dominican Republic, Fiji, Italy, New Zealand	19
Belgium, Brazil, France, Mauritius, Peru	20
Cyprus, South Africa, Soviet Union	21
Chile, Cuba	22
Germany (F.R.), Madagascar, Philippines, United Kingdom	23
Puerto Rico, Sri Lanka, Netherlands	24
Panama	29
Mexico	33
Colombia	40
Hawaii	44
Continental United States	70

The logic behind these intensities of introduction is sometimes unclear. The presence of many island states (irrespective of region) and of European countries in the list of major receivers of exotic species is understandable as both categories generally have impoverished fish faunas. It is difficult to explain the large number of introductions into the countries of North, Central and South America. Here there are numerous species capable of fulfilling most of the functions for which fish are usually introduced, and which have themselves been exported to other regions for these purposes. The rationale for these introductions clearly lies elsewhere.