Experiences throughout the world have shown that a number of problems may arise following the introduction of a new species. The impact of introductions has been examined in detail in many individual countries (see for example Rosenthal, 1976; Contreras and Escalante, 1984; Taylor et al., 1984; Baigun and Quiros, 1985; Eldredge, 1986) who identify the following principle negative effects.
Degradation of host environment
The effects of introductions of aquatic organisms on the environment are frequently surprising especially as the new species may adopt a niche that differs completely from that occupied in its native range. The most striking example of such behaviour is the penetration of a Micronesian marine atoll by Oreochromis mossambicus (Lobel, 1980). This species is normally adapted to moderate salinities as it inhabits estuaries in its normal habitat but its penetration into more saline areas was unexpected. Similar flexibility has also enabled other species of cichlid to expand outside of temperature ranges that would normally be limiting. Thus, in the Southern United States, species that are normally tropical have established themselves in sub-tropical or even warm temperate regimes (Noble, 1980).
Unexpected shifts in habitat of these types are not necessarily detrimental to the environment. Indeed they may result in occupancy of waters which would otherwise remain unoccupied due to the inadequacy of local species. However, more severe effects have been recorded in some cases. For example the common carp, through its habit of rooting around in the bottom, has the reputation of muddying the waters it occupies. This shades out macrophytes, chokes benthic invertebrates, and through the more rapid recycling of phosphate contributes to accelerated eutrophication. As a result the composition and abundance of the native fish fauna has been altered, as in India, where species of the genus Schizothorax have disappeared from waters to which carp had been introduced, together with the fisheries based on them (Jhingran and Sehgal, 1978).
These effects are rarely noted in areas where the carp has been established for some time, as for example in Europe, and McClaren (1980) concluded that such effects were correlated with the excessive numbers of the species that develop shortly after Introduction. Moreover, population expansion by this and other lowland species often follows upon modification of the environment by man usually through dam construction. In these cases native rheophilic species would probably not be able to survive the changed conditions even in the absence of better adapted competitors (Minckley and Mefee, 1987).
Ctenopharyngodon idella has been introduced into many areas of the world with the intention of eliminating submersed and emergent vegetation. It usually performs this task adequately but, through selective feeding on more tender species may favourize the development of tougher vegetation which is even more of a nuisance. Furthermore, the removal of submersed vegetation which forms the spawning substrate of phytophilous fishes may cause a reduction in numbers of such species. Concern has also been expressed in several areas over the elimination of weed beds which form the principal protective refugia of young fish and certain amphibia, and the feeding habitat of waterfowl.
Other examples of modification to the habitat are produced by burrowing forms such as Procambarus clarkii and Eriocheir sinensis, both of which are judged pests due to their habit of constructing galleries in levees and pond banks.
Disruption of host community
Fish assemblages receiving an introduction may be altered through competition between the new and the existing faunistic elements, by direct predation on the native species or by other aggressive effects.
Some introductions have proved so effective that the new species has been able to out compete existing fishes resulting in a considerable reduction in their populations or even in their complete disappearance. For example, in Tashkent in the U.S.S.R. a number of fishes including - Opsariichthys uncirostris; Hypseleotris swinhonis; Hemiculter leucisculus; Hemibarbus eigenmanni; Percottus glehni and Pseudorasbora parva - which were accidentally introduced together with Ctenopharyngodon idella, resulted in declines in local species through superior growth and fecundity (Rosenthal, 1976). Similarly, native species have been unable to compete with introduced tilapiine cichlids in some southern states in the United States of America (Noble, 1980), Cichlasona managuense displaced local predators in El Salvador, Poecilia reticulata has been blamed for the decline in numbers of local cyprinodonts in Uganda and also in the southwestern United States, and Lepomis auritus has supplanted Alburnus alborellus in some Italian oligotrophic lakes.
Competition is not United to trophic interactions but also to such other ecological limiting factors as breeding space, as in the case of Lake Victoria where Introduced Tilapia zillii and indigenous Oreochromis variabilis shared the same nursery habitats to the detriment of the endemic species (Welcomme, 1967). Aggressive behaviour on the part of the introduced species can also alter distribution patterns of native fishes as shown by studies on interactions between brown trout and indigenous salmonids in the U.S. (Taylor et al. 1984).
One of the recurrent themes in reports of the impact of introductions is the elimination by predation of local species. These are sometime localized in distribution and sufficiently rare as to cause concern for possible extinctions. Recently the introduction and spread of Nile perch (Lates niloticus) in Lake Victoria has attracted particular attention. This predatory species has possibly led to the extinction of over 300 species of haplochromine cichlid and is considered by many as a major disaster (Barel et al., 1985, Rybbink, 1987). While evidence from fishing does indicate that the Nile perch has eliminated many species it is too early to make a definitive evaluation of its impact. In an earlier case, where Cichla ocellaris was thought to have removed several species of fish from Gatun lake, Panama (Zaret and Paine, 1973), the supposedly eliminated species later recolonized the lake from refugia. Although the parallels are not exact some of the Lake Victoria species may have survived in a similar manner. Another case of severe disturbance to an endemic species flock was Lake Titicaca. Here Odontesthes bonariensis and Salmo gairdneri were implicated in the decline of stocks of native Orestias and Trichomycterus species. The precise mechanism for the decline was obscured by the possibility of predation, heavy fishing pressure on both introduced and native stocks, competition for food as the diet of introduced and native species is similar. Some native species have declined or disappeared, particularly Orestias cuvieri which was described by Tchernavin (1944-and is now no longer caught. Orestias pentlandi, Trichomycterus rivulatus, and T. dispar have also become extremely rare (Hanek, 1982), however, Orestias species still make up 942 of the 6 000 tons of fish caught in the lake.
Rainbow trout have been implicated in the disappearance or decline of small native species in many countries. In addition to the effects on stocks in Lake Titicaca they have been implicated in the disappearance of other Andean Orestias and Trichomycterus species in Colombia and Chile. They have also been involved in the reduction of native salmonid populations in Lake Ohrid, Yugoslavia (Nijssen and de Groot, 1974), Schizothorax species in Himalayan rivers, Oreodaimon gnathlambae in Lesotho, Trachyistoma euronotus and Sandelia capensis in South Africa (Jackson, 1960), Protroctes oxyrhynchus and Galaxias gracilis in New Zealand (McDowall, 1984), and other galaxiids in Australia. Although brown trout have not acquired the same reputation as rainbows for the elimination of native species McDowall (1984) describes the disappearance of Galaxias divergens and G. argenteus from waters in which S. trutta had been introduced.
Another predator that has been blamed for the disappearance of local species is Micropterus salmoides. This is said to have caused the decline of Alburnus alborellus, Esox lucius, Perca fluviatilis and introduced Lepomis species in some Italian lakes, although the disappearance of perch and pike may have been through competition. The species is also blamed for the destruction of local fish stocks in Lakes Atitlan and Calderas in Guatemala; stocks of Chirostoma ester, several species of Goodeidae and other native species in Lago de Patzcuaro and other lakes in Mexico; several species of cichlid, themselves introduced into Lake Naivasha, Kenya, and a riverine catfish Amphilius platychir in some Zimbabwean waters. The effects of predation by M. salmoides have possibly been avoided in some other water bodies by the contemporaneous introduction of forage fish such as Lepomis species.
Feeding by large carnivorous species on adult prey is not the only type of predatory interaction. Many smaller species can prey on juvenile fishes and some cyprinodonts, particularly Gambusia affinis are said to feed on the eggs of other species. Although the impact of this does not seem to have been quantified, nor any cases of damage to a specific species cited, concern was expressed by many correspondents as to possible damage arising from this habit.
The quality of the stock of fish in any water body can also deteriorate through stunting by the introduced species. Stunting is a process whereby the population of a species expands rapidly, producing large numbers of individuals which mature and breed at a much reduced size. This behaviour appears in natural waters and in aquaculture ponds and considerably diminishes the usefulness of the population for sport or commercial purposes. Fish species which have been reported as producing stunted populations include:- Alburnus alburnus; Carassius auratus; Lepomis cyanellus; L. gibbosus; L. macrochirus; Oreochromis mossambicus; O. niloticus; Perca fluviatilis and Tilapia rendalli. Stunting is not confined to fish species as two crustaceans, Macrobrachium amazonicum and M. rosenbergii also show this behaviour.
It is this aspect of several of the tilapiine cichlids which has most seriously limited their usefulness for establishing fishable populations in natural waters and for aquaculture. Stunted populations also tend to suffocate already established species by denying them living space and, in extreme cases, by creating oxygen deficient conditions in the water. They also prevent introduction of new, more suitable species for the same reasons. Oreochromis mossambicus has been the most troublesome in this respect and the elimination of this species has been attempted in many areas, generally without success.
Genetic degradation of host stock
Ecological studies of the racial composition of fish in large water bodies such as the Great Lakes of North America point to the considerable local adaptation to local conditions of the various stocks within any one lake. Because of this newly introduced races or stocks of fish are more likely to reduce than to enhance genetic fitness of local stocks to their own environments. In such circumstances 'hybrid vigour' may not be an advantage for, although as is well known, intense inbreeding leads to loss of viability in any breeding line, this is unlikely to occur under natural conditions where the brood stock contains a number of parental lines. Hybridization or crossing of genetic lines may be effective in aquaculture where limited brood stocks can lead to loss of vigour, or where desirable strains can be produced by judicious mixing of characters.
Fishes are generally more plastic in their potential for interbreeding than are mammals. Thus fewer crosses between fish species result in sterility, and hybrids may be produced spontaneously and persist under natural conditions. Introduced individuals may thus interbreed with closely related native forms or with individuals of other introduced species especially where one of the introduced forms is rare relative to the other. Under the stresses involved in introduction normal behaviour patterns may be broken down and hybrids formed from species or even genera that co-exist naturally and which do not normally hybridize. One example reported by Maciolek (1984) was a cross between Micropterus salmoides and Lepomis macrochirus in at least two Hawaiian reservoirs. If the native stock, or one of the introduced stocks, has characteristics that are especially useful for fisheries or aquaculture these are apt to be diluted through such crosses and the purpose of the introduction negated.
In Africa and Latin America there has been a widespread breeding of different genetic strains, particularly of the tilapiine cichlids, which has already resulted in a considerable mixing of the gene pool (Wohlfarth and Hulata, 1981). Other genera of cichlid such as Cichlasoma hybridize with equal facility under wild conditions and other families have shown similar behaviour. For example, in the Cyprinidae Groot (1985) comments on the extensive intermingling of the many strains of common carp following on the introduction or transfer of different varieties. In the Salmonidae concern has been expressed in Europe on the movements of hatchery reared fish because individuals escaping into the wild carry with them genetic characteristics that are less adaptive than those of the wild stock. In the Poeciliidae the introduction of Xiphophorus helleri, X. maculatus and X. variatus into Mexico led to genetic swamping of the native X. couchianus which is now an endangered species. The uncertainty as to the origins and characteristics of local stocks following extensive hybridization of fish in the wild is particularly inconvenient for the aquaculturist who requires predictability of growth, yield and nutrient demand of the fish he rears.
Introduction of disease and parasites
One of the most persistent risks inherent with movements of living organisms around the world is that pathogens and parasites associated with the organisms be spread to new hosts in the receiving area. This is particularly important where the organisms are cultured and are thus concentrated in such a way as to increase their susceptibility to disease. Fish and crustacean species have proved no exception to this and many examples of outbreaks of new diseases or parasite infestations following introductions of new species are now on record.
In contrast to some environmental and populational impacts. Introductions do not have to be definitive for a disease to be introduced. The recent appearance of nematode parasites of the genus Anguillicola in Europe is an example of this. The parasite, which is endemic to Australian and Asiatic species of Anguilla, was introduced into Europe together with imported oriental eels intended for human consumption and not for stocking into natural waters. Nevertheless the nematode escaped and has since spread rapidly through the waters of Northern Europe. Another introduced disease, probably originating from ungutted Pacific salmon carcasses imported for consumption, is IHN (infectious haemato-poietic necrosis), which has only recently been recognized in France (Anon., 1987). These examples of introductions of diseases from fish never intended for introduction into natural waters indicate the potential dangers of transporting any biotic material over national frontiers.
Many of the diseases of salmonids that infect hatchery reared fish and have also established reservoirs in wild populations are introduced. Furunculosis appeared both in Europe and South America following the introduction of rainbow trout from Western North America (Snieszko, 1973). Similarly, infectious dropsy of cyprinids spread rapidly around continental Europe in the 1930's after carp were transferred from Yugoslavia without adequate sanitary controls. More recently, Michel et al. (1986) traced the sudden appearance of Yersinia ruckeri, the causative agent of enteric redmouth disease, to uncontrolled shipments of a North American minnow Pimephelas promelas for bait.
These examples are drawn mainly from Europe where the state of knowledge of fish hygiene is relatively advanced and controls are enforced. However these and other diseases could also have been imported into other parts of the world and so far remain unreported. Certainly a range of disease organisms and parasites are potentially transferable should adequate standards of hygiene and quarantine not be enforced (Shotts and Gratzek, 1984; Hoffman and Schubert,1984).
Impacts of introductions are not limited to biological and ecological effects but may also have socio-economic implications. This is particularly apparent when an introduced species which is not favoured for consumption displaces a more desirable species from a fishery. A typical example of this occurred in India where Oreochromis mossambicus were introduced into reservoirs which they colonized but were not accepted by local populations. This has proved to be a typical pattern with certain tilapiine cichlids whose poor flavour and small size has rendered them generally unpopular.
The introduction of Nile perch into lake Victoria emphasized another set of economic problems. In the first place the fishery on Lake Victoria had been based on small cichlids whose capture required small mesh gill nets. The need for the fishermen to re-equip with nets of the appropriate size to capture Nile perch exceeded the possibilities of many of the poorer fishermen and a shift from subsistence fisheries pursued by small scale fishermen to more commercial operations for export, financed from urban centres occurred. Furthermore the technology to process and consume the larger more oily fish posed considerable difficulties and has resulted in a degree of deforestation to provide firewood for processing. These changes may have been only transitional but have produced a marked shift in the rural economy of the region.
Aquaculture is a high risk industry and where local economies become dependent on it financial vulnerability is high. Disease is perhaps the major threat to intensive rearing operations but other problems arising from the genetic integrity of the stock may also be significant. Modern aquaculture depends heavily on exchange of genetic material through transfers and introductions, therefore especial surveillance is needed to ensure that risks are minimized.