MOTIVES FOR INTRODUCTIONS

The different motives for performing introductions are listed in Table 6. This listing is far from complete. There is a high percentage (23%) of introductions where the reasons were unknown. There are also some 10% of the recorded introductions where appearance in natural waters is listed as accidental. In these records the original motive for the introduction of the captive stock is not recorded. Furthermore, in many cases where an objective is defined for carrying out a particular introduction it is unclear. For example a reported motive such as 'stocking into reservoirs' conveys little information as to what the species is supposed to accomplish once it is in the water body. It might be supposed that the person responsible for planning the introduction was clearer in his own mind as to the reasons for the action, but a surprising number of introductions appear to have been performed for what today would be considered trivial motives. Among these, nostalgia of displaced peoples for familiar fauna to surround them would seem to rank fairly highly. Many of the earlier movements of species following colonization may have been made on this basis, even though they were seemingly irrational in that adequate local species already existed or that the species was poorly adapted to its destination. Similarly, simple dietary preferences or sporting prejudices may have motivated movements of some species such as common carp or trout. In many cases introductions have been carried out for more than one motive thus the totals in Table 6 do not correspond to those of other tables.

The motives for which exotic fish species have been moved from country to country have changed with time (Table 7 and 8). To better illustrate these trends a difference has had to be drawn between the absolute number of introductions made for any purpose (Table 7) and the relative importance assigned to any motive in any decade as indicated by the percentage (Table 8).

Introductions made for aquaculture have been the most numerous. They have always comprised a significant proportion of the total at any time but have grown in importance in more recent decades. Since the beginning of the 1970's introductions made for this purpose have accounted for well over half of all introductions made. Sport fishing has provided the second major motive for introduction with a relatively constant number of introductions per decade. Relatively introductions for this purpose have declined in importance since the 1950's. Introductions made for the improvement of capture fisheries in lakes, reservoirs and rivers increased in number from 1950 to 1979, when they overtook sport fishing as the second most important motive. The use of fish species for control of unwanted organisms has a long history but most of the introductions made for this purpose have been concentrated into the decades from 1950 to 1979.

Table 6
Purposes for which species have been introduced

Purpose			No. of introductions
Aquaculture			495
Sport			191
Improvement of wild stocks			165
	To establish new food fisheries	45
	To fill a 'vacant niche'	33
	Stocking natural waters	28
	Forage for predators	21
	Restoration of fisheries	16
	Establish a wild stock	8
	Control stunted species	2
Accident			139
	Escape or release from aquaria	73
	Diffusion through water ways	20
	Escape or release of bait fish	11
	Introduced with other species	11
	Unspecified	10
	From aquaculture installations	9
	In ballast water of ships	4
	Live transport for consumption	1
Ornament			130
Control of unwanted organisms			82
	Aquatic vegetation	36
	Mosquitos	35
	Snails	5
	Blooms of phytoplankton	5
	Other fish	2
Dietary preference			1
Unknown: including introductions made by private persons			312

Three categories are somewhat more problematic. Most of the introductions of fish species for ornament have been made by private individuals or have resulted from escapes into the wild. The date at which the introduction occurred is frequently unknown and trends are difficult to identify. The date of introductions arising from various types of accident are likewise generally unknown, although there does appear to have been a slight trend for numbers of accidental introductions to have risen over the years although as a percentage they have declined considerably. Introductions which were made for motives which are unknown have also tended to rise numerically until the 1960's but as a percentage of all introductions they reached a peak in the 1940's and have since declined. The most substantial reduction in this category since 1970 indicates that introductions of exotic species have been more carefully documented in recent years.

Table 7
Changes in purpose of introductions expressed as numbers introduced by decade for each major category of use

Decade	Aquaculture	Sport	Fishery improvement	Ornament	Control	Accident	Unknown
1850	1			1		1	1
1860	2	5	1	1			1
1870	3	9	6	1		3	2
1880	10	7	3			3	5
1890	16	14	6	3		1	4
1900	16	16	10	4	2	1	5
1910	10	7	3		2	2	3
1920	8	14	6	2	5	12	8
1930	12	17	14	2	6	7	11
1940	15	15	8	1	5	4	12
1950	79	19	35	1	15	5	27
1960	100	18	25	4	23	22	27
1970	115	17	22	1	16	10	12
1980	43	4	5			3	6
Total	430	162	144	21	74	74	124

Table 8
Changes in purpose of introductions expressed as percentage within each decade for each major category of use

Decade	Aquaculture	Sport	Fishery improvement	Ornament	Control	Accident	Unknown
1850	25.0			25.0		25.0	25.0
1860	20.0	50.0	10.0	10.0			10.0
1870	12.5	37.5	25.0	4.2		12.5	8.3
1880	35.7	25.0	10.7			10.7	17.9
1890	36.4	31.8	13.6	6.8		2.3	9.1
1900	29.6	29.6	18.5	7.4	3.7	1.9	9.3
1910	37.0	25.9	11.1		7.4	7.4	11.1
1920	14.5	25.5	10.9	3.6	9.1	21.8	14.5
1930	17.4	24.6	20.3	2.9	8.7	10.1	15.9
1940	25.0	25.0	13.3	1.7	8.3	6.7	20.0
1950	43.6	10.5	19.3	0.6	8.3	2.8	14.9
1960	45.7	8.2	11.4	1.8	10.5	10.0	12.3
1970	59.6	8.8	11.4	0.5	8.3	5.2	6.2
1980	69.4	6.5	8.1	0.0	0.0	4.8	9.7

Aquaculture

Ninety eight species of fish have formed the subject of international introductions associated with aquaculture (Table 9). Many of these have been introduced into relatively few countries often for purposes described as experimental. Relatively few species have been widely distributed, only 10 species have been introduced into more than 10 countries and only 4 into more than 30.

There has been an historical trend in the species introduced (Table 10). Early international movements of species, up to 1900, mainly involved fresh water salmonids such as Salmo gairdneri, Salmo trutta and various species of Salvelinus which were introduced into temperate regions for aquaculture associated with the maintenance of sport fisheries as well as for limited food production. More recently salmonid introductions have involve anadromous forms which are being used for mariculture in cages. Common carp reached their maximum popularity in the decades between 1910 and 1940 and were successively replaced by tilapias (1950 - 1979) and Chinese carps (1960 - 1980) as preferred species. Current trends point to an increasing number of crustacean species which are being introduced around the world for the rapidly expanding brackish water shrimp culture.

Table 9
List of species introduced for aquaculture and the number of countries receiving them, listed in numerical order

Species	Number of countries
37	1
15	2
8	3
7	4
5	5
6	6
3	7
1	9
Ictalurus punctatus	11
Micropterus salmoides	12
Oreochromis aureus; Oreochromis macrochir	13
Oreochromis hornorum	14
Tilapia rendalli	15
Aristichthys nobilis; Hypophthalmichthys molitrix	21
Oreochromis niloticus	26
Oreochromis mossambicus	31
Ctenopharyngodon idella	35
Cyprinus carpio	41
Salmo gairdneri	44

Introduced species have played a major role in the development of aquaculture across the world. The early expansion of aquaculture in Europe through the monastic tradition was based on a species, Cyprinus carpio, that was exotic to most of the countries involved. Subsequently, commercial intensive culture of rainbow trout in Europe, cage culture for Atlantic and Pacific salmon, tropical culture of tilapia species in Brazil, Thailand, Cuba, Sri Lanka, Chinese carp culture in South East Asia and brackish water shrimp culture have all been based on species introduced from elsewhere. The failure to deploy what are often judged equally desirable local species for aquaculture has often been cited as a criticism for this policy. However, there is a well known trend for food producers to cut risk by using technologies that are already established. This would explain the relatively narrow selection of species used in contemporary aquaculture. There is now a tendency to explore the potential of local species, such as Colossoma in Latin America, which, should they prove successful for culture, may well set of a new round of movements involving such species.

Table 10
Percentage by major species groups introduced for aquaculture during each decade since 1850

Decade	Salmonids	Common carp	Tilapia	Chinese carp	Crustacea	Others
1850						100.0
1860	50.0					50.0
1870	33.3	33.3				33.3
1880	75.0	8.3			8.3	8.3
1890	84.6	7.7				7.7
1900	50.0				10.0	40.0
1910	20.0	30.0		40.0		10.0
1920	30.0	20.0				50.0
1930	16.7	25.0	8.3			50.0
1940	20.0	20.0	26.6			33.3
1950	8.9	7.6	53.8	2.5		26.9
1960	7.7	8.7	33.9	32.0	2.9	14.6
1970	5.5	5.5	33.0	22.0	7.3	26.6
1980	7.8		21.0	7.8	21.0	42.1

Species originally introduced for aquaculture eventually escape from the confinement of their ponds often but not always to colonize natural waters. Therefore any introduction made for aquaculture must be thought of as a potential addition to the wild fauna in the receiving country.

Management of Inland Waters

The second category of introductions are those aiming at the manipulation of wild, or modified stocks in natural water bodies. Stocks of this kind are used for sport or recreational fishing and for a variety of food fisheries ranging from subsistence to fully commercial. The two major categories are frequently difficult to differentiate and the same stock may also be exploited by both sport and food fishermen. Most correspondents reporting introductions were clear in listing 'angling' or 'sport' as reasons for introductions but difficulties arise in categorizing motives such as introduction of forage species for predators introduced for recreational purposes. Here only introductions listed as having been made for sport are placed in that category.

Sport

Seventy eight species have been recorded as having been introduced for sport (Table 11). A great number of these are either salmonids or larger predators having the fighting qualities sought by sports fishermen. Furthermore sports fishermen show two distinct tendencies. There are those which are attached to some traditional target species, particularly salmonid fishermen. This motive was the prime driving force behind the widespread dissemination of rainbow and brown trouts throughout the world in the earlier decades of this century (1900 -1930). It also accounts for the spread of centrarchid species between 1930 and 1950. There are also those that seek diversity and a number of species have been introduced to increase the variety of fish available to the recreational fisherman.

Many of the introductions made originally for sport fisheries have subsequently been adopted for aquaculture or have formed the basis of stocks which are exploited by subsistence or commercial fishermen.

Table 11
List of species introduced for sport fisheries and the number of countries receiving them, listed in numerical order

Species	Number of countries
28	1
10	2
8	3
1	4
Salmo salar	6
Micropterus dolomieui	7
Salvelinus fontinalis	15
Salmo trutta	20
Micropterus salmoides	25
Salmo gairdneri	42

Improvement of wild stocks

From Table 6 it can be seen that a range of motives are cited for introducing fish species for the improvement of wild stocks. The major motivation, however, is to introduce some element that is perceived as lacking to the fauna of a water body. This is usually termed 'to fill a vacant niche' or some variant of it. Although not strictly in line with the niche concept, which sees the niche as a property of the organism, the idea of a vacant niche is used to describe the perception that there are resources within a water body which are not being used efficiently for lack of a suitable species. This applies particularly in faunistically poor regions such as cool temperate areas affected by glaciation during the ice ages, islands east of the Wallace line or high altitude mountain lakes. Not surprisingly most of the introductions made for this motive have been made into countries having these types of water. It also applies in new habitats such as reservoirs or regulated rivers, where the indigenous fauna lacks elements competent to establish themselves in the new water body. A good example of this latter is the introduction of tilapiine cichlids into reservoirs advocated by Fernando and Holcik (1982). Allied to the above motive are those introductions made to replace faunistic elements lost through environmental change. The introduction of new faunistic elements into the Great Lakes of North America was undertaken for this reason. Extensive introductions of new crayfish species into European waters have also been made in an attempt to reconstitute stocks after the native species were virtually wiped out by disease.

Often the introduction of one species produces the need to introduce further species. Following introduction of major predators into fish communities which are not adapted to heavy predation the decline in native species is such that it has often been assumed to be necessary to introduce a forage species more closely adapted to survive alongside the predator. Classic examples of this are the introductions of Lepomis species to mitigate the impacts of black bass introductions, the use of tilapias or Cichlasoma as forage for Cichla ocellaris and the introduction of Bairdiella icistia to provide a prey species for Cynoscion xanthulus in the Salton Sea. Conversely, predatory species have been introduced into many areas to control stunted populations usually of tilapias and sunfishes. A further example of this type of serial introduction occurs where herbivorous fishes such as Ctenopharyngodon idella or common carp, which exert an eutrophicating influence on the water, convert primary producers from macrophytic to phytoplanktonic organisms. The resulting algal blooms can achieve nuisance proportions and Hypophthalmichthys molitrix has been introduced to correct for this. These types of complementary introduction have been formalized in aquaculture as polyculture systems and further introductions can be anticipated as knowledge of the eco-manipulation of small water bodies for extensive culture increases.

Several successes have followed the introduction of species as a foundation for fisheries, although negative effects have also been produced. The introduction of the pelagic clupeid Stolothrissa tanganyikae into Lakes Kivu and Kariba and its accidental diffusion downstream to Lake Cahora Bassa have led to the establishment of substantial stocks of fish. These form the basis of important fisheries which total some 20 000 tons annually. Other examples of important local fisheries which have been established through introductions are: tilapias (principally Oreochromis niloticus) in lakes and reservoirs in South East Asia and Latin America, Cichlasoma and Micropterus species in Central American lakes, Odontesthes bonariensis and Salmo gairdneri in Lake Titicaca and Punt Jus gonionotus in rivers of Philippine and Indonesian islands.

Table 12
List of species introduced for improvement of commercial and subsistence fisheries and the number of countries receiving them, listed in numerical order

Species	Number of countries
38	1
17	2
54	34
Coregonus lavaretus; Micropterus macrochirus;
Micropterus dolomieui; Micropterus salmoides;
Oreochromis niloticus; Pacifastacus leniusculus	5
Cyprinus carpio; Lepomis cyanellus	6
Hypophthalmichthys molitrix; Oncorhynchus tshawytscha	7

One minor but possibly growing motive for introductions is to protect a species which is endangered within its own habitat. Several introductions of Hucho hucho have been made for this purpose in Europe as have introductions of Gila orcutti and G. bicolor in the United States and Canada. This type of introduction tends to be made more within national boundaries thus this list is somewhat limited.

Ornament

The records for introductions of ornamental species are almost certainly incomplete. This category may be divided into two groups. Firstly large species introduced for stocking into natural waters or ornamental ponds. The main Species used for this purpose is the goldfish Carassius auratus auratus which has achieved a wide distribution by reason of its popularity for rearing in ornamental ponds. Another sub-species Carassius auratus gibelio has been introduced into some countries for food fisheries.

The second group consist of small, usually tropical species, that have been disseminated around the world by the aquarium fish trade. These species which number several thousand (see Conroy, 1975; UNCTAD/GATT, 1979) are a problem because they are rarely recorded as introduced. It is generally assumed that fish of this type can not survive in temperate climates and thus do not constitute valid introductions. This point of view is largely correct, although such species have maintained populations for many years in artificial warn waters such as discharge canals from power stations and in naturally warned waters such as hot springs. The diverse fauna recorded from the hot springs of the Banff National Park, Alberta, Canada are evidence to the persistence of such populations. In the tropics aquarium fish are either used by aquarists or are cultured in hatcheries for export. Ornamental fish species from all over the tropical world are cultured in Brazil, Colombia, Florida (U.S.A.), Peru and Singapore, and the practice is probably more widespread than these reports indicate. Escapes from rearing installations are common and are probably responsible for the large number of small exotics reported from Colombia and Florida. However, it would appear that in most cases such escapes have gone unreported or even unnoticed and that the penetration by small species into tropical ecosystems is probably much greater than official statistics would indicate. So far only 38 species have been recorded as having been introduced for ornamental purposes and most of these usually form part of the aquarium fish trade. Detailed analysis of the waters adjacent to rearing installations would doubtless increase this number considerably.

Table 13
List of species introduced for ornament and the number of countries receiving them, listed in numerical order

Species	Number of countries
32	1
4	2
1	3
Carassius auratus	17

Control

Fish have been used for many years to control undesirable organisms in the aquatic environment. An extensive literature exists on the various aspects of such control and a joint WHO/FAO/UNEP panel of experts meets to consider and evaluate the effectiveness of the various species. Reviews by this panel, for instance that of Petr (1987) summarize the literature available. Most of the earlier introductions for this purpose were made with the intention of controlling mosquitos and other vectors of water borne diseases but later control of aquatic vegetation became more important.

Mosquitos

Twelve species have been introduced for mosquito control of which 9 were introduced to one country only. More extensive introductions have been made with:

Poecilia latipinna introduced into 3 countries
Poecilia reticulata introduced into 4 countries
Gambusia affinis introduced into 21 countries

Control of mosquito larvae by small larvivorous fish species is highly effective and is replacing the more costly and environmentally dangerous control with insecticides. The introduced Gambusia is now being replaced by local species in many areas. Further dissemination of Gambusia is thus limited and a large number of small, native larvivorous fishes are being investigated for this role. An extensive literature exists on this topic, see for instance Gerberich and Laird (1968) and Haas (1984).

Snails

The efficiency of fish in controlling the snail vectors of schistosomiasis is less well known. In large reservoirs it appears to be almost ineffective despite the presence of numerous molluscivorous species. Greater success is claimed for more limited aquatic environments and in fish ponds Jhingran and Gopalakrishnan estimated a 64 - 98% reduction in snail populations using Astatoreochromis alluaudi. Currently the value of this species and a second, Haplochromis mellandi, is being made by the FAO African Aquaculture Development and Coordination Project. Astatoreochromis alluaudi has been introduced into only 5 African countries so far to control snail vectors of schistosomiasis but if trials prove successful the species may receive a much wider dissemination. Mylopharyngodon piceus has also been introduced into reservoirs in Israel to control gastropods without any need for control of schistosomiasis vectors.

Aquatic vegetation

Numerous species of fish eat aquatic weeds and a range of specialized phytophagous habits have been described which could be used to construct complex herbivorous communities (Shireman, 1984). Up till now this potential remains largely unexploited and only a few of fish have been introduced specifically to control aquatic vegetation although a number of other species may utilize this food source incidentally. Specifically, the following four species were recorded as having been introduced to control aquatic plants:

Oreochromis mossambicus introduced into 2 countries;
Tilapia zillii introduced into 2 countries;
Tilapia rendalli introduced into 4 countries;
Ctenopharyngodon idella introduced into 28 countries.

Grass carp Ctenopharyngodon idella have been successfully used for control of aquatic macrophytes in many countries and probably provides the only practical method for control where the use of herbicides is overly costly or undesirable (Negonovskaya, 1981; Shireman and Smith, 1983; Shireman,1984). This species normally has the advantage that it does not breed in the waters to which is introduced thus permitting periodic adjustments in the number and sizes of fish present. It is also considered a valuable edible species in many areas of the world and can play a useful secondary role in food supply. The use of this and other species for the control of a range of aquatic weeds is reviewed by Petr (1987a).

Blooms of phytoplankton

Hypophthalmichthys molitrix has been introduced into five countries to control heavy blooms of phytoplankton in natural waters.

Accident

About 10 percent of introductions into natural waters were unplanned and resulted from some form of accident or private initiative. It is likely that this proportion is even higher as many of the introductions whose motives are reported as unknown may also lie within this category. This relatively large percentage of unintentional introductions illustrates the difficulty of containing an introduced species within limited environments such as ponds or aquaria and support the principle that a specie must be regarded as introduced into a country once a breeding pair have crossed the national boundary. Other sources of new species are, the introduction of fry of non-target species along with those of a species whose introduction was intended, the escape or release of bait fish, and the transportation of eggs, juvenile or adult fish in the ballast water of ships. This last phenomenon is common in marine and brackish water species (see Carlton.1985) but some freshwater species appear to have been transported in this way. Once established in one country some species have diffused through freshwater systems in different parts of the world. For example carp moved naturally down the Uruguay river from Brazil to invade Uruguay and Argentina, the mitten crab diffused from an original site in West Germany to occupy much of Northern Europe and Stolothrissa migrated down the Zambezi river from Lake Kariba to establish itself in Lake Cahora Bassa. This mobility of species once they have been introduced illustrates the need for international agreement to limit the spread of undesirable species and to reduce risks of introduction of ones that are judged to be useful.